THE PALEONEUROLOGY OF MANIDENS CONDORENSIS (ORNITHISCHIA: HETERODONTOSAURIDAE) BASED ON CT SCANS

BECERRA, MARCOS G.; PAULINA-CARABAJAL, ARIANA; POL, DIEGO

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

Paleoneurology in ornithischian dinosaurs has been studied in species from most clades. However, outside Genasauria, few details on cranial nerve foramina have been addressed in Lesothosaurus and Heterodontosaurus, existing a complete lack of descriptions of endocranial cavities (brain, inner ear, vasculature) in early ornithischians. The complete skull of the holotype specimen of Manidens condorensis MPEF-PV 3211 (housed at the Museo Paleontológico Egidio Feruglio)?from the Early Jurassic of Patagonia?was micro-CT scanned, enabling the 3D reconstruction of brain and inner ear endocasts. The olfactory bulbs are oval, long, and slightly divergent. The olfactory tracts are undivided and proportionally longer than in Leaellynasaura, Dryosaurus, and Dysalotosaurus. The exit foramen for the caudal middle cerebral vein is assumed to be located at the parietal-supraoccipital suture, as in Heterodontosaurus. Retrodeformation of the parietals suggests a seemingly conspicuous dorsal eminence. The cast of the pituitary fossa is round, wide, and shallow. Relatively large passages for the cerebral branches of the internal carotid arteries enter the pituitary fossa separately. A conspicuous thin flocculus projects posterolaterally through the anterior semicircular canal on each side of the skull, more developed than in other bipedal ornithischians. The cranial passages for the optic, oculomotor, and trochlear nerves (II, III, and IV, respectively) are hard to differentiate given the overall damaged nature of the braincase. The trigeminal nerve (V) seems to exit the braincase through a single foramen, although separation of the ophthalmic branch is hard to define. The roots of the abducens nerve (VI) are on the anteroventral surface of the brain endocast, facing the pituitary, although their path and exit foramina are damaged, making impossible to stablish if there is a connection with the pituitary fossa. The facial and vestibulocochlear nerves (VII and VIII) origin together posteroventrally to cranial nerve V, with cranial nerve VII exiting laterally throughout the prootic and the cranial nerve VIII entering the otic capsule. As in Heterodontosaurus, three exit passages enclosed in a common region of the braincase may include the fenestra ovalis, and the passages for the jugular vein, and the glossopharingeal (IX) and vagus (X) nerves. Three passages exiting lateral to the occipital condyle can be easily traced on the posterior region of the medulla oblongata, corresponding to the accessory (XI) and hypoglossal (XII) nerves. The inner ear is partially reconstructed, preserving thin semicircular canals (if compared to other ornithischians), with the anterior semicircular canal being longer than the posterior one, the crus commune located below the dorsalmost portion of the anterior semicircular canal and flexed caudally, and with the anterior and lateral semicircular canals forming bulging ampullae, features shared with Heterodontosaurus. The inner ear is lacking the foramen vestibularis and the lagena in both sides. Here, we report the paleoneurology of one of the earliest-diverging ornithischian dinosaurs. Further studies on early species of the clade such as Heterodontosaurus and Lesothosaurus are needed to better understand the early evolution of sense development and its divergence from the common ancestor of Dinosauria.