BODY SIZE TRENDS IN THE MESOZOIC TRIGONIID BIVALVES OF KUTCH, INDIA

Body size has been argued as one of the important traits of any organism as it can be directly correlated to a number of evolutionary and environmental parameters. Here, we investigate long-term body size trends of the Mesozoic bivalves, collected from the Kutch basin, India, spanning from the Middle Jurassic (Late Bathonian) to Lower Cretaceous (Aptian). Body size of trigoniid bivalves remained relatively similar from the Bathonian to Tithonian, followed by a statistically significant increase across the Jurassic – Cretaceous boundary and a subsequent statistically significant decline in the Lower Cretaceous. Evolutionary models based on maximum likelihood estimates support an overall stasis rather than any directional trend. Trends in the Mesozoic trigoniid bivalves of Kutch do not support the hypothesis of Cope’s rule which has been described as increase in body size over geologic time as a result of either an directional increase in both maximum and minimum size ranges (sensu stricto), or an increase in overall variance (sensu lato). This lack of support for Cope’s rule is pervasive across different taxonomic levels. Individual family – level clades or genus – level analysis also reflected similar trends unveiled in order – level analysis. The biggest change of trigoniid body size occurred across the Jurassic – Cretaceous mass extinction boundary. This significant increase is also associated with faunal turnover and ecosystem reconfiguration, including a release of predation pressure from the disappearance of a number of predatory groups, as a result of mass extinction.