Desmostylians were marine mammals whose mode of life and evolutionary relationship with other similar mammal groups continue to be debated by paleontologists and evolutionary biologists.
Their geologic range is late Oligocene (about 25 m.y. ago) to late Miocene (about 4.5 m. y. ago), a span of about 20 million years. They lived in coastal waters of the northern Pacific from Japan to Mexico. Some complete specimens of these rare animals have been found in California.
The best modern analogy for what a desmostylian looked like in terms of shape and body mass is a hippopotamus. Desmostylians were up to 1.8 m (6 feet) in length and weighed up to 200 kg (400 pounds). Six genera are currently known.
Some experts have reported that desmostylians probably walked along the shallow ocean floor, possibly ate oysters and other shelled animals, and could walk onto beaches. In recent years, other experts, however, have concluded that desmostylians might have been strictly aquatic and herbivores.
Their feet were very wide and long. It seems that their wide feet might have served as paddles, which certainly would have been effective for swimming. In addition, levels of stable isotopes in their tooth enamel suggest an aquatic environment. They had inturned feet, and the feet could not be turned without rotating the whole leg. This would have been a severe limitation if they tried to walk on land.
They had short but stout tusks probably adapted for bottom feeding. The structure of their cheek teeth (molars) is unique among mammals. Their molars look like tubes welded together, with each peg-like tooth covered with thick enamel. This feature is how the name desmostylid was derived: desmos = linked, chain; stylus = pillar. Their molars are arranged in a cluster, which would have been ideal for crushing their food. It seems that their cheek teeth were replaced cyclically, as in elephants.
The above picture on the right shows a life-sized plaster replica of a cluster of teeth. The replica is 7 cm = 2.75 inches in width.
The above picture on the left shows a 43-mm (1.75 in.) long tooth whose exterior enamel is gray. The other tooth (two views) is 45-mm long and has been cut in half, lengthwise. Its exterior enamel is black, and its interior, which has been polished, shows the root and core of the tooth.
Some experts believe that desmostylians are closely related to sirenians (manatees) and elephants, whereas other experts maintain desmostylians are closely related to horses and other perissodactyls. Both groups of experts have used cladistics techniques in their analyses. Thus, the subject of desmostylians is an on-going source of study and debate, even though complete specimens have been found.
My connection with the subject matter concerns my field work many years ago on the lower Miocene Vaqueros Formation of California. This formation contains many shallow-marine invertebrate fossils (oysters, turritellas, shark teeth, etc.). Among these remains are very rare individual teeth of desmostylians.