Status of “Trigonia ventricosa” (Bivalvia) from the Upper Jurassic–Lower Cretaceous of Kutch, western India: Kitchin's unfinished synthesis

Kitchin first reported Trigonia ventricosa (Krauss) from Kutch in 1903. The populations he studied came from different localities and stratigraphic horizons, but erroneous information on Kutch geology initially led him to believe that all his material was obtained only from Late Tithonian strata. He noted, however, that populations of different areas had distinct morphological characters and biofacies associations and later expressed uncertainties about the homogeneity of the Kutch species. Trigonia ventricosa was later included in Pisotrigonia van Hoepen (subfamily Pterotrigoniinae). Our field investigation and study of numerous specimens collected from different localities as well as the material examined by previous workers (Kitchin and Cox) reveal that populations of “T. ventricosa” are, in fact, found at four distinct stratigraphic levels ranging from Late Tithonian to Aptian in age; they differ both morphologically and morphometrically, allowing the recognition of four distinct species, some of which are temporally separated by as much as seven million years. The species, in ascending stratigraphical order, are Pisotrigonia kitchini sp. nov., P. umiensis sp. nov., P. ventricosa (Krauss) and P. ghuneriensis sp. nov. The Pterotrigoniinae are apparently polyphyletic since Pterotrigonia has myophorellian ancestry while Pisotrigonia has megatrigonian affinities; they are stratigraphically heterochronous and evolved initially in two different palaeobiogeographic provinces. During the Valanginian, Pterotrigonia and Pisotrigonia shared the same biogeographic area and showed character displacement. Pisotrigonia species in Kutch evolved anagenetically with initial increase in body size followed finally by size decrease. The mode of evolution is punctuational and was induced by heterochrony.