First 3D reconstructions of the brain and inner ear of the Mongolian ankylosaurids Tarchia and Talarurus based on CT scans

PAULINA CARABAJAL, A.; LEE, Y-N.; KOBAYASHI, Y.; LEE, H-J.; CURRIE, P. J.

Mendoza

Congreso; International Paleontological Congress; 2014

International Paleontologica Asociation

Ankylosaur braincase and endocranial morphology is in general poorly known, and only the cranial endocast and inner ear of Euoplocephalus, an ankylosaurid from North America, and the cranial endocasts of Sauropelta and another unnamed nodosaurid from Japan have been described in detail. The first 3D reconstructions of the brains and inner ears of the Mongolian Upper Cretaceous ankylosaurids Talarurus and Tarchia are presented here, based on the CT scans of two beautifully preserved skulls. The specimens were recovered by the Korea-Mongolia International Expedition, and were prepared and CT scanned in South Korea at the Hwaseong City laboratory and the Korea Institute of Geoscience and Mineral Resources (KIGAM). The general morphologies of the brains of the two Mongolian taxa resemble that described for Euoplocephalus in being anteroposteriorly short, and in having a globose forebrain, and large internal carotids that transversely enter the distal end of an elongated pituitary fossa. The size, number and disposition of cranial nerves I-XII are also similar. For example, all branches of CN V exit through a single foramen; CN VI does not enter the pituitary fossa, and there are two branches for CN XII even though they exit the braincase externally through a single foramen. In lateral view, the shape of the brain is slightly sigmoidal, with low angles between the hindbrain, midbrain and forebrain. The inner ear has a dorsoventrally short labyrinth with robust semicircular canals and an extremely elongate lagena, indicating that hearing was an important -- if not the most important -- sense, as in Euoplocephalus. Some differences can be observed in the brains of both taxa. Talarurus is characterized by the presence of a small, strongly-projected dorsal expansion that is not developed in Euoplocephalus or Tarchia. Tarchia in turn, is characterized by the presence of a flocculus. Within saurischian dinosaurs, the flocculus is present in theropods and absent in most sauropods except in Spinosphoraurus, and few rebbachisaurids. The presence of flocculus in Tarchia suggests some development of gaze stabilization, probably related to fast movement of the head and the club tail during alert or defense activities.