Revision of the pachycormid fish Saurostomus esocinus Agassiz from the Early Jurassic (Toarcian) of Europe, with new insight into the origins of suspension‐feeding in Pachycormidae

The Early Jurassic Posidonienschiefer Formation in south‐west Germany (Toarcian) records the first appearance of the stem‐teleost group Pachycormidae in the fossil record. However, most pachycormid taxa remain poorly diagnosed or undescribed, making questions of the morphological underpinnings of trophic diversification in the group difficult to address. Here we re‐describe Saurostomus esocinus, a large (≤1710 mm) pelagic pachycormid, based on material from the type formation. Additional material of Saurostomus is identified in near‐coeval strata in Luxembourg, France and the UK, demonstrating a broad European distribution. We assign a neotype, and provide the first formal diagnosis and cranial reconstruction for this genus. Historical misconceptions regarding taxonomic identity of Saurostomus have caused numerous specimens to be conflated with the better known genus Pachycormus. In south‐west Germany, Saurostomus is stratigraphically distributed across the tenuicostatum and serpentinum Zones, encapsulating six ammonite subzones, with an individual from the paltum subzone representing the oldest stratigraphic occurrence of Pachycormidae. Stout recurved teeth, powerful pectoral fins, a streamlined body, a pre‐caudal scaly keel and remnants of coleoid gut contents indicate fast swimming capabilities and a pelagic teuthophagous ecology for Saurostomus esocinus. Cladistic analysis retains Saurostomus as sister to a clade comprising Ohmdenia and the suspension‐feeding pachycormids. Several character states associated with the evolution of suspension‐feeding are shared with Saurostomus, indicating S. esocinus, rather than Ohmdenia, to be the most basal pachycormid to have transitional suspension‐feeding characteristics. The divergence of the suspension‐feeding and macrophagous pachycormids therefore pre‐dates the paltum subzone of the lower Toarcian.