Monday, November 3, 2014

The evolutionary history of walruses, part 5: what did tusks evolve for?

 This is the last text-heavy post in the walrus evolution series; one last post remains, which will be up in a few days. I've been busy with job applications and thesis writing, and finalizing and submitting three short manuscripts on various subjects.

Sexual dimorphism in the walrus - male in the background, female in the foreground. Photo from

Tusks in Odobenus rosmarus

Tusks in the modern walrus Odobenus rosmarus occur in both sexes, but are generally larger and longer in males – and like most other pinnipeds, they are polygynous (a single male mates with multiple females) and sexually dimorphic (males are larger than females). The walrus is restricted to the Arctic – and owing to this, tusks were usually assumed to have something to do with ice. For example, walruses tend to use their tusks to assist in hauling out onto ice, leading many to originally propose that tusks evolved for this purpose. Other workers erroneously identified tusks as being used for excavation of mollusks on the seafloor. However, observations by Francis Fay (1982) and Edward Miller (1975) indicate that a use in feeding or haul out behavior is unlikely. Miller (1975) studied aggressive behavior in male walruses, and observed that tusks perform a central role in male interactions. Most interactions consist of tusk threat displays – the aggressor leans his head back so that the tusks are horizontal and pointing toward the target. If the target is somewhat submissive the aggressor will perform a “stabbing” motion. In more aggressive interactions the aggressor strikes the target with the tusks using the same downward stabbing motion, typically striking the hindquarters, back, or neck. These strikes commonly draw blood but Miller (1975) doubted that many cause serious injury (similar to elephant seal combat). Tusks were also frequently used to parry strikes close to the face. Predictably, walruses preferentially threatened smaller males; perhaps more adorably, juvenile males that even lacked tusks performed play fighting that was similarly ritualized. Strikes tended to follow visual threats, and Miller (1975) indicated that ritualized aggressive behavior like this is fundamentally similar to that seen in sea lions, who perform visual displays (prior to striking) by similarly leaning back and opening the mouth to show the canines. Interestingly, the pattern of scarring is completely opposite to the pattern of observed tusk strikes: scarring is mostly present on the anterior neck region, and Miller (1975) attributed this to several reasons: 1) his observations were on land and 2) during the summer. He hypothesized that during the breeding season, more intense “face to face” combat on ice (or more likely, in the water, as some rare anecdotes suggest) is the origin of anterior scarring. So, the relatively violent behavior that Miller (1975) described is not even that which is known to cause the most scarring on walruses – which seems to suggest that walrus breeding behavior might be a bit terrifying and may give the elephant seal a run for its money.

Walrus tusk display and combat. Threat displays frequently prelude tusk strikes. Photo from

            Many earlier workers (see Fay, 1982: 134-135 and references therein) concluded that walruses dug prey items out of seafloor with their tusks, and this was based primarily on observations of tusk abrasion in dead animals. At least one early study suggested that walruses scraped the seafloor with their tusks in a posterior direction, but later revised to a side-to-side motion as no abrasion exists on the posterior side of the tusks. Some early reports did cast doubt upon these hypotheses, as occasional individuals were identified as lacking tusks but of otherwise healthy appearance. Fay’s (1982) classic study re-examined the abrasion patterns, and concluded that the primary direction of sediment-tusk interaction was from proximal to distal (e.g. base of tusk to tip), which indicates that tusks are passively dragged through the sediment during benthic foraging. Fay (1982) also indicated that tusks are frequently used for locomotion – including hauling out onto sea ice, and even during aquatic sleeping with the tusks hooked over the edge of an ice floe (like a swimmer resting at the edge of a pool). However, he suggested that these were secondary functions and that by far and away the most significant functions were all social in origin. He hypothesized that because all/most pinnipeds are polygynous, the capability for tusk development is probably universal among the group but extreme canine enlargement is probably only possible once a pinniped lineage has made the shift from piscivory (fish eating) to suction feeding. Notably, most toothed whales with tusks (beaked whales, narwhal, Odobenocetops) are all either known or inferred to be suction feeders.

 Abrasion of walrus tusks - figure from Fay (1982). Abrasion is focused on the anterior side of the tusk, indicating passive dragging of the tusks through sediment during foraging rather than active digging.

Temperate and Subtropical tusked walruses

Further eroding ice-related hypotheses for the evolution of tusks in walruses are discoveries of fossil walruses that inhabited drastically warmer waters than the extant Odobenus rosmarus. The earliest known temperate tusked walrus was Alachtherium, which for the past 130 years was known from Belgium, and in the late 1990’s was also reported from the northwestern coast of Africa (Geraads 1997). Subsequently, additional discoveries indicated more occurrences of Alachtherium from Japan and the eastern USA as far south as Florida, and records of the toothless odobenine walrus Valenictus from southern California and even Mexico (Deméré, 1994). Fossils of Valenictus from San Diego and the Imperial Desert indicated to Deméré (1994) that walrus tusks evolved long before walruses became ice-bound in the Arctic, and that tusks are thus “structures with history”.

 Life restoration of Odobenocetops by Smithsonian artist Mary Parrish.

The walrus-faced whale Odobenocetops: implications for tusk use

The 1990 discovery of a bizarre fossil mammal, named in 1993 by Christian de Muizon as Odobenocetops, led to a reinterpretation of tusk function in walruses. Odobenocetops was collected from late-Miocene strata of the Pisco Formation of Peru and initially accidentally misidentified as a walrus; I’ve been told that an early SVP abstract with this mistake can be found. LACM Curator Emeritus takes credit for setting the record straight and asking those involved “why does the skull have premaxillary sac fossae?” These fossae, for the uninitiated, are unique to odontocetes (toothed whales), and Muizon (1993) named it as a new genus and species in a new family, Odobenocetopsidae, which he and others (Muizon et al., 2002) considered to be a sister clade to the Monodontidae – the family that includes the beluga and narwhal (and the fossil belugas, Bohaskaia and Denebola). I won’t go into too much detail irrelevant to the tusks, but Odobenocetops only possesses two teeth: asymmetrical left and right tusks that are posteriorly directed and set into elongate, columnar alveolar processes, and exhibits a deeply concave palate. These features and their similarity with the modern walrus indicated a similar mode of feeding. However, the occurrence of similar tusks in a completely different type of marine mammal that independently evolved benthic suction feeding for mollusks begs the question: did tusks really evolve for social purposes? Muizon et al. (2002) conclude that the orientation of the tusks is a bit too coincidental, and that the alveolar processes likely behaved as “sled runners” to stabilize and properly orient the head of Odobenocetops as it trawled the ocean floor for molluscan prey. They conceded that the asymmetry of the tusks (the left tusk is barely erupted while the right tusk is very long – up to 1.35 meters in Odobenocetops leptodon; Muizon and Domning, 2002) indicates that such a function was not optimized in Odobenocetops, and it likely reflects a social function like the tusk of the narwhal.

Seafloor foraging of a walrus. From this paper by Levermann et al.

Speaking of tusked cetaceans… what the heck is the narwhal tusk for?

This is a bit of a convenient topic to tack on here; I’d like to revisit it in more detail in the future since some interesting papers have come out in recent years on the topic. The narwhal (Monodon monoceros) is also sexually dimorphic, and possesses a pair of tusks, generally only the left tusk erupts from the soft tissue. Rarely males will possess an erupted right tusk. Although formerly considered an incisor, recent CT studies indicate that the tusk is embedded entirely within the maxilla and is therefore the canine tooth; a series of other vestigial postcanine teeth also form (Nweeia et al. 2012) but rarely erupt from the skull or soft tissues (and are therefore detectable only using CT imaging). Sexual tusk dimorphism is a bit more extreme than in the walrus: only 15% of female narwhals ever possess tusks that erupt from the soft tissue, and the tusks are always smaller and shorter than those of males. Significantly, narwhals do not appear to be polygynous. The narwhal tusk is conspicuously “spiraled” (presumably for structural rigidity) and exhibits dentine tubules exposed on the surface of the tooth – which suggests some ability to sense water temperature and salinity (Nweeia et al. 2009). In contrast, in mammals that masticate their food the dentine tubules do not extend to the outer margin of the tooth; indeed, toothaches may be caused by dentine tubules being exposed to the oral environment when a cavity forms. Additionally, a pulp cavity extends along the entire length of the tusks. Field experiments which consisted of exposing a small section of tusk to high salinity solution resulted in rapid head movements and breathing in several different individuals. These observations lead Nweeia et al. (2009) to propose that the narwhal tusk fulfills a sensory function. 

Male and female narwhals underwater. There are surprisingly few underwater photos of narwhals, although this is generally true of most arctic marine mammals and I for one don't blame photographers: it's damned cold! Photo by Paul Nicklen, National Geographic.

However, the above arguments follow for the narwhal: the tusks are indeed dimorphic, and if these functions are not important for females (85% of females lack erupted tusks, making sensory functions useless for nearly half of the species), they probably do not reflect the main purpose of the tusk. The extreme sexual dimorphism strongly indicates a social role, and another recent study (Kelley et al. 2014) has found a strong correlation between narwhal tusk size and testes mass – confirming the sexual/social importance of tusks. More observations of tusk use in the narwhal is necessary, but males have been observed rubbing or slapping tusks together, and broken tips of tusks have been found embedded in other male narwhal heads (and, heads of belugas) – indirect evidence of narwhal combat. Similarly, underwater observations of walrus and narwhal behavior and combat are rare or lacking altogether. 

Adorable bonus photo (by Paul Nicklen, Nationa Geographic/Getty Images).
What about other walruses?

Thus far, almost all discussions of tusk evolution in walruses have either been confined to the modern species, or daresay even cetaceans like Odobenocetops. Obviously, the former is a necessary starting point, and the latter merits consideration – but, what about extinct walruses? The only serious consideration of tusk evolution using fossil walruses was Deméré (1994), who (as outlined above) remarked upon tusks in walruses (e.g. Valenictus) from temperate and subtropical latitudes. An important question that hasn’t really been asked before is: who had the first tusks? The answer is remarkably easy and quick: the dusignathine Gomphotaria pugnax, which is 2-3 million years older than the earliest known tusked odobenine fossils. Tusks in Gomphotaria are quite a bit different in morphology than modern Odobenus: the tusks are short and procumbent, lack globular dentine, and a smaller pair of lower tusks are present; similar double-tusks are seen in Dusignathus (particularly D. seftoni). There is some variation even amongst the odobenines: Protodobenus has thickened maxillae and large canine roots, but the emergent canine crowns are barely proportionally larger than that in a sea lion; tusks are absent in Aivukus, and short, curved, and procumbent (forward inclined) tusks are present in Alachtherium/Ontocetus and Valenictus (although somewhat longer but no less precumbent). Morgan Churchill and I discussed a few of these points in our paper on Pelagiarctos (Boessenecker and Churchill, 2013). This pattern tells us several things: 1) “Sled runner” tusk function would have only really been present in the modern walrus, as most earlier forms had somewhat procumbent tusks that would not have been aligned with the seafloor; 2) tusks do not really seem to be correlated with any subset of the marine environment, and association with ice likely reflects a relatively recent (e.g. Pleistocene) adaptation of Odobenus to high latitude environments; and 3) tusks evolved in several directions in the last 8 million years, which if anything signifies sexual selection and recalls horn and antler diversity amongst small clades of sexually dimorphic and selective ungulates.

