Gastropods

Gastropods are the largest and most diverse class of mollusks. They live in marine, freshwater, and terrestrial environments and have exploited the widest variety of habitats and have developed a remarkable range of feeding strategies. Gastropods first appeared in the early Cambrian and reached their peak diversity in the Cenozoic.

Gastropods may have a calcareous shell or be entirely soft bodied. They have a well-developed head and sensory organs and an expanded muscular foot (Fig. 9.3). In terrestrial gastropods the gills are lost and the mantle cavity is modified into an air-filled "lung". The defining characteristic of gastropods is torsion. At the embryonic stage, the visceral mass is rotated through 180°, bringing the mantle cavity to the front. In this position water currents flow more easily into the cavity. The shell coils to accommodate the gut, which is more manageable when coiled.

Inhalant siphon: mantle extension used to direct water to gills (within —^Jjjfy that closes shell

Sensory organs is withdrawn

Head Mouth Foot: flattened muscular foot used for locomotion

Fig. 9.3 General gastropod morphology.

There are three main subclasses of gastropods (Table 9.2). Although the classification is based on soft-part anatomy, mainly on the nature of the respiratory system, fossil gastropods are assigned to the different subclasses on the basis of shell shape.

Ecology/evolutionary history

Cambrian gastropods were typically marine, herbivorous grazers with low, coiled shells. By the Carboniferous, forms with a siphonal notch were common, indicating the presence of a siphon and therefore an infaunal mode of life. Palaeozoic gastropods generally occupied shallow water environments and were greatly affected by the end-Permian extinction.

During the Mesozoic, prosobranchs diversified and deep-burrowing, long-siphoned prosobranchs originated in the Cretaceous. These forms dominate the gastropod fauna today. Carnivorous gastropods were important predators in the Cenozoic. The Cenozoic also saw the radiation of gastropods into freshwater environments and the appearance of planktic opisthobranchs. Radiation of the air-breathing pulmonates into the terrestrial environment began in the Jurassic. Today, gastropods are one of the most common groups of organisms living in the sea, in fresh water, and on land.

The aragonitic shell of most gastropods, which does not preserve well, and the high level of convergence shown by the group (that is, the large number of distantly related species that look alike because they share the same life habit) makes the evolution of fossil gastropods very difficult to elucidate.

Table 9.2 The main subclasses of gastropods.

Subclass

Respiratory system

Shell morphology

Habitat

Examples

Prosobranchiata (Lower Cambrian-Recent)

Gill in front

Cap shaped or conispiral

Mostly marine

Limpets, winkles, and whelks

Opisthobranchiata (?Carboniferous-Recent)

Gill behind, due to detorsion

Shell lost or very reduced

Marine

Sea-slugs and sea hares

Pulmonata (Mesozoic-Recent)

Mantle cavity modified into "lung"

Where present, conispiral or planispiral

Terrestrial

Land snails and slugs

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Responses

  • Savanna
    What forms gastropods fossils?
    8 years ago
  • kisanet abel
    How to classify fossil gastropods?
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  • prima
    How to classify shell fossils?
    27 days ago

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