Differential diversification through the K-Pg boundary, and post-crisis opportunism in longirostrine crocodyliforms

Abstract

The evolution of the crocodyliforms through the K-Pg crisis has often been evaluated, but each time, the crocodyliforms were considered as forming a homogeneous group. I considered here, the evolution of two longirostrine taxa from the Campanian to the Thanetian: tethysuchians and gavialoids. The gavialoids are almost restricted to Laurasian continents, where tethysuchians form most of the Gondwanan crocodyliform fauna. This segregation can be compared with climatic distribution: tethysuchians are restricted to hot climatic areas, where gavialoids are restricted to a northern, warm temperate climatic belt from where tethysuchians were almost excluded. This suggests that gavialoids were more tolerant of cooler climates than tethysuchians. The tethysuchians could have been excluded from the European continent by the existence of a cool European oceanic current, whilst on the contrary, the presence of a proto-gulfstream along north-east American coast could have allowed the presence of some tethysuchians in marine realm in this area. Both gavialoids and tethysuchians strongly diversified after the K-Pg crisis, particularly on Gondwanan continents, and mostly with tethysuchian species. Most of these tethysuchians were non-marine during the Maastrichtian, and the number of marine species strongly increases after the K-Pg crisis, whilst the number of non-marine ones remains nearly constant. This rise of marine diversity compared to non-marine forms is congruent with previous hypotheses suggesting that the crocodyliforms did not suffer from the K-Pg crisis, but on the contrary, benefited from the extinction of large marine reptiles to diversify after the crisis mainly in this environment. So, if the history of the crocodyliforms on the whole is important, the evolution of each group should be considered separately, as their evolution could be influenced by regional environmental conditions and factors.