Class Gastropoda

The gastropods, the "belly-footed" mollusks, are the most varied and abundant of the mol-luskan classes today. The group includes the snails and slugs, forms both with and without a calcareous shell. During a history spanning the entire Phanerozoic, gastropods evolved creeping, floating and swimming strategies together with grazing, predatory and parasitic trophic styles.

Most gastropods are characterized by torsion in which the mantle cavity containing the gills and anus, excretory and reproductive openings comes to lie above the head (Fig. 13.11). The advantages of this arrangement are unclear. In fact, torsion seems to be distinctly disadvantageous because it involves the loss of one of the gills and/or development of a peristomal slit allowing separation of inhalant and exhalant currents. The first larval stage, the trochophore, is usually fixed. However, the second, veliger, phase is free-

swimming and unique to the mollusks. During development, the head and foot remain fixed but all the visceral mass, the mantle and the larval shell are, in effect, rotated through 180°. The process of torsion is characteristic of the Gastropoda, although in some groups there may be secondary reversal. The coiling of the gastropod shell is unrelated to the rotation of the soft parts. Following torsion, the mantle cavity and anus are open anteriorly and the shell is coiled posteriorly in an endo-gastric position, in contrast to the exogastric style of the "monoplacophoran" grade shell.

The gastropod shell is usually aragonitic, usually conical with closure posteriorly at the pointed apex, and open ventrally at the aperture. Each revolution of the shell or whorl meets adjacent whorls along a suture, and the whorls together comprise the spire. Tight coiling about the vertical axis generates a central pillar or columella. The aperture is commonly oval or subcircular and is circumscribed by an outer and inner lip. The head emerges at the anterior margin of the aperture, where the aperture may be notched or extended as a siphonal canal supporting inhalant flow through the siphons. Material is ejected



Gastropoda Cypr
Soft to firm substrate
Shallow Burrowers

Figure 13.9 Life modes of bivalve mollusks: (a) shallow and deep burrowers into soft to firm substrates, (b) epifaunal swimming, attached or resting on soft to firm substrates, and (c) boring into hard substrates. (From Milsom & Rigby 2004.)

Figure 13.9 Life modes of bivalve mollusks: (a) shallow and deep burrowers into soft to firm substrates, (b) epifaunal swimming, attached or resting on soft to firm substrates, and (c) boring into hard substrates. (From Milsom & Rigby 2004.)

Box 13.5 Rudists: bivalves disguised as corals

The rudists were aberrant heteroconch bivalves that range in age from the Late Jurassic to the Late Cretaceous and occupied the Tethyan region. During a relatively short interval they developed a bizarre range of morphologies, and although many groups apparently mimicked corals, the rudists were probably not reef-building organisms. The rudists were inequivalved with a large attached valve, usually the right valve of conventional terminology, and a small cap-like free valve. Virtually all rudists had a single tooth flanked by two sockets in the attached valve, and two corresponding teeth and a socket in the free valve. The valves functioned with an external ligament and pairs of adductors attached to internal plates or myophores. Three growth strategies have been identified (Fig. 13.10). Elevators had tall conical shells with a commissure raised above the sediment-water interface to free the animal from the risk of ingesting sediment. The elevators were thus similar to solitary corals, suggesting a possible reef-building strategy. Clingers or encrusters were flat, bun-shaped forms that usually adhered to hard substrates. The recumbents had large shells, extending laterally extravagantly over the seafloor like large calcified bananas. The rudists occupied carbonate shelves throughout the Tethys region, with their larvae island hopping around the tropics, often growing together in a gregarious habit; clusters or clumps probably trapped mud in molluskan-rich structures. As noted above it now seems likely that the rudists were never true reef-building organisms although they came close to fulfilling that mode of life.

Thomas Steuber (University of Bochum) has developed a comprehensive database on rudist bivalves together with spectacular pictures of rudist accumulations. Study of this comprehensive database, and a smaller dataset that can be used to reconstruct ancient paleogeographic associations at http://www.blackwellpublishing/paleobiology/, can be used for a variety of exercises. The small dataset investigates the biogeography of Campanian rudists, emphasizing their relationship to the paleotropics (Tethyan province) on

Bivalves Rudist
Figure 13.10 Rudist growth strategies: encrusters (A, B, H and I), elevators (C, D and E) and recumbents (F, G). (From Skelton, P.W. 1985. Spec. Pap. Palaeont. 33.)

through the exhalant slit in the outer lip. During ontogeny the inactive track of the slit is successively overgrown with shell material to form the selenizone, the calcified track of the slit band separating the siphons from the mouth.

