Significance of new species of Cryptorhynchia (Brachiopoda) from the Middle Jurassic of Kutch,India

Cryptorhynchia is a brachiopod genus, until recently known only from the late Bathonian. Two new species C. karuna and C. jhooraensis are here described from the middle Bathonian at Kutch, western India and record the earliest known occurrences of the genus. They constitute ancestor-descendant lineages with the two existing younger species C. pulcherrima and C. rugosa respectively. Evolution in both cases shows ‘parallel’ trends and the descendants are scaled-down versions of their respective progenitors. Statistical analyses reveal that this evolutionary miniaturization involves allometry-induced heterochrony, especially progenesis, and is marked by a rapid speciation event compatible with the punctuational model. Evolution appears to be anagenetic and may be attributed to the unstable nature of environment where onshore innovation produced smaller descendants that adopted r-strategy. Facies distribution and functional morphology suggest that species of Cryptorhynchia were well adapted to shallow, unstable carbonate shelf. However, they disappeared suddenly at the Bathonian - Callovian boundary which was marked by a global transgression. The possible causal factors of their extinction may be the consequent changes in bathymetry and substrate condition.