San Francisco Bay Region Geology and Geologic Hazards
Stories > East of Livermore - Mesozoic (dinosaur-age) marine fossil (ichthyosaur)
East of Livermore - Mesozoic (dinosaur-age) marine fossil (ichthyosaur)The only fossil of a large Mesozoic animal that has been found in the San Francisco Bay region is a small part of an ichthyosaur preserved in chert of the Franciscan Complex near Corral Hollow east of Livermore (the fossil was actually found in a chert boulder that had been washed into Corral Hollow from the nearby chert outcrops in the northern Diablo Range).
Although ichthyosaur looks like a fish or dolphin, it was really a marine reptile.
Fossils form when the remains of plants and animals, or traces of their lives such as footprints, are buried in sediments and preserved when the sediments are converted to sedimentary rocks. Sometimes the original animal or plant material is preserved, other times only an imprint or mold is preserved, and sometimes the original material is replaced by new material in the same shape as the original. The ichthyosaur fragments found in Franciscan Complex chert had been completely replaced by silica.
Learn more about ichthyosaurs.