Centennial Museum gecko logo

Desert Diary
Fossils/Fossil Confusion


Superficial similarities often mislead the unwary. The novice fossil hunter often forgets that similarities, as differences, may be only skin deep, and assign every vaguely clam-like fossil to the mollusks, the group that contains such shellfish as clams, oysters, and mussels. Many, however, are brachiopods—members of an entirely different group. A basic difference is that the oyster-like mollusks have right and left shells, while the somewhat similar-looking brachiopods have a top and bottom shell.

Confusion between groups of fossil shellfish isn't limited to those two groups. Every snail-like fossil shell doesn't belong to a snail. An extinct group of mollusks quite common in some of our Chihuahuan Desert mountains superficially look like snails, but belong to an entirely different group of mollusks: namely, with the octopuses and squids. These ammonites contain a series of chambers within the shells, a new chamber being formed to house the squid-like creatures as they outgrew the current ones. Snails, though, have a major advantage over ammonites: they survived the great extinction at the end of the age of dinosaurs.
pen and ink

Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.


Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.

external and internal views of a fossil ammonite

An external view (left) and a longitudinal section of a fossil ammonite are shown. Both surfaces have been ground and polished for display. The internal cavities are partially filled by mineral deposits, but most of the original chamber walls are still visible. Photograph by A.H. Harris.