The stem group teleost Pachycormus (Pachycormiformes: Pachycormidae) from the Upper Lias (Lower Jurassic) of Strawberry Bank, UK

摘要

Three-dimensionally preserved specimens of the pachycormiform neopterygian Pachycormus from the Lower Jurassic (Upper Lias) of Strawberry Bank, Ilminster, UK, are described for the first time, and we present new and detailed morphological descriptions of the skull, trunk and fins of these specimens. The cranial material is characteristic of Pachycormus macropterus, with its operculum being twice as long as it is deep and the ventral angle lying towards the posterior half of the bone, and the preorbital portion occupying up to one-third of the skull. The description of the caudal fin conflicts with previous descriptions of caudal fins assumed to belong to Pachycormus. This raises the question of whether the other caudal fins belong to different species of Pachycormus, or whether it could be a case of intraspecific variability and is in conflict with morphometric data suggesting a monospecific status for Pachycormus. Recent phylogenetic analysis indicates that Pachycormus is sister taxon to Ohmdenia, which is an intermediate form between Pachycormus and the later giant suspension-feeding pachycormids. An alternative phylogenetic analysis reveals Pachycormus to be among the most basal pachycormiforms due to its incorporation of characters present in both hyper-carnivorous and suspension-feeding clades. We suggest future research on these specimens such as CT scanning of the skull may offer vital data regarding pachycormiform morphology and actinopterygian evolution, particularly the sister group relationship of pachycormiforms to the successful teleost fishes. In the future it may be important to focus on caudal fin characters of articulated specimens to identify possible cryptic species diversities within Pachycormus and also other members of Pachycormiformes.