Irradiated by LabRat
In a post about the affability of manatees, Peter asks:
. Fascinating creatures . . . I’d love to know how they evolved, and what particular ecological niche they came to occupy.
Manatees and dugongs are Sirenians, an old order of placental mammals that has its first roots in Paenungulata, one of the earliest groups of placental mammals to diversify away. It’s a quirky clade in that there are only three groups alive today that descend from this group- the Sirenians, the Proboscideans (elephants), and the Hyracoideans, or hyraxes. The Sirenians and the Proboscideans are the more closely related of the living groups, and are grouped together along with another extinct group in the Tethytheria, which evolved along the shores of the Tethys ocean that existed between the Gondwana and Laurasian supercontinents, around where the Indian ocean and India itself are now.
What would became Sirenia started wandering into the water at around the same time some other famous aquatic mammals did, during the early Eocene, the period about ten million years after the extinction of the dinosaurs that can also be described as “1001 Nifty Things You Can Do With A Mammal”, evolutionarily speaking.
The Eocene was a period somewhat warmer than now, but more crucially, also featured a much more moderate temperature gradient from pole to pole- it might not have been dramatically warmer overall, but there was far less truly cold water, almost no ice, and there were far more shallow seas between the continents, which had not yet drifted all that far apart. The Sirenians were, effectively speaking, simply grazing animals that found good fodder in rivers and these shallow seas; like the cetaceans, they started off as robust four-legged animals that spent most of their time in the water (much like modern hippos, ecologically if not genetically speaking) and slowly lost their ability to function on land, as well as hind limbs, over time. Between their choice of habitat and their dense, heavy skeletons, the Sirenian fossil record is one of the smoother examples of major anatomical transition out there.
Global cooling was not kind to order Sirenia, and with the exception of Steller’s sea cow (which seems to have become extinct in short order after its official discovery, because mariners treated it exactly as the name implies) the extant Sirenians were reduced to a small slice of their original diversity, eating seagrasses in tropical oceans and river grasses in rivers and estuaries.
So, there you have it: they’re grazing animals that evolved out of a process of following the fodder, during a period when “into the water” seemed like a really good idea to more than one mammal group, and the niche they fill is, essentially, that of legless, amiable hippos. Were it not for the invention of the extended ocean journey for hominids, and subsequently the powerboat, it would be a pretty idyllic life.