Revision of platypterygiine rostral material from the Northern Apennines (Italy): New insights on distal neurovascular anatomy and tooth replacement in Cretaceous ichthyosaurs

In 2016 a new Cretaceous ichthyosaur rostrum fragment (IG 251372) was found near Gombola, Modena Province, northern Italy. Two rostral fragments and a partial humerus were previously reported from the same locality. Despite their fragmentary nature, all these finds are relatively well preserved, with just slightly deformed shape and teeth still in place. Here we describe IG 251372, relying on CT-scan technology to allow visualization of the fossil internal structure. The new specimen has been ascribed to the sub-family Platypterygiinae based on the sub-rectangular profile of the tooth roots. Tooth crowns are robust and show a similar density of enamel ridges to many other Cretaceous ichthyosaurs historically referred to the genus Platypterygius. A detailed taphonomical analysis was performed on the material from Gombola. The ichthyosaurs previously ascribed to Platypterygius are now identified only to the subfamily level. CT-scans revealed an anterior neurovascular network within both the premaxilla and the dentary, here interpreted as mandibular and maxillary/ophthalmic divisions of the trigeminal nerve. Axial sections and videos show how these intraosseous channels emerge within the dental and premaxillary fossae. Cross section imaging of tooth roots showed an alternating pattern of tooth resorption and eruption, providing additional information on dental replacement in ichthyosaurs. IG 251372 has been attributed to the Albian-Cenomanian by means of nannofossils contained within the matrix, making it the first ichthyosaur from the Northern Apennines to be directly dated.