Braincase and endocast anatomy in porolepiforms (Sarcopterygii: Dipnomorpha): evidence from the Early Devonian (Emsian) ‘porolepidid’ Durialepis edentatus

Porolepiforms represent one of the key lineages of early lobe-finned fishes. They include two taxonomic groupings: the paraphyletic ‘porolepidids’, characterised by rhombic scales and extensive development of cosmine on dermal bones; and the monophyletic, more nested holoptychiids, characterised by round scales and lack of cosmine. Braincases and endocasts represent important sources of information bearing on sarcopterygian interrelationships, but the level of detail available for these structures is highly uneven. Among porolepiforms, the braincase and endocast is best known for the Middle Devonian holoptychiid Glyptolepis groenlandica, but even here there is lack of detail compared to the most completely understood examples from other sarcopterygian lineages. Here we address this deficiency through micro-computed tomographic study of the intact neurocranium of Durialepis edentatus and Early Devonian (Emsian) ‘porolepidid’-grade porolepiforms from the Eifel region of Germany. We present some revisions to external details of the braincase reported by a prior study, the most significant of which is identification of the occipital arch. Scans reveal substantial new data on internal structure of the braincase. The endocranial cavity of the ethmosphenoid division closely agrees with details reported for Glyptolepis and Powichthys, and we agree with past studies that argue for a close relationship between the latter genus and porolepiforms. Durialepis edentatus shows a dense network of canals in the snout comparable to the rostral tubuli of several other sarcoptergians. We also provide the first model of the posterior cranial endocast in a porolepiform. Drawing on additional tomographic data for Glyptolepis paucidens and an unnamed Scottish holoptychiid, we review patterns of character distribution in porolepiform braincases and endocasts, amplifying support for clades including Porolepiformes + Powichthys and for Holoptychiidae.