More Seal Than Otter
Although the skeleton of Puijila darwini appears generally otter-like, characteristics of the skull indicate that the animal is actually an early relative of pinnipeds (true seals, sea lions and the walrus).
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- Four Lower Incisors
Most carnivores have six lower incisors (three on each side). Puijila has only four, as seen in pinnipeds today.
- Size and Position of Two Upper Molars
Each upper "end-molar" in pinnipeds is very small and located slightly toward the midline of the skull. This molar pattern is normal for pinnipeds, but unusual in other mammals. Puijila's molars follow the pinniped pattern.
- Large Infraorbital Foramen
The infraorbital foramen is an opening in the skull below the eye socket. The opening allows nerves and blood vessels to reach from the back of the head to the front of the snout, including the whiskers. The opening tends to be larger if these snout structures are large or specialized. This is true of modern seals, whose whiskers are very well developed (in some seals, the whiskers can sense water-borne vibrations from the movements of prey). The large infraorbital foramen in Puijila may correspond with the presence of well-developed whiskers, suggesting enhanced sensitivity of the snout.
- Large Eyes
Seals have large eyes, which are useful for seeing prey under water. Puijila was not a deep diver, but its large eyes would have been useful for hunting in water, especially during dark Arctic winter months.
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Not an Otter
Although some of these characters are seen in other carnivores, it is only in pinnipeds that they all appear together.
Otters, for example, have a large infraorbital foramen, just as pinnipeds do. Otters, however, also have relatively small eyes, and they don't have the pinniped tooth characters.
Otters and pinnipeds are only distantly related to one another. Otters belong to the musteloid group, which also includes weasels, wolverines, badgers and skunks. Pinnipeds are an entirely separate lineage. Ongoing research will determine where Puijila fits in the pinniped family tree.
The fact that Puijila's skeleton is otter-like is an example of evolutionary convergence (when unrelated lineages acquire the same biological trait).