Page 355 - Introduction to Paleobiology and The Fossil Record

342 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
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- commonly lacking shells. Pteropods and sea
rally coiled, hyperstrophic with a robust oper- slugs are typical opisthobranchs.
culum and ranged from the Ordovician to the
Devonian. The pleurotomariines have vari-
ably shaped shells, usually conispiral. They
dominated shallow-water Paleozoic environ-
ments, 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 inter- environments have thick shells and are com-
tidal 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 diversifi ed 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 fi nal
whorl.
The order Neogastropoda contains coni-
spiral, commonly fusiform, shells with a siph- organization, forming a series of parallel-
onal notch; most of the order is carnivorous
and members dominated marine environ-
ments 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
The subclass Pulmonata contains detorted
gastropods, with the mantle cavity modifi ed
as an air-breathing lung. The group probably
ranges in age from the Jurassic to the present,
and is characteristic of terrestrial environ-
ments. 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 diver-
sity 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 refl ect its trophic
function (Wagner 1995). In general terms,
however, gastropods occupying high-energy
monly 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 ani-
mal’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
evolving 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-