The Early Triassic ammonoid recovery: Paleoclimatic significance of diversity gradients

Ammonoids recovered much faster than other marine shelly invertebrates after the end-Permian mass extinction. Based on a refined global data set at the basin level, we investigate the paleobiogeographical global latitudinal and longitudinal diversity patterns in terms of climatic changes during the Early Triassic. Such analysis differs from already published qualitative or quantitative studies in that it estimates faunal patterns and endemicity at an ecologically meaningful level of spatial resolution, i.e. at the basin level. During the Early Triassic, the global first order trend in increasing ammonoid diversity was accompanied by a progressive change from cosmopolitan to latitudinally-restricted distributions. This led to the emergence of a clear latitudinal diversity gradient during most of the Smithian and Spathian stages, which entails increased steepness of the Sea Surface Temperature gradient during the late Early Triassic. However, two brief episodes of ammonoid cosmopolitanism combined with low global diversity interrupted the first order increasing trend at the very beginning of the Smithian and at its very end. The longitudinal analysis of Smithian distributions indicates a westward decrease of diversity within the Tethys, which faded away during the Spathian. Analysis of endemicity indicates a rapid biogeographical maturing and structuring of faunas concomitant with the edification of the latitudinal diversity gradient.