Late Lower Kimmeridgian-Lower Tithonian Virgatosphinctins of India: Evolutionary succession and biogeographic implications

Abstract

A very significant late Lower Kimmeridgian to Lower Tithonian virgatosphictin ammonoid fauna is examined under the rigorest possible stratigraphic precision at Ler-Katrol (Kachchh), India for evolutionary succession and biogeographic implications. The study allows the recognition of at least 12 successive faunas which have been organised into 8 zones and 12 subzones along with their relative precise correlations with Western Tethys. The principal single lineage worked out is Torquatisphinctes intermedius (Alterneplicatus Zone to Acanthicum Zone)-Pachysphinctes bathyplocus (Bathyplocus Zone)-Katroliceras katrolensis (Katrolensis Zone) (up to close of Kimmeridgian)-K. pottingeri (Pottingeri Zone). K. pottingeri in turn gives origin by cladogenesis to two short lived lineages, which include new species one in each lineage marking the termination of the stock at the close of Lower Tithonian with a probable lineage discontinuity in the basal Tithonian Hybonotum Zone. Among the other salient features are: 1. There is a major stratigraphic break at the base (=Vail's subcycles LZA 4.3-4.4). The start of the fauna corresponds to the eustatic rise of LZA 4.5, while the termination of the lineage is correlated with the termination of LZB 1.1. 2. K. pottingeri, known widely in the Indo-East-African province, is precisely dated for the first time as post-Hybonotum Zone, in association of very significant although also not well dated Himalayan Aulacosphinctoides (A. uhligi, A. willisi, A. tibetanus etc.). Aulacosphinctoides in the section is also found older than Virgatosphinctes. 3. The rare presence of non-virgatosphinctin West Tethyan species indicates expansion events related to eustatic rises, which permits correlation with the Western Tethys of the otherwise near endemic Indian faunas. Nothing is found in common with the SE. Pacific faunas, suggesting continued absence of a direct marine connection via the east and south coastal region of Africa. © 1993.