A 3D braincase of the early jawed vertebrate Palaeospondylus from Australia

Palaeospondylus gunni Traquair, 1890, is represented by thousands of similarly preserved articulated fossils from the Achannaras quarry (∼390 Mya) in Caithness, Scotland. With radically different interpretations of its structure, it has been assigned to almost all major jawless and jawed vertebrate groups. Here we report a new and older species of Palaeospondylus from the Early Devonian of Australia (c. 400 Ma), investigated using high-resolution computed tomography. Its 3D-preserved braincase demonstrates a combination of primitive gnathostome features including an anteriorly positioned transverse cranial fissure of uncertain homology, a large dorsal fontanelle and a small hypophysial fossa. Contrary to recent interpretations of P. gunni, the new braincase shows that Palaeospondylus lacks both a postorbital process and an intracranial joint. Our new Australian species extends the history of Palaeospondylus back some 10 million years prior to its occurrence in Scotland. The newly identified neurocranial characters have been coded into a phylogenetic analysis that places Palaeospondylus as a sister group of the Chondrichthyes, but some neurocranial characters could indicate a phylogenetic position within the gnathostome stem group.