San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Paleontology

Introduction
Imperial Formation
Palm Spring Formation
The Ethics of Fossil Collecting

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Paleontology and Our Local Desert

Imperial Formation

In the Fish Creek and Coyote Mountains are preserved a thick sequence of sandstone and siltstone strata which geologists have named the Imperial Formation. These rocks are thought to have been deposited approximately 5-7 million years ago in a northern extension of the proto-Gulf of California.

The Imperial Formation locally contains abundant remains of marine invertebrate fossils. Most distinctive are remains of scallops, pen shells, oysters, whelks, sea urchins, and corals. In all, over 200 different fossil marine invertebrate species are known from the Imperial Formation.

Recent field work by staff of the Natural History Museum have recovered rare remains of marine vertebrates from these rocks including remains of bat ray, white shark, tiger shark, giant barracuda, pufferfish, sea cow, and baleen whale. Remains of an early walrus have also been collected from the Imperial Formation.

One of the more fascinating aspects of the Imperial Formation fossils has been the recognition of certain molluscan species having modern counterparts living today in the Caribbean Sea. In fact, some workers estimate that as many as 16% of the Imperial Formation fossil molluscs represent Caribbean species. The remainder of the fossil molluscan species are related to eastern Pacific or Gulf of California forms.

What does this mean? It means that 5-7 million years ago when the Imperial Formation was being deposited, the proto-Gulf of California had a direct connection with the Caribbean Sea. This connection was through the area of modern day Panama, prior to elevation of the Isthmus and the formation of a land connection between North and South America. Additional paleontologic and geologic evidence confirms this hypothesis of an open connection between the two oceans.

From this example we can see how our local fossil deposits lend support to general ideas of earth history and help to define the changes in geography which have occurred -- another reminder of the dynamic nature of the world we inhabit.

Thomas A. Deméré, Ph.D.
Curator, Department of Paleontology