The Evolution
When the supercontinent Pangaea broke apart about 180 million years ago, small basal mammals*)such as the shrew-like Eomaia
Image: Wikipedia
already crawled in the shadow of the dominant dinosaurs. On the separating continents Gondwana*)Africa, South America, Australia, Antarctica, India and Laurasia*)now Europe, Asia except India, North America they developed into the very divergent groups of mammals we know today. In Africa for instance, which was isolated for many million years, arose 100 Million years ago the weird clade of the Afrotheria: the elephants, hyraxes, aardvarks, and seacows. Afrotheria therefore are the oldest group of the Eutheria or Placental Mammals
Approximately 65 million years ago a ten kilometer wide asteroid hit the Gulf of Mexico, as consequence of this catastrophic event the dinosaurs died out. This was the beginning of the ongoing "Age of Mammals", they filled the ecological niches vacated by the dinos, and their numbers exploded.

Age of Mammals:
Epoch Key of the Cenozoic Era
Tertiary Quaternary
Paleocene Eocene*)dawn of modern mammals Oligocene Miocene Pliocene Pleistocene Holocene
10 000-

The origin of the sirens is uncertain.
"The last common ancestor of Proboscids and Seacows presumably lived in the Late Cretaceous Epoch over 65 Ma ago. As the early fossil proboscids resembled todays hippos, and even todays elephants have characteristics that point to an aquatic ancestor, scientists believe that seacows turned to a purely aquatic life, while elephants colonized dry land."*)translated from "Marina und andere Elefanten, Sonderausstellung des Phyletischen Museums Jena"
The most archaic, about 50 million year old, sirenian fossil was found 1855 in Jamaica, consisting of a skull, mandible, and some vertebrae. R. Owen named it Prorastomus sirenoides.
In the “Encyclopedia of Marine Mammals” of 2002 we read,
"This occurrence in the western tropical Atlantic is surprising, as tethytheres*)taxon below afrotheria, include proboscids and sirens are thought to have evolved in the eastern Tethys."
Eocene fossil finds in northern America suggest that these (still) land dependent animals have migrated into the New World via the Atlantic (and not over the Bering-Landbridge), over a landbridge via the then tropical Greenland*)The hypothetical North Atlantic land bridge (NALB–connecting northern Europe and eastern North America).
In 2001, also in Jamaica, a team under Daryl Domning
Design of the skeleton of the Pezosiren
Composite skeleton of Pezosiren portelli
(after D. Domning)
excavated a near complete skeleton of a Prorastomid of early middle Eocene, slightly younger than the foregoing. Domning named the animal Pezosiren portelli. It was a pig-sized animal with a length of about two meters, which still possessed front and hind legs. It was obviously capable to walk on land. Other characteristics, however, suggest that it spent most of its time in the water, eating seagrasses.
From there on the seacows became a very successful animal order, which in the Oligocene populated all warm shallow seas in great varieties and numbers. Very soon the tail end evolved into a powerful horizontal fluke, and the rear extremities regressed.

Cladogram*)Cladogram = diagram to show relationships between organisms based on similarities of the Sirens

Kladogramm of the sirens
 (public access image, Prorastomus depiction added by H. Rothauscher)
The sirens always were herbivores with a very narrow diet breadth. In the course of the millions of years their skulls and dentures adapted to their actual main food*)The bottom-feeding dugong for example developed down turned snouts.
    Manatees, who feed on floating and submerged fibrous herbs, have a straight snout, and repeatedly replace their worn teeth.
. The upper lips of all Afrotherians are trunk-like organs, on seacows they are very versatile tactile and grabbing tools. Initially all sirens were of approximately the same size as today's manatees and dugong. Their heavy skeletons are very durable and many fossils have been found to verify their evolution.
Someday in early Miocene the Caribbean Dugongid species Dusisiren reinharti entered the North Pacific through the then open Central American seaway, and without branching developed into the new dugongid subfamily of the Hydrodamalinae.
Toward the end of the Miocene the climate deteriorated and the earth climate cooled. Seagrass, the main food of the sirens, disappeared in the northern Pacific, and large brown algae were spreading instead. Our Hydrodamalinae adapted to the changed food supply and the colder habitat.
For thermal efficiency they developed thick blubber and a bark like skin. They became cold tolerant, huge, and bulky animals. At the same time their molars regressed with the soft kelp diet.