Systematic revision and palaeobiogeography of Perisphinctes Waagen (Ammonoidea) from the Oxfordian of Kutch, India: Stratigraphic and evolutionary implications

Perisphinctes Waagen, 1869 is an important genus of the Oxfordian. In many areas the taxon has been excessively and subjectively split. Based on the material collected in Kutch (India), including the type specimens, we have grouped eight species previously described as distinct into one biological species, Perisphinctes indogermanus Waagen, 1875. It is characterized by strong sexual dimorphism. Macroconch shows high intraspecific variability. Microconch is previously described from Kutch in different generic names. Thirteen species of Dichotomosphinctes Buckman, 1926, Perisphinctes Waagen, 1869 and Lithacoceras Hyatt, 1900 are considered here as microconch which is lappeted. The precise age of P. indogermanus is disputed. A detailed stratigraphic analysis of several sections in this study suggests that the species ranges from early to middle Oxfordian. Perisphinctes indogermanus is distinct from other contemporary species of the world and abundant in the Indo-Madagascan Province. Kutch population of P. indogermanus has developed virgatotome style of ribbing in juvenile stage. During the middle Oxfordian marine transgression, the species along with many other ammonites of deeper habitat of the mainland basin reached the shallow shelf of eastern fringe of the Kutch Sea. Being peripherally isolated, it soon underwent an adaptive radiation that resulted in the origination of a new family Ataxioceratidae Buckman, 1921 whose members have neotenously retained the virgatotome ribbing in adult stage. The ataxioceratids later migrated to Europe and flourished during the Kimmeridgian.