EVOLUTIONARY INSIGHTS INTO THE BODY SHAPE OF MESOZOIC TRIGONIIDA BIVALVES FROM KUTCH, INDIA: A GEOMETRIC MORPHOMETRICS STUDY

Trigoniids, often referred to as the 'cockles' of the Mesozoic, stand out as one of the predominant components within the marine Mesozoic sequence of Kutch. In this study, we employed landmark geometric morphometrics to investigate the temporal pattern of shape evolution for 10 Trigoniida species across the Middle Jurassic to the Early Cretaceous strata of the Kutch basin. The mean shape for each time bin, derived from landmark morphometrics on the external surface, reveals a significant change in the morphology of trigoniid shells over time, with a maximum shift during the Oxfordian – Kimmeridgian transition. In the Bathonian – Oxfordian assemblages, older trigoniid forms exhibit elongation along the dorso-ventral direction, featuring a broad and linear posterior margin. In contrast, within the younger Kimmeridgian – Berriasian forms, elongation occurs along the anterior-posterior direction, with a rounded and wedge-shaped posterior near the ventral margin.

The temporal variations in trigoniid shape exhibit a strong correlation with environmental factors, where older sub-triangular forms are associated with low-energy depositional settings, while younger rostrate/sub-rostrate forms are linked to high-energy environments. The Kutch basin, during the Middle Jurassic to Early Cretaceous period, underwent frequent sea-level fluctuations associated with tectonic changes. Therefore, long-term trigoniid shape change can be, at least partially, attributed to frequent sea-level fluctuations and subsequent changes in substrate. The unstable nature of the environment prompted trigoniids to undergo rapid diversification, leading to the emergence of various short-lived species characterized by distinct shapes. Accordingly, this long-term shape change may also arise from genus and species-level taxonomic turnover, with new taxa originating from a more rostrate form. These findings provide a valuable understanding of ecological and evolutionary changes observed among the Mesozoic trigoniids of the Kutch basin.