A new species of the Late Jurassic pachycormiform fish <i>Orthocormus</i> and comments on the beta taxonomy and phylogenetic systematics of the Pachycormiformes

Abstract Pachycormiformes, a clade of stem-teleost actinopterygians of the Jurassic and Cretaceous, have been actively researched in recent years; however, the relationships between certain Jurassic and Cretaceous pachycormiform taxa remain the subject of debate. Here, we describe a new species, Orthocormus gushchinae sp. nov., from the Upper Jurassic of Russia. Orthocormus is a member of the macrocarnivorous pachycormiform lineage, and is generally seen as ancestral to the Cretaceous swordfish-like Protosphyraena. The pectoral fin of the new specimen shows features hitherto considered unique to Protosphyraena nitida and thus unambiguously links Jurassic and Cretaceous macrocarnivorous taxa. The great similarity of the pectoral fin of P. nitida to the new species suggests its basal phylogenetic position relative to other species of Protosphyraena and, thus, placement in a separate genus, Erisichthe, as originally introduced by Cope. Our phylogenetic analysis confirms the prevailing hypothesis of two major pachycormiform lineages. However, the interpretation of these lineages in terms of Linnaean taxonomy appears more complex than previously assumed. After reviewing historical family-group names and their original definitions, we propose that the order Pachycormiformes be regarded as consisting of the superfamily Pachycormoidea, which contains two families: Pachycormidae and Protosphyraenidae. Finally, we introduce PhyloCode-compliant phylogenetic definitions for Pachycormiformes and its subclades.