The Devonian actinopterygianCheirolepisAgassiz

Abstract

The structure of the fishes constituting the Devonian genus Cheirolepis is investigated. Articulated remains from the Scottish Old Red Sandstone and the Canadian Upper Devonian were studied and two species are recognised, the Middle Devonian C. trailli and the Upper Devonian C. canadensis. Recently described fragmentary material from Europe assigned to the genus is best regarded as incertae sedis. Cheirolepis is the earliest actinopterygian genus with extensive material. Neurocranial remains are described, with a crossopterygian-like parasphenoid. There seems to have been a mobile rostral region with several small bones between premaxilla and postrostral extending to the anterior corner of the small dermal orbit. New palatoquadrate and pectoral girdle material is described. The apparently primitive nature of the head skeleton is related to the cranial dynamics and the likely mode of life. The small scales and the elongate-fusiform body shape are a corollary of the method of swimming. The ecology of the fishes is touched upon. A taxonomic investigation of the two species was carried out but although interesting variation in the scale-row numbers came to light no changes at the specific level were deemed necessary. Revised generic and specific diagnoses are given.