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

Clams of Champions:
The San Diego Formation

Clams of Champions | More Photos

Scott Rugh, SDNHM Paleontology Specialist

San Diego Formation, photo by Hugh Wagner, May 1998 The shell bed from which we collected the clams and other fossils in Balboa Park was deposited during the Pliocene Epoch of the Cenozoic (2 - 3 million years ago). This deposit is called the San Diego Formation.

The San Diego Formation occurs from the south slope of Mount Soledad in San Diego County to Rosarito in northern Baja California, Mexico. These deposits indicate that a large, open, crescent-shaped bay -- roughly the size of modern-day Monterey Bay -- existed during the Pliocene in an area that now includes the heavily populated area of Tijuana, Mexico, and the southwestern corner of San Diego County from San Ysidro to Pacific Beach. In this area, when construction for a new housing development or business complex occurs, fossils of the San Diego Formation are often exposed -- so it is no surprise that the Hall of Champions basement construction revealed new Pliocene marine fossils.

The fossils from the Hall of Champions and the other collecting sites in the San Diego Formation have provided valuable information about what life was like in the Pliocene embayment. The species of invertebrate fossils, such as mollusks, echinoderms, and crustaceans, indicate that the bottom of the Pliocene bay was mostly sandy, with a few rocky areas.

Most of the invertebrate species found are the same or similar to today's species. But included are a few species, such as Dosinia ponderosa and Mitha xantusi, now found in the more tropical Gulf of California -- indicating the climate was slightly more tropical than that of today.

Besides these invertebrate fossils, the fossil bones and teeth of fish and marine mammals are found at many locations in the San Diego Formation. The marine mammals include several species of baleen whales, toothed whales, fur seals, walruses, and sea cows, many of which represent extinct species.

San Diego Formation. Photo Credit: Hugh Wagner, 14 May 1998