New data on the inner skull cavities of Diplocynodon tormis (Crocodylia, Diplocynodontinae) from the Duero Basin (Iberian Peninsula, Spain)

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Inner skull cavities provide key characters to elucidate taxonomic, phylogenetic, and palaeobiological inferences in reptiles. Herein, an integral description of the extinct alligatoroid Diplocynodon tormis is presented based on its holotype, recovered from the Middle Eocene site of “Teso de la Flecha” (Salamanca, Spain). It is an almost complete skull only missing some bones of the posterior part of the basicranium. A computed tomography (CT-scan) allowed the creation of a 3D model that includes the paranasal air sinuses in association with the nasal cavity, the dorsal region of the forebrain, including the cerebral hemispheres and the olfactory tract and the bulbs, the maxillary and mandibular branches of the trigeminal nerve, and the nasopharyngeal duct. Based on this, an anatomical description is provided and compared with both extant and extinct members of Alligatoridae and Crocodyloidea, including the previously published neuroanatomical description of one of the paratypes of this species. Neurosensorial (such as olfactory ratio and visual acuity) and cognitive capabilities (such as Reptile Encephalization Quotient) were estimated for the holotype and paratype of D. tormis and compared to those of other crocodylians. The inner skull cavities of D. tormis are similar to those of other crocodylians. Interestingly, the polarity of some characters in their brain, paranasal sinuses, and pharyngotympanic sinus system support its position as a basal alligatoroid. Neurosensorial and cognitive estimations also concur with a medium-sized basal alligatoroid. Nonetheless, further studies including other fossil crocodylians are required to better understand the evolutionary patterns of the inner skull cavities that have been relevant during the evolution of this successful archosaurian lineage.