Gastropod Fossils
Fossil Gastropods
Gastropods are the largest class of mollusks with more than 65.000 living species. They are an evolutionary success story since the early Paleozoic Era. This coupled with a shell that is easy to fossilize means that fossil gastropods are abundant in the fossil record. Gastropod means "stomach foot". These animals have a single muscular foot. Other caracteristics of gastropods are tenticles, eyes, a spiral shell, and a feeding organ called a radula, (tongue covered with thousands of tiny teeth to tear apart food). Gastropods can be carnivorous (meat-eaters) or herbivorous (plant eaters).
Gastropods and the Fossil Record
There are over 15,000 species of extinct gastropods that we know of in the fossil record. We know of them because of the many shapes and sizes of shells that they have left behind. Gastropod shells have three layers. A thin, colored outer layer A thin “mother of pearl” inner layer; and A thick calcareous middle layer.
In addition to fossilized shells, molds and casts are very common fossils of gastropods. These fossil types happen when the actual shell dissolves leaving an impression of the shell.
The first gastropods that are commonly accepted showed up during the Ordovician Period
Primary Groups of Gastropods
- Prosobranchs (Subclass Prosobranchia)
- Opisthobranchs (Subclass Opisthobranchia)
- Pulmonates (Subclass Pulmonata)
Gastropod Diversity
The first true gastropod fossils that everybody agrees on were from the Ordovician Period. However early Paleozoic Gastropod fossils are not well preserved.
Fossils with good preservation come from the Silurian Period. Several different species are identified giving evidence of diversification.
By the Carboniferous Period, now extinct gastropods moved onto land and freshwater environments. Even so Paleozoic gastropods are mostly primitive and not related directly to modern forms.
During the Mesozoic Era ancestors of modern gastropods appeared.
The Gastropod Diversity spindle Chart on the right shows relative diversity throughout the Phanerozoic. While there are ups and downs gastropod diversity has steadily increased over time, with the greatest numbers of gastropods happening at the present time. Gastropods seem to be somewhat resistant to the mass extinctions that have decimated other groups of plants and animals.
