A New Large †Pachycormiform (Teleosteomorpha: †Pachycormiformes) from the Lower Jurassic of Germany, with Affinities to the Suspension-Feeding Clade, and Comments on the Gastrointestinal Anatomy of Pachycormid Fishes

Pachycormiformes is a diverse clade of stem-teleost actinopterygian fishes with a stratigraphic range from the Lower Jurassic (Toarcian) to Upper Cretaceous (Maastrichtian). The Toarcian Posidonienschiefer Formation in SW Germany records the earliest occurrence of †Pachycormiformes in the fossil record, offering unique and crucial insight into the clade’s origins and early adaptive radiation in the Early Jurassic. However, Early Jurassic taxa remain poorly studied with the taxonomic diversity and stratigraphic/geographic distributions insufficiently defined, thus masking the early part of this evolutionary radiation. Here, we report a new genus and species of pachycormid fish from the Posidonienschiefer Formation identified by phylogenetic analysis as falling in an intermediate position between Saurostomus and Ohmdenia at the base of the suspension-feeding clade. The new taxon shows a unique suite of cranial and postcranial characters. Several synapomorphies of the suspension-feeding clade, notably, the morphology of the hyomandibula, elongation of the skull, and reduced squamation are shared with the new taxon. The intestinal tract is exceptionally preserved, providing one of the most complete examples of pachycormid gastric anatomy. A comparison of the gastrointestinal anatomy of the new genus with other pachycormiforms indicates extensive taxonomic variation within the clade, in the configuration of both the midgut and spiral valve, potentially related to trophic divergence. The results highlight an underestimated high diversity and the rapid acquisition of trophic specializations in Pachycormiformes much earlier in the clade’s evolution than previously considered.