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Sirenians, very successful group of aquatic mammals, have flourished throughout the majority of the Cenozoic. Originally thought to be related to whales, Sirenians are actually much more closely related to elephants. Historically, Dugongs have been much more numerous and diverse worldwide, but nowadays there is only 1 species of Dugong left and several species of Manatees. Of course how they evolved from hippo or pig-like mammals took time, but once they near- mastered the aquatic life, they quickly radiated out of Africa.
Phylogeny of Sirenians
2 Species of Modern Sirenians (Notice the tail differences and how Dugongs have more downward facing skulls (for feeding on the bottom))
Pezosiren
An amphibious mammal, these early Sirenians from the early Eocene (50 MA) of Jamaica resembled hippopotamuses more than manatees or dugongs. While they had functioning legs that could be used for walking on land, they had trademark Sirenian features such as a long, barrel-shaped trunk lined with heavy, dense ribs, downward skull deflection and sirenian ear features. Reached (at least) up to 6.5 feet in length.
Prorastomus
Another transitional Sirenian (lived ~40+ MA) of the Mid Eocene, these have been known to science for a few centuries (unlike Pezosiren, described in the early 2000's). They also had thick, dense ribs and a clearly Sirenian skull. About 5 feet in length.
Protosiren
Although these Sirenians of the Mid-Late Eocene still had hind limbs, they were shortened and had weak joint connections. They were likely mostly, if not fully, aquatic. Their skull further reflects becoming a more advanced Sirenian. Lived ~40 million years ago.
Metaxytherium
A diverse Miocene genus, the majority of FL fossil dugongs come from the Mid-Late Miocene dugong M. floridanum. Their fossils can be found in at least 8 different counties in Florida but likely found world-wide in Miocene deposits. Like all sirenians they were herbivorous, but likely ate the invertebrates attached to seagrasses for protein. Reached up to 9-11.5 feet (3-3.5 meters) in length.
More Details Coming Soon!
Dugongid sp. Late Pliocene
(From Domning paper, see reccommended papers page for more)
“The only Late Pliocene dugongid known in the region is a Dugong-like dugongine with large but not bladelike tusks. Its immediate ancestry is unclear, but it may have stemmed from one of the undescribed small dugongines mentioned above. It presumably did not feed on rhizomes of the larger seagrasses, many of which by then (following the closure of the Central American Seaway) are here assumed to have died out in the New World. In the apparent absence of sirenian competitors other than trichechids, it may have been a comparative generalist, consuming leaves and small to mid-sized rhizomes. By the end of the Pliocene, dugongids seem to have died out in the western Atlantic altogether. Trichechids, however, in the form of Ribodon, known from North Carolina, had apparently expanded their range outside South America by the Pliocene, and with the end of that epoch they became the only surviving western Atlantic±Caribbean sirenians.”
Stellar Sea Cow
By far the largest sirenian in modern times, they grew to enormous sizes to survive in the harsh Arctic. Reaching up to 26-30 feet (8-9 meters) in length and weighed up to 8-10 tons (and reaching up to 8 feet in diameter), they were sociable animals that protected their young and each other. Even with thick hide and plenty of strength, native arctic hunters were able to hunt them at ease. It is estimated that when Stellar first described them in the mid 18th century, there were only a couple thousand remaining and the species became extinct 27 years later.
Unlike most sirenians, they had no true teeth. Instead, they had hard bristles on their upper lip for manipulating kelp and 2 keratinous mouthplates on their lower jaw for grinding.
Modern Dugongs
Coming soon! (see HOL notebook notes + Mio story)
Manatees
(The only sirenians left in the Gulf of Mexico, originated in the Amazon region)
Coming soon!
Sources
25 Fossils Book (by Donald Prothero)
https://www.floridamuseum.ufl.edu/florida-vertebrate-fossils/species/metaxytherium-floridanum
https://etb-whales.blogspot.com/2012/03/origin-of-sirenians.html?m=1
Yes, whales evolved from land mammals too but that's yet another long, complex process!
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