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.
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.
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.