Of all the evolutionary transitions that have ever taken place few have received as much attention as the origin of whales. (See here, here, here, here,and here for a few of my posts on the subject.) The story of how terrestrial hoofed mammals gave rise to the exclusively aquatic leviathans has been highlighted in headlines over and over again, but other marine mammals have not received the same amount of public attention. In the case of pinnipeds (seals, sea lions, and walruses) this may be at least partially due to the fact that their origins have been difficult to tease out.
It has long been known that seals and sea lions are carnivoran mammals closely related to weasels or bears, but just how pinnipeds first became adapted to a semi-aquatic life has been more difficult to figure out. This is largely due to the fact that their fossil trail stopped rather abruptly. The 24-22 million year old Enaliarctos, the oldest pinniped well-represented by fossils, was already a sea-lion like creature that swam in the sea. Surely there had to be even older fossils connecting it to its progenitors, but for years the trail was cold.
The gap between Enaliarctos and its forebears did not represent a real void in nature. Instead it pointed to a gap in our knowledge. That gap has now been partially filled with the announcement in the journal Nature of Puijila darwini, an early Miocene seal that represents a more terrestrial stage of pinniped evolution.
Bear in mind, however, that Puijila was probably not ancestral to Enaliarctos or living pinnipeds. Given that evolution is a branching process it can be extremely difficult to tell whether you are dealing with, as T.H. Huxley put it, “fathers” and “sons” (linear types) or “uncles” and “nephews” (intercalary types). Puijila appears to be one of the latter, a side branch that preserved some of the tell-tale traits than can inform us about pinniped evolutionary transitions. Indeed, the significance of Puijila is that it is representative of a stage of pinniped evolution that scientists could only hypothesize about previously. It is the fossil confirmation of the idea that seals did evolved from fully terrestrial ancestors.
Puijila was found on Devon Island, a chilly island well inside the Arctic Circle in Nunavut, Canada. About 21 million years ago there was a freshwater lake there where rabbits, rhinoceros, antelope, and other creatures sometimes came to drink.Puijila also lived in and around the lake. It was about three feet long with a long tail and broad, flat foot bones that, as in many groups of aquatic mammals, indicate that it had webbed feet. It did not have flippers, but its webbed feet certainly would have helped it get around in the water.
Overall Puijila was very similar in many respects to the extant river otters of North America, yet it differed in some important ways. It had upper arms and shoulders, for instance, that were slightly more expanded and robust than the same bones in otters. These differences would have provided more area for muscle attachment, and this could mean that Puijila was propelling itself with its webbed front and hind feet (possibly with an emphasis on the front feet).
So how does Puijila relate to other mammals? Given that it lived just prior to Enaliarctos and a radiation of marine pinnipeds it is probably more of a “persistent type” of early pinniped than an actual ancestor of creatures like Enaliarctos. Still, the phylogenetic analysis grouped it with Enaliarctos and another pinniped called Potamotherium. From what is known of these genera and the locations in which they have been found the authors of the paper suggest that pinnipeds may have originated by about 33 million years ago somewhere in the Arctic.
If the authors of the paper are correct the transition of pinnipeds to an aquatic mode of life would have started with Puijila-like mammals that lived in freshwater. Eventually, though, these amphibious mammals would have extend their range to the sea shore where they would have been further adapted to life in the water into forms more like Enaliarctos. Whether this hypothesis is correct, however, will rest on further studies of the fossil evidence.
Puijila is also important to illustrating the contingent nature of evolution. Even though it is extremely otter-like it did not swim like an otter. It primarily used its limbs to swim and, it probably did not incorporate up-and-down motions of its spinal column like transitional whales (i.e. Rodhocetus) did, either. If Puijila really does represent an important transitional stage in pinniped evolution, then, they way it swam can tell us much about why pinnipeds swim by using their limbs while other marine mammals exhibit different types of locomotion. The past is the key to the present.
I truly hope that Puijila gets the public attention it deserves. It is a wonderful, nearly-complete transitional form that answers some of our evolutionary questions while raising new ones. I hope it will inspire vertebrate paleontologists to look into pinniped origins with renewed vigor, and perhaps in a few decades we can talk about transitional pinnipeds with as much excitement as that with which we discuss transitional cetaceans.
Rybczynski, N., Dawson, M., & Tedford, R. (2009). A semi-aquatic Arctic mammalian carnivore from the Miocene epoch and origin of Pinnipedia Nature, 458 (7241), 1021-1024 DOI: 10.1038/nature07985