Carpus in Mesozoic anurans: The Early Cretaceous anuran Genibatrachus from northeastern China

Abstract

The carpus (wrist) of fossil frogs is rarely preserved, because it consists of tiny skeletal elements that ossify only during the postmetamorphic life stage. The structure of the carpus is comparatively well-known in the temnospondyl ancestors of the Anura, but its changes during the transition to their anuran descendents are unknown due to the absence of transitional, presumably paedomorphic forms. The Early Cretaceous Genibatrachus from northeastern China is among the best-documented Mesozoic anurans, both regarding the number of preserved individuals and the representation of developmental stages. The latter aspect is especially important, because in its early developmental history, the anuran carpus is represented by cartilaginous nodules which may be the subject of various, often multiple fusions. Only later do the nodules or the fused elements ossify, enabling them to be preserved in fossils. This is why the carpus of adult fossil frogs is simpler than the foregoing larval period of cartilaginous carpus, not recordable in fossils. Nevertheless, the early development of the carpus may be reconstructed from the morphological details of its ultimate constituents. In this way, Genibatrachus may fill the gap in the evolutionary sequence of the carpus between Paleozoic temnospondyl amphibians and modern frogs.