Callovian–Oxfordian hecticoceratins from western India: Their biostratigraphic and palaeobiogeographic implications

The Kutch Basin of western India is famous for its rich assemblages of the Callovian–Oxfordian ammonites. The family Oppelidae Douvillé is the second most diverse ammonite group after perisphinctids during the Middle–Upper Jurassic. Hecticoceratinae is the most diverse subfamily within Oppelidae and has wide palaeobiogeographic (near cosmopolitan) and temporal distributions (Bathonian–Oxfordian). Some species were well time-diagnostic and thus help in interprovincial correlation. The taxonomy of the subfamily Hecticoceratinae of Kutch was in a state of flux until recently. It was not revised since Spath’s (1927–1933) great contribution. Many genera and species were morphogenera or morphospecies and they again suffer from excessive subjective splitting. It was therefore badly needed for a comprehensive taxonomic revision of the subfamily with modern aspects of systematics i.e., sexual dimorphism and population dynamics. A lithostratigraphic framework has already been well documented in the Kutch Basin of western India. A high resolution biostratigraphy incorporating stage-intrastage fossil assemblages have been used in interbasinal correlation based on the Callovian–Oxfordian hecticoceratins. Near cosmopolitan distribution of many hecticoceratin genera were widely used for biostratigraphic zonation as well as an understanding of the palaeobiogeographic pattern. The phylogeny of the subfamily Hecticoceratinae has been used to construct the cladograms depicting area relationships among different provinces during the Callovian–Oxfordian.