The moral of the story is this: there is a difference between what a structure evolved for and what its current function(s) is/are; when walrus tusks first evolved, there was no extensive pack ice and walruses inhabited temperate and subtropical latitudes. The walrus tusk continues to serve an important role in social behavior, but has been used for other purposes (locomotion, sleeping) and is thus an exaptation of sorts. This point can be extended to the narwhal: simply because the narwhal tusk can be sensitive to salinity and temperature does not mean that it evolved for that purpose. In both cases the evidence of sexual dental dimorphism is the most significant, and the evidence rather overhwhelmingly supports a social or sexual origin of tusks in both Arctic species.


R. W. Boessenecker and M. Churchill. 2013. A Reevaluation of the Morphology, Paleoecology, and Phylogenetic Relationships of the Enigmatic Walrus Pelagiarctos. PLoS One 8(1):e5411.

Deméré, T.A. 1994. Two new species of fossil walruses (Pinnipedia: Odobenidae) from the upper Pliocene San Diego Formation. Proceedings of the San Diego Society of Natural History 29:77-98

Geraads, D. 1997. Carnivores du Pliocene terminal de Ahl al Oughlam (Casablanca, Maroc). Géobios 30(1):127-164

Fay, F.H. 1982. Ecology and biology of the Pacific walrus Odobenus rosmarus divergens Illiger. North American Fauna 74:1-279.

Kelley, T.C., Stewart, R.E.A., Yurkowski, D.J., Ryan, A., and Ferguson, S.H. 2014. Mating ecology of beluga (Delphinapterus leucas) and narwhal (Monodon monoceros) as estimated by reproductive tract metrics. Marine Mammal Science (Online early: DOI: 10.1111/mms.12165

Miller, E.H. 1975. Walrus ethology 1. The social role of tusks and applications of multidimensional scaling. Canadian Journal of Zoology 53: 590-613.

Muizon, C. de. 1993. Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru. Nature 365-745-748.

Muizon, C. de., and Domning, D.P. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134: 423-452.

Muizon, C. de., Domning, D.P., and Ketten, D. 2002. Odobenocetops peruvianus, the walrus-convergent delphinoid (Mammalia: Cetacea) from the early Pliocene of Peru. Smithsonian Contributions to Paleobiology 93: 223-261.

Nweeia, M.T., Eichmiller, F.C., Nutarak, C., Eidelman, N., Giuseppetti, A.A., Quinn, J., Mead, J.G., K’issuk, K., Hauschka, P.V., Tyler, E.M., Potter, C., Orr, J.R., Avike, R., Nielsen, P., and Angnatsiak, D. 2009. Considerations of anatomy, morphology, evolution, and function for the narwhal dentition. In Krupnik, I., Lang, M.A., and Miller, S.E. (editors), Smithsonian at the Poles: contributions to International Polar Year science. 223-240.

Nweeia, M.T., Eichmiller, F.C., Hauschka, P.V., Tyler, E., Mead, J.G., Potter, C.W., Angnatsiak, D.P., Richard, P.R., Orr, J.R., and Black, S.R. 2012. Vestigial tooth anatomy and tusk nomenclature for Monodon monoceros. The Anatomical Record 295:1006-1016.

Thursday, October 23, 2014

Paleontology "kickstarter": relationships of Allodesmus

A mount of the partial holotype skeleton of Allodesmus kelloggi, a junior synonym of Allodesmus kernensis (at the Natural History Museum of Los Angeles).

My colleague Reagan Furbish - a master's student under Dr. Annalisa Berta at San Diego State University, has started a kickstarter campaign to fund some of her master's thesis research. Reagan is studying the phylogenetic relationships of the fossil pinniped Allodesmus: controversy exists whether or not it was more closely related to true seals (Phocidae) or to sea lions (Otariidae) and walruses (Otarioidea). She needs to visit several museums including the American Museum of Natural History in New York, the Smithsonian Institution in Washington D.C., and the Natural History Museum of Los Angeles. Reagan asked me to post this here - if any readers happen to have a few spare bucks they can part with, think about sending some in Reagan's direction! I will note that recently, my colleague Rachel Racicot successfully funded some future CT work on the bizarre porpoise Semirostrum using an kickstarter. Go donate!

Here's the link:

Sunday, October 19, 2014

Beautiful new Oligocene dolphin in the prep lab

 A couple of months ago the Haugh's quarry triage project started - we had dozens of unopened plaster jackets from Haugh's Quarry in South Canterbury. About a dozen medium to large size jackets were prepped out, including this one, which was found to have a beautifully preserved odontocete skull, mandible, and partial postcranial skeleton from the upper Oligocene Otekaike Limestone.

Here's a view of the skull and mandible; the gray pieces at the upper right are parts of a large echinoid.

And a mandible with numerous in situ teeth - with the exception of a handful of specimens, most fossil odontocetes in the Otekaike Limestone have teeth that are mostly disarticulated.

And the skull has quite a nice dentition as well! Unfortunately, the tip of the rostrum is missing.

Here it is with a little more preparation. The teeth are quite a bit narrower and more needle-like than other Otekaike Limestone odontocetes.

Here's some nice postcrania...

And more teeth; a single tooth resembles Microcetus hectori (a taxon currently being redescribed by Yoshi Tanaka). There are some tusk-like teeth that were found loose - almost all of the Oligocene odontocetes from NZ appear to have had procumbent tusk-like anterior teeth. Provisionally the skull is similar to odontocetes that have been identified previously as "dalpiazinids". It should be a beauty when preparation is done!

Friday, October 10, 2014

How should we report the age of fossils? Pitfalls and implications for paleontologists.

A few years ago during a talk I was watching at a conference (the details are better left unstated), I realized that there is quite a variety - both in terms of methodology (or lack thereof) and quality of format - of ways to report the geologic age of fossils. Ever since I have wanted to write up a post since the topic has been nagging at the back of my mind. Paleontologists often frustrate geologists for poor understanding of certain major geological principles - something I never really understood when I was an undergraduate student; at Montana State U., we had a pretty strong background in geology. Further to the point, my undergraduate Dave Varricchio is a specialist in taphonomy, and hammered into our poor little heads the importance of geology in our field - which was subsequently galvanized by a master's thesis in the subject, my adviser for which was (gasp!) a sedimentologist (Jim Schmitt). After attending various conferences, however, statements about geology and taphonomy in various talks have left me slack-jawed, and I unfortunately understand why paleontologists get ribbed about it.

I'll start by saying this:  we study fossils because they give us unique insight into the history of life on earth, and we risk missing the true impact of paleontological data if we are careless about it. Now that that's out of the way, here's some of the topics I'm going to touch on: 1) basics of geologic age, 2) sources of age data, 3) recently proposed best practices for reporting ages for molecular clock data, 4) dates from the paleontologic and stratigraphic literature, 5) pitfalls of the paleobiology database, and 6) some suggestions for how to present age data for paleontologists. I'll try to keep this informative and useful instead of a rant, but there will be specific points where I just won't be able to help myself. 

As a quick note: while I am perhaps conversant in stratigraphy and geology, I am by no means an expert (I spend far more time looking at bones!), so anybody who knows better than I do - if you have anything to add to this, whether it be corrections, hate mail, tweaks to suggestions or even additional suggestions - your input can help improve this.

Basics of Geologic Age

I think that since this blog attracts readers with a wide variety of experience, some very basic geology is worth retreading what some have probably/hopefully read elsewhere. The geologic time scale was first assembled during the early 19th century in western Europe primarily based upon biostratigraphy. Pioneering geologists like William Smith used index fossils to identify zones that he identified as being the same age, and in 1815, Smith published the first large geologic map - looking at the beautifully exposed bands of strata in England, it's no wonder it formed the basis for the early understanding of geologic time and stratigraphy.

Charles Lyell introduced the terms Paleozoic, Mesozoic, and Cenozoic, to replace the Primary, Secondary, and Tertiary "periods" (as they were known then; Tertiary is the only one that lingers on). Lyell also introduced epoch divisions for the Cenozoic, and coined the epochs Pliocene, Miocene, and Eocene - at first, these formed the only divisions of the Cenozoic. Eocene roughly translates to "new dawn", referring to the dawn of the "new" Cenozoic invertebrate fauna; Miocene translates to "less new" and Pliocene as "more new", and the latter roughly means "continuation of the recent", referring to the relatively "young" aspect of the invertebrate fauna. These epochs were originally defined by Lyell based upon what percentage of invertebrates were extant: only 3.5% of Eocene mollusks and 17% of Miocene mollusks were extant; on the other hand, the "older Pliocene" was defined upon a mollusk fauna composed of 30-50% extant taxa. Lastly, the "newer Pliocene" was based upon mollusk faunas that were 90-95% extant. Subsequently, additional epochs were added, and over the next century, piecemeal advances in biostratigraphy clarified the stratigraphic distribution of many invertebrates and a geologic time scale was cobbled together across Europe.
What is obviously missing thus far is the sense of time; dating of radioisotopes was not discovered until the early 20th century and not perfected until the nuclear age. Until radiometric dating was employed, geologists worked effectively in a vaccum of numerical age data - all work was done in an understanding of relative time; Lyell would never learn that the Eocene was far longer than the Pliocene, for example, or that the Miocene-Pliocene boundary was only about 5 million years ago - but the Cenozoic in total had a 65 million year duration.

The geologic time scale updated for 2014. [From]
This abstract idea of time - otherwise referred to as Geochronology - regardless of absolute dates, has always been separated from chronostratigraphy. Chronostratigraphy can be viewed as the rock record itself with all of its imperfections. This dichotomy may sound a bit strange for the uninitiated, and indeed, it seems strange to many paleontologists (this is actually not a jest). Chronostratigraphic units are in a sense material units, while Geochronologic units are immaterial and refer only to periods of time. For example, the Paleogene period in geochronology is a period of time from about 65 to 25 mya, while in chronostratigraphy the Paleogene system comprises all rocks deposited during that time period. This dichotomy serves to highlight epistemological differences between the two frameworks (e.g. they are defined based upon different sets of criteria). Why the difference? Further to the point: the rock record, as reflected by the deposition of sediment, is like a barcode through time: surprisingly little time is represented by packages of sediment, and the majority is locked up instead in erosional surfaces (e.g. unconformities). Because one framework is purely based upon rocks (Chronostratigraphy) and the other is purely time (Geochronology), we need to be careful about using terms like upper versus late: upper denotes a position within the stratigraphic column, whereas late refers to time. This is why if you write something like "the late Miocene Santa Margarita Sandstone" geologists will either yell at you or make fun of you. This would be better written as "the upper Miocene Santa Margarita Sandstone" as we're discussing a rock unit; however, it would be acceptable to say "the Santa Margarita Sandstone is of late Miocene age". I'll admit that I've been slack about this before and this slipped through in at least one article, and I've gotten considerably more careful about it after being chewed out. A bunch of super-helpful writing tips for some of these confusing issues is given by Owen (2009).

