A new onychodont (Osteichthyes; Sarcopterygii) from the Middle Devonian of Morocco and its bearing on early osteichthyan evolution

Abstract

A new onychodont sarcopterygian is described from the Middle Devonian (Eifelian) of Morocco. Selenodus aquesbiae gen. et sp. nov. is characterized by a unique suite of traits among onychodonts such as a large maxilla with a rounded posterior expansion and a straight ventral margin, the presence of a single supraorbital, and a reduced opercular series. It also displays certain features commonly found in onychodontids and early sarcopterygians, like a lateral rostral contacting the orbit through a narrow posterodorsal projection, numerous sclerotic plates surrounding the orbit, the occurrence of an accessory row of small denticles labially lining the single row of teeth from the upper and lower jaws, a prearticular ornamented with small rounded tubercules, the postotic lateral line canal passing through the supratemporal, tabular and lateral extrascapular bones of the skull roof, and dermal bones and rounded scales ornamented by small dentine tubercules capped with enamel. All the fossil remains belong to a single individual representing a relatively large fish (around 1 m of estimated length based on the skull dimensions) for which the snout, cheek, and skull roof have been partially reconstructed. The well-preserved material comprises mainly the dermocranium but two branchial elements are known, adding to our knowledge of early osteichthyan endoskeleton. A phylogenetic analysis places Selenodus in an unresolved polytomy among onychodonts and coelacanths, enabling discussion of evolutionary scenarios and character combinations among early osteichthyans. The close relationship between onychodonts and actinistians is firmly established and suggests that all onychodont taxa are stem coelacanths. Selenodus represents the first occurrence of onychodonts from Africa and furnishes key information not only on onychodont morphology and interrelationships but also on their palaeobiogeographical distribution and Devonian faunal affinities between Euramerica and Gondwana.

http://zoobank.org/urn:lsid:zoobank.org:pub:3FEB765B-E203-4015-BB48-20B108509F3B

Keywords:

• Devonian
• Gondwana
• histology
• Onychodontida
• phylogeny
• stem coelacanth

Acknowledgements

I warmly thank Gaël Clément, Philippe Janvier, Daniel Goujet, Hervé Lelièvre, Alan Pradel and Guillaume Lecointre (MNHN, Paris), Per Ahlberg (Uppsala University), Zerina Johanson (NHM, London), Jenny Clack (University of Cambridge), John Long (Flinders University), Jing Lu (IVPP, Beijing), Matt Friedman (University of Michigan), Sam Giles (University of Birmingham), Martin Brazeau (Imperial College, London), Martin Rücklin (Naturalis Biodiversity Center, Leiden), Pierre Guériau (University of Lausanne), Fabrice Fack and Vincent Dupret for fruitful discussions on sarcopterygian diversity, onychodont anatomy and phylogenetic analyses. Florian Witzmann (MfN, Berlin) and Zerina Johanson (NHM) generously provided access to fossil material under their care. Séverin Morel (MNHN) prepared the thin sections, and François Meunier and Damien Germain (MNHN) allowed microscope access. SEM pictures were taken by Philippe Loubry (MNHN) and Marc Herbin and Zora Gabsi (MNHN) assisted with the acquisition of X-rays images. The constructive remarks of two anonymous reviewers improved the paper. I also acknowledge the funding by the Société des Amis du Muséum national d’Histoire naturelle for the study of the collections at the Museum für Naturkunde (MfN), Berlin. Per Ahlberg (Uppsala University) and Paulo Brito (Universidade do Estado do Rio de Janeiro) kindly supported my application to a Synthesis grant to study the Onychodus collections at the Natural History Museum (NHM), London.

Supplemental material

Supplemental material can be accessed here: http://dx.doi.org/10.1080/14772019.2019.1655495.

Associate Editor: Zerina Johanson