Cranial morphology of the Carboniferous rhizodontid Screbinodus ornatus (Osteichthyes: Sarcopterygii)

Well-preserved material is described of several specimens of Screbinodus ornatus, a small (approximately 1.5 metres long) rhizodontid sarcopterygian, from the Visean (Mississippian, Carboniferous) of Scotland. This species shows numerous 'typical' rhizodontid characteristics, such as post-parietals with a median 'tail', pectoral lepidotrichia with elongated basal segments, large symphysial tusks on the dentary and a 'reverse' overlap relationship between the cleithrum and clavicle. It also shows several characters unknown in other rhizodontids, such as the loss of the extratemporal and subopercular, and the probable exclusion of the postorbital from the orbital margin. A phylogenetic analysis finds that rhizodontids are the sister group to all other members of the tetrapod total group (with the exception of Kenichthys). This confirms that resemblances between derived rhizodontids and digited stem-tetrapods such as Acanthostega (noted by several previous authors) are due to convergence. A second analysis finds that Screbinodus is a derived rhizodontid, and part of a monophyletic Carboniferous radiation.