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

摘要

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.