Chronostratigraphy (left) versus Geochronology (right). From Owen (2009).

One last point: capitalization of time period modifiers. Some periods and epochs are formally subdivided, others are not; if a period of time has been formally subdivided, then it should be capitalized. For example, the Late Cretaceous is a formally subdivided period of time; the middle Miocene is not. Some confusion exists about the Pliocene and Pleistocene; some stratigraphers such as John Van Couvering have designated various international stages (e.g. Zanclean, Piacenzian) and their boundaries as defining partitions within the Pliocene and thus formally subdivided the Pliocene, and only some partitions of the Pleistocene have been formally subdivided. Further complicating matters is that according to the Geologic Time Scale 2012 the Pliocene and some of the Pleistocene subdivisions are formally designated (and thus can be capitalized), but the International Committee for Stratigraphy ( do not recognize these as being formally subdivided (except for the Upper and Middle Pleistocene). So it's admittedly a huge headache, but the moral of the story of this: paleontologists frequently screw this up too, but then again so to geologists: there is disagreement within stratigraphy about these things, so as long as you cite a particular framework (e.g. GTS 2012 or, or others) and follow that consistently, you'll be fine. This is perhaps not totally relevant to some of the specific points I'll be making, but is good ground to cover as relevant background, as these issues are often a source of confusion.

Sources of Age Data in Geology and Paleontology

With that out of the way, what sources of age data do we have at our disposal? My list is far from comprehensive, but these are the major methods used.

1) Radiometric dates. More often than not these are ash/tuff beds that have been dated using Ar/Ar or K/Ar from feldspars, although occasionally we get interbedded basalts that are even easier to date. Additionally, albeit less common, are radiometric dates from glauconite (that's right, ordinary greensand lends itself towards radiometric dates, randomly). These are generally quite good, and can be recalculated using updated decay constants. There are other types of dating methods under this category, such as U-Pb series dating.

A cute infographic showing the basics of radiometric dating (in this case, the isotope is Carbon-14). [Borrowed from]

2) Fission track dates. Elements undergoing radioactive decay like Uranium-238 leave little damaged tracks in the crystal lattice of minerals like zircon each time the uranium undergoes spontaneous fission. Because the decay rate of U-238 is known, the number of fission tracks can reliably tell us how long ago the zircon cooled from a magma. This is best used as a dating method when fresh zircons are sampled from an ash bed and were formed during that eruption.

Fission tracks in a zircon. [Ironically, this is borrowed from a creationist website:]

3) Ash correlation. Not all ash beds are datable - indeed, many ash beds lack phenocrysts large enough to sample by radiometric means or execute a fission track count. However, because every volcano has punched through a slightly different suite of rocks, every eruption has a distinct chemical fingerprint which can be used to tie non-dated ash beds to dated basalts or proximal ashes (e.g. an ash bed may have larger phenocrysts within it close to the volcanic vent). Ash fingerprinting - otherwise known as tephrochronology - is immensely important in Cenozoic marine and terrestrial rocks in the western U.S., as volcanism was fairly active during the Cenozoic.

A tephrochronologic correlation web for the late Neogene of Northern California. 
From Powell et al. (2007).

4) Magnetostratigraphy. Little iron rich grains in sediment align themselves with the earth's magnetic field - just like iron filaments on a sheet of paper adjacent to a bar magnet as illustrated in literally every science textbook ever made. Every so often the earth's magnetic poles switch, and when that happens the grains will point in the opposite direction - so a geologist (specifically a magnetostratigrapher) can take hundreds of oriented samples from a section of rock and identify which sections have normal or reversed polarity - this data can then be organized into a vertical "barcode" which can be calibrated using microfossils (see below). This vertical barcode can of course be matched to stripes on the Atlantic seafloor parallel to the mid-Atlantic Spreading Ridge; as new oceanic crust cools at the ridge, iron-rich grains also align with the magnetic field, resulting in magnetic stripes along the entire seafloor. These stripes go back to the mid-Mesozoic (as much of the pre-Cretaceous Atlantic oceanic crust has been subducted). In concert with biostratigraphy, a robust paleomagnetic framework exists and can be applied to virtually any strata on earth (generally Cretaceous or younger), provided that some biostratigraphically useful fossil content is preserved in the strata in question. A bonus is that because mid Atlantic seafloor spreading is more or less continuous (and rates of change in spreading are well-known), paleomagnetism can be a good way to tell if a significant hiatus in deposition exists; indeed, paleomagnetic work identified that a single bonebed in the Purisima Formation (=Bonebed 6 of my thesis/PLOS One article) recorded a 1 million year gap in deposition.

Paleomagnetism of the mid Atlantic spreading ridge and within a sediment core: matching the vertical barcode to the horizontal barcode is essentially the main principle of magnetostratigraphy. [Images borrowed from and]
5) Biostratigraphy. This is the oldest method for age determination and is a form of relative dating. Whereas during the 19th and early 20th century fossils told us how old the rocks were in a sense relative to other strata, we now have a whole host of absolute dates determined from the other four methods which have allowed assignment of well-constrained dates to biostratigraphic boundaries. Biostratigraphic zones (aka biozones) are historically defined based upon a number of zones, including A) taxon range zones (e.g. range of a single species), B) concurrent range zones (overlap range of two species), C) assemblage zones (overlap range of several species), D) interval zones (defined based on the bracketing of two bioevents, such as the extinction of two separate species at different times), and E) acme zones (defined based on abundance of a particular species; very susceptible to taphonomic processes). Taxa useful for biostratigraphy include microfossils such as diatoms (siliceous algae), benthic and planktic foraminifera (calcareous protists), calcareous nannoplankton (aka coccolithophores), radiolarians (also siliceous, but protists), dinoflagellates, conodonts (Paleozoic and Triassic only; actually a phosphatic element of early vertebrates); useful macroinvertebrate groups include bivalves, gastropods, and particularly ammonites for the Mesozoic - echinoderms have also been used in places. In terrestrial settings mammals are frequently used (e.g. North American Land Mammal Ages) as well as pollen. Useful fossils (index fossils) need to be widespread, easy to identify (and preserved well enough to be easily identifiable), and have a rate of morphological change that permits reasonable biostratigraphic subdivision. In other words, a hypothetical brachiopod that has changed little since the Triassic, only lives in deep sea environments, and is usually too poorly preserved to identify would be a pretty shitty choice for an index fossil. On the other hand, foraminifera tend to live everywhere in the marine realm and are widespread and evolve somewhat rapidly, and are therefore one of the most useful groups.

 Examples of biozone types and definitions from the North American Stratigraphic Code.

So what sort of age data do these provide? The first three generally all provide classical "dates" in the sense that it is presented as a midpoint with error bars: 25 ± 0.2 Ma, for example. The third sort is an ash correlation, so while the date is not intrinsic to the section of rock in question, it is chemically unique to an ash or lava bed somewhere else that has been dated, and thus the date can be applied to the correlative ash bed in our section. The last two are a bit of a mixed bag. Many boundaries of magnetozones are well-dated, so if we have the Gauss-Matuyama chronozone boundary preserved in our section, we can slap a date of 2.58 Ma to that magnetic reversal in our section. Many biozone boundaries - particularly for marine microfossils - have known absolute dates established based upon other methods. If a zone boundary is reflected in our section, we can use that; however, if a fossil in question just has "Boringmicrofossil taxon subzone B" attached to it, then the range of that subzone must be used - let's say the zone is 45.5-48.7 Ma in age, for example. What if our fossil is just above or below a well-dated biozone boundary, but is not bracketed by another date or biozone boundary? In that case, we're stuck with a range as well - whatever biozone our fossil belongs to, from whichever side of the zone boundary it came from. To be totally honest, most absolute ages for biozone boundaries are derived from methods reporting error bars, but are rarely reported as such and thus for biozone boundary ages it's not necessary to report a "±" range as few biostratigaphers bother. Lastly, what if our fossil occurrence has a biozone determination that lacks absolutely dated zone boundaries? Well, for the time being we're out of luck until a nice geologist fixes it for us.

An example of a robust set of dates for a vertebrate fossil: a fossil of the bony toothed bird Pelagornis from the Purisima Formation in California dated to ~2.5-3.35 Ma, from Boessenecker and Smith (2011). The fossil occurred in a stratum bracketed by two ash beds, and remains the youngest well-dated pelagornithid from the Pacific Basin.

Ideally, we would have radiometric dates that bracket a fossil occurrence nicely - for example, a fossil of Pelagornis that N. Adam Smith and I reported in 2011 was bracketed by a 2.5 ± 0.2 Ma ash below and a 3.35 ± 0.05 Ma ash above; using the midpoints of each results in an age of 2.5-3.35 Ma for the fossil. Alternatively, we could be slightly more conservative and utilize the endpoints (e.g. 2.3-3.34 Ma in this example). Rarely does this happen, and often we need to cobble together dates from various sources and dating methods in order to get the most tightly constrained age determination possible. For example, our fossil may be 10 meters below a dated ash, but 50 meters above the next dated ash - and a foraminiferal zone boundary may be 10 meters below our fossil. In which case, it's totally fine to use that zone boundary and the upper ash. Or, perhaps microfossils associated with the fossil itself have a substantially younger maximum age than the lower ash bed; in that case, it's fine to use the endpoint of that zone as a maximum age control, and the overlying ash date for the minimum age. We have to use whatever age control we can get our hands on - and age determinations for fossils are best regarded as ranges. Rarely do we ever get a date right from a fossil - radiocarbon dating in the Pleistocene is a notable exception (or, a fossil preserved directly in an ash bed, or a greensand unit with a K/Ar date) - and in these cases, a midpoint with a "±" is appropriate. If, for the purposes of graphical portrayal of geologic age, a range is needed (or for methodical purposes), the range of error could perhaps be used (e.g. 67 ± 0.5 Ma would be come 67.5-66.5 Ma). In general, one could use the minimum and maximum endpoints no matter what: the message here is that consistency is important, even if our age data have different sources. Lastly, it is necessary to point out that the most up-to-date papers on the geology and stratigraphy of a particular locality should be read and cited.

