Dimorphism and a new record of<i>Barroisiceras</i>De Grossouvre (Ammonoidea) from the Coniacian of Bagh, central India

Abstract

Barroisiceras de Grossouvre was hitherto unknown in India. B. onilahyense Basse, which elsewhere belongs exclusively to the Coniacian, has been recorded from the Bryozoan Limestone Formation of central India. This find, as well as the presence of another time-diagnostic Coniacian ammonite, Placenticeras kaffrarium Etheridge and two inoceramid bivalves, Inoceramus (Mytiloides) incertus Jimbo and I. (Platyceramus) mantelli (Barrois) Mercey in the underlying horizons, help in resolving the long standing problem of the age of the Bagh Group. B. onilahyense shows wide intraspecific variability and overlaps morphometrically with the late Turonian type species, B. haberfellneri (Hauer). They may be conspecific. The present species has been found to be sexually dimorphic, a phenomenon previously not well recognised in the subfamily Barroisiceratinae. As in many Late Cretaceous ammonites, dimorphism is manifest mainly by differences in relative adult sizes and in strength of ornamentation, the microconch being smaller and more strongly ornamented.