New light on the braincases of <i>Ventastega curonica</i> and <i>Acanthostega gunnari</i>

Abstract The braincase and middle ear region underwent a dramatic reconfiguration during the fish–tetrapod transition, involving the loss of the intracranial joint and associated structures, as well as the loss of the lateral commissure and the transformation of the hyomandibula into a stapes. The earliest examples of the tetrapod condition are seen in Devonian stem tetrapods. Complete otoccipital regions have only been described from two genera, Ichthyostega and Acanthostega , which differ greatly from each other. Here we present the first complete otoccipital region of Ventastega , together with new data on Acanthostega that modify the published reconstruction. In lateral view, both braincases are similar in shape, with large fenestrae vestibuli and anteroposteriorly short, but tall, exoccipitals. However, in occipital view the exoccipital-basioccipital complex of Ventastega resembles that of Eusthenopteron and differs substantially from Acanthostega . Remarkably, both braincases contain an arcual plate, a sarcopterygian fish characteristic known in taxa with an intracranial joint, such as Eusthenopteron and Latimeria , but never before seen in a tetrapod. The hypophysial region of Acanthostega contains an interorbital foramen similar to that in post-Devonian tetrapods, contrasting with Ventastega which has a solid braincase wall with small foramina similar to the condition in Eusthenopteron . The suture pattern of the skull roof of Ventastega is fully resolved and the presence of an intertemporal bone is confirmed.