Reconstructing a fossil walrus, part 1: the enigmatic Dusignathus santacruzensis

For a recent manuscript project I found myself in need of a cranial reconstruction of the strange pinniped Dusignathus santacruzensis. D. santacruzensis was named from the Purisima Formation in 1927 by Remington Kellogg, the father of modern marine mammal paleontology. Research and interviews conducted by F.A Perry have successfully relocated the type locality, which evaded Kellogg and later forays by E.D. Mitchell in the early 1960's. The holotype specimen consists of a few cranium fragments including a partial maxilla bearing a procumbent canine, a squamosal, and a fragment of the 'vertex' of the skull (a term usually relegated to cetaceans, but utilized for odobenids by Demere [1994]), as well as both dentaries. The 'exploded' nature of the cranium is actually fairly literal; interviews by F.A. Perry indicate the collector 'poked it with a stick and the skull exploded', and only some of the cranium fragments were recovered. The dentaries are thus far the most distinctive element of the taxon; they are very robust, also with a procumbent canine, curious postcanine teeth with anterior and posterior wear facets, and a sinuous ventral border of the dentary (the first and last are synapomorphies of the Dusignathinae - Demere [1994]). Mitchell (1975) depicted the first reconstruction of the cranium of D. santacruzensis.

Mitchell's 1975 reconstruction of Dusignathus santacruzensis.

Unfortunately, no more cranial remains of Dusignathus santacruzensis have been recovered from the Purisima Formation since. Trust me, this isn't for lack of trying - dozens of dedicated amateurs and professionals (myself included, for the last few years) have been scouring the coastal exposures of the Purisima Formation nonstop since the 1970's. Plenty of odobenid postcrania have been collected, in addition to an edentulous odobenine walrus cranium (Barnes and Perry, 1989).

Former display at the Santa Cruz Museum of Natural History of casts of the Dusignathus santacruzensis holotype (without squamosal) based on Mitchell's reconstruction. Done by my colleague, Frank Perry. I apologize for the reversed image.

A gigantic pinniped skeleton was unearthed in the late 1980's from the Capistrano Formation, and was named Gomphotaria pugnax (Barnes and Raschke, 1991). This very strange animal was initially perceived as being very different from Dusignathus in terms of its mandibular morphology. Gomphotaria, which had a 40cm+ skull, bore two stout, worn, procumbent tusks - in the cranium and in the mandible.

Gomphotaria pugnax, a pinniped straight from hell.

Collections from the late Pliocene San Diego Formation of southern California included two new species of walruses - the extremely bizarre toothless Valenictus chulavistensis (which deserves several posts by itself), and the 'bizarrer' Dusignathus seftoni, both described by Tom Demere (1994a), the curator of SDNHM. D. seftoni is known from several crania and jaws, a partial skeleton, and a handful of postcranial elements. This animal had a cranium generally similar to the well known Imagotaria, and very similar to the 'killer walrus' Pontolis, and Gomphotaria. A trait shared with Gomphotaria were the possession of upper and lower (but less procumbent) tusks, also highly worn. The dentary shared similarities with both taxa; the sinuous ventral border, but it had a gigantic lower canine.

Referred crania of Dusignathus seftoni, described by Demere (1994).

Although the holotype of D. santacruzensis does not have very large canines, Demere (1994b) suggested that the specimen represents a female. Indeed, the canines are relatively small for any male pinniped, although the skull fragments do preserve a sagittal crest. The root of the lower canine is also exceptionally long, suggesting some potential.

The holotype right dentary of Dusignathus santacruzensis, from
Repenning and Tedford (1977).

Sure enough, in March 2008 I collected (from near the type locality) a pair of associated tusks (one upper, one lower) - one is straight, and the other curved posteriorly and laterally; these are adult teeth (based on root pulp cavity closure), and lack the highly abraded and parasagittally oriented tusks of Gomphotaria, and instead have tusks more similar to D. seftoni (albeit unworn). Additionally, these tusks are substantially smaller than in Gomphotaria, which was a monster in comparison (although dwarfed by Pontolis magnus, another dusignathine, one of the largest carnivorans of all time, possibly only smaller than elephant seals). These are best identified as male tusks of D. santacruzensis. These tusks are larger than any male D. seftoni specimen, and indicate an animal that is pretty damn sexually dimorphic. One of the tusks exhibits curvature in a parasaggital plane, but also lateral curvature. Tom Demere has successfully convinced me that the curved tusk is the lower canine, very similar to D. seftoni.

Unfortunately, dentaries of female D. seftoni are not yet known, and male dentaries of D. santacruzensis are not yet known. I guess I just have to keep looking in the Purisima Formation!

Coming up - the actual reconstruction process I used, now that the essential (albeit convoluted) backstory is done.

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

Demere, T.A. 1994b. The family Odobenidae : a phylogenetic analysis of fossil and living taxa. Proceedings of the San Diego Society of Natural History 10:99-123.

R. Kellogg. 1927. Fossil Pinnipeds from California. Contributions to Palaeontology from the Carnegie Institution of Washington 27-37

Mitchell, E.D., jr. 1975. Parallelism and convergence in the evolution of the otariidae and phocidae. In Biology of the Seal, p. 12-26.

Repenning, C. and Tedford, T., 1977. Otarioid seals of the Neogene. USGS Professional Paper 992.