Gastropod opercula are classified here on a new morphogenetic basis, which distinguishes three main types: (1) flexiclaudent spiral (mostly multispiral) operculum, the shape of which does not coincide with that of the aperture, (2) rigiclaudent spiral (usually paucispiral) operculum, the shape of which fits that of the aperture, and (3) rigiclaudent concentric operculum, also aperture-fitting. The first type fits by flexing into the aperture and is secreted when the soft parts are partly or wholly extended (i.e., when the operculum is not in a closed position). The other two types do not flex upon retraction (except at the very margin) and grow when the operculum closes over the aperture, with or without rotation. A study of opercular types at the family level confirms the systematic and evolutionary significance of opercula. Types 1 and 2 are the only ones present in archaeogastropods, Type 1 being predominant. Opercula (if present) in Neritopsina are always rigiclaudent. Within Caenogastropoda, Type 2 predominates; the only flexiclaudent spiral opercula are found in certain basal cerithioidean families. Concentric opercula are predominant in higher neotaenioglossans and exclusive in neogastropods. Except for one family, opercula in Heterostropha are always rigiclaudent spiral. Morphological, systematic, and histological criteria point to the flexiclaudent spiral operculum as the ancestral form. This leads us to propose the “periostracum shaving” model in prosobranchs to account for the origin of this kind of operculum. According to this model, in the earliest trochospiral gastropods the periostracum ceased to serve a shell-formation function at the band of overlap between whorls (the parietal band). The periostracal band was then extruded from the shell to constitute an incipient operculum, taking on the appearance of a spiral strip coiling opposite to the shell. The parietal segment of the periostracal groove migrated toward the epipodium and became independent from the rest of the mantle. The concomitant development of an opercular disc allowed the successive turns of periostracal strip to seal together. In this way, a spiral operculum emerged, coiling counterclockwise without matching the aperture shape. During the course of prosobranch evolution, rigiclaudent spiral opercula emerged several times from the ancestral flexiclaudent type, although they were always restricted to apertures with a spiral-shaped outer (labral) edge. Such opercula enlarged the range of shell morphologies for which the operculum constituted an efficient protective barrier to include those of neritoidean or naticoidean type. The onset of calcification in opercula took place with the rigiclaudent type. Concentric opercula also evolved independently from rigiclaudent spiral opercula in several gastropod groups, thus further broadening the spectrum of apertures and, hence, of shell morphologies using opercula for protection. From the standpoint of adaptation, the concentric type was probably the only one available to neogastropods having long and wide siphonal canals.

ABSTRACT. Fossil bivalves bearing oblique ribs ®rst appeared in the Mid Ordovician but their diversity remained low during the Palaeozoic. The diversity soon increased after the Early Triassic, peaking in the Early Cretaceous. The Palaeozoic±Mesozoic record is dominated by burrowing bivalves (mainly pholadomyoids and trigonioids), which developed oblique ribs with symmetric pro®les, probably adapted for shell reinforcement, although there are indications that the ribs of trigonioids also enhanced burrowing ef®ciency. After the Paleocene, the main groups of burrowing bivalves were veneroids (primarily tellinoideans and lucinoideans) and nuculoids, which generated oblique ribs of the shingled type, adapted to increase burrowing ef®ciency. The inferred change in function at the Mesozoic/ Cenozoic boundary can be correlated with an increase in mean mobility of the bivalve faunas bearing oblique ribs through time. This implies a major ecological cause for the observed temporal patterns, which forced bivalve faunas to burrow more rapidly and ef®ciently. In particular, either the Phanerozoic increase in the diversity of durophagous predators or the accelerating rate of sediment reworking (both being a consequence of the Mesozoic Marine Revolution), or both, could have provided the necessary evolutionary force.KEY WORDS: functional morphology, evolutionary morphology, diversity, macroevolution, Mesozoic Marine Revolution, oblique ribs, bivalves.M O S T rib patterns in bivalves are of the commarginal (often called concentric) or radial (longitudinal) type. The former are secreted periodically by extrusion of the entire mantle margin, while the latter are secreted continuously by specialized areas distributed intermittently along the mantle margin. A third, less common pattern is the oblique (Seilacher 1972) or discordant (Stanley 1969) type, in which ribs migrate along the margin with growth. Several varieties can be differentiated according to whether there are one (single) or more (divaricate) branches, whether ribs maintain a constant angle to the growth lines, and whether they are composed of discrete elements (Text-®g. 1). A total of 176 living species displaying such patterns has been recognized, with tellinoideans, unionoideans and veneroideans being the most speciose (Checa 2002). Antimarginal ribs of the Ostreoidea and Plicatuloidea remain perpendicular to the shell margin during growth and can be classi®ed as oblique. They are produced because the bivalve mantle margin grows allometrically, and the extra length adjusts by producing folds (Checa and Jime Ânez-Jime Ânez 1999; pers. obs. 2002). Therefore, they are morphogenetically unrelated to other types of ribs, including oblique ribs. Accordingly, they are excluded from the present paper.From the standpoint of their function, oblique ribs in bivalves are usually assumed to perform a burrowing-related function (Stanley 1969(Stanley , 1970 Seilacher 1972 Seilacher , 1973. Seilacher (1972), in applying the paradigmatic method (Rudwick 1964), found three essen...

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Hutchinson's ('89) road-holding model states that spiral ornaments of the snail shell (keels and low-curvature areas) dictate the growth path of the subsequent whorl, which in turn gives the signal for attachment of the next whorl. Experiments were performed with two species of the terrestrial snail Sphincterochila in order to test the role of the external keel in determining the correct coiling of successive turns. Experiments substituted a ridge made of silicone for the keel. This ridge ran either (1) abapical or (2) adapical of the original keel. In mode (1), subsequent growth continued by taking the false keel as the adapical limit of the whorl. In only very few instances of mode (2) did the whorls extend incipiently slightly adapical of the path of the original keel. Our results confirm that the keel is an important reference for the coiling strategy of the snail, although the keel itself probably does not constitute the reference, but rather the two flat ramps into which the keel divides the outer lip of the aperture.

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