The most recent theory of the origins of Hippopotamidae suggests that hippos and whales shared a common semi-aquatic ancestor
that branched off from other artiodactyls around 60 million years ago. This hypothesized ancestral group likely split
into two branches around 54 million years ago. One branch would evolve into cetaceans, possibly beginning about 52
million years ago with the proto-whale Pakicetus and other early whale ancestors collectively known as Archaeoceti, which
eventually underwent aquatic adaptation into the completely aquatic cetaceans.
The Anthracotherium magnus, an anthracothere from the Oligocene, already bore similarities to the modern hippopotamus.
The other branch became the anthracotheres, a large family of four-legged beasts, the earliest of whom in the late Eocene
would have resembled skinny hippopotamuses with comparatively small and narrow heads. All branches of the anthracotheres,
except that which evolved into Hippopotamidae, became extinct during the Pliocene without leaving any descendants.
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