Body Size Trends in Trigoniida Bivalves From the Mesozoic Kutch, India

Abstract

Although empirical testing of Cope's rule, the tendency for size to increase over time, has received significant attention in the last few decades, there is no consensus about the applicability of this rule across taxonomic levels. In the present study, we investigate the distribution of body size of Trigoniida bivalves, at order-, family-, genus- and species-level, through the Middle Jurassic and Early Cretaceous of the Kutch region in India. Our data suggest that the body size of Trigoniida bivalves did not vary significantly in the Middle–Late Jurassic, followed by an increase after the Jurassic–Cretaceous mass extinction boundary and a reduction in the late Early Cretaceous. Changes in relative sea-level and associated sedimentary facies composition generally exhibit poor correlation with the overall stasis, or no net body size change, displayed by Trigoniida bivalves. Body-size analysis across taxonomic hierarchy reveals that order-level trends are not a simple aggregation of trends at lower taxon levels. An important observation of our study is the body-size increase immediately in the aftermath of the Jurassic– Cretaceous mass extinction, a deviation from the general observation that size reduction occurs in post-extinction communities. We argue that this increase may be result of both ecological competition and evolutionary faunal turnover.