Introducing Palaeontology by Patrick N. Wyse Jackson

Author:Patrick N. Wyse Jackson
Language: eng
Format: epub
Publisher: Dunedin Academic Press Ltd

Figure 57 A. The living gastropod Buccinum undatum, the Common Whelk, showing protrusible soft parts; B. An empty shell of Buccinum with shell terminology outlined.

A after Treatise on Invertebrate Paleontology, Part I, Mollusca 1 (1960). Geological Society of America, Boulder, Colorado/University of Kansas Press, Lawrence, Kansas.

B after Clarkson, E.N.K. 1998. Invertebrate Palaeontology and Evolution. Blackwell Science, Oxford.

The head is the mobile, anterior part of the body that bears the mouth, eyes and one or two pairs of tentacles. Situated just inside the mouth is a long muscular movable rasping mechanism that is composed of many minute horny teeth arranged in transverse rows on a tough, flexible ribbon. This mechanism, called the radula, functions for grazing food from rocky surfaces (Figure 101D) and tearing food into particles, and has been used by taxonomists as an aid to classifying living gastropods. Eyes are borne at the tips of the tentacles of some gastropods, but in others they are situated near the base of the tentacles. The gastropods are hermaphrodite and respire by means of gills situated within the body.

The calcareous or aragonitic shell is the structure on which palaeontological study of the group must be based. Most gastropod shells show a wide range of form and structure which can be used in identifying species and genera. Shells of gastropods may be divided into two groups in general: those with little or no coiling (patelliform) such as the modern-day limpet Patella, and those which are partly or very distinctively coiled. Of the coiled forms, some show a flattened test (planispiral) as in Straparollus or Phanerotinus (Figure 58A) from the Mississippian, while in others the coiling can be drawn out into a high spire as in Turritella (Figure 58B). Others, such as the modern top shell, have a trochiform shell with a pyramidal spire and a flat base.

The majority of gastropods are aquatic, and most species live in shallow marine waters. Some have become adapted to brackish and freshwater environments, and two groups, by modifying their breathing apparatus, have invaded the land. Today gastropods are found in a variety of environments. Phanerotinus developed large triangular flattened flanges along its lateral margins, and these suggest that the shell lay on a muddy substrate with the flanges distributing the weight so that the animal did not sink into the mud.

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