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 and 2) During more than 500 million years of evolution, they developed shells with various shapes and ornament, ranging in size from about mm up to more than 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