One of the most fascinating aspects of the study of marine mammals are the origins of the land to sea transition in various groups. Often marine mammals are so adapted for marine existence, that it is difficult to identify what group (fossil or modern) they descended from. Such has been the case with the origin of whales, for example. While I'm certainly interested in cetaceans, there has been a ton written about them, and I'm currently more intrigued by pinnipeds.
While modern cetaceans don't necessarily look like their artiodactyl relatives, pinnipeds generally retain enough features that allows them to be readily identified as carnivoran mammals, straight down to the dog-like bark of California sea lions. Certainly, this should make identifying the terrestrial progenitors of pinnipeds an easier task. Or does it?
The prevailing opinion throughout much of the 20th century was that the Pinnipedia are diphyletic; the sea lions and walruses formed the "Otarioidea", who descended from the Ursidae, and the true seals (the Phocidae) descended from mustelid (or musteloid) ancestors. In 1973, Ed Mitchell and Richard Tedford described
Enaliarctos mealsi from the Early Miocene Pyramid Hill member of the Jewett Sand in Kern County, California.
Enaliarctos still bore an ursid-like shearing dentition, which is absent in all modern pinnipeds (which have generally homodont postcanine teeth).
Enaliarctos was purported to be the common ancestor of the Otarioidea.
Later work (which at some point will be detailed on here) by Andre Wyss (UC Santa Barbara) and Annalisa Berta (San Diego State University), proposed a drastically different phylogeny of the pinnipeds, which suggested that pinnipeds were instead monophyletic, and that walruses (Odobenidae) were more closely related to the true seals (Phocidae), forming a new group, the Phocomorpha. Dozens of molecular phlyogenetic analyses have unequivocally supported pinniped monophyly, chucking the old diphyletic view out the window. The major tenets of the new studies are the following: 1) all pinnipeds descended from a common ancestor; 2)
Enaliarctos is basal to the Otariidae, Odobenidae, and Phocoidea; 3) the Otarioidea is paraphyletic, as odobenids and desmatophocids are more closely related to the phocids; and 4) pinnipeds were derived from an ursid (bear) like ancestor sometime during the Late Oligocene, probably in the Northeast Pacific, where the oldest pinniped fossils are known.
Putative seal femora from the Late Oligocene of South Carolina, from Koretsky and Sanders (2002). In 2002, Irina Koretsky and Al Sanders reported on some partial femora from the Late Oligocene of South Carolina. According to Koretsky and Sanders, these femora are most similar to those of extant phocids, and list several features they share in common. However, they argued that the Late Oligocene age of these, as well as the occurrence of these in deposits of the Atlantic Coastal Plain, indicate that true seals were already present in the Atlantic Ocean when the very primitive Enaliarctos was just appearing in the Pacific Ocean. Thus, the monophyletic origin of pinnipeds was not supported by this fossil, and it appeared that true seals evolved in the Atlantic separately from the Otarioidea, which are (during the Oligocene and the Miocene) restricted to the North Pacific.
There are, of course, some issues with this study and its inherent implications, irrespective of the incomplete nature of the fossils. For starters, the fossil record is notoriously crappy; in fact, the Early Oligocene marine record is the poorest of all with regards to marine mammal fossils. Very few cetaceans are known from the Early Oligocene worldwide, for example, primarily due to the scarcity of marine rocks for this time, due to low sea levels caused by the Eocene-Oligocene climate crash. Pinniped fossils just "show up" after sea levels rise and deposit more marine sediments during the late Oligocene, in both the Pacific (Enaliarctos) and Atlantic (Phocid femora). So - it is entirely possible for even more primitive pinnipeds to be found in earlier sediments (or, as "lazarus taxa" in Late Oligocene rocks).
The completeness of these fossils requires additional scrutiny. Some of Koretsky's other work focuses on the major elements of the fore- and hind-limb of true seals (and lower jaws) but generally placing low importance on cranial material (Koretsky and Ray, 2008). Koretsky and Sanders (2002), however, did not compare these specimens with femora of Enaliarctos, or basal odobenids such as Proneotherium and Neotherium (or an unnamed basal odobenid described by Naoki Kohno early on, ~1990). Additionally, now that the putative stem-pinniped Puijila darwini has been described, it's femora should be compared with these specimens as well. These could very well turn out to belong to something more like Enaliarctos or Puijila that we don't yet have a record of in the Atlantic. However these fossils are interpreted, their Late Oligocene occurrence (if the provenance is accurate) is intriguing, and further field investigation of Late Oligocene sedimentary rocks of the Atlantic Coastal Plain (and elsewhere!) should be considered.
References:
Koretsky, I.A. and A.E. Sanders, 2002. Paleontology of the Late Oligocene Ashley and Chandler Bridge Formations of South Carolina, 1: Paleogene pinniped remains; the oldest known Seal. Smithsonian Contributions to Paleobiology 93: 179-183.
Koretsky, I.A., and Ray, C.E. 2008. Phocidae of the Pliocene of Eastern USA. In: C.E. Ray, D. Bohaska, I.A. Koretsky, L.W. Ward, and L.G. Barnes (eds.), Geology and Paleontology of the Lee Creek Mine, North Carolina, IV. Virginia Museum of Natural History Special Publication 14: 81-140.
The skull is somewhere down in that trench the excavator dug. I imagine that's Dave Maloney up front, I can see one of the two lead engineers (both pretty cool guys). You can see that this fossil was only about a foot or so down below the wave cut platform; I must have walked right above this thing hundreds of times.
Here's all the bone that was poking out initially. Pretty ugly if you ask me. This is what you have to start with from heavy machinery - it often fractures a small zone of the bone. Waves and the fiercest winter storms are decidedly more gentle, and can expose bones that look nice enough to warrant collection. Nevertheless...
Hey, there's something there, all right! This is one of the first photos I saw of the thing, and I got pretty excited, and nervous; it looked like the trench had cut obliquely across the rostrum.
And further posterior at this stage of the excavation. You can see the occipital condyles to the left; a bit of the right side is gone, and it isn't clear what angle the cut goes to - obliquely to the left across the entire skull, it would seem.
Wow, I'm impressed - the left side of the skull looks beautiful. You can see much of the left maxilla, left frontal, and left squamosal here.
Holy cow! Here's the money shot. This thing is just beautiful! Fortunately, the trench went exactly parallel to the long axis of the skull, and didn't even cut across the rostrum; the anterior tip of the rostrum is intact, and aside from some minor damage to the left maxilla, is more or less complete (on the left side).
Here Dave Maloney (left) and Karl Heiman (right) pose while excavating Karl's beautiful new discovery.
At last, the plaster jacket is on the fossil. Apparently not two minutes after 5:00 Karl and Dave got the jacket on, a large wave came in and filled in the hole, which is now the size of a swimming pool. The specimen is probably now on its way (or already) to the prep facility. Luckily for these guys, the sediment is really soft and separates from the bone easily, and should make for a really easy prep job.