The gastropod shell is normally oriented with the aperture facing forward and the apex facing upwards. If the aperture is on the right-hand side, the shell is coiled clockwise in a so-called dextral mode; sinistral shells have mantle flap shel

mantle flap inhalent penis I siphon""""^

mantle flap mantle flap inhalent penis I siphon""""^

siphonal canal operculum opercular lobe foot shel

tentacle proboscis-

mouth tentacle proboscis-

mouth siphonal canal operculum opercular lobe foot patelliform patelliform convolute discoidal discoidal digitate convolute turbinate digitate yj turbinate turreted

Figure 13.12 Gastropod shell shapes.

bi-conical bi-conical spiral ornament inductura on inner (= columellar) lip aperture siphonal canal growth line axis of coiling

spire body whorl callus whorl over , . umbilicus spiral ornament inductura on inner (= columellar) lip aperture spire

growth line operculum

siphonal canal growth line

selenizone slit umbilicus selenizone slit axis of coiling

umbilicus exhalent notch , plamspmd hyperstroplhi£

inhalent siphonal notch

exhalent notch , plamspmd hyperstroplhi£

aperture sinistral denticle inhalent siphonal notch aperture sinistral denticle conisprial

Figure 13.11 Gastropod morphology: (a) annotated reconstruction of a living gastropod, (b) annotated shell morphology of three gastropod shell morphotypes, and (c) main types of gastropod coiling strategy.


Figure 13.11 Gastropod morphology: (a) annotated reconstruction of a living gastropod, (b) annotated shell morphology of three gastropod shell morphotypes, and (c) main types of gastropod coiling strategy.

the opposite sense of coiling. The shell surface is commonly modified by strong growth lines, ribs, tubercles and projections. Many gastropods have an operculum covering the aperture.

Gastropods developed a variety of shell shapes. Eight different morphologies ranging from the simple patelliform to the complex digitate shell are illustrated in Fig. 13.12 as a sample of the large amount of exoskeletal variation in the group.

Main gastropod groups and their ecology_

Gastropods have been divided into three classes largely based on information from their soft parts. Three subclasses are traditionally defined on the basis of the radula and their respiratory and nervous systems, although some of the groups may not be truly monophyletic: (i) the Prosobranchia are fully torted with one or two gills, an anterior mantle cavity and cap-shaped or conispiral shells; (ii) the Opisthobranchia are untorted (having gone through torsion followed by detorsion) with the shell reduced or absent, and the mantle cavity posterior or absent; and (iii) the Pulmonata are untorted with the mantle cavity modified as a lung, and the shells are usually conispiral. Fossil taxa are usually assigned to these categories on the basis of similarities in shell morphology with their living representatives.

The prosobranchs are mainly part of the marine benthos with a few freshwater and terrestrial taxa. The primitive members of the group, the Eogastropoda, are marine, mainly grazing herbivores with cap-shaped or low-spired forms and include a diverse set of superfamilies including the following groups. Macluritines have large, thick shells lacking a slit-band; for example Maclurites is planispi-rally coiled, hyperstrophic with a robust oper-culum and ranged from the Ordovician to the Devonian. The pleurotomariines have variably shaped shells, usually conispiral. They dominated shallow-water Paleozoic environments, although today the group is restricted to deeper-water settings. Pleurotomaria had a trochiform shell with a broad selenizone; the older Ordovician-Silurian Lophospira had a turbinate shell. The trochines are typical of rocky coasts, grazing on algae; Paleozoic taxa, for example the Ordovician-Silurian Cyclo-nema, were probably scavengers, whereas some, such as the Devonian Platyceras, are commonly attached to the anal tubes of cri-noids and were parasites. The patellines, such as the limpets like Patella, have cap-like shells and they graze on algae on rocks in the intertidal zone. The euomphalines were mainly discoidal, such as Euomphalus, which ranged from the Silurian to the Permian.

The murchisoniines were a more advanced group that ranged from the Ordovician to the Triassic, possessing high-spired shells with a siphonal notch. Murchisonia is a long-ranging genus (Silurian-Permian).

