Geology Reference
In-Depth Information
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Further reading:
K124
Skeletal mineralogy and microstructure:
The calcar-
eous shell (ostracum) is covered by an organic layer
(periostracum). Most gastropod shells consist com-
pletely of aragonite; a few families have a mixed min-
eralogy, with an outer layer of calcite and an inner layer
of aragonite. Conchiolin forms a thin external cover
over the shell (periostracum). The whorls are composed
of one to several layers of calcium carbonate, which
are distinguished by differences in mineral composi-
tion and microstructure. The most abundant micro-
structure in aragonitic layers is crossed-lamellar, char-
acterized by layers of crystals oriented in different di-
rections. Other common microstructures are the nor-
mal prismatic found in calcitic layers (fine calcite prisms
oriented normal or inclined to the outer surface), com-
plex crossed-lamellar, and nacreous (aragonite crystals
arranged in thin sheets and separated by sheets of or-
ganic material; e.g. some Archaeogastropoda). Photo-
graphs of gastropod microstructures can be found in
Majewske (1974).
Small fragments of gastropods are distinguished
from bivalves by the existence of multiple layers of
well-developed crossed-lamellar microstructures, small
composite prisms, and nacreous layers associated with
layers exhibiting dissimilar microstructures.
Preservation:
Most gastropods are preserved as
molds and casts (Pl. 89/8). Shells composed of arago-
nite are subject to two main fates: they are either dis-
solved to form complete or partial molds that may be
subsequently filled with cement, or they may be trans-
formed during recrystallization to Low-Mg calcite by
the simultaneous volume per volume dissolution of ara-
gonite and precipitation of calcite along intervening so-
lution films. In recrystallized shells some trace of the
original microstructures can be retained (Pl. 89/4, 7).
Students may confuse small gastropods with foramin-
ifera but the tests of the latter consisting of calcite are
rarely preserved as casts.
10.2.4.3 Gastropods
Classification:
Three subclasses are differentiated
based on the respirative system comprising the Proso-
branchia, with more than half of all gastropods, the
Archaeogastropoda, Mesogastropoda and Neogastro-
poda (Cambrian to Holocene), the highly diverse
Opisthobranchia (Mississippian to Holocene) and the
terrestrial Pulmonata (Pennsylvanian to Holocene). The
Opisthobranchia and Pulmonata have aragonitic shells.
Most calcite-bearing shells belong to the Prosobranchia.
Gastropods are abundant constituents of limestones
(Fig. 10.43). They are known throughout the Phanero-
zoic, but are most abundant in the Mesozoic and Ceno-
zoic.
Morphology:
Most gastropods have an external uni-
valved, usually unchambered shell that is usually coiled.
Coiling may be either in a single plane or a helical spi-
ral about an imaginary axis. The whorls are tightly
wrapped about the preceding whorl to form a central
pillar (columella, Pl. 89/5), or the whorls are closely
wrapped to form a central cavity. The exterior of the
shells may be smooth or ornamented with ribs, nodes
and spines.
Ecology:
Gastropods live in marine, freshwater and
terrestrial environments. Most gastropods are mobile
benthic organisms, but some have become adapted to
sedentary life and to pelagic life (pteropods).
