This chapter was lasted updated by the author on January 24, 2018.
J. R. Hendricks. 2017. Gastropoda. In: The Digital Encyclopedia of Ancient Life. http://www.digitalatlasofancientlife.org/learn/gastropoda/
– 11.1 Gastropoda ←
–– 11.1.1 Shell morphology
–– 11.1.2 Caenogastropoda
–– 11.1.3 Heterobranchia
–– 11.1.4 Neritimorpha
–– 11.1.5 Vetigastropoda
–– 11.1.6 Patellogastropoda
–– 11.1.7 Cocculiniformia
–– 11.1.8 Neomphalina
–– 11.1.9 Paleozoic gastropods
–– 11.1.10 Gastropods in the news
Phylum Mollusca, Class Gastropoda
Common names of representatives: snails, slugs, and limpets.
Habitat(s): marine (salt water), freshwater (lakes and streams), and terrestrial (on land).
Feeding type(s): herbivorous (algae or plant eaters), carnivorous predators (meat eaters/hunters), detritivores (feed on dead organic matter), scavengers (feed on dead animals), and parasitic (feed on living animals).
Geological range: Cambrian to today.
Clade defining feature(s): torsion (see below), head present, shell (if present) univalved.
In terms of total diversity (~70,000 living species; Brusca and Brusca, 2003) and range of habitats occupied, gastropods are one of the most evolutionarily successful groups of animals and include snails and slugs. They occupy the world’s oceans, freshwater lakes and streams, and terrestrial ecosystems, including many backyards. Some are algae-eating herbivores, while others are venomous hunters of fish. Their strong, univalved shells have left behind a rich Cambrian to Recent fossil record that has been the focus of many paleobiological studies.
The name “Gastropoda” comes from the Greek roots “Gastro” (= stomach) and “pod” (= foot). Snails were given this name because many have the appearance of crawling around on their stomachs.
Hear pronunciation of “gastropod:”
While gastropods have many different shell shapes (or no shells at all) and lifestyles, they are united by undergoing a process called torsion during their development. Initially, the mantle cavity (which holds the gills and sensory and excretory structures) and anus are positioned at the posterior end of the animal. Later in development, these structures are rotated counterclockwise up to 180 degrees, placing the mantle cavity and the organs it houses–as well as the anus–right above the animal’s head! Further, torsion causes the gut to become U-shaped and important nerves to cross over one another. Why gastropods undergo torsion remains unclear, but they all do it. To complicate matters, some gastropods (e.g., opisthobranchs) undergo torsion early in development, but then the process reverses itself back to nearly the starting condition later in development. Importantly, torsion is not related to the coiling of the shell, and the foot of the animal is unaffected by the process.
Groups of Extant Gastropoda
Recent molecular phylogenetic investigations of gastropod relationships have upended the traditional taxonomic classification scheme for the Class Gastropoda. While we now know that the older classifications (which include groups like “Archaeogastropoda” and “Mesogastropoda”) are no longer tenable, creating a new, stable classification for the Gastropoda remains very much a work in progress. This is in part because molecular sequence data have not yet resulted in phylogenetic hypotheses that consistently show the same relationships among the major groups of gastropods (this is due in part to variations among analyses in both the genetic regions and organisms sampled).
What has dramatically improved, however, is our understanding of which groups of extant species constitute the major clades of Gastropoda. The following seven clades are now generally accepted (click on each for more information):
A phylogenetic hypothesis of relationships for the five major clades (excluding Neomphalina and Cocculiniformia) was published by Zapata et al. (2014, fig. 3) and is redrawn (and simplified) here:
It is important to note that many generalized forms within the Gastropoda have independently evolved multiple times. That is, they are evolutionarily convergent. Notable examples include include “slugs” (gastropods lacking shells into which they can fully retract) and “limpets” (gastropods with cap-like shells). Thus, “slugs” and “limits” do not constitute monophyletic groupings; rather, they are polyphyletic in origin.
Gastropod Diversity Patterns
The graphs below illustrate broad patterns of gastropod diversity over the span of the Phanerozoic Eon, both at the genus and family levels. These data were downloaded from the Paleobiology Database Navigator. Note the broad overall trend of gastropod diversity increasing over time.
Examples of different kinds of Paleozoic gastropods may be viewed here.
Also visit the following Digital Atlas of Ancient Life pages to find out more about gastropod fossils from particular regions of the United States:
References for this Chapter
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