Seals, sea lions, and walruses (oh my!) are widespread, successful, and invariably charming. In this episode, we examine the adaptations, mysterious origins, and diverse but spotty fossil record of this fascinating group of aquatic mammals, the Pinnipeds.
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Carnivorans of the sea
Pinnipeds are a group of aquatic mammals that includes three living families: Phocidae (earless seals), Otariidae (sea lions and fur seals), and Odobenidae (walruses). More than 30 species of pinnipeds live today in waters all over the world. Unlike whales and dolphins, all pinnipeds have the ability to move around on land, though some (like sea lions) are better at it than others (like earless seals).
Though there’s plenty of diversity within the group, they share a number of traits that contribute to their aquatic lifestyles, including streamlined bodies, simple cone-shaped teeth (common among aquatic carnivores), eyes and ears specialized for sensing in and out of water, and whiskers (vibrissae) that help them feel their surroundings underwater.
Pinnipeds belong to the Order Carnivora, which also includes cats, dogs, and many other carnivorous mammals. Specifically, pinnipeds are part of the subgroup Arctoidea, which includes ursids (bears) and mustelids (weasels and their relatives), though there is some uncertainty as to which of those groups pinnipeds are more closely related to; studies of their anatomy traditionally link them with bears, while DNA studies typically link them with weasels.
Pinnipeds of the Past
Pinniped fossils are known from ocean deposits all over the world, though their fossil record is spotty, leaving lots of open questions about their evolution. There have been a few notable fossils pointed to as possibly close relatives of pinnipeds, including the bear-like Kolponomos and the otter-like Potamotherium and Puijila, all from the Miocene Epoch, but their exact relationships are still uncertain.
Based on genetic evidence, the pinniped lineage probably originated around 50 million years ago. By the Miocene Epoch, two extinct groups of pinnipeds had evolved. Enaliarctids, including Enaliarctos from the west coast of North America, were similar to living seals and sea lions, but still had the specialized carnassial teeth common to carnivorans (those teeth are lost in modern pinnipeds). And Desmatophocids included the first pinnipeds to reach large sizes, up to 3 meters (10 feet) long, including Allodesmus from fossil sites around the Pacific Ocean.
We see the earliest members of the living pinniped groups, the earliest seals, sea lions, and walruses, appear around 20-15 million years ago, including several well-known ancient species from California, Japan, Africa, and more. The fossil record reveals that pinnipeds of the past were diverse and widespread like their living cousins. Even walruses, which today are limited to a single species, were more diverse in the past, with fossils representing a variety of lifestyles and multiple origins of large body size (as in the giant Pontolis) and large canine tusks (as in the “semi-tusked” Osodobenus).
More to Explore
The origins and evolution of Pinnipeds:
Pinniped Evolution (technical, PDF)
The Origin and Evolutionary Biology of Pinnipeds: Seals, Sea Lions, and Walruses (technical, paywalled)
Some recent pinniped fossil discoveries:
Eomonachus, a new species of seal from New Zealand
Three new walruses from California, including “semi-tusked” Osodobenus
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