Allometric shell growth in infaunal burrowing bivalves: examples of the archiheterodonts Claibornicardia paleopatagonica (Ihering, 1903) and Crassatella kokeni Ihering, 1899.

Damián Eduardo Perez,María Belén Santelli

doi:10.7717/peerj.5051

Abstract

We present two cases of study of ontogenetic allometry in outlines of bivalves using longitudinal data, a rarity among fossils, based on the preserved post-larval record of shells. The examples are two infaunal burrowing bivalves of the southern South America, Claibornicardia paleopatagonica (Archiheterodonta: Carditidae) (early Paleocene) and Crassatella kokeni (Archiheterodonta: Crassatellidae) (late Oligocene–late Miocene). Outline analyses were conducted using a geometric morphometric approach (Elliptic Fourier Analysis), obtaining successive outlines from shells’ growth lines, which were used to reconstruct ontogenetic trajectories. In both taxa, ontogenetic changes are characterized by the presence of positive allometry in the extension of posterior end, resulting in elongated adult shells. This particular allometric growth is known in others infaunal burrowing bivalves (Claibornicardia alticostata and some Spissatella species) and the resulting adult morphology is present in representatives of several groups (e.g., Carditidae, Crassatellidae, Veneridae, Trigoniidae). Taxonomic, ecological and evolutionary implications of this allometric growth pattern are discussed.

Highlights

According to the Gould-Mosimann school, ‘allometry’ is the association between size and shape
Size of an organism can be determined by its own biological growth, and in these cases, allometry is the covariation between shape and growth through its life-span
Studies on ontogenetic allometry mainly use ‘‘cross-sectional’’ data

Summary

Introduction

According to the Gould-Mosimann school (defined by Klingenberg, 1998), ‘allometry’ is the association between size and shape. Size of an organism can be determined by its own biological growth (or ontogeny), and in these cases, allometry is the covariation between shape and growth through its life-span. This allometry is known as ‘‘ontogenetic allometry’’ (Klingenberg, 1996a; Klingenberg, 1998). Studies on ontogenetic allometry mainly use ‘‘cross-sectional’’ data (each individual is measured at a single stage, and an average allometric trajectory is estimated from a composite sample from many individuals).

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