Evolutionary morphology of oblique ribs of bivalves

Abstract

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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