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Allodesmus

FOSSIL FIELD GUIDE

Allodesmus
Extinct seal-like mammal
Family: Desmatophocidae

Time
Miocene Epoch

Place
Southern California, northern Baja California

In Our Region
Fossil remains of this genus have been collected in Orange County in southern California and near Ensenada in Baja California. Thousands of isolated bones of Allodesmus have been found in the Miocene-age Sharktooth Hill Bonebed near Bakersfield, suggesting that this extinct pinniped lived in large social groups.

Description
Allodesmus was a large seal-like pinniped that lived in the north Pacific Ocean region during the middle and late Miocene 15 to 10 million years ago (mya). At least three species of Allodesmus are known from California and one from Japan during this time period.

Allodesmus fossil reconstuction.
Allodesmus fossil reconstuction.

Allodesmus had a large, narrow skull like living leopard seals (Hydrurga leptonyx), but its teeth were relatively simple with bulbous-shaped crowns. It swam with its forelimbs like living sea lions and could rotate its hindlimbs forward and "walk" while on land. Allodesmus had very large eyes which helped it to see underwater, but unlike sea lions, the ears of Allodesmus were only somewhat adapted to hearing underwater.

Allodesmus displayed sexual dimorphism, with males being considerably larger than females. This pattern suggests a harem-style reproductive strategy for Allodesmus, with individual bull males controlling large groups of females at seasonal breeding beaches.

Ecology
The teeth of Allodesmus vary only in size; they are all alike in shape and are well-suited for fish eating. An active swimmer and predator, Allodesmus shared the sea with sharks, rays, turtles, whales, and in shallow waters, the bizarre hippo-like desmostylians.

Scientists think that Allodesmus may have been hunted as prey by Carcharodon megalodon, as small marine mammals provide many meals for modern great white sharks.

Suggested Reading
Berta, Annalisa and James L. Sumich. 1999. Marine Mammals: Evolutionary Biology. San Diego: Academic Press.


Text: Dr. Tom Deméré
Illustration: Doug Henderson
Fossil photograph: François Gohier