MIDDLE JURASSIC AMMONITE BIOZONATION IN WESTERN INDIA: GLOBAL IMPLICATIONS

Discovery of Indonesian Middle Jurassic ammonites in Kachchh (Western India) has broadened the biogeographic extension of the Indonesian fauna from Central Nepal to W. India, indicating an overlap of the SW Pacific subrealm fauna with that of the East Pacific in Bathonian times. Clearly, a seaway across the Peri-Gondwana platforms existed around 167 Ma, well before the long held view in Lower and Middle Callovian times (~156 Ma). Late Middle Bathonian Indonesian ammonites Macrocephalites bifurcatus transient bifurcatus [m] and M. bifurcatus trans. intermedius [M] have now been recorded from the core of the Jumara Dome (Kachchh), enabling the precise dating of the core bed. This discovery documents the arrival of the coeval Indonesian fauna in Kachchh and also records the first co-occurrence of the Macrocephalitid fauna with Procerites (Gracilisphinctes) arkelli and Micromphalites (Clydomphalites) sp. within the entire Tethyan Realm. Presence of Xenocephalites in Indonesia, New Zealand, Antarctica and South America and its co-occurence with Macrocephalites in the former three, indicates a seaway between them. Additionally, the Lower Bathonian Morphoceras record from Argentina confirms the Submediterranean (W. Tethyan) influence in the E. Pacific subrealm and its faunal overlap with the SW Pacific. The new biogeographic extension supports the idea that a land-locked pattern in the M. Jurassic never existed in the Antarctic as in the Arctic. The extension of the S. American Eurycephalitinae (Eurycephalites/Xenocephalites) in the SW Pacific subrealm confirms a seaway and attests to a probable divergence of the Eurycephalitids with the Macrocephalitids in Middle or Lower Bathonian times. The present record also bridges the gap in the Bathonian-Callovian biogeography. This partial endemism and the overlap between faunal subrealms may be due to paleoclimatic factors at high latitudes or to larval dispersal, facilitated by sea level change