Description: Gastropods are the most diverse and abundant type of mollusks, with nearly 35,000 living and 15,000 fossil species identified so far. The group includes snails, slugs, conchs, whelks, and limpets. Like the familiar snail, most gastropods have a single coiled shell (slugs being a notable exception). A variety of fossil gastropods occur in the Pennsylvanian and Permian rocks of eastern Kansas.
These common Kansas fossil gastropods show some of the variation in size, shape, and external markings. From left to right, top to bottom: Turritella (base of the Church Limestone Member of the Howard Limestone, Pennsylvanian, Marion County); Worthenia (shale below the Stanton Limestone, Pennsylvanian, Elk County); Soleniscus (Florena Shale Member of the Beattie Limestone, Permian, Cowley County); Baylea (Shawnee Group, Pennsylvanian, Jefferson County); Meekospira? (Stranger Formation, Pennsylvanian, Douglas County); Bellerophon (Lawrence Shale?, Pennsylvanian, Douglas County).
The earliest undisputed gastropods date from the Late Cambrian Period, around 500 million years ago. Some paleontologists think gastropods are even older, based on a small, shelly fossil called Aldanella, known from Lower Cambrian rocks, but others think Aldanella is a worm. Either way, by the end of the Cambrian, gastropods were abundant and diverse, and they continue to be so up to the present day.
In a typical shelled gastropod, such as the snail, the soft body parts include those that normally extend outside the shell--the head and foot--and those that remain inside--the mantle and visceral mass. Because the fossil record of gastropods is based almost exclusively on the shells, paleontologists rely on living gastropods for information about the anatomy of their ancient relatives.
The gastropod head bears the most obvious sensory organs, at least one pair of hornlike tentacles that are generally located above the mouth. Most also have two eyes, situated along or near the base of the tentacles. The mouth is usually located on the underside of the head or at the end of a long retractable snout. Inside the mouth is a feeding structure called the radula, made up of numerous tiny teeth (as many as 250,000) that scrape against a horny plate in the upper part of the mouth to shred food.
The foot is typically a broad, tough, muscular structure with a flattened base; it moves along a surface by means of small undulations. In land snails or slugs, the foot contains mucous glands that secrete a slimy substance that facilitates progress over a dry surface; a similar kind of lubricating substance is also secreted by some marine snails.
Inside the shell, the mantle is a heavy fold of tissue that covers the visceral mass and lines the inside of the gastropod shell. The mantle also secretes the shell. The visceral mass fills the shell cavity (which can be a very long, narrow, twisted space) and contains the heart, gonad, digestive gland, liver, kidney, and excretory organs.
The gastropod shell forms a hollow, twisted or coiled cone that increases in width from its initial point, called the apex, to the opening from which the head and foot protrude. In many gastropods, a platelike cover, the operculum, is attached to the foot and closes the shell opening when the head and foot are retracted within the shell.
The shells of different gastropods vary enormously, and these variations are used to distinguish one species from another. Most gastropod shells are coiled. This coiling may be in one plane, similar to the shells of coiled ammonoids. Other gastropod shells may be coiled in such a way as to produce spires of varying heights. The outer surface of the shells may be ornamented with ridges, grooves, bumps, spines, or other markings.
This Pennsylvanian gastropod, which probably belongs to the genus Meekospira, has a shell with a very high spire. It was collected from the Leavenworth Limestone Member of the Oread Limestone in Douglas County.
A series of raised bumps spiraling out from the apex distinguishes these three specimens of Trepospira, collected from the shale below the Stanton Limestone in Elk County.
The earliest gastropods were exclusively marine, but by the Mesozoic Era, about 248 million years ago, many had adapted to terrestrial and freshwater environments. During their long history, gastropods have developed many different ways of obtaining food. Some are carnivores, while others are herbivores, omnivores, deposit feeders, scavengers, suspension feeders, and parasites. Some carnivorous gastropods use their radula to rasp through the shells of other gastropods or bivalves, drilling a neat, round hole, through which they inject a muscle relaxant. Fossil shells with drill holes are evidence of gastropod predation, probably dating back to the Devonian Period (415 to 360 million years ago).
The long fossil record and present-day abundance and diversity of gastropods attests to their evolutionary success. Over time, they have withstood a number of major extinction events that wiped out other creatures.
In Kansas, fossils of marine snails are common in the Pennsylvanian and Permian rocks of the eastern part of the state and in the Cretaceous rocks farther west. Fossils of terrestrial and freshwater snails are also common in some Pleistocene deposits in northwestern and northeastern Kansas.
Straporallus (Amphiscapha) is a common fossil in the Pennsylvanian rocks of eastern Kansas. This specimen is embedded in a chunk of limestone taken from the Drum Limestone of Montgomery County.
Stratigraphic Range: Upper Cambrian to Holocene.
Taxonomic Classification: Gastropods belong to the kingdom Animalia, phylum Mollusca, class Gastropoda.
Boardman, Richard S., Cheetham, Alan H., and Rowell, Albert J., 1987, Fossil Invertebrates: Boston, Blackwell Scientific Publications, 713 p.
Clarkson, E. N. K., 1979, Invertebrate Palaeontology and Evolution, 3rd Edition: London, Chapman and Hall, 434 p.
Levin, Harold L., 1999, Ancient Invertebrates and Their Living Relatives: Upper Saddle River, New Jersey, Prentice Hall, 358 p.
Moore, Raymond C., Lalicker, Cecil G., and Fischer, Alfred G., 1952, Invertebrate Fossils: New York, McGraw-Hill Book Co., 766 p.
Moore, R. C., ed., 1960, Mollusca 1, Part I; in, Treatise on Invertebrate Paleontology: Boulder, Colorado and Lawrence, Kansas, Geological Society of America and The University of Kansas, 351 p.
Text by Liz Brosius, Kansas Geological Survey. Drawing of gastropod at top of page by Alan Kamb, KU Natural History Museum, Invertebrate Paleontology. Other illustrations by Jennifer Sims, Kansas Geological Survey; photographs by John Charlton, Kansas Geological Survey.