Desmostylia––Ancient Sirenians (Manatees) of the North Pacific

December 11, 2014

This area of southwest Washington and northwest Oregon was underwater for many, many millions of years, which means that marine animals 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 species that formerly lived here.

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 our 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 Desmostylian, see

http://www.thisviewoflife.com/index.php/magazine/articles/10-million-year-old-desmostylian-roamed-ancient-pacific

Natural History Museum of Los Angeles has great photos of skeletons and animal reconstructions. 

Megalodon: An ancient shark that makes the Great White Shark look small

Written February 4, 2013, published March 2013.  Photos of teeth from a private collection, all photos by Kathleen Sayce.

Many animals that formerly lived on earth have modern analogs, animals living today that look and behave very like those ancient animals, though they might not be direct descendants. It’s as though the giant cats, bears, wolves, and sharks of the world recur again and again, slightly reconfigured each time. One ancient mega-tooth shark, Megalodon (mega for big, odon for tooth) has a small analog in the great white shark. 

Great White Sharks are big as predatory sharks get today, growing to twenty feet long, weighing up to 4,200 pounds (2.1 short tons). [Some older records of much larger Great Whites are based on inferences of size and not direct measurements.] Body shape and weight of these sharks help estimate the size of fossil Megalodon skeletons. Compared to Megalodon, they are in the second tier for size: Megalodon grew to 67 feet long, and a weight of 114 short tons. 

Megalodon lived from the late Oligocene (28 million years ago) into the start of the Pleistocene (2 million years ago), for 26 million years. Great Whites first appeared during the mid Miocene, so both species overlapped for millions of years. Even when young, Megalodon Sharks were so much larger that Great Whites were probably prey. They ate fish and marine mammals when small, but when more than 40 feet long probably had to shift to whales to get enough protein with each meal. Fossil whale bones have been found with Megalodon tooth marks on them. In some cases, the sharks simply bit the whales in half. Megalodon jaws were up to seven feet wide when open, large enough for a tall man to stand inside, so they could easily catch and eat whales.  Today, a Great White Shark can eat a Harbor Seal in two or three bites. The equivalent for an adult Megalodon was eating a Gray Whale in two or three bites. 

Fossil Megalodon teeth have a characteristic wide triangular shape and serrated edge. All were collected near Bakersfield, Cal. and are in a private collection. 

Megalodon teeth were known long before skeletal fossils were found. Sharks grow many teeth over each life, growing, shedding and replacing them continuously, several hundred teeth per shark. Already hard, teeth easily fossilize, and can be found millions of years later. Megalodon teeth are large, up to seven inches from base to tip, and serrated to improve slicing ability. They turn up in rocks, in marine sediments, and in soils all over the world. Initially they were thought to be fossil dragon or snake tongues, and were called glossopetrae, or tongue stones.  During the late Renaissance a Danish naturalist named Nicolaus Steno correctly identified these as fossil shark teeth. 

From these widespread fossils, found all over the world, we know that Megalodon were cosmopolitan, living throughout the world’s oceans. Like all top predators, their presence determined the structure of the marine communities in which they fed. As they grew, they moved from small to large fish, to marine mammals like seals and porpoises, and then to larger and larger whales. 

This Megalodon tooth is almost five inches wide and tall; the largest teeth known for this species are seven inches tall. Next to it, a fossil Mako shark tooth is two inches tall. Great White Sharks have teeth similar in size to Makos, up to two and one half inches tall. 

The first glacial maximum of the Pleistocene, with shrinking oceans, falling sea levels, and expanding ice sheets, also reduced whale populations due to changes in nutrient cycling that affected the entire food web. These changes left large Megalodon adults starved for food, and impacted the warm shallow seas where juvenile Megalodon lived. Many shallow seas simply drained away as more and more water was locked up on land in continental and montane glaciers. Great White Sharks, being much smaller, with less than one third the length and one fiftieth the body mass of Megalodon, adapted to these changes and survived in colder oceans with smaller prey.   

A small tooth (1.5 inches wide) shows the serrated tooth edge that is distinctive to Megalodon, and which gave it good slashing ability. 

The tooth that was photographed for this article came from California, and was found east of Bakersfield by a private collector. At one time the area was a large shallow sea, and it is known for a large variety of marine fossils. So far as I know, Megalodon fossils have not yet been found in Pacific County. If someone has a Megalodon tooth from this area, I would like to know about it, and I promise to keep your name out of the paper. 

However, we can deduce the historic presence of this great mega-tooth shark without local fossils.  For many millions of years this area was under water, first as deep ocean and later as an ever shallower warm sea.  Megalodon Sharks swam over this part of the planet for millions of years. We see modern Great White Sharks as awesome for their size, speed and predatory behavior. Yet Megalodon was a shark that other sharks avoided, including Great Whites, because they too were food for this top predator.