Osteostracans are a Silurian-Devonian clade of jawless stem gnathostomes, and, as the putative sister group to jawed vertebrates, play a fundamental role in the study of the origin of jaws. Despite this, the timing and modality of their early diversification are poorly studied and understood. Here, their early evolutionary radiation is reconstructed using Bayesian tip dating methods. Using a revised and expanded phylogenetic dataset, I estimate that osteostracans originated between 450 and 443 million years ago, in the Late Ordovician. The emergence of Osteostraci is succeeded by a peak in rates of morphological evolution between 443 and 440 million years ago, after the Late Ordovician mass extinction. Following this early burst, major lineages appeared in a relatively rapid time frame, with a particularly rapid radiation taking place between 433 and 429 million years ago, after the Ireviken Biogeochemical event. Results presented in this study show how the Silurian and Devonian morphological diversity of osteostracans was the result of successive and diverse adaptive responses to climate change and extinctions of competing taxa. Further, the condensed pattern of divergences recovered among internal branches possibly explains low support and the different phylogenetic results retrieved in this study compared with previous analyses.

Osteostracans, Tip-dating, Phylogenetics, Evolutionary rates, Evolutionary radiation