Prorastomidae – Ancestor of the Gentle Manatee

Adult ManateeSirenians, sometimes called sea cows, are the order of mammals to which manatees and dugongs belong. They live in coastal marine water and rivers and estuaries, and are large and peaceful animals. So called for the mythological Sirens, who lived on islands and would lure sailors towards them to drown, Sirenians are nothing at all like their namesake, so don’t worry. The explorers who observed manatees swimming next to the ship at first thought that they were mermaids and the association stuck.


Prorastomidae, also known as “terrestrial sirenians”, are an extinct family of sirenians from Jamaica. There are only two species in the family Prorastomidae, Pezosiren and Prorastomus.

Fossil of Early Manatee AncestorPezosiren might have looked a little bit like a hippopotamus, and certainly had a lifestyle similar to that which the hippo enjoys today. Discovered in 2001 in Jamaica by Daryl Domning, the skeletal structure of Pezosiren was similar to that of manatees and dugongs. Except, however, for the legs and feet capable of walking on land. Today, sirenians have flippers instead of feet, and the skeletal structure of their legs are different from Prorastomidae. The legs of Pezosiren were more similar to a dog or a bear.

The fossil found by Domning was nearly complete. The site in Jamaica had yielded plenty of new discoveries, such as a prehistoric rhinoceros and primates, but even still, it’s rare for the first fossil discovered of a species to be so whole. It is considered a transitional form between an entirely terrestrial mammal and an entirely aquatic one. Pezosiren lived at a time when sea levels had dropped and many islands and land bridges in the Gulf of Mexico and Caribbean were formed. Shortly afterwards, though, the area flooded once more and created a space for modern Sirenia to live.


Pezosiren Art of AdultWhile Pezosiren was a fairly recent discovery, we’ve known about Prorastomus since 1855, since it was described by Richard Owen. The fossil remains were much more sparse, and most of the information about what kind of animal Prorastomus was is from the skull. Their teeth tell that they lived mainly on land and ate soft plants and bulbs. It was originally thought that Prorastomus originated in Africa and travelled across the Atlantic, but since the initial discovery, it has been concluded that the species originated in the Americas.

These two species, the only ones in the family Prorastomidae, are the oldest known Sirenians. They lived about 50 million years ago and lived throughout most of the Eocene Epoch, which ended 40 million years ago. By the time that Prorastomidae went extinct, dugongs already existed and modern-day Sirenians have existed ever since.


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