Finally, the precise systematic position of the bellerophontines is still unresolved; they were planispirally-coiled shells with a well-developed slit, ranging in age from the Cambrian to the Triassic. The long-ranging Bellerophon was very common in the Early Carboniferous.

The order Mesogastropoda consists of pro-sobranchs that have lost the right gill and usually have conispiral shells with siphonal notches. These taxa have diversified in marine, freshwater and terrestrial environments. Turritella is a high-spired, multiwhorled shell with strong ribs and a simple aperture, whereas Cypraea is involute with the earlier whorls completely enclosed by the final whorl.

The order Neogastropoda contains coni-spiral, commonly fusiform, shells with a siph-onal notch; most of the order is carnivorous and members dominated marine environments from the Tertiary onwards. Neptunea has a large body whorl and a short siphonal canal whereas Conus is biconical with a narrow aperture and a siphonal notch.

The subclass Opisthobranchia includes marine gastropods with reversed torsion and commonly lacking shells. Pteropods and sea slugs are typical opisthobranchs.

The subclass Pulmonata contains detorted gastropods, with the mantle cavity modified as an air-breathing lung. The group probably ranges in age from the Jurassic to the present, and is characteristic of terrestrial environments. Planorbis has a smooth, planispiral shell with a wide umbilicus whereas Helix is smooth and conispiral and Pupilla has a smooth pupiform shell.

The gastropods show a considerable diversity of form across the entire class (Fig. 13.13). It is difficult to relate given morphotypes to particular life modes although the overall morphology of the shell can reflect its trophic function (Wagner 1995). In general terms, however, gastropods occupying high-energy environments have thick shells and are commonly cap-shaped or low-spired, whereas shells with marked siphonal canals are adapted to creeping across soft substrates. Carnivores are usually siphonal whereas herbivores have complete apertural margins and commonly grazed on hard substrates. Thin-shelled taxa are typical of freshwater and terrestrial environments.

Gastropod evolution_

There is no general agreement on the origin of the gastropods. Currently the group is thought to have been derived from a mono-placophoran-type ancestor by torsion and development of an exogastric condition, where the shell is coiled away from the animal's head. An origin from among coiled forms such as Pelagiella may link the mono-placophoran grade through the Tommotian Aldanella to the gastropods.

The monophyly of the gastropods has been questioned. It is possible that many of the traditional groups, for example the archaeo-gastropods, mesogastropods, opisthobranchs and pulmonates may be grades of gastropod organization, forming a series of parallelevolving clades. In particular the archaeogas-tropods have been shown to be polyphyletic and they are no longer considered to be a natural grouping. Nevertheless, the neogas-

Silurian Biology

Figure 13.13 Some gastropod genera: (a) Murchisonia (Devonian) (x1.25), (b) Euomphalus (Carboniferous) (x0.5), (c) Lophospira (Silurian) (x0.5), (d) Patella (Recent) (x1), (e) Platyceras (Silurian) (x1), (f) Neptunea (Plio-Pleistocene) (x0.6), (g) Viviparus (Oligocene) (x0.8), and (h) Turritella (Oligocene) (x1). (Courtesy of John Peel.)

tropods appear to comprise a unified group derived from either advanced eogastropods or primitive mesogastropods during the Late Mesozoic.

Most Paleozoic gastropods were probably herbivores or detritus feeders. Drill holes in brachiopod shells, however, suggest that a few genera were carnivores and some, such as Platyceras, were parasites. The class became more important during the Late Paleozoic and the Mesozoic when many more predatory groups evolved. However, during the

Cenozoic, gastropods reached their acme with the neogastropods in particular dominating molluskan nektobenthos.

Gastropods are not particularly good zone fossils, although nerineid gastropods are stratigraphically useful in parts of the English Middle Jurassic in the absence of ammonites. Gastropods are generally associated with particular facies and few rapidly evolving lineages are known in detail. Nevertheless, microevolutionary sequences in the genus Poecilizontes from the Pleistocene of Bermuda, described in detail by Stephen Jay Gould, suggest that new subspecies evolving by allo-patric speciation arose suddenly by pedomor-phosis (see p. 145). These rapid speciation events, separated by intervals of stasis, are strong supportive evidence of the punctuated equilibrium model of microevolutionary change. Moreover, in a classic study of Late Tertiary snails from Lake Turkana, Kenya, Peter Williamson (1981) suggested there had been punctuated changes in 14 separate lineages (see also p. 123).

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