San Francisco Bay Region Geology and Geologic Hazards

Stories > Why Are There No Dinosaur Fossils in the San Francisco Bay Region?

Why Are There No Dinosaur Fossils in the San Francisco Bay Region?

Dinosaur-age rocks in the region all formed in the ocean.

Dinosaur fossils can only be found in sedimentary rocks that formed at the time the dinosaurs were living (the Mesozoic era, 251-65 million years ago). The shapes of dinosaur fossils suggest to geologists that the dinosaurs were land animals. However, all Mesozoic sedimentary rocks in the region were deposited in an ancient ocean, and so there can be no dinosaur fossils. How do we know that the Mesozoic sedimentary rocks were deposited in the ocean? Because fossils that have been found in them consist exclusively of marine animals. The oldest land-animal fossils found in the region are Miocene-age (23-5.3 million years old) mammals, including primitive horses, mammoths, and hippopotami. One of the most recognizable fossils found in the region is the California state fossil, the Pleistocene-age (1.8 million to 11,500 years old) Smilodon californicus, better known as the Ice Age saber-toothed cat.

image collage - description below
Examples of fossils from the San Francisco Bay region. Mesozoic fossils are all marine, such as (J) Jurassic "clams" (mollusks, genus Buchia), (K) a Cretaceous ammonite, and (L) a Cretaceous ichthyosaur similar to the one pictured here. Land-animal fossils are all Cenozoic in age, such as (M) part of the Miocene beaver skull shown in this drawing, (N) a Pleistocene saber-toothed cat, and (O) a Pleistocene mammoth similar to the one in this drawing.



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