Desmostylia––Ancient Sirenians of the North Pacific

May 21, 2017

This area of southwest Washington and northwest Oregon was underwater for many millions of years. Ancient marine mammals lived here along with fishes and a wide range of invertebrates, even though we do not have fossils from every square mile to look at today. So we look around the Pacific Rim to learn about the diversity of former species.

One of the strangest animals from our watery past is Desmostylia. A chunky, stout aquatic mammal of shallow waters and shorelines, it is distantly related to modern manatees, which are Sirenians. 

Formerly much more common in geologic time, Sirenians include three living species of manatees, one dugong, and the recently extinct Steller's sea cow. Their closest living relatives are elephants and hyraxes. Fossil Sirenian species in the Desmostylia group lived from the Oligocene to the late Miocene, about 25 million years, ending about 7 mya (millions of years ago). 

By the Miocene this area was a shallow sea with several river deltas and emerging mountain ranges, and with extensive swamps along the eastern edge, near the position of the modern Cascade Range. Climate was warmer in the Miocene, tropical to subtropical, and sea level was a couple of hundred feet higher.

Desmostylia fossils, including full skeletons and partial bits of bones, teeth and skulls, have been found around the North Pacific, from the south end of Japan, through Siberia, the Aleutian Islands, Pacific Northwest, south to the south tip of Baja California. Teeth make particularly good fossils because they are hard and slow to break down. Desmostylia has interesting large molars, along with more typical mammalian tusks and canine teeth. These teeth have been described as bundles of columns, which gives them their name, from the Greek desmos (bundle) and stylos (pillar).

These mammals were aquatic, and from isotopic analysis of teeth and bones, we know that they were marine. Other marine mammal features include retracted nostrils (tightly closed when underwater), and raised eye sockets (to see better at the surface). Stocky and stout, they weighed up to 440 pounds and were about six feet long, with a heavy shovel-shaped head and large strong teeth, short strong legs, and broad feet. You can see a complete desmostylian skeleton of at the Natural History Museum of Los Angeles County. This individual lived 10 million years ago, towards the end of the Miocene. The museum has also done reconstructions of living animals, to give us an idea of what they were like.

There are no modern analogs to these mammals. For size comparisons, black bears and wild boars (feral pigs) can grow to 400 pounds or more in size. Hippopotamuses weigh up to 3,300 pounds, and live in freshwater, though some populations live in mangrove swamps. Manatees weigh up to 1,300 pounds, and live entirely in water. We could think of Desmostylia as a small hippo, in a sense, though they are not closely related.

With broad grinding molars, Desmostylians were herbivores. In marine and estuarine waters, what did they eat? Sea grasses and seaweeds, including kelps, are the mostly likely food plants. These plants live in shallow saltwater in large, dense stands. There was another powerful reason to stay in shallow water: Megalodon cruised the open waters of the world's warm oceans and seas. Desmostylia were about the right size to this huge shark to be like chicken nuggets to us.

Imagine if today 400-pound, six-feet-long marine herbivores grazed eelgrass beds in Willapa Bay. They'd jostle with the seals for haul out space, or sprawl in the marshes around the edges, and graze down the eelgrass stands at mid to high tide. Water quality might be an issue. Herbivores tend to produce a lot of poop, about five to seven times the volume, based on body size, that carnivores do. Today, hippos are one of the most dangerous animals we live around. Desmostylia might be similarly dangerous––placid until someone gets too close, and then those large teeth come into action, and oops, there's another ex-kayaker or ex-hiker. It would definitely make boating on the bay lively!

For more information, and good reconstructions of a Desmostylia, see

http://a-fragi.blogspot.com/2011/07/desmostylus-2010.html  where a Japanese sculptor, Hirokazu Tokugawa, has done very nice reconstructions of this fascinating paleo marine mammal.