Systematics, endemism and phylogeny of Indian proplanulitins (Ammonoidea) from the Bathonian–Callovian of Kutch, western India

Abstract

Spath (1931) described five genera namely Sivajiceras Spath, Obtusicostites Buckman, Hubertoceras Spath, Kinkeliniceras Buckman and Cutchisphinctes Spath from the Upper Bathonian and entire Callovian of Kutch, western India and included 33 species within them. He grouped these genera within the subfamily Proplanulitinae of the Boreal Province. But however the palaeobiogeographic distributions of the Kutch forms suggest that they were restricted within the Indo-Madagascan Province. Callomon (1993) expressed doubts about the phylogenetic affinity of Kutch genera and inferred that proplanulitins of Kutch were unrelated to Boreal Proplanulites and constitute an endemic lineage. In the present study, we made a thorough systematic revision of proplanulitin taxa of Kutch in the light of intraspecific variability and sexual dimorphism. Our study reveals that Spath’s (1931) work suffered from excessive and subjective splitting. Many species described by him were nothing but variants of a single, variable, biological species. The genus Hubertoceras was considered as the microconch of Obtusicostites. Cutchisphinctes was excluded as it appears to be unrelated. We finally described two species within each genus, Sivajiceras, Obtusicostites and Kinkeliniceras from the Callovian. We grouped these Kutch genera within a new subfamily Sivajiceratinae which is distinct from the existing subfamily Proplanulitinae. The oldest genus Sivajiceras evolved from contemporary Procerites during the Late Bathonian of Kutch (Roy et al. 2007). Procerites was a European genus and perhaps gave rise to Proplanulites in the Boreal Europe. We here envisaged that Sivajiceratinae and the European Proplanulitinae show evolutionary convergence of some characters since they evolved from a common ancestral stock.