A reconstruction of the skeleton of Puijila by Alex Tirabasso of the Canadian Museum of Nature
- Puijila has four lower incisors, rather than six as seen in most extant carnivorans. However, seals and sea lions today also have four lower incisors.
- Puijila's upper end-molars are small and located towards the midline of the skull. This is a strange arrangement for most mammals, but typical of pinnipeds.
- Like modern seals, Puijila has a large, well-developed infraorbital foramen, a hole below the eye socket in the front of the skull. This hole allows nerves to reach the front of the skull, and is well developed in species that have well developed or sensitive whiskers...like seals.
- While Puijila looks otter-like, it has large eyes, like a seal.
- Puijila has long, flattened toe bones, which are typically found in web-footed mammals.
- The limbs and tail of Puijila suggest that it swam via its four limbs, like seals and sea lions, rather than a combination of tail and limb strokes, like otters do.
Puijila's otter-like features, on the other hand, can be chalked up to convergent evolution. Because Puijila and otters were inhabiting the same sorts of environments, the sleek, otter-like body plan was the one best. In fact this conclusion is vindicated by other fossils in the fossil record. Potamotherium, another early seal, was once regarded as a sort of otter! However, it must be said that Puijila is not the ancestor of seals and sea lions. Puijila dates from the same time as Enaliarctos, about the Late Oligocene-Early Miocene time. Instead, Puijila shows us how seals and sea lions took to the sea, evolving from otter-like forms that thrived on the Arctic coast, spreading south via more oceanic forms into the Pacific and Atlantic oceans, respectively.
This amazing find also answers another nagging question about pinniped evolution, do seals and sea lions form a polyphyletic, or artificial, grouping, much like the now defunct classification term "pachyderms". The evidence Puijila gives us emphatically says no to this idea. Recent molecular and DNA analysises have given weight to the theory that seals and other pinnipeds do form an actual taxonomic group, but there are still doubters. Scientists have been arguing for quite some time on these matters. While one group supports the idea that seals and sea lions form a natural group, descended from bears; the other side suggests that seals and sea lions evolved convergently, the former evolving from the otters and the latter evolving from the bears. This side cites the fact the fossil Potamotherium as evidence for this theory, saying that it is the real ancestor of true seals, as well as the fact that sea lions are mostly known from the Pacific Ocean, while true seals are mostly an Atlantic phenomenon. Potamotherium has been reclassified as a very otter-like seal, adding weight against the concept of pinniped polyphyly, but Puijila strikes the final blow. Its features suggest a common ancestry with seals and sea lions, showing that pinnipeds are indeed a monophyletic grouping.