A quick note that doesn't fit elsewhere nicely: most strata are diachronous. That means that the age of the rock unit changes laterally. This makes sense because a given rock unit is generally a mappable unit of similar lithology, generally produced by a single depositional environments; depositional environments tend to move around through time (think of beaches during periods of sea level rise or fall). What this means is that geologic/stratigraphic data from the same locality (or close nearby) is preferable as it will decrease the chance of using dates that are too old or too young, owing to diachroneity.

Time transgressive strata, using beach deposited sand as an example here. [From]
Otherwise, it is important to note that this is all empirical: species and their geologic age are not abstract units existing in some intangible ether - the range of a species, or genus, is defined based upon the youngest and oldest occurring specimens identifiable to the taxon in question. This is all based on specimens - physical objects somebody dug out of a hole somewhere that are reasonably assigned to a given taxon, and are sitting in some museum collection someplace (which we can go and visit!). All fossil specimens sitting in a museum drawer were once part of the rock record - which technically makes paleontologists earth scientists. Sometimes specimens are misidentified, which may throw a wrench or two into the machine. The point is this: geochronology, chronostratigraphy, and biostratigraphy are dynamic and based upon real data, and refinements in methodology and framework "anatomy" (e.g. zone boundaries) as well as new dates are constantly being published. More on this below.

Best practices for molecular clock calibrations

Up to this point the discussion has been mostly academic with little application outside paleontology or geology. However, many enterprising biologists have over the past two decades used certain fossils as calibration points for molecular clocks; these clocks, if properly calibrated using paleontological evidence, can provide powerful estimates for the dating of certain nodes on a phylogeny. The problem with all this is that some paleontologists have been notably sloppy in publishing age data for fossils; most biologists are complete novices when it comes to geology, and lack the background to discern whether age data sounds "bullshitty" or not. I won't cite a specific example so as to avoid offending anyone, but here's an example of a bad geologic age determination for an important fossil with specifics modified or omitted: "XXX part of the XXX Formation, middle XXXocene, correlative with the XXX North American Land Mammal age, ca XX-XX Ma (citations of paleontologic papers that didn't actually provide primary age information for the locality)". First off: whenever we see "circa", it means it's an estimate; estimates are fine, so long as they're understood to be such (which means it must be obvious from the text). Secondly, in this case no actual land mammals were found in that unit; the author(s) wished to indicate that the formation in question was thought to be broadly correlative with the XXX NALMA, but not actually based upon any information. The problems with this are twofold: at the time of writing, the author(s) of the paper in question had access to up-to-date papers on the stratigraphy of the unit which actually constrained the time fairly well - and this was either unknown, or simply ignored out of laziness. The second problem is that fairly irrelevant papers were cited. What might a biologist see when reading this, however? If I were a biologist I'd say "look, so and so said this fossil was about 35-37 million years old, I can't really argue with him, so let's go with that." The author in particular regularly publishes papers that do not cite geological papers published after 1990. It sounds silly, but unfortunately this is the state of affairs.

Because of issues like this and other lapses in quality on behalf of certain molecular clock papers - Jim Parham and a huge lot of other authors (2012) published a fantastic paper advocating a set of "best practices" for identifying extinct taxa for molecular clock calibration points. Various studies have failed to identify individual fossil specimens, anatomical criteria (e.g. synapomorphies) for explaining why such fossils were identified as relevant for calibration, the formations they come from, or detailed age and locality information. Without this information, it is not necessarily possible to track down the rationale behind the use of certain criteria - indeed, certain molecular calibration points have even been based upon unpublished fossil specimens (that other researchers are not allowed to see) where the rationale for selection boils down to"one of our coauthors is a paleontologist and this is their opinion". Well, that doesn't sound very repeatable. So, Parham et al. (2012) outlined 5 recommended steps to ensure that a "readily auditable chain of evidence" could be established for each fossil selected as a calibration point. These steps designed by Parham et al. (2012) include:

1) Museum numbers of specimen(s) that demonstrate all the relevant characters and provenance data should be listed. Referrals of additional specimens to the focal taxon should be justified.

2) An apomorphy-based diagnosis of the specimen(s) or an explicit, up-to-date, phylogenetic analysis that includes the specimen(s) should be referenced.

3) Explicit statements on the reconciliation of morphological and molecular data should be given.

4) The locality and stratigraphic level (to the best of current knowledge) from which the calibrating fossil(s) was/were collected should be specified.

5) Reference to a published radiosotopic age and/or numeric timescale and details of numeric age selection should be given.

In retrospect, none of these sound very demanding and are altogether reasonable - points 4 and 5 are italicized because they apply specifically to this topic. In general these "best practices" emphasize repeatability. Last year I published a paper on the baleen whale Herpetocetus from the Pleistocene and included a phylogeny with the stratigraphic ranges of various mysticetes. In order to firmly establish the duration of different mysticete lineages, I spent some time reading the most recent literature and compiled some notes; these notes turned into a supplementary appendix for the paper that included a short (~1 paragraph) justification for each species shown in the cladogram (Boessenecker 2013B: appendix). Altogether this entailed an extra few days of work - well worth it for getting a publication that will hopefully be useful, or at least blatantly transparent.

Dates from the paleontologic and stratigraphic literature

Should we use/read/cite up-to-date stratigraphic literature? It feels asinine to even ask the question. If you answer 'no', you're lazy. Many paleontologists are satisfied with papers published as long ago as the 1960's for stratigraphic information about particular fossil occurrences. Well, one of the great things about paleontology is that we're a small field and because we're a historical science, many early papers will always be relevant, unlike fields such as physics and medicine where some studies may be totally irrelevant and outdated within a decade (or less) of being published. Lets say we wanted to cite the age of some fossil that Cope or Marsh described - we surely wouldn't rely upon a Victorian account of local geology and take Ed or Charles's statement on the matter at face value. Victorian era publications are, effectively by default, outdated in terms of stratigraphic knowledge. Why is that? Could it be, perhaps, that geologists are still poking around hillsides, badlands, coastal cliffs, and roadcuts, collecting ever more data? Last I checked, geology is still a pretty dynamic field (with far more researchers than paleontology, for starters) and hundreds of journals crank out new publications every week. If it makes sense to double check for up-to-date stratigraphy for Victorian-era discoveries, than it follows to do the same for fossils published in the 1960's, 1970's, 1980's, and 1990's. Hell, you should make sure to double check up to date stratigraphy even for something you published a year ago (this week I found out that one of my published age determinations is off by about 0.4 Ma, thanks to an overlooked piece of data). It's not like we don't have magical internet machines and google to find this sort of stuff - it's not that hard to find, thanks to the power of the internet. Parham et al. (2012) eloquently point out that "Any numeric age is merely the best current estimate and can be refined through time." I'll give a series of "what if?" questions and some answers, since there are numerous issues to tackle:

What if a paleo article doesn't cite any geology articles, but gives an estimate of the age?

           Shame on them. Use google, georef, or web of science to search for the formation name. Using what papers you can, try to find stratigraphic data for that locality.

What if I can only find stratigraphic data for a different locality?

            Sometimes this can't be avoided. If it really can't be avoided, make sure to state that available evidence is from a separate locality.

What if I can't find any information about that formation at that locality?

            Stratigraphic terminology has changed a lot, with formations being promoted to Group status or demoted to members, or completely renamed in some cases. For example, the Drakes Bay Formation at Point Reyes in Northern California is now known to actually be three different formations already known from other localities (Santa Margarita Sandstone, Santa Cruz Mudstone, and Purisima Formation). Again, use google; see if papers on regional geology make any reference to nomenclatural changes.

What if the guy who described this fossil said it was Pliocene, but other papers indicate the fossil is Miocene?

            Just like lithostratigraphy, chronostratigraphic stages and boundaries have been modified and refined through time. In many papers published pre-1980, the Pliocene included much of what is now late Miocene. Read the available literature, and use whatever is most careful and up-to-date. Also, use google (again).

What if the paper that reported the fossil reported some sort of new data associated with the fossil?

            Perfect! A best-case scenario as this means the separation between the fossil and geologic data is minimized. In this case, please cite whatever data they report. If biostratigraphic data is involved, double check that that zone is still in use. Also, in the case of diatom biochronology in the North Pacific, the names of different zones have changed; they used to have roman numerals (I, II, III etc.) but in the 1980's were switched to taxon-range and concurrent-range zones using the names of the various diatom species.

What if the aforementioned example is sort of old?

            Then you should double check the biostratigraphic literature to see if refinements have been made to the boundary dates and zone definitions. If an absolute date is involved, it may be prudent to recalculate the age using updated constants. Personally I don't feel qualified to do this myself, but for my personal research purposes most of the absolute dates I've cited are less than a decade old or have already been recalculated by others. Refinements in radiometric dating methods mean that old dates ought to be updated.

What if age data is available only at the level of the formation?

            It happens unfortunately - sometimes a given unit is not well studied and lacks internal age determinations, and perhaps the formation corresponds to a single biozone or magnetic chronozone, or has a date from the top and base. Well, there's not much else you can do other than accept formation-level data; sometimes it's not a big deal as many units are thin and record a short period of time - but you're a bit hosed if it's a long-duration unit like the Monterey Formation.

What if the locality is virtually unstudied, but the author said it was a certain epoch?

            This is sort of an almost worse case scenario, but for some poorly studied localities, perhaps "Pliocene" (or better yet a stage, like "Maastrichtian") is all you're going to get; in that case, you can use the known age boundaries for the epoch (or other period of time). Often the original author doesn't have a hope in hell of knowing exactly how old it is, but this is still defensible as we're leaving a "citation trail".

What if I got these neat dates off of the Paleobiology Database?

            Much of the other concerns listed above apply to dates from the PBDB. Read the following section.

What if I’m putting together an analysis that requires “binning” of age data for different taxa into interval time bins? I’ve already binned it so I might as well use the binned age data for reporting the actual age (e.g. in a figure showing lineage ranges or a time-calibrated phylogeny showing lineage duration).

For heaven’s sakes don’t do that either! Binning is great for paleobiologic analyses of diversity data, but why bother with converting the real age data into something less accurate? Let’s say, for example, that you have an unusually long time bin, based on a long stage: a lineage that ends at the beginning of the time bin and another lineage that begins at the end of the time bin, but do not actually overlap in time would be assigned the same time bin. Based upon the modified data, this would artificially make it appear that the two taxa lived at the same time. Although technically defensible as it can be applied consistently, we should be reporting the actual geochronologic age of particular fossils.

Should I publish citations of which stratigraphic articles I looked at, so that the reader can tell the difference between the citation for the fossil occurrence and the geologic age if they come from different papers?

An emphatic YES! This should always be done so as not to leave the reader guessing (guessing ≠
science). Here are two examples of how you can organize such data.

