Time to grow up: the PETM climatic event favoured metamorphosing salamanders (Urodela, Salamandridae)

Currently, little is known about the early evolution and geographical origin of Salamandridae, the most species-rich family of Palearctic salamanders. The description of new fossil material from the early Palaeogene of the Paris Basin provides new insight into the history of amphibians and for the first time allows the inclusion of the oldest salamandrid taxa into a phylogenetic framework, based on a new morphology-based character matrix. By comparing the evolutionary trajectories of the two main Nearctic groups of Cenozoic amphibians, urodeles and anurans, it emerges that whereas frogs were hardly affected by the Palaeocene-Eocene Thermal Maximum (PETM), salamanders responded in Western Europe with a diversification event, corresponding to a spread of metamorphosing, likely terrestrial, salamandrids. Among Salamandridae, the Palaeocene Koalliella genzeli is retrieved as the sister taxon of the extant, Italian endemic genus Salamandrina, clarifying the plesiomorphic nature of the features that, in morphology-based phylogeny, traditionally group Salamandrina with the newts. Two Eocene taxa herein described replaced Koalliella in the early Eocene assemblages, being respectively sister taxa of Salamandrinae, the 'true' salamanders, and Pleurodelinae, the newts. The PETM is, therefore, the main driving event leading to the first radiation of the Salamandridae, ultimately ending up being the most diverse extant clade of Palearctic salamanders. All in all, our study provides new insights into (i) the worldwide diversification of the family Salamandridae and (ii) the evolution of the whole amphibian fauna in Western Europe.