A new close mammal relative and the origin and evolution of the mammalian central nervous system
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Mammals are distinguished by the presences of the cerebral neocortex, and