This is just a tiny part of an enormous table I put together for my 2013 Geodiversitas paper. Geochronologic ages are frequently derived from non paleontological sources. From Boessenecker (2013A).

In this case the table includes hard dates for fossil occurrences where the paleontologists provided reasonable dates, and one exception where several sources are cited thanks to refinements in stratigraphy. From Churchill et al. (2014).

To wrap up this section, I'll include another quote from Parham et al. (2012) that explains the point behind all this: "Anatomically trained fossil systematists may not be able to retrieve [geologic age] data any more easily than molecular systematists, but by listing the specimen numbers, rock units, and ages in a standardized way, others may check the claim, thus facilitating the refinement of numeric dates over time."

Pitfalls of the PaleoBiology Database

The PBDB is an excellent reservoir of data, and is used frequently for analyses of diversity and paleobiogeography. For the uninitiated, the PBDB is a database recording various types of data directly from the paleontological literature, foremost of which are geochronologic age and the taxonomic identification of fossil occurrences. The problem with dates provided by the PBDB is that the quality is only as high as the investment by the data enterer: most fossil occurrences just use the date from the original article the data is taken from. In some rare cases primary stratigraphic literature is cited; rarer yet are cases where unpublished stratigraphic data are supplied.

Because the PBDB is just an aggregator of data – data primarily from the paleontological (rather than stratigraphic) literature – it suffers from many of the same problems I’ve already discussed earlier. In addition to possessing these limitations, it also suffers from having outdated age determinations – including the shift in recognition of the Miocene-Pliocene boundary from ~12 Ma to 5.33 Ma, and also recording dates from old paleontological articles that cited stratigraphic works that are long out of date. Because of the lack of double-checking the stratigraphic literature, most of the age data on the PBDB should be assumed to be as accurate to the time at which the article was published. Another issue is binning: unless pinpointed dates are given in a paleontological article, if a stage or epoch was given – hard numbers for the boundaries are given. This is not necessarily a problem because I advocate that above (e.g. so and so said the fossil is “Pliocene”, so the fossil is assigned an age of 5.33-1.81 Ma). However, it’s important to evaluate the manner in which age was determined for each occurrence. I strongly recommend that age data from the PBDB be double checked using the stratigraphic literature. Much of the PBDB is “reasonably” accurate, but reidentifications in the literature are not always recorded accurately – which will affect age ranges. I don’t expect PBDB data enterers to bother with the stratigraphic literature as it would multiply the amount of work involved tenfold – but if someone is serious about getting accurate ages for fossils, the PBDB is an excellent tool and a great bibliography, but should not be accepted at face value.

Suggested Methodology

1) Familiarize yourself with the stratigraphic literature for a given fossil occurrence. Old paleontological publications are almost guaranteed to require citations of more recent stratigraphic studies (and many do a piss-poor job of even citing current geological work at the time of publication!).

2) Double check that subsequent paleontological papers have not reidentified the fossil in question. Is it really a fossil of the walrus Pontolis, or is it an Imagotaria?

3) If the age of a fossil occurrence was given using biostratigraphic zones, they are probably out of date! Find the most up to date zone boundary ages available in the literature. Similarly, old radiometric dates can (and should) be recalculated (although often these dates can be found recalculated in more recent geological literature).

4) Justify the geochronologic age range that is being reported. Now that you’ve read the stratigraphic literature, cite it! Other researchers need to know how you arrived at your age determination – as Parham et al. (2012) stated, a readily auditable chain of evidence” is the goal. At the bare minimum, at least include a table showing a citation for the fossil occurrence (primary paleontologic literature) and a citation for the geochronologic age from the best source possible (occasionally within paleontologic literature, but most often from the stratigraphic literature). The distinction between the two is very important! If age data are important to your study, then an appendix with a short description of the occurrences and age - filled with both of the aforementioned citation types – is a good idea.

5) Always vet fossil occurrences on the PBDB in a similar manner as you would for published occurrences.


Boessenecker, R.W. and N.A. Smith. 2011. Latest Pacific basin record of a bony-toothed bird (Aves, Pelagornithidae) from the Pliocene Purisima Formation of California, U.S.A. Journal of Vertebrate Paleontology 31(3):652-657.

Boessenecker, R.W. 2013. A new marine vertebrate assemblage from the Late Neogene Purisima Formation in Central California, Part II: Pinnipeds and Cetaceans. Geodiversitas 35:4:815-940.

Boessenecker, R.W. 2013. Pleistocene survival of an archaic dwarf baleen whale (Mysticeti: Cetotheriidae). Naturwissenschaften 100:4:365-371.

Churchill, M.M., Boessenecker, R.W., and Clementz, M. 2014. Colonization of the Southern Hemisphere by fur seals and sea lions (Carnivora: Otariidae), revealed by combined evidence phylogenetic and Bayesian biogeographic analysis. Zoological Journal of the Linnean Society 172:200-225.

Owen, D.E. 2009. How to use stratigraphic terminology in papers, illustrations, and talks. Stratigraphy 6:106-116.

Parham, J.F., and many other authors. 2012. Best practices for justifying fossil calibrations. Systematic Biology 61:346-359.

Powell, C. L., II, J. A. Barron, A. M. Sarna-Wojcicki, J. C. Clark, F. A. Perry, E. E. Brabb, and R. J. Fleck. 2007. Age, stratigraphy, and correlation of the late Neogene Purisima Formation, central California coast ranges. U.S. Geological Survey Professional Paper 1740:1–32.

Wednesday, September 24, 2014

The evolutionary history of walruses, part 4: the odobenines and the evolution of the modern walrus

The Walrus (Trichechus rosmarus) is a very fat, clumsy brute, much uglier than his picture, with a coarse, oily skin all wrinkled and scarred; long, protruding tusks; bristly whiskers and scuffling flippers that barely serve to move his bulky body over the land. In the water he is more at home, and though it does not require a high degree of strength and skill to dig clams, that being his daily occupation, yet he is able to keep very fat on the fruits of his industry and has much leisure to swim about or doze on ice floes and sea beaches.” – Dane Coolidge, Birds and Nature 10:2, September 1901

First introductory note: I have varied so far in writing in a chronological order in terms of the history of research or phylogenetic order (e.g. up the cladogram one node at a time). However, because the majority of non-odobenine odobenids (e.g. “Imagotariinae” and Dusignathinae) were not recognized as walruses until the 1970’s, only the odobenines were recognized as walruses during the early history of fossil walrus research. Because of this, the story of odobenines can largely be told in chronological order.

Second introductory note: This is by far and away my longest post ever on coastal paleo, so bear with me – save a half hour, or bookmark the page and come back. More has been published on odobenines than the rest of the odobenids combined, so there is quite a lot to summarize – and I think it’s quite a fascinating story.

The unfortunate taxonomic history of Alachtherium

The first fossil walrus was described in the mid 19th century by Du Bus (1867) and named Alachtherium cretsii based upon a well-preserved mandible from the lower Pliocene Scaldisian sands of Belgium. This mandible shares several features with modern Odobenus including an elongate mandibular symphysis and small coronoid process, a lower canine that is reduced to the same size as the cheek teeth, and incisors that are positioned anterior to the canine and in line with the toothrow (rather than medial to the canine). However, the mandible differs in its much larger size, having an upturned and unfused symphysis, and primitively retaining a fourth lower premolar (lost in Odobenus, which only has p1-3). Van Beneden (1877) later referred a partial braincase and humerus to the species, but Rutten (1907) thought the braincase and mandible were incompatible and erected a new taxon for the braincase, Trichechus antverpiensis*. This braincase is also larger than modern Odobenus rosmarus, and differs principally in having a rectangular dorsal margin in posterior view. Further unnecessary complications arose when Hasse (1910) described some partial skulls and postcrania of several individuals from the slightly younger, upper Pliocene Merxemian sands, which he named Alachtherium antwerpiensis (note: antverpiensis versus antwerpiensis) as he also considered the new material incompatible with the type.

*Note that early workers often included walrus in the genus Trichechus, which is the genus that the manatees belong in; most recent works do not discuss the errors of earlier workers, and a bit of searching on google has failed to enlighten me any further. I assume that superficial similarities such as blubber and a short muzzle as well as bottom feeding contributed to the confusion of earlier workers. However, Linneaus originally got it right by naming the species Phoca rosmarus – obviously not a phocid in the modern sense, but Linneaus placed practically all pinnipeds within the genus Phoca, so at least he recognized the pinniped affinities of the walrus.

Beautiful illustrations of the holotype mandible and referred braincase (in posterior view) of Alachtherium cretsii, (Pliocene Scaldisian Sands, Belgium) from Van Beneden (1877). Thanks to Olivier Lambert for the excellent scan of this work.

Before we return to the complicated taxonomic history of Alachtherium, another discovery was made around the turn of the century in Virginia: a new walrus, also based on a lower jaw, was named by Berry and Gregory (1906) as Prorosmarus alleni. The fossil jaw was collected from the lower Pliocene Yorktown Formation, and is similar to both Odobenus and Alachtherium cretsii in having a less-upturned ramus as in the former, but again having an unfused symphysis as in the latter as well as primitively retaining a lower fourth premolar, which (along with the lower molar) is missing in Odobenus.

The holotype mandible of Prorosmarus alleni, (Pliocene Yorktown Formation, Virginia) photographed at the Smithsonian.
For the rest of the 20th century, work on Pliocene walruses assignable to Alachtherium was monopolized by European researchers. Van der Feen (1968) described some new cranial material which he assigned, without explanation, to T. antverpiensis, which he placed in Odobenus (as Odobenus antverpiensis); this was done without explaining why the fossil was not assignable to either species of Alachtherium. Later work by Erdbrink and Van Bree (1990) figured and described a beautiful, complete, and gigantic skull dredged from the seafloor off the Dutch coast which they similarly identified as Odobenus antverpiensis. Erdbrink and Van Bree (1986, 1990) considered virtually all Pliocene walruses to belong in the genus Odobenus and assigned all specimens from the North Atlantic to O. antverpiensis, identifying other species Trichecodon huxleyi (see below), Trichecodon koninkii (see below), and Alachtherium cretsii as nomina nuda* as well as (rightfully) questioning the generic distinctiveness of Prorosmarus.

The skull of Alachtherium cretsii (see below) reported by Erdbrink and Van Bree (1990, as O. antverpiensis) is large (a bit larger than extant Odobenus), and bears large canine alveoli indicating the presence of tusks that are slightly anteriorly sloping, unlike the vertical tusks of extant Odobenus. The skull also has a somewhat elongate rostrum (as opposed to the blunt and inflated rostrum of Odobenus), and possesses more teeth than Odobenus: the modern walrus lacks incisors and an upper molar, and these teeth are primitively retained in Alachtherium. A single postcranial feature unites Alachtherium with the modern walrus: a deltoid insertion on the humerus that is separated from the deltopectoral crest (in other pinnipeds and non-odobenines, the deltoid insertion is positioned on the crest and not easily identifiable). As it turns out, humeral morphology is fairly diagnostic in walruses (more on this below).

