San Francisco - Mesozoic (dinosaur-age) marine fossil (ammonite)

Ammonite fossil from Cretaceous sedimentary rocks of the Franciscan Complex, found south of the Golden Gate in San Francisco.

Jurassic and Cretaceous sedimentary rocks of both the Great Valley and Franciscan basement complexes contain fossils of the ancient group of mollusks, ammonite. Ammonites may be ancient relatives of the modern squid and octopus.

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.

Learn more about fossils at the USGS or by visiting the Paleontology Portal Web site.