The following brief summary has been adapted from a recent summary by Reynolds and Odell (1991). Much of what is known about the evolution of sirenians has been assembled by Daryl Domning, the world's foremost expert on this topic.

Manatees and dugongs are marine mammals of the order Sirenia, and despite similarities in body shape, adaptations and habitat, they have no evolutionary relationship with the other major orders of marine mammals, such as the the Cetacea (whales, dolphins and porpoises), Pinnepedia (seals, sea lions and walrus), or Carnivora (sea otters and polar bears). Paleontological evidence as well as recent biochemical evidence, reveal that Sirenians, together with the Proboscideans (elephants), Hyracoidea (hyraxes) and Tubulidentata (aardvarks) represent four living orders of mammals that are sometimes lumped together as "subungulates", which derived from a primitive ungulate ancestral stock.

The mammals in these four orders all lack a clavicle, and have nails or hooves instead of claws. Sirenian evolution is not fully understood. They likely originated in Eurasia and/or Africa, but spread into tropical South America by the middle Eocene (45-50 mya). The sirenians reached a peak of diversity during the Oligocene and Miocene epochs (55 mya). The earliest animal with a manateelike appearance (Potamosiren) dates from the Miocene epoch (13-16 mya). Early fossil record begins with early Eocene genus Prorastomus, structurally close to the common ancestor, already adapted to at least a partially aquatic lifestyle "Sirenians have a long history, first appearing on earth some 50 mya, and their family tree has included denizens of cold as well as warm waters.

West Indian manatees are, geologically speaking, relative newcomers to the Caribbean. For millions of years, their cousins the dugongs dominated the tropical Western Hemisphere. These ancient dugongs were abundant as well as diverse ( 30-5 mya) at least three, probably more kinds of dugongs lived together in the Caribbean. They had similar diets to the manatees. Today no single place supports more than a single species of sirenia. Much of the age of mammals promoted this wide diversity of sirenians. Various combinations of anatomy and behavior possibly allowed manatees and dugongs to share the available marine plant foods.

Extinct dugongs (Dusierion) (like their living cousins) had tusks (digging rhizomes of coastal marine sea grasses (ie. Thalassia- turtle grass) inaccessible to manatees w/o tusks. & kelp)Another extinct genus = Metaxytherium = (no tusks) unspecialized feedero= graed leaves of sea grasses. Sea-grass beds supported diverse species of plants until about 2-3 mya, and they supported a contingent of large tusked rhizome eater. Turtle grass is a climax species, but heavy foraging, would enhance plant diversity and productivity of other grasses that could support less capable diggers such as tiny-tusked Tetaxytherium. 2-3 mya a major ecological upheaval, Caribbean saw extinction of may shallow-water molluscs and some of the plant life. Upheaval due to plate movements and mountain building: Isthmus of central America was completed, separating the Caribbean and Pacific & disrupting water circulation and salinity that produced the mass extinction of Caribbean invertebrates; Dugongs from the area could have disappeared at the same time.

At this time manatees made their first appearance in Caribbean and in North America. They evolved in rivers of SA and only at this time spread north to Caribbean. At this time they evolved renewable teeth to withstand wear of griding sea grasses. Perhaps disappearance of Dugongs, manatees filled the vacuum (Domning, SA; "West Indian Tuskers") There are three living species of manatee (Family Trichechidae) and one species of Dugong (Family Dugongidae) (Fig. 1)). The Florida manatee (Trechcus manatus latirostris) is one subspecies of the West Indian manatee. The antilian manatee (Trichechus manatus manatus) is the other West Indian subspecies. West Indian manatees occupy coastal and estuarine waters throughout the Caribbean.

The Amazonian manatee (Trichechus ininguis) lives entirely in the Amazon river and its tributaries. A third species of manatee, the West African, (Trichechus senagalensis) occupies the coastal waterways of the West African Continent. Dugongs (Dugong dugon) occupy the marine coasts of east Africa, the Indian subcontinent, Malaysia, Indonesia and northern Australia. A fifth, and largest Sirenian of recent times, the Stellars sea cow (Hydrodamalis gigas), was also a member of the family Dugongidae. These enormous sirenians had evolved further to occupy the colder waters along the shores of the eastern Pacific ocean. These animals survived off the Aleutian islands until 1872, when the last animal of this species was killed for food , 28 years after being first sighted. Although there is only one living species of dugong, ancestors resembling them were diverse and widely distributed in the fossil record. Sirenians were most diverse in the Miocene (5-25 mya) when tropical conditions were widespread.

The most widespread genus was Meta xytherium, which is considered ancestral to a subfamily of dugongids which included the Stellars sea cow. Metaxytherium lived world wide= Mediterranean, Caribbean and western Pacific (SA & NA). (Sea Cow Family Reunion) Manatees and Dugongs separated 40 mya. Extinct forms of Dugong which led to Stellars sea cow were Metaxytherium - Dusisisren jorland- Dusisiren dewana- Hydromales cuestae- Hydromalis gigas (Stellars). Body size gradually became large, finers and wrist bones reduced, tusks reduced and lost, snout less downturned, and loss of teeth. Dusisiren was significantly different from Caribbean and Mediterranean fossil sirenians of Metaxytherium and closer to Stellars. Stellars ancestors lived in North Pacific (eastern & western= Japan). Dusisiren - Hydromalis was linear over 20 my from Miocene onward. Dusisiren shared Pacific coast with Dioplotherium allisoni. Dioplo had large tusks, & downturned snout (70 degrees) resembling living dugongs & so fed on different niche than Dusisiren ( & later Hydrodamalis) which had 45 degreee snout deflections The latter differed on soft kelps and in time lost their teeth (large kelps existed in California at mid Miocene. Dusisiren also had teeth.

Dioplotherium disappeared with colder pacific climes and rugged coast line and loss of sear grasses to which they had specialized. In cold Kelp waters Dusisiren specialized on kelp, float in turbulent waters off exposed rocky shores, increased body size & thicker skin and blubber conserved heat. This increased buoyancy which esd advantage most kelp biomass lay near surface. Also floating with back out of water reduced conductive heat & received radiant solar warmth & permitted entry into shallower water for foraging & escape from shark and whale predation (sea birds removed parasites). ie plenty of selective pressures for voluntary & obligatory floating. But Hydromalis had to keep swimming into current, so = increased flexibility of neck (Ohead side to side), also clawlike forelimbs could push away from rocks, pull against wave surges & detach plants.

Hydromalis survived only in vicinity of two islands that had never before been reached by humans. (Bering island) The four living sirenians are similar in several respects. They all are marine herbivores, have specialized dentition and horny plates in the mouth which aid in crushing ingested plants, have streamlined, fusiform bodies, lack an externally visible neck and pelvic appendages, have a large flukelike tail whose actions provide thrust, braking and steering, have relatively small pectoral flippers, have large heavy bones, sparse body hair, as well as many internal organ specializations adapted to a herbivorous, marine existence. Sirenians are far less abundant today than they were millions of years ago.