*Nomen nudum means a “naked name”: a name that has been proposed but an insufficient description or diagnosis has been used. In all three cases these names satisfy the minimum requirements for being valid names under ICZN rules and thus this was a bit of a taxonomic faux-pas on behalf of Erdbrink and Van Bree. However, they were right to question the distinctiveness of T. huxleyi and T. koninckii (see below), but Alachtherium cretsii is clearly a good name with a well-preserved, readily diagnosable (and, historically diagnosed and well-figured) type specimen. However, the plot thickens – so read on…

Well-preserved skull of Alachtherium cretsii dredged from the North Sea offshore of the Netherlands (Pliocene), from Post (2004).

In 1994, San Diego Natural History Museum curator Tom Deméré published two papers, one of which described the dusignathine Dusignathus seftoni (see previous post) and also the bizarre odobenine Valenictus chulavistensis (see below). The second paper was a phylogenetic analysis and revision of the Odobenidae, and he summarized much of the prior work on North Atlantic Pliocene walruses. He indicated that the large skull assigned to “Odobenus antverpiensis” by Erdbrink and Van Bree (1990) uniquely shares a rectangular outline of the braincase in posterior view with braincases figured by Van Beneden (1877), Rutten (1907), and Hasse (1910), previously assigned to either T./O. antverpiensis (Rutten, 1907) or Alachtherium antwerpiensis (Hasse, 1910). Deméré (1994) concluded that insufficient evidence existed to distinguish between two (or three, for that matter) species of Alachtherium, and synonymized all with Alachtherium cretsii, noting that Prorosmarus alleni may also fall victim to synonymy. In defense of this lumping, Deméré (1994) noted that all the material is relatively larger than extant Odobenus and the toothrow of the skull and mandible are both sinuous and matching in profile. He further indicated that two species of Alachtherium may be a defensible hypothesis considering that the material reported by Hasse (1910: named as A. antwerpiensis) is late Pliocene in age as opposed to the early Pliocene age of the holotype A. cretsii mandible. Deméré (1994) further noted that the skull reported by Erdbrink and Van Bree (1990) has four postcanine teeth as opposed to five in the A. antwerpiensis skull described by Hasse (1910), and that the former specimen retains a medial incisor whereas it is lost in the latter.

            A curious recent development was a popular article by Klaas Post (2004) who agreed with studies by Deméré (1994) and Kohno et al. (1995) in assigning Pliocene North Atlantic walruses to Alachtherium cretsii. Despite this rather sober taxonomic opinion, Post (2004: page 70) explained that “even Americans (Deméré, 1994) and the Japanese (Kohno et al., 1995)” have contributed to the “babylonian confusion” of North Atlantic walrus taxonomy. Those dastardly North Americans and Japanese! As it happens, this was nothing more than an unfortunate accident in word choice by Post (2004; see below) and difficulty in translating Dutch to English. Kohno and Ray (2008) humorously responded to these comments:

“If Post’s (2004:70) characterization of the study of Pliocene odobenines as complete Babylonian confusion is correct, then he deserves some of the confusion for it. He repeated previous generic misspellings, Trichechodon (p.70) and Obdobaenus (p.73), in allusion to Trichecodon huxleyi Lankester, 1865. He did not mention Alachtherium antwerpiensis Hasse, 1909, but did attribute A. antverpiensis (Rutten, 1907) to Hasse, while not citing Rutten at all, even though published in Amsterdam. Although he noted (p.70) that even Americans and Japanese had meddled into discussion of Pliocene walruses, he neglected to cite two of the most prominent recent transgressions in support of his case…Our paper, based on Pliocene fossils from the eastern United States, may well be perceived as yet another transgression into European affairs. The notorious disregard by marine mammals for political boundaries, though an intractable problem in conservation, has made their fossils far more interesting than would extreme provincialism. We do not share Post’s pessimism about the status of knowledge of Pliocene walruses, but feel rather that much progress has been made through the contributions of all who have focused on the fossils, irrespective of nationality and in spite of multiplicity of languages.”

UPDATE: I've been informed by a European colleague that the wording used by Post (2004) was accidental and not meant to convey any nationalistic issues, and instead was intended to simply express the variety of researchers from different regions weighing in on the taxonomy of Alachtherium. I've been informed that Kohno and Post get on quite well and have exchanged casts, and Post has even lived in Japan for a time. Thus it seems abundantly clear to me from my correspondence that no ill-will was intended and it was simply a case of "lost in translation". That all being said, lapses in citations and the like (as pointed out by Kohno and Ray, 2008) are arguably acceptable for a popular article. With this caveat, I've left the unedited quotations above.

The holotype of Pliopedia pacifica (left; latest Miocene Paso Robles Formation) and the referred forelimb and braincase (right; latest Miocene-earliest Pliocene Etchegoin Formation, California), from Kellogg (1921) and Repenning and Tedford (1977).

The Santa Margarita Walrus

In March 1909, Mr. Robert Anderson found some large pinniped bones in a conglomerate about a mile southeast of the dinky town of Santa Margarita in the California coast ranges. Later, this unit would later be named the Paso Robles Formation; in the vicinity of Santa Margarita, it overlies the type section of the Santa Margarita Sandstone, and is correlative in age with the Purisima Formation (latest Miocene and Pliocene). The Paso Robles Formation was deposited on the west side of the proto-coast ranges of California, which during the latest Miocene formed a large island separating the Temblor Sea* to the east (where the Etchegoin/San Joaquin Formations were deposited), which was connected to the Pacific by a straight to the north (where the Purisima Formation was deposited) and another straight to the south (where the Pismo Formation was deposited). Pliopedia pacifica was originally named from a fragmentary forelimb, which Kellogg (1922) tentatively assigned to the Otariidae, but also recognized some walrus features. Repenning and Tedford (1977) reported another partial skeleton including a braincase, complete humerus, radius, and ulna from the Etchegoin Formation (i.e. from within the Temblor Sea). They curiously referred Pliopedia, along with Valenictus imperialensis (keep reading) to the Dusignathinae, despite correctly identifying that Pliopedia had an Odobenus-like braincase and a deltoid tubercle separate from the deltopectoral crest of the humerus. The braincase is similar to Odobenus, Alachtherium, and Valenictus chulavistensis (see below) in lacking a sagittal crest and having a nuchal crest expanded into a crescent-shaped muscle attachment surface. Barnes and Raschke (1991) subsequently removed the Etchegoin Formation specimen from Pliopedia and cited unpublished research on a toothless odobenine from the Purisima Formation, which as of yet is still incomplete. Deméré (1994a) dismissed this removal; I have reservations about why the specimen was removed, and I believe that Repenning and Tedford (1977) were correct in their identification. Because of the derive humeral and cranial morphology, Deméré (1994a) placed Pliopedia within the Odobenini (see below) and indicated that more complete remains would likely show that this walrus bore a pair of enlarged tusks. Pliopedia, despite being poorly known (no additional material has been discovered since the “pinniped bible” was published), demonstrates that a single species of walrus inhabited both east and west shores of the coast range in the Pliocene. I would absolutely love to conduct some fieldwork in the Kettleman Hills (type section of Etchegoin and San Joaquin formations, along the west side of I-5 in California between Lost Hills and Kettleman City) and search for additional walrus material.

*The Temblor Sea takes its name from the Temblor Range (or possibly the Temblor Formation, or perhaps both are derived from the nearby Temblor Range). The Early Miocene age Pyramid Hill marine mammal assemblage from the Jewett Sand and the middle Miocene Sharktooth Hill assemblage from the Round Mountain Silt, both in the vicinity of Bakersfield, California, were deposited along the eastern shore of the Temblor Sea.

The holotype humerus of Valenictus imperialensis from the Pliocene Deguynos Formation of Imperial County, photographed at LACM.

Subtropical walruses? The Isla Cedros and Imperial walruses

In 1961, Ed Mitchell named a new genus and species of walrus from an isolated humerus collected from the early Pliocene Imperial Group of southern California. For the uninitiated, the Imperial Group is exposed in the Imperial desert near El Centro, which is located just north of the US-Mexico border, southwest of the Salton Sea, and east of San Diego. The Imperial Group was deposited in a rapidly subsiding basin and appears to have hosted a subtropical warm-water invertebrate fauna, preserved within the proto-gulf of California. The humerus was collected in 1949 from the Coyote Mountains, and subsequent visits to the locality by Ed Mitchell and others failed to yield any additional fossils; in the subsequent 50 years, only a handful of additional bones have been collected from the same formation. Mitchell (1961) correctly identified this specimen as a walrus, and as such this fossil represented the first explicitly recognized walrus from pre-Pleistocene rocks in the North Pacific. At the time, the absence of pre-Pleistocene walrus remains in the region led other researchers to propose that walruses immigrated to the Atlantic via the Panama seaway during the early Miocene. It seems a bit ridiculous now, but they really were operating in a vacuum of information. Mitchell (1961) was thus the first to identify that walruses did in fact have a North Pacific evolutionary heritage, and suggested that a center of origin may yet be identified in the North Pacific for walruses (as had already been identified for the Otariidae). Mitchell also pointed out the rather robust and strange construction of the humerus, including the huge knob-like medial entepicondyle, and suggested that Valenictus had a powerful flipper stroke and probably did not swim in a manner similar to otariids. The most interesting aspect of Valenictus imperialensis, aside from its strange morphology, is that it – like Pliopedia – was found in sediments deposited within a large embayment. Further making matters interesting is that Repenning and Tedford (1977) reported a somewhat younger partial humerus from the upper Pliocene San Joaquin Formation. The San Joaquin Formation overlies the Etchegoin Formation in the Kettleman Hills, and marks the final phase of marine sedimentation in the Temblor Sea; at the end of the Pliocene, uplift of the Sierra Nevada caused a massive influx of sediment shed westward into the San Joaquin Basin, which in concert with a Plio-Pleistocene fall in sea level, caused the Temblor Sea to dry up, forming the modern day southern San Joaquin valley. A last vestige of the Temblor Sea is the shallow and freshwater Tulare Lake west of Bakersfield, which has been mostly emptied by 20th century irrigation. The San Joaquin Formation is overlain by the estuarine and nonmarine uppermost Pliocene and Pleistocene Tulare Formation (which is fossiliferous and has yielded scrappy terrestrial mammals).

