A 3D braincase of the early jawed vertebrate <i>Palaeospondylus</i> from Australia

<i>Palaeospondylus gunni</i> 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 <i>Palaeospondylus</i> 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 <i>P. gunni</i>, the new braincase shows that <i>Palaeospondylus</i> lacks both a postorbital process and an intracranial joint. Our new Australian species extends the history of <i>Palaeospondylus</i> 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 <i>Palaeospondylus</i> as a sister group of the Chondrichthyes, but some neurocranial characters could indicate a phylogenetic position within the gnathostome stem group.