Manatees and dugongs are both commonly referred to as “seacows.” Both belong to order Sirenia, which ranges from the early Eocene (50 million years ago) to Recent. Sirenians originated in the now tectonically closed-up Tethys Sea region (between Europe and Africa/Middle East) of the Old World. During the Miocene, the western European Tethys Sea was rich in sirenians (the former eastern Tethys region was tectonically uplifted land during this time). Although there are about 10 extinct genera, today there are only two genera.
Manatees:
Manatees belong to family Trichechidae. Trichechus is only living genus, with three species. The dental formula for manatees is: 0/0, 0/0, indefinite/indefinite. That means there are no incisors and no canines. The indefinite cheek “teeth” consist of only low growths (transverse ridges) that somewhat resemble teeth. These cheek “teeth” are continually replaced from the posterior.
Manatees are characterized by having evenly rounded tail flukes, which are not notched. The bodies of manatees are often covered with algae and can have barnacles growing on them. Manatees have thickly bristled upper lips.
Three views of Trichechus = the only extant genus of manatee.
Manatees today are approximately 4 m long and weigh about 900 kg. They have a large tail, short neck, small front flippers, no hind flippers, no external ears, small eyes, and a blunt mouth with thick bristles overhanging the lips. The large horizontal, spatulate tail is the only means of locomotion. Their thick bones of manatees add weight.
Manatees are found today in rivers, mangroves, estuaries, and shallow-coastal areas in tropical and subtropical Atlantic and Gulf of Mexico/Caribbean, Central America waters (especially southern Florida). These gentle animals graze slowly and quietly (they have a slow metabolic rate) on aquatic plants. They are slow swimmers and can stay submerged for up to 15 minutes. They are known to rest on the banks of rivers. See the diagram at the end of this post for a summary of the geologic history of manatees.
Dugongs:
Dugongs strongly resemble manatees, but dugongs are larger (up to 8m long), overall heavier, have a split tail (two horizontal “fins”), and the anterior part of the snout of the dugong is downturned and without the oversized bristles around the upper lips. The dental formula for dugongs is 1/1, 0/0, 2–3/2–3 = 12–16. The tusk-like incisors of dugongs are useful for rooting up vegetation.
Dugongs today inhabit tropical coastal seas around the Indian Ocean. They seldom invade rivers. Their geologic history is summarized in the diagram below. North Pacific dugongs apparently arrived there from the Caribbean Sea region. When this happened is not well known but it had to happen when there was an open seaway between the Caribbean and the Pacific Ocean. This condition existed best during the Eocene but became increasingly less viable toward the middle of the Cenozoic (e.g., Miocene and Pliocene times).
Hydrodamalis = the geologically youngest genus of dugong. The last dugong in the North Pacific was H. gigas [= Stellar’s sea cow]. It was the only dugong adapted to living in really cold waters. It lived in cold waters in the Bering Sea region of the Arctic Ocean, where it fed on the fronds of kelp (= large stalked algae). This dugong was discovered in 1741. Twenty-seven years later, in 1768, this docile creature became extinct because of overhunting by humans.
The geologic histories of manatees and dugongs are summarized in the following diagram.
References Used:
en.wikipedia.org
Domning, D.P. and H. Furusawa. 1994. Summary of taxa and distribution of Sirenia in the North Pacific Ocean. Island Arc, vol. 3, issue 4, pp.506–512.
https://online library.wiley.com/doi/10,1111/j.1440-1738.1994.tb00129.x
Savage, R.J.G. and M.R. Long. 1986. Mammal evolution, an illustrated guide. British Museum of Natural History. 259 pp. [out-of-print?, but a very helpful book].
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