Meandrina sp.
Dichocoenia sp.
Extinct brain coral
Family: Meandrinidae

Time
Miocene Epoch

Place
Imperial County

In Our Region
Anza-Borrego Desert State Park

Surface of a brain coral fossil. SDSNH catalog no. 11926

Description
These corals belong to the stony corals, the group that is responsible today for the building of the major coral reefs throughout the world's tropical oceans. These corals are colonial marine organisms, with the soft body tissues of the animal encased in a skeleton of calcium carbonate, which is left when the animal dies. Brain corals are so-called because the skeletal pattern is reminiscent of the wrinkles in a human brain.

In the Coyote Mountains of eastern San Diego County fossil remains of these coral are usually discovered as fragmentary coral heads. However, occasionally small complete patches of fossil reef are found, still attached to the ancient bedrock of the sea floor.

Species related to these two species of fossil corals occur today only in the Caribbean Sea. For example, Melandrina bowersi is an extinct species, but Melandrina labyrinthiformis, found today in the Caribbean, looks very similar to it. In the same way, Dichocoenia merriami is an extinct species, but has many similarities to D. stokesi which occurs in the Caribbean. How can we explain this phenomenon?

During the late Miocene and early Pliocene, the eastern part of the Pacific Ocean, the Gulf of Mexico and the Caribbean Sea were all linked together to form one continuous marine environment, since the land barrier of the Isthmus of Panama did not yet exist. This means that marine invertebrates such as corals, which have larval forms that occur in the plankton and move with ocean currents, could easily spread from off the coast of California to the Caribbean Sea. With elevation of the Isthmus in the mid-Pliocene the brain corals became extinct in the Pacific while surviving in the Caribbean.

Suggested Reading
Jefferson, George T. and Lowell Lindsay. 2006. Fossil Treasures of the Anza-Borrego Desert. San Diego: Sunbelt Publications.

Text: Margaret Dykens and Lynett Gillette
Illustration: Doug Henderson
Fossil photograph: François Gohier

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