San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]
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Frequently Asked Questions about Paleontology
Answered by Tom Deméré

What is a fossil?
How deep do you have to dig to find fossils?
Where can I see fossils in San Diego County?
Did dinosaurs ever live in San Diego County?
How do you know how old a particular fossil is?



What is a fossil?

Fossils are the remains and/or traces of prehistoric life. The critical factor is age. Fossils have to be older than 10,000 years, the generally accepted temporal boundary marking the end of the last Pleistocene glacial event. Fossil remains include bones, teeth, shells, and wood. Fossil traces include footprints, burrows, impressions, molds, casts, and coprolites.

How deep do you have to dig to find fossils?

Fossils are found in the sedimentary rock layers in which they were originally buried. So, the question really is, where do these layers occur? The sea cliffs at Torrey Pines State Reserve provide excellent exposures of 48-50 million year old fossil-bearing sedimentary rocks. The fossils in these rocks occur right at the surface because erosion has done the "digging" for us.

Where can I see fossils in San Diego County?

Since fossils occur in sedimentary rock layers, wherever sedimentary rocks are found there is a potential for fossil discovery. As mentioned above, the sea cliffs at Torrey Pines State Reserve offer good opportunities to observe fossils. These fossils generally consist of Eocene-aged (48-50 million years old) shells of estuarine snails and oysters. Because the fossils are in a state reserve there is no collecting allowed. However, this protection ensures that we and future generations will continue to be able to discover and enjoy these paleontological remains.

Did dinosaurs ever live in San Diego County?

Yes. Dinosaur remains have been found in Cretaceous-aged (70-75 million years old) sedimentary rocks as exposed at Sunset Cliffs, La Jolla Bay, and Carlsbad. The types of dinosaurs represented by these local fossils include hadrosaur (duck-billed dinosaur) and nodosaur (armored dinosaur).

How do you know how old a particular fossil is?

The age of a fossil can be determined by several means. One method, called superposition, is based on the position of a fossil in a stacked sequence of sedimentary rock layers. The fossils in the lowest layers are older than the fossils in the upper layers. This method does not give an absolute age for the fossil, only a relative age (older or younger). Absolute age of a fossil is determined by radiometric dating -- a method that relies on the natural radioactivity of certain elemental isotopes. Different isotopes have different rates of radioactive decay and these rates are constant. This constancy of decay rates serves as a "radiometric clock" that allows geochronologists to analyze samples and measure the relative quantities (ratios) of "parent" to "daughter" isotopes. These ratios provide a means for determining when the "radiometric clock" started and therefore the age of the rock layers associated with the fossil.

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