Phylogenetic implications of the systematic reassessment of Xenacanthiformes and ‘Ctenacanthiformes’ (Chondrichthyes) neurocrania from the Carboniferous–Permian Autun Basin (France)

Abstract

Four complete isolated neurocrania excavated during the nineteenth century in the Autun Basin (Saône-et-Loire, France) were used by Heyler & Poplin (1982) to erect two species: the symmoriiform Bibractopiscis niger and the xenacanthiform Orthacanthus commailli. However, the specimens have not been mechanically prepared and only a portion of their anatomy is available, complicating their systematic attribution. We revise their systematic status by using X-ray computed microtomography and new comparative material from the Bourbon l’Archambault Basin (Allier, France). This method allows access to the hidden anatomy of the specimens and new systematic revision: we propose O. commailli to be left in open nomenclature as ?Xenacanthimorpha. The specimens attributed to B. niger encompass two distinct morphologies: the holotype, one paratype and a neurocranium from the Bourbon l’Archambault Basin are identified as cf. Triodus sp. The last paratype is left in open nomenclature as ‘Ctenacanthiformes’ indet. We included these neurocrania in geometric morphometric and cladistic analyses of a large sample of Palaeozoic chondrichthyans. Based on the results we propose an evolutionary scenario for the neurocranial transition between the orders ‘Ctenacanthiformes’ and Xenacanthiformes. This transition mostly affects the ethmo-orbital region. The revision of the diversity of these chondrichthyan faunas calls into question potential differences in trophic structures between the different European Carboniferous–Permian basins.

Keywords:

• Xenacanthiformes
• ‘Ctenacanthiformes’
• neurocranium
• cladistics
• geometric morphometrics
• Carboniferous–Permian transition

Acknowledgements

The authors thank the Rhinopolis Association, the Gannat Town Hall and the Conseil Départemental de l’Allier for having funded the excavation campaigns in the Buxières-les-Mines locality during which the specimen MHNE.2021.9.1 was found, and for providing storage space in which the Buxières-les-Mines fossils are housed. We thank René Kindlimann (Aathal, Switzerland) for access to his collection. We thank Florent Goussard (MNHN) and Céline Salaviale (LGL-TPE) for their help in the 3D reconstructions, and Philippe Loubry (MNHN) for the photographs of the specimens from the Autun Basin. We thank two anonymous reviewers and the associate editor, Martin Brazeau, for their remarks that have greatly helped to improve this manuscript. The first author was funded by the Doctoral School ED 341 – E2M2 Évolution Écosystème Microbiologie Modélisation, Université Claude Bernard Lyon 1, France.

Supplemental material

Supplemental material for this article can be accessed here: http://dx.doi.org/10.1080/14772019.2022.2073279

Associate Editor: Martin Brazeau