First hypsocormine pachycormid in the Early Jurassic and its evolutionary implications for Pachycormidae (Actinopterygii: Teleosteomorpha)

Abstract An isolated lower jaw of a morphologically distinct actinopterygian fish from the Early Jurassic Posidonienschiefer Formation of Holzmaden (SW Germany) is identified as belonging to the pachycormid subfamily Hypsocorminae based on the presence of an anteriorly inflated coronoid plate, an enlarged coronoid tusk, and granular dermal bone ornamentation. Previously, the first occurrence of hypsocormine pachycormids was from the Middle Jurassic onwards (Callovian–Maastrichtian). This new report pushes the evidence for the clade back into the Toarcian stage of the Lower Jurassic, closing a 17-million-year ghost lineage hypothesized on phylogenetic grounds. Both Asthenocorminae + Hypsocorminae, the two main pachycormid clades, are now recognised in the Toarcian, with their emergences coinciding with the first appearance of Pachycormidae in the fossil record. The split between Hypsocorminae and Asthenocorminae is therefore strongly suggested to have occurred earlier than is currently documented in the fossil record, and further indicates a probable ghost lineage of Pachycormidae prior to the clade’s first appearance in the Toarcian. We interpret the presence of an enlarged coronoid tooth (tusk) to be plesiomorphic in Pachycormidae, with this feature being modified in Hypsocorminae + Simocormus spp., but secondarily lost in derived Asthenocorminae.