Gastropods

Hexaplex tripteroides, (a caenogastropod) from the Palaeogene (Eocene) of southern England

Gastropods (formally Gastropoda) make up a large group (a class) of molluscs. They have a muscular foot, eyes, tentacles and a special rasp-like feeding organ (the radula) composed of many tiny teeth. Most gastropods have a coiled or conical shell, which may be extremely reduced in some species or lost entirely as in slugs. Gastropods evolved early in the Cambrian, but since the Palaeogene they have become the most common of molluscs, inhabiting aquatic and terrestrial environments. We focus here on those shelled forms that are normally found as fossils: the Prosobranchia, Pulmonata and, although rather rare, Opisthobranchia.

Gastropods: their biology

Gastropods can be recognised by their large foot, tentacles, coiled shell and the presence of torsion (the body is twisted round so that the anus, reproductive organs, mantle cavity and gills all point forwards). Gills occur in most aquatic forms, but in land snails, part of the mantle cavity is closed off to form a lung. Some marine gastropods, especially those that live on a muddy sea floor, have a tube (siphon) protruding from the front of the shell through which clean water is drawn into the mantle cavity.

Caenogastropod
Archaeogastropod
Pulmonate gastropod

The shell, which is the part that may be fossilised, is constructed in three layers: a thin, coloured outer layer, a thin mother-of-pearl inner layer and the thick calcareous middle layer. The shell may be planispirally coiled, but more usually it is helicoidal forming a spire with the original juvenile shell (protoconch) preserved at its apex. Sometimes there is a hollow tube-like canal that holds the siphon during life. Many species carry a horny lid (operculum) on their foot to close the aperture of the shell after retracting inside.

Planispiral
Globose
Fusiform

Trochiform
Turreted

The shell of Volutispina luctator showing some of its different parts.

Prosobranchs

Prosobranchs have strong torsion in both males and females. They inhabit marine and fresh water habitats.

There are two groups of prosobranchs. Archaeogastropods have distinctive gills, two auricles in the heart and some have paired gills and kidneys. Caenogastropods have only one gill, one kidney, one auricle in the heart and sometimes a siphon and proboscis.

The archaeogastropod Emarginulina crassa, from the Pliocene of southern England, has a simple conical shell. The slit in the front was for exhalant water currents that washed waste away from the head.
Pleurotomaria gigantea, an archaeogastropod from the early Cretaceous of southern England. It has a trochiform shell. Modern species are primitive in that they have paired gills.
Euomphalus pentangulatus, an almost planispiral archaeogastropod found in Ireland, inhabited tropical seas in early Carboniferous times.
Clavilithes macrospira, a caenogastropod from the Eocene of southern England. The fusiform shape is due to the presence of a long siphonal canal. The siphon was used to draw clean water across the gill.

Pulmonates

Galba longiscata (a basommatophore) from southern England lived in fresh waters during the Palaeogene (Eocene to Oligocene). This thin-shelled gastropod grazed on plants growing around lake shores.

Pulmonates inhabit terrestrial environments, although a few have returned to live in fresh water. They have a lung in the mantle cavity, generally lack an operculum and are hermaphrodites (there are no separate males and females).

There are two groups of pulmonates. Basommatophores, have a single pair of tentacles with eyes at the base of each, and are often found in fresh water environments. Stylommatophores have two pairs of tentacles, with the eyes at the end of the posterior pair, and the shell may be reduced or lost altogether.

Opisthobranchs

Diacria trispinosa, a pteropod that swam in the ocean waters of the North Atlantic during the Quaternary. Sometimes millions of pteropod shells accumulate to form an ooze on the ocean floor.

Opisthobranchs may have a coiled shell, but some have lost the torsion characteristic of gastropods and have become bilaterally symmetrical. Their foot is fin-shaped and used for swimming and their shells are very small, thin and fragile, and in some species it has been lost entirely. It is for this reason that these gastropods are very rarely found as fossils. They live in marine environments and an example is the pelagic (open sea) pteropod or 'sea butterfly'.

Gastropods: their environment

The caenogastropod Purpuroidea lived in high energy, shallow water conditions during the mid Jurassic.
The primitive, almost bilaterally symmetrical archaeogastropod, Bellerophon, found in Leicestershire, lived in shallow tropical waters during the early Carboniferous.
Poleumita lived in the warm, clear, marine waters that covered western England during the Silurian.
Anchura carinata, an early Cretaceous caenogastropod from Folkestone, south-eastern England, lived on the muddy sea floor and ate small particles of detritus.
The caenogastropod <em>Turritella sulcifera</em>, from Hampshire, southern England, searched for food by burrowing into the muddy sea floor during the Palaeogene (Eocene).
Pulmonate, fresh water <em>Planorbina</em> from the Oligocene of the Isle of Wight. Living species of this basommatophorean gastropod are able to secrete threads which are attached to objects and used by the animal to ascend and descend through the water.
The land snail Helix nemoralis (a pulmonate stylommatophore) from the Quaternary of Cambridgeshire was active in cool, wet periods, but sheltered in damp soil or in shade during hotter, drier times.
During the Late Jurassic Bourguetia saemanni from Malton, Yorkshire, occupied shallow water habitats where it grazed on algae.
Natica multipunctata, a carnivorous Pleistocene caenogastropod from southern England. Having secreted acid to soften the shell of its molluscan prey, modern Natica bores a hole with its radula to reach the flesh inside.
Gastropods: their environment

Gastropods inhabit all aquatic environments from the deepest oceans, where they may live beneath five kilometres of water, to small shallow, fresh water ponds. They are one of the few invertebrates to have colonised the land and can live at altitudes of 6000 metres above sea level. As they can live in so many different environments, they have become the most diverse type of mollusc.

