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

Abstract

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.

Death by ammonite: fatal ingestion of an ammonoid shell by an Early Jurassic bony fish

“…Our data suggest that Pachycormus was an obligate piscivore as a juvenile and became facultatively teuthophagous at standard lengths ≥300 mm. Unlike Saurostomus and Germanostomus, which frequently consumed loligosepiid and belemnoid coleoids (Cooper & Maxwell, 2022; Cooper et al 2022), teudopseid vampyropods were more common prey items for Pachycormus (Table 1). The ammonite in the gut of SMNS 52472 represents a unique exception to the soft-bodied coleoid diet of similar-sized Pachycormus individuals.…”

“…Occasionally, small actinopterygians ( Dapedium and Pholidophorus ) are found preserved trapped inside of an ammonite body chamber (Jäger, 1990; Fraaye & Jäger, 1995); although whether these fishes were scavenging on the rotting ammonite, feeding on smaller scavengers or seeking refuge remains undetermined (Fraaye & Jäger, 1995; Jäger, 2005). Wild (1994) reported alleged ammonite remains inside the gut of Saurostomus esocinus ; however, these remains represent fragments of the diplobeliid squid Clarkeiteuthis conocauda (Cooper & Maxwell, 2022). An immature S. esocinus with a small (2 mm diameter) ammonite larva mixed among larger prey likely represents unintentional bycatch, rather than evidence for Saurostomus feeding on zooplankton (Cooper & Maxwell, 2022; fig.…”

“…The specimen was examined using a Leica light microscope and photographed with a Nikon Series DMC-FZ72 camera with compact 60X optical macro-lens. Measurements are defined based on Cooper and Maxwell (2022, suppl. fig.…”

“…Pachycormid diets and feeding strategies are poorly known: gut contents are known for few taxa and most of these observations are anecdotally based only on a single individual. Early Jurassic genera have been described as teuthophagous (Přikryl et al 2012; Cooper & Maxwell, 2022) or consuming both soft-bodied coleoids and small actinopterygians (Aldinger, 1965; Cooper et al 2022); gastric contents in Middle and Late Jurassic species consist exclusively of small teleosteans (Vetter, 1881; Viohl, 1990; Martill, 1991; Lambers, 1992; Liston, 2008; Maxwell et al 2020). Ammonites were previously theorized to have contributed to the diets of some pachycormids, although no direct evidence has been published (Wild, 1994; Cooper & Maxwell, 2022).…”

“…Early Jurassic genera have been described as teuthophagous (Přikryl et al 2012; Cooper & Maxwell, 2022) or consuming both soft-bodied coleoids and small actinopterygians (Aldinger, 1965; Cooper et al 2022); gastric contents in Middle and Late Jurassic species consist exclusively of small teleosteans (Vetter, 1881; Viohl, 1990; Martill, 1991; Lambers, 1992; Liston, 2008; Maxwell et al 2020). Ammonites were previously theorized to have contributed to the diets of some pachycormids, although no direct evidence has been published (Wild, 1994; Cooper & Maxwell, 2022). Here, we report a specimen of Pachycormus macropterus (de Blainville, 1818) from the Early Jurassic of Germany, representing the first direct evidence of an actinopterygian fish consuming a Mesozoic ammonoid.…”