One of these things is not like the other…

First row: Walrus (Odobenus rosmarus) skeleton
Second row: Hooded seal (Cystopkora cristata) skeleton
Third row: Dugong (Dugong dugon) skeleton, Brazilian sea lion (Otaria flavescens) skeleton.

*Skulls depicted are of species in the same genus as the skeleton.

Sirenia (manatees, dugongs, and sea cows) and Pinnipedia (the seals, walruses, and sea lions) are often seen as very similar, but they came from very different lineages.

While both came from land mammals (just like all sea mammals), the pinnipeds evolved from a bear-like ancestor, who returned to the sea around 28 MYA. They’re Caniformidae, or dog-like Carnivora.

The sirens evolved from the same ancestor as the hyraxes and elephants, and returned to the sea around 50 MYA. They’re only distantly related to Cetaceans and Pinnipeds.

Vergleicheende Osteologie. Edward D’alton, 1821.

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