Chondrenchelys problematica, from the Viséan (Holkerian) of Scotland, is the earliest holocephalan known from extensive cranial and postcranial material. Here, we provide a comprehensive new description of this taxon using three new specimens, in which we observe many morphological features for the first time. Much of the cranial morphology is closer to that of living chimaeroid holocephalans than was previously appreciated. For this reason, we provide original figures illustrating the chondrocranium of a hatchling Callorhinchus milii demonstrating these similarities. In Chondrenchelys, although the jaw articulation is positioned at the posterior margin of the orbit, the high-walled lamina orbitonasalis and densely-mineralised antorbital crest provide evidence for forward rotation of the jaw adductor musculature. Preserved foramina for the efferent superficial ophthalmic nerves show that the sensory organs on the rostrum were enervated in a manner similar to modern sharks, with the ophthalmic nerves not enclosed in an ethmoid canal, as in modern holocephalans. The conjunction of numerous distinctly holocephalan features with those that are otherwise general to Chondrichthyes demonstrates a decoupling of several of the structural conditions that characterise the distinctive morphological complex of the extant holocephalan skull. The anguiliform postcranium is more elongate than previously reconstructed, and it is now clear that the axial skeleton extended beyond the posterior extremity of the elongate dorsal fin. Morphological characters are reviewed with a view to further phylogenetic analyses. We recommend using the appearance of Chondrenchelys at 336.5 Ma as a hard minimum age for the last common ancestor of elasmobranchs and chimaeroids, because of its secure association with other holocephalans, and current uncertainties concerning elasmobranch stem lineage membership.
