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.
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