Neuroanatomy of the abelisaurid theropod Viavenator exxoni

Viavenator exxoni, Museo Municipal Argentino Urquiza

The Abelisauridae represents the best-known carnivorous dinosaur group from Gondwana. Their fossil remains have been recovered in Argentina, Brazil, Morocco, Niger, Libya, Madagascar, India, and France. The group was erected by Jose Bonaparte with the description of  Abelisaurus Comahuensis. These theropods exhibit spectacular cranial ornamentation in the form of horns and spikes and strongly reduced forelimbs and hands. In South America, braincase remains are known for Carnotaurus sastrei, Abelisaurus comahuensis, Aucasaurus garridoi, Ekrixinatosaurus novasi, Skorpiovenator bustingorryi, Eoabelisaurus and Viavenator exxoni.

The holotype of Viavenator exxoni (MAU-Pv-LI-530) was found in the outcrops of the Bajo de la Carpa Formation (Santonian, Upper Cretaceous), northwestern Patagonia, Argentina. Cranial elements of this specimen include the complete neurocranium: frontals, parietals, sphenethmoids, orbitosphenoids, laterosphenoids, prootics, opisthotics, supraoccipital, exoccipitals, basioccipital, parasphenoids and basisphenoids. The cranial endocast of Viavenator measures 157.7 mm from the olfactory bulbs to the foramen magnum, with a volume of approximately 141.6 cm3. The general shape of cranial endocast is elongate and narrow, similar to Aucasaurus and Majungasaurus. The widest part of the cranial endocast of Viavenator is at the level of the cerebral hemispheres. Four blood vessel foramina are recognized in the braincase: the caudal middle cerebral vein, the rostral middle cerebral vein, the cerebral internal carotid artery and the sphenoid artery.

Figure 1. Rendering of the type braincase of Viavenator exxoni (MAU-Pv-LI-530) in dorsal (A,B), and right lateral (C,D) view. Adapted from Carabajal y Filippi, 2017.

The forebrain of Viavenator include the olfactory tracts and olfactory bulbs, the cerebral hemispheres, optic nerves, the infundibular stalk, and the pituitary body. The CT scans show that the olfatory tracts are undivided. The olfactory bulbs are oval and are separated by a median septum at the anterior region of the sphenethmoids. The optic lobes are not clearly defined. The visible features of the hindbrain in the cranial endocast include the cerebellum, medulla oblongata, and cranial nerves V–XII. The cerebellum is not clearly expanded in the endocast; however, the floccular process of the cerebellum is well defined. The general morphology of both, brain and inner ear of Viavenator is markedly similar to that of Aucasaurus.
Neurosensorial capabilities of extinct animals can be inferred in part based on the relative development of certain regions of the brain. The flocculus of the cerebellum plays a role in coordinate eye movements, and tends to be enlarged in taxa that rely on quick movements of the head and the body. The flocculus of Viavenator is particularly large compared with Majungasaurus, suggesting that Viavenator relied more on quick movements of the head and sophisticated gaze stabilization mechanisms than the African form. The dimensions of the auditory sensory epithelium of Viavenator is similar to Majungasaurus, suggesting that they had similar hearing capabilities. In large dinosaurs, hearing was restricted to low frequencies with high frequency limit below 3 kHz.

References:

Paulina-Carabajal, A., Filippi, L., Neuroanatomy of the abelisaurid theropod Viavenator: The most complete reconstruction of a cranial endocast and inner ear for a South American representative of the clade, Cretaceous Research (2017), doi: 10.1016/j.cretres.2017.06.013

Leonardo S. Filippi, Ariel H. Méndez, Rubén D. Juárez Valieri and Alberto C. Garrido (2016). «A new brachyrostran with hypertrophied axial structures reveals an unexpected radiation of latest Cretaceous abelisaurids». Cretaceous Research 61: 209-219. doi:10.1016/j.cretres.2015.12.018

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