Global phylogeny of Diplocynodontinae and its implications for crocodylian evolution

Diplocynodontinae represents the only crocodylian lineage that survived the Eocene-Oligocene transition in Europe and is known for its long endemic evolution in the continent. Current phylogenies place Diplocynodontinae as an early diverging member of Alligatoroidea, a group that otherwise originated in North America during the Late Cretaceous. This phylogenetic position is incongruent with the spatiotemporal distribution of fossils and leaves the origin of Diplocynodontinae enigmatic. In this study, we critically reassess the evolutionary relationships of Diplocynodontinae through first-hand revision of key taxa and homologies placing, according to previous phylogenies, the group into Alligatoroidea. We find that several synapomorphies supporting the alligatoroid position represent irreproducible or poorly defined character states. Parsimony analyses of a revised dataset results in several most parsimonious trees with an alternative position for Diplocynodontinae, along the stem lineage of crown-group crocodylians. The traditional basal alligatoroid position is persisting in the remaining trees but the synapomorphies supporting this placement are poorly known for outgroup taxa. A stem-crocodylian position for Diplocynodontinae is more congruent with stratigraphy and European vertebrate paleobiogeographical patterns, and would clarify the origin of the clade as a result of an early Paleogene dispersal from North America. Furthermore, phylogenetic patterns of extinction across the Eocene-Oligocene transition likely need to be reconsidered together with our current hypotheses of early alligatoroid morphology.