A few years later, a curious pinniped was dug out of the badlands of the south end of Isla Cedros off the Vizcaino Peninsula of Baja California by UC Riverside paleontology expeditions led by paleontologist Frank Kilmer. They collected an enormous volume of fossils which eventually led to the naming of various marine mammals like the fur seal Thalassoleon mexicanus, the false killer whale Praekogia cedrosensis, the porpoises Piscolithax boreios, Piscolithax tedfordi, Albireo whistleri, and Parapontoporia pacifica, and the early pilot whale-convergent beluga Denebola brachycephala. These fossils came from deposits of the Almejas Formation – and the strange new pinniped, despite lacking tusks, had several skull and postcranial features that allied it with the modern walrus – and Repenning and Tedford (1977) named it Aivukus cedrosensis (the genus name is Inuit for ‘walrus’). Aivukus has an elongate rostrum, small canines, highly worn teeth, reduced incisors, but an Odobenus-like basicranium and postcrania. The mandible of Aivukus was thought to have some similarities with Prorosmarus alleni, leading Repenning and Tedford (1977) to hypothesize that it was directly ancestral to Prorosmarus. Aivukus represents the most southerly described walrus, at about 28˚ latitude, and like Valenictus imperialensis, demonstrates that walruses inhabited subtropical waters. The southern occurrence of Aivukus led Repenning and Tedford (1977) to hypothesize that it or something similar immigrated to the Atlantic via the still-open Panamanian Seaway to give rise to Prorosmarus, Alachtherium, and eventually Odobenus during the Pliocene and Pleistocene.

Cast of the holotype skull of Aivukus cedrosensis from the late Miocene Almejas Formation of Baja California, photographed at the USNM.

The toothless Chula Vista walrus and a makeover of Valenictus

In the late 1980’s more discoveries were being made in the hills around San Diego, California. The Pliocene San Diego Formation had long been known to host a magnificent invertebrate fossil assemblage but had also produced a fair number of birds including several species of the flightless auk Mancalla, the fur seal Callorhinus gilmorei, the longirostrine dolphin Parapontoporia sternbergi, and baleen whales like Balaenoptera davidsonii. Subdivision-scale housing construction was booming in the 1980’s, and these construction operations often scraped off bedrock, uncovering vertebrate fossils in the process. Paleontological mitigation began in Orange County in the 1970’s and San Diego followed shortly thereafter (I’m not exactly sure on the timing of mitigation in San Diego or LA county, to be perfectly honest). This led to a figurative explosion in the amount of fossil vertebrates collected, and now institutions housing mitigation-derived collections like the Natural History Museum of LA, the San Diego Natural History Museum, and (most significantly) the Cooper Center are packed to the brim with exciting collections of marine vertebrates (99% of which are undescribed!).

The holotype skull of Valenictus chulavistensis (Pliocene, San Diego Formation, California), photographed at the SDNHM.

Valenictus chulavistensis was one of those discoveries. A couple of tusks from the San Diego Formation were originally thought to represent some sort of weird proboscidean; a strange, toothless mandible found later yielded no further clues. A partial skeleton was subsequently discovered, including a fragmentary odobenine walrus skull with the same type of tusk, a humerus with the same strange morphology as Valenictus imperialensis, and the strange mandible, as well as a bunch of other postcranial bones – confirming that Valenictus imperialensis was indeed a tusked odobenine walrus (Deméré 1994b). Weirder yet, the mandible and skull lacked any teeth aside from the upper canine. A nearly complete but smaller (and therefore younger) male skull was found and designated as the paratype for the species; this second skull, along with an isolated juvenile maxilla, demonstrated that the lack of teeth in the adult holotype specimen was not some weird pathologic condition. Since the early 90’s, four additional skulls have been found, and none of them exhibit any non-canine teeth.


The more completely preserved paratype skull of Valenictus chulavistensis; this is the skull figured in Demere (1994b).

Deméré (1994b) explained that studies of walrus feeding show that modern walruses do not use their teeth during feeding, and rather only clack their teeth together as a form of underwater communication. The classic study of walrus biology and behavior by Francis Fay (1982) examined feeding behavior and showed that walruses have a powerful ability to generate oral suction by using their tongue as a piston against the deeply vaulted palate. The poor defenseless clam, after being unearthed (typically by water jetting, using the opposite of suction; walruses do not use their tusks for “digging”) is manipulated into place by the walrus’ fleshy lips, and the suction generated is sufficient enough to suck the soft tissues right off of the shell (other observations by Fay included a walrus feeding on a small phocid seal it had presumably killed, and was just sucking the flesh right off of the bone). Valenictus chulavistensis shares a vaulted palate and was just as well-adapted for suction feeding as Odobenus. Tooth loss in Valenictus is therefore analogous to tooth loss in suction-feeding beaked whales, and is a remarkably derived condition amongst pinnipeds. In fact, it’s also worth pointing out that Odobenus is evolving towards tooth loss: it’s already lost its medial incisors, the fourth premolar, and upper and lower molars.

The holotype mandible of Valenictus chulavistensis...

Another curious feature is the highly dense, pachyosteosclerotic nature of the postcranial bones. The bones have a reduced medullary zone (osteosclerosis) and inflated proportions and cortex relative to other pinnipeds (pachyostosis). Modern Odobenus bones are slightly denser than other pinnipeds, and the skull in particular is extraordinarily dense – but the postcrania of Valenictus are massive, dense, and very heavy (and not just because of fossilization). SDNHM visits can actually be sort of a pain if I’m interested in photographing Valenictus bones because they’re so damn heavy (but chicken scratch compared to baleen whales, I’ll add). Dense bones are thought to act as ballast, and the denser bones of Valenictus suggests it had a unique ecology with respect to other odobenines. Valenictus is known from the proto-gulf of California, the Temblor Sea (Deméré, 1994a,b actually reidentified the San Joaquin Fm. specimen as a specimen of V. chulavistensis), and the Pliocene San Diego embayment, and was a benthic feeder. Deméré (1994b) suggested that benthic feeding in relatively warm waters would have favored increased bone ballast. Barnes (2005) preliminarily reported additional pachyosteosclerotic Valenictus occurrences from southern Baja California, and suggested that the hypersaline environments in some of these embayments would have also fostered adaptations towards overcoming greater buoyancy. Finally, some new specimens from the Purisima Formation near Santa Cruz include a femur and a complete skull (collected recently by high school student and avid amateur paleontologist Forrest Sheperd), both identifiable as Valenictus – demonstrate that Valenictus also existed somewhat further north at the terminus of the northern connection of the Temblor Sea.

...And the holotype humerus of Valenictus chulavistensis, photographed at the SDNHM. Note the similarities with Valenictus imperialensis (above).

Deméré (1994a) also conducted the first phylogenetic analysis of walruses, as I’ve alluded to in earlier posts. This analysis confirmed the monophyly of the Odobeninae, and recovered Aivukus as the earliest diverging odobenine. Alachtherium was the next diverging odobenine, which in turn was sister to an Odobenus + Valenictus clade. Demere (1994a) importantly noted that amongst walruses, only Alachtheirum, Valenictus, and Odobenus possessed globular dentine, and named the tribe Odobenini to unite the long-tusked odobenines together. In a phylogenetic and morphological context, Deméré (1994a) argued that Valenictus is actually more derived than the extant walrus Odobenus rosmarus – and certainly, given details of the dentition and postcranial skeleton, he makes an excellent case. For whatever reason, Valenictus went extinct at the end of the Pliocene. If you want to read up more on that, I wrote a bit about Plio-Pleistocene marine mammal extinctions in my recent Geodiversitas monograph (Boessenecker, 2013).


One last Valenictus chulavistensis - a composite skeleton (all San Diego Formation material) on display at the SDNHM; the paratype skull can be seen upside-down behind the skeleton.

More records of the Odobeninae from Japan

Another important Pliocene odobenine was published in the 1995 special volume of The Island Arc by Hideo Horikawa, in which he named the small, primitive odobenine Protodobenus japonicus. Protodobenus lacks tusklike canines, and it is unclear if it possessed globular dentine. It did on the other hand possess an Odobenini-like deep, robust rostrum, and retained the primitive number of teeth. Protodobenus lacked extreme dental wear and also had a flattish palate, suggesting it was incapable of effective suction feeding and likely subsisted on fish. In this context, it’s unclear why it evolved such a deep rostrum; damage to the skull shows that the canines have elongate roots, and in the Odobenini, the inflated rostrum accommodates the enormous canine roots. More on this in the next post…

An important contribution towards the evolutionary history of odobenines was published by Kohno et al. (1995), who reported several tusks from the Pliocene of Japan. Following the definition of Deméré (1994), they identified a number of tusks with globular dentine from lower and upper Pliocene localities in Japan, which they identified to the tribe Odobenini. Most of these have an oval-shaped cross section, are more highly curved and tapering than Odobenus rosmarus, and some possess longitudinal fluting – and therefore compare well with tusks of the Alachtherium-Ontocetus-Prorosmarus-Trichecodon morphotype (see below). Critically, they identified a single tusk of Odobenus sp. from the upper Pliocene (see section on Odobenus for more on this). The importance here is that, based on tusks, a minimum of two species of Odobenini appear to have coexisted in Japan during the Pliocene.


The skull and mandible of Protodobenus japonicus - the first deep-snouted walrus, from the early Pliocene of Japan.


The first record of Alachtherium from outside the North Atlantic: a skull of Alachtherium sp. from the early Pliocene of Japan. From Kohno et al. (1998).

Alachtherium from Japan… and Africa?!

A couple of surprising occurrences of the Pliocene walrus Alachtherium were reported in the late 1990’s. In 1997, Denis Geraads (who specializes in African mammal paleontology) described a new species of Alachtherium from the upper Pliocene of Morocco, based on a fragmentary skull, a partial mandible, and a partial humerus. Although the species is founded upon material that is of dubious diagnostic value, it clearly represented Alachtherium and nonetheless demonstrates that walruses formerly inhabited the northwestern shoreline of Africa.

The following year, Kohno et al. (1998) described a fantastically preserved skull from the early Pliocene of Japan they identified as Alachtherium sp. This skull doesn’t quite have the rectangular outline in posterior view like Alachtherium cretsii, but has a similarly short, curved tusk with an oval cross section, a slightly longer rostrum, and a full complement of postcanine teeth (preserved only as alveoli, unfortunately). This discovery indicates that Alachtherium was present in both the Atlantic and Pacific during the Pliocene, and likely used the Arctic portal as a means for dispersal, which had recently opened up at 5 Ma (Marincovich, 2000; see below).

The referred humeri, tibia, and mandible of Alachtherium africanus, from Geraads (1997).

The Lee Creek walruses and a taxonomic solution for Alachtherium

Various researchers including Deméré (1994a), Post (2004), and Kohno and Ray (2008) have preferred a single species assignment for North Atlantic walrus remains. In fact, although using a weird taxonomy, Erdbrink and Van Bree (1999) also preferred a single species, “Odobenus antverpiensis”, which others (Deméré, 1994a; Kohno and Ray, 2008) cogently argued was clearly a junior synonym of Alachtherium cretsii.

