Tracking manatee evolution over 47 million years

Only four species of manatees, or “sea cows,” remain on Earth – all of which are vulnerable to extinction. Fossil evidence suggests that many different kinds of sea cows lived in the past, located along the coasts of every continent but Antarctica. A new paper offers the most complete story yet of these unique creatures’ ancestry.

“The earliest known fossil sea cows are about 47 million years old, and those animals lived along the coasts of northern Africa in the proto-Mediterranean Sea,” said study co-author Steven Heritage of the Duke Lemur Center Museum of Natural History.

“Our analysis found that this first appearance was about 11 million years after the sea cow lineage diverged from their closest living relatives, the elephants.”

While modern manatees have no hind limbs and are strictly aquatic, the oldest known fossil sea cows had four limbs and could walk on land.

The study assembled the largest dataset of living and fossil species, combining genetics, anatomy, geography, and geologic ages. The team analyzed models for the ancestry of sea cows and biogeography that identify the ages and directions of their migrations.

“Our models suggest that the direct ancestors of manatees evolved within continental South America” said study co-author Erik Seiffert.

The migration of manatees into the Caribbean and  North America was a relatively recent event. “In a sense, manatees are newcomers to these West Atlantic ecosystems,” said Seiffert.

This migration corresponds with the Eocene-Oligocene boundary, a time of plummeting global sea levels, a rapidly cooling climate, and extinctions of numerous animal species.

The ancestral sea cows later faced a steep decline and eventually disappeared. But the Western lineage from the Oligocene gave rise to many species that have persisted for tens of millions of years, sometimes living in communities of multiple species.

The analysis also found that at least three Caribbean sea cow lineages migrated into the Pacific Ocean during the Miocene epoch, between 23 and 5 million years ago, before Central and South America were connected.

One descendant was the gigantic Steller’s Sea Cow, a cold-adapted species that measured up to 33 feet. European naturalists who explored the Bering Sea during the mid-1700s observed a living Steller’s Sea Cows, which was hunted to extinction shortly thereafter.

The highest diversity of sea cow lineages occurred around 22 and 16 million years ago. Over the past nine million years, this number has dropped precipitously resulting in the few species that remain today.

The study is published in the journal PeerJ.

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By Katherine Bucko, Earth.com Staff Writer