Abstract
The early evolutionary history of acanthodians, a paraphyletic group of Palaeozoic stem chondrichthyans, is reconstructed using a comprehensive and updated morphological phylogenetic dataset consisting of 429 characters scored for 87 articulated taxa. Results of a time-calibrated Bayesian analysis suggest a Middle Ordovician origin for total-group Chondrichthyes, coinciding with the end of the Great Ordovician Biodiversification Event, and an Early Devonian age for the most recent common ancestor of crown Chondrichthyes. Further, major acanthodian lineages are inferred to originate towards the end of the Late Ordovician. This supports a diversification model in which interordinal cladogenesis occurred in the Middle-Late Ordovician and most intraordinal cladogenesis occurred in the Silurian. Divergence dates and short internodes inferred by the tip-dating analysis indicate a relatively rapid pattern of successive cladogenetic splits for major groups of acanthodians that diverged from the chondrichthyan stem in the Darriwilian and Sandbian. These speciation events possibly produced daughter lineages that remained morphologically similar to their ancestors before undergoing other cladogenetic splits, as suggested by the relatively low rates of morphological evolution (number of morphological changes per million year) reconstructed for the same branches. The lack of synapomorphies resulting from such a compressed pattern of divergences is likely the cause of low branch support and the topological instability that characterizes acanthodian relationships. The results presented in this study confirm that underlying evolutionary processes such as rapid radiations, in conjunction with the fragmentary nature of the fossil record, can further negatively impact phylogenetic reconstruction based on morphological data.
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