Aquatic gastropods evolved in the Cambrian and began to colonise all the marine habitats. Some crawled over the sea bed, others burrowed into the mud and sand, while yet others preferred to attach themselves to firm surfaces. More recently some began to float or swim and pelagic (open sea) species evolved. These are known as pteropods or sea butterflies.

During the Carboniferous, gastropods began to live in fresh water and terrestrial snails probably evolved from these species. Life out of the water brought two big problems: how to breathe and how to prevent drying out. They solved the first problem by evolving lungs. As they require high humidity and wet conditions to be active, gastropods solved the second problem by aestivation.

As gastropods are classified mainly by the soft parts of their body — the parts that are not preserved — it may be difficult to place fossils into subclasses and orders.

Aestivation is the opposite of hibernation. Creatures that hibernate go to sleep during cold winter months and wake up in the warmer spring and summer months. Animals that go to sleep during hot, dry periods and wake up during colder, wetter months are said to aestivate.

Gastropods: their geological history

Biological events in gastropod history

The first gastropods evolved from an unknown bilaterally symmetrical mollusc ancestor in the early Cambrian, but they became common during Palaeozoic times. Between the Cambrian and Devonian, gastropods were entirely marine, but by the Carboniferous some had entered non-marine waters and land snails may have evolved by the late Carboniferous.

At the end of Permian times there was a mass extinction event, and gastropods did not escape. However, with the Mesozoic, many new species evolved, including high spired, burrowing forms and some gastropods grew to an enormous size (e.g. some of the cowry shells). Gastropods colonised marine, brackish and freshwater habitats as well as the land by this time.

The maximum development of the gastropods has been in the last 65 million years. This was a time of rapid evolutionary radiation of benthic (bottom-dwelling) species and, starting in the early Eocene, pelagic pteropods evolved as well. Terrestrial gastropods became particularly common during the Palaeogene and it was probably at this time that shell-less gastropods also developed, but they are not found as fossils.

In all about 105 000 living and 15 000 fossil gastropod species are known.

Slugs and snails and puppy dog tails

Gastropods for the wealthy ...

Send for Fulvius Lupinus! There must be a way of making these things edible!

In Roman times, one of the greatest signs of wealth was the purple toga. The dye was extracted from the gastropod Murex which is found in parts of the Mediterranean Sea. Purple dye was very expensive and only the very wealthy could afford clothing of this colour. In fact only certain people were permitted to wear it. Roman senators were allowed to have a purple stripe on their toga, but only the emperor could dress entirely in purple.

... and for the gourmet

The Roman or edible snail (Helix pomatia), Britain's largest land snail, grows up to 10 cm in length.

Snails were one of the favourite foods of the Roman gourmet and they appeared on the menus of feasts marking special occasions. It was a Roman called Fulvius Lupinus who first discovered that snails tasted best when they were fattened up on milk until they became so large that they could not retract into their shell.

Patella, a limpet that lives around the coast of Great Britain.

Other kinds of sea snails, like winkles and whelks, are also good to eat and so are limpets. The limpet Patella (an archaeogastropod) has a simple conical shell and lives attached to rocks. One of the traditional recipes of the Scottish Isle of Colonsay is limpet stovie, a layered bake of limpets and potatoes.

Seaside rock

Limpets cause about 30% of the erosion along the coast of Sussex, England. They eat chalk as they graze on the algae and hollow out places to shelter during low tide. Each limpet eats almost 6 grams of rock a year and when you consider how many millions of limpets there are, this amounts to a lot of chalk! The rocks of the Sussex foreshore are being lowered by up to 1.5 mm per year and this can contribute to damaged sea defences and landslides.

Bigfoot

Filholia elliptica, from the Oligocene of southern England, is believed to have laid some of the largest known fossil gastropod eggs, which are up to 30mm long.

The biggest living gastropod is the 'sea hare' Aplysia californicus, which is found off California and known to grow to over 7 kilograms. By comparison, the 27cm long African Giant Snail (Achatina fulica), the largest land snail, weighs only half a kilogram. Britain's largest land snail — the edible snail — is only about one third of this. Gastropod eggs are usually small, but the largest (which have a calcareous shell) are laid by some of the giant tropical snails and may be up to 45mm long. Huge fossil gastropod eggs have been found in Palaeogene (Oligocene) rocks of southern England.

3D model

LIVCM 8.7.70.AC – Holotype
Load 3D model (LIVCM 8.7.70.AC – Holotype)

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