In 2008, Naoki Kohno and Clayton Ray published their monograph of walrus remains from the Pliocene Yorktown Formation in the long-awaited Lee Creek IV volume (seriously, if you’re interested in marine mammals from the east coast, do yourself a favor and buy a copy through the Virginia Museum of Natural History). First, they laid out all the prior taxonomic arguments, which are summarized here in bullet format for convenience:

-Ontocetus emmonsi is a Pliocene walrus from the Yorktown Formation, originally named as a cetacean in 1859 by Joseph Leidy, based on a partial tusk
-Trichecodon huxleyi from the upper Pliocene Red Crag (UK), named in 1865 by Lankester, was based on an isolated tusk similar to Ontocetus emmonsi
-Alachtherium cretsii named by Du Bus (1867) from lower Pliocene Scaldisian sands of Belgium based on well-preserved mandible
-Trichechodon koninckii was named from a fragmentary tusks by Van Beneden (1871), also from the lower Pliocene Scaldisian sands of Belgium, later identified by many later authors as non-diagnostic and a nomen nudum or nomen dubium
-Prorosmarus alleni named from incomplete mandible from Yorktown Fm. by Berry and Gregory (1906)
-Trichechus antverpiensis erected by Rutten (1907) for partial skull originally referred to A. cretsii by Van Beneden (1877)
-Alachtherium antwerpiensis named by Hasse (1910) for other cranial material from Pliocene of Belgium
-Trichechus antverpiensis recombined as Odobenus antverpiensis by Van der Feen (1968), followed by Erdbrink and Van Bree (1986, 1990, 1999)
-T./O. antverpiensis and Alachtherium antwerpiensis synonymized with Alachtherium cretsii by Deméré (1994a), followed by Post (2004)
-Alachtherium africanus named by Geraads (1997) from fragmentary cranial elements from the Pliocene of Morocco

The holotype tusk fragment of Ontocetus emmonsi - and the "parent" specimen of virtually all North Atlantic Pliocene walruses (=Alachtherium cretsii, Alachtherium antwerpiensis, Trichecodon huxleyi, Trichecodon/Odobenus antverpiensis, Prorosmarus alleni), photographed at the USNM.

Kohno and Ray (2008) further considered Prorosmarus alleni to be relatively similar to the type mandible of A. cretsii, and figured a new mandible that is intermediate between the two, indicating that the perceived absence of an upturned symphysis is probably an ontogenetic feature. They also pointed out that the holotype tusk of Ontocetus emmonsi is a walrus, and that it and the holotype tusk of Trichecodon huxleyi both share an oval, transversely compressed cross section, longitudinal fluting, greater curvature, and are more tapered in contrast to the more elongate, straighter, smoother tusks of Odobenus rosmarus, which also have more of a circular cross section. Most significantly, they identify that all tusks found in sediments of Pliocene age yielding remains of Alachtherium/Trichecodon/Prorosmarus all conform to this morphology. They showed some bivariate plots of Atlantic Pliocene walrus tusks, and showed conclusively that all of these tusks conform to similar proportions, and cluster together to the exclusion of Odobenus rosmarus (Kohno and Ray 2008: fig. 27). In light of this information, they synonymized all Pliocene Atlantic walruses with Ontocetus emmonsi.

An adorable Ontocetus emmonsi juvenile maxilla (complete with mini-tusk!) from the Pliocene Yorktown Formation, described and figured by Kohno and Ray (2008), photographed at the USNM. Seriously, that is a cute fossil.

It’s admittedly a controversial decision, and some other pinniped workers have expressed the notion that Alachtherium cretsii is a better name as it is founded upon a complete mandible that inherently preserves more morphological information and is thus certainly more diagnostic than a tusk. I’m on the fence; on one hand, if the tusk morphology proves in the long run to really be that distinctive (and so far, it seems to), then Ontocetus emmonsi works; on the other hand, an isolated fragmentary tusk may not be diagnostic and in cetacean paleontology most species named off of isolated teeth were shit-canned a long time ago. My mind isn’t completely made up, and I think both camps in favor of Ontocetus emmonsi or Alachtherium cretsii have decent arguments.

The fossil record and biogeography of Odobenus

Pleistocene fossils of the modern walrus Odobenus have been widely reported from coastlines and the sea floor from both sides of the North Pacific (e.g. California, British Columbia, Japan) and North Atlantic (Maritime provinces of Canada, eastern USA – New Jersey to Georgia, and the UK and Netherlands) as well as the Arctic (Canada). Historically, many of the Pleistocene tusks, crania, mandibles, and other remains were assigned to Trichecodon huxleyi by earlier workers (e.g. Rutten, 1907); Demere (1994a) recombined it as Odobenus huxleyi, remarking that it was possibly diagnosable based on possessing a thin cementum layer in the holotype tusk; however, Kohno and Ray (2008) indicated that the tusk is identical to Ontocetus emmonsi (regardless, most material referred to T. huxleyi does appear to represent Odobenus rather than Alachtherium/Ontocetus). Fossils assignable to the extant genus Odobenus are widely reported from Pleistocene deposits in the Northern Hemisphere; in North America, fossils of Odobenus rosmarus have been dredged from as far south as the San Francisco Bay in California (Harington, 1984) and Georgia (Sanders 2002). These southerly records likely reflect southward latitudinal expansion of the natural range of Odobenus rosmarus during cold glacial periods. Furthermore, trace fossil evidence has recently been identified from the Olympic Peninsula in Washington, USA, indicating the presence of suction/jet-feeding walruses during the late Pleistocene (Gingras et al. 2007).

A Pleistocene skull figured and referred to Odobenus huxleyi (=Odobenus rosmarus), and the braincase of Alachtherium cretsii/Ontocetus emmonsi named as the new species Trichecodon antverpiensis by Rutten (1907).

That’s all neat, but not very surprising: modern Odobenus rosmarus is predominantly Arctic in distribution but occurs at the fringes of the North Pacific and extensively in the Northernmost Atlantic, and we know it was pretty damn cold during parts of the Pleistocene, facilitating southward migration during cold periods. But how old is the Odobenus lineage? And where the hell did it come from?

Cross-sections of Pleistocene and modern Odobenus tusks, with the illustration of the broken cross-section of the type specimen of Hemicaulodon effodiens, a junior synonym of Odobenus rosmarus. From Ray (1975). Note the distinctive core of globular dentine. This is a typical record of isolated Odobenus tusks from Pleistocene sediments.

At the time of writing the pinniped bible, Repenning and Tedford (1977) were sort of at a loss for the more recent evolutionary history of the modern walrus. Most of the remains were Pleistocene in age, and more or less confined to the above described regions: mostly in the North Atlantic and fringes of the Arctic. Based upon their discovery of the tuskless odobenine Aivukus cedrosensis from Baja California (which they also presumed was phylogenetically close to Prorosmarus alleni based on mandibular similarities; of course we now know that Aivukus and “Prorosmarus” had widely disparate skull morphology), they hypothesized that the ancestor of all tusked walruses (=Odobenini of modern usage) dispersed to the North Atlantic prior to the closure of the Panamanian isthmus. Following this, the extant Pacific walrus (Odobenus rosmarus divergens) reinvaded the northernmost Pacific late in the Pleistocene. This hypothesis began to unravel upon the discovery of tusked walruses like Valenictus chulavistensis from California (confirming that the genus Valenictus was assignable to the Odobenini rather than Dusignathinae), Protodobenus from Japan, and another toothless walrus from the Purisima Formation of California, all indicating that tusked walruses persisted in the North Pacific long after the disappearance of Aivukus (Kohno et al., 1995). A new extinct species of Odobenus was named by Tomida (1989) which he named Odobenus mandanoensis, from the middle Pleistocene of Japan. It proportionally differs from extant Odobenus and appears to have been slightly larger; although fragmentary, it appears to genuinely reflect a separate species (Deméré, 1994a). 

An isolated tusk fragment from the Purisima Formation (this specimen may be seen on display at the Santa Cruz Museum of Natural History), with globular dentine; the distinctive dental tissue identifies this tusk to the Odobenini. This and another specimen are from approximately the Miocene-Pliocene boundary, and therefore constitute some of the oldest records of the Odobenini.

One of the most fascinating advances was the discovery tusks and crania assignable to Odobenus from the upper Pliocene of Japan. Aside from the aforementioned tusk described by Kohno et al. (1995), a nearly complete skull of Odobenus sp. with tusk dredged from the Sea of Okhotsk was reported by Miyazaki et al. (1992), who found that it was associated with late Pliocene microfossils. These finds indicate that while Valenictus was hanging out in warm waters along the California and Baja California margin, and while Ontocetus/Alachtherium was proliferating across virtually the entire North Atlantic, the modern walrus had already evolved in the western North Pacific.

The curious referred mandible of Odobenus "koninckii", identified here as Odobenus sp., from the Pliocene Scaldisian Sands of Belgium, from Van Beneden (1877).

A single commonly overlooked fossil from the early Pliocene of Belgium indicates that perhaps neither of these two possibilities are likely. Although the name Trichechodon koninckii is defunct and useless, a single mandible apparently from the Pliocene Scaldisian sands of Belgium referred to T. koninckii and figured by Van Beneden (1877) bears a non-upturned symphyseal region and a fused symphysis, two features unique (amongst the Odobenini) to Odobenus. Deméré (1994a) pointed out that this specimen reflects a primitive Odobenus that retains a fourth lower premolar, a canine that is slightly larger than the premolars, as well as a sinuous outline of the mandible in dorsal view; the specimen is of apparent Pliocene age, and appears to indicate that Odobenus can be tracked to the early Pliocene in the North Atlantic, a bit older than the late Pliocene of the western North Pacific. What could this suggest? Perhaps it suggests a third option, that ancestral Odobenus had, like today, a circum Arctic distribution that extended as far south as Japan and Belgium during the Pliocene, facilitated by the lack of extensive ice sheets. Such a distribution may have pre-adapted Odobenus for Pleistocene glaciation. It’s possible, but other hypotheses are equally likely, and we need more walrus fossils with better dates to get a more complete picture.

Whatever happened, we know the following take-home points: 1) until the Pleistocene, tusked walruses (Odobenini) enjoyed a much wider variety of habitats and happily existed as far south as Baja California, Florida, and Morocco, and Pliocene fossils of the genus Odobenus are found at latitudes that would have been temperate during the Pliocene; 2) Sometime during the past 2 million years, a lineage within the genus Odobenus transformed from a temperate species (as was typical of Pliocene Odobenini) into the Arctic glacially-adapted specialist we know today.

The diversity of tusked walruses (Odobeninae); note the much smaller size of Aivukus and Protodobenus, and the gigantic size of Ontocetus emmonsi/Alachtherium cretsii.

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