PHYLOGENY OF THE LATE JURASSIC-EARLY CRETACEOUS SUBGENUS ESELAEVITRIGONIA (BIVALVIA) OF KUTCH, INDIA, AND PALEOBIOGEOGRAPHIC CONSTRAINTS

摘要

Three of Kitchin's (1903) trigoniid species, Trigonia trapeziformis, T. spissicostata, and T. cardiniiformis, have been frequently examined taxonomically, but their phylogenetic relationships remain uncertain. Taxonomic designations have ranged from grouping them within a single subgenus to separating them into different subfamilies. Principal factors affecting the previous studies include: excessive weighing of certain features from a typological perspective; phylogenetic relationships established on the basis of plesiomorphic characters which are applicable even at the family level. Although they are endemic to Kutch, India, species' ancestries were rarely sought from the regional taxa, a situation exacerbated by inadequate stratigraphic information. This study reveals that all the three species are both geographically and stratigraphically distinct, and their distribution is linked to major regression events. Since the latest Jurassic, the Kutch Basin experienced shallowing. The unstable environment triggered a substantial diversification of new forms. The present species are close morphologically to some endemic species of different genera. Detailed morphologic, morphometric, and cladistic analyses reveal affinities between ‘Eselaevitrigonia’ trapeziformis and Indotrigonia smeei, as well as ‘E.’ spissicostata and Opisthotrigonia retrorsa. It is believed that E. cardiniiformis evolved from ‘E.’ spissicostata. In each case, speciation took place in a very shallow, high-energy, nearshore environment and proceeded through various heterochronic processes. The pattern is consistent with a punctuated model of evolution. Gondwana fragmentation and rise of eustatic sea level during the Aptian opened up the central Indian oceanic corridor, prompting the spread of ‘Eselaevitrigonia’ to the Austral Province.