Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1015

380 FOSSIL INVERTEBRATES/Gastropods

larva, and a similar larval type is developed in all
molluscan groups. The trochophore larvae may be
free swimming, as in the ancient gastropod groups
(Patellogastropoda and Archaeogastropoda), or may
occur in egg capsules, as in more advanced gastropods. The last larval stage is termed veliger, which
typically bears two ciliate paddles (velum), sometimes
subdivided into several lobes. If free-swimming gastropod larvae use planktic organisms for their nutrition, their development is termed planktotrophic.
Marine gastropods with such development have
small eggs, but numbering over half a million. Planktotrophic larvae may stay planktic for several months
and thus can be carried for long distances by oceanic
currents. The gastropods, however, developed another ontogenetic strategy in which their larvae were
not dependent on an external food source, but on
the yolk of their eggs. Gastropods with such a nonplanktotrophic development (lecithotrophic) typically produce fewer eggs, which are relatively large.
The larval stages end with a metamorphosis that
involves anatomical and physiological reorganization
of the larval body into the juvenile, post-larval body.
Terrestrial and freshwater gastropods have simplified their development, and their embryonic and
larval stages are fixed on egg capsules or the female
body (direct development). Such ontogenetic changes
considerably decreased their dispersal potential.

The Gastropod Shell
Gastropods are not only one of the most diverse
animal groups, but the morphology of their shells is
extremely varied (Figures 1 and 2). During more than
500 million years of evolution, they developed shells
with various shapes and ornament, ranging in size
from about 1 mm up to more than 1 m (Eocene
Campaniloidea, Caenogastropoda). The shell and its

ornament may be broadly linked to the mode of
gastropod life (e.g., origin of limpet-shaped shells in
unrelated gastropod groups). Generally, the most
ornate shells occur in tropical marine environments,
but freshwater and terrestrial gastropods are often
less ornate.
Protoconch and Teleoconch

In shell-bearing gastropods, the shell grows during
almost the whole of their ontogeny. The part of the
shell formed during the embryonic and larval stages is
called a protoconch (Figure 3), and that growing after
metamorphosis is termed a teleoconch. The main gastropod groups differ in their early development, which
is reflected in their protoconch morphology. The
more ancient gastropod groups (Patellogastropoda

and Archaeogastropoda) have the simplest shell
ontogeny and their protoconchs have only an embryonic shell (protoconch I), which is followed by a
teleoconch (Figures 3B and 3F–3H). On the other
hand, the protoconchs of more advanced gastropods
(Neritimorpha, Caenogastropoda, and Heterobranchia) consist of an embryonic shell (protoconch I)
and a subsequent larval shell (protoconch II). In
most caenogastropods, the larval shells have different
ornament from the teleoconchs (Figures 3K and 3L),
and both shells are coiled in the same direction (such a
condition is termed homeostrophic; Figure 4). In
contrast, in the Heterobranchia with planktotrophic
development, the protoconchs are coiled in the opposite direction to the teleoconchs (Figures 3J and 4).
Such shells are termed heterostrophic. The Neritimorpha form typical, strongly convolute protoconchs
during planktotrophic development, which are

homeostrophic (Figures 3A, 3N, and 4). Higher gastropods with non-planktotrophic development (some
marine, freshwater, and terrestrial gastropods) have
simplified their early ontogeny and thus also the
morphology of their protoconchs. The latter strategy
is documented from the Devonian (400 Ma).
Operculum

The majority of gastropods have a lid-like structure
(operculum) to close their aperture. This operculum is
present in all living gastropods during their larval
stages, but is lost in some adults (e.g., limpets and
the majority of terrestrial gastropods). The operculum is mostly horny (corneous) and may be tightly
(multispiral) or loosely (paucispiral) coiled or concentric. Some gastropod groups have calcareous opercula, and the oldest operculum known is from the
Ordovician (Macluritoidea).
Shell Structure

Most gastropod shells are composed of an outer organic layer (periostracum) and an inner, mostly much
thicker, calcified layer. The colour pattern typical of
many gastropod shells (Figure 1) is formed by different organic pigments which are limited to the periostracum and the uppermost calcified layer. This shell
feature, sometimes reflecting the mode of life, has
been known since the Palaeozoic (Figure 5). The
inner layers of gastropod shells consist of minute
calcium carbonate crystals (aragonite or calcite) in
an organic matrix. There are over 20 structural
types of gastropod shell and, in general, more ancient
groups exhibit more diverse shell structures. The
Patellogastropoda (Eogastropoda) had the most complex shell structure. On the other hand, the majority
of the higher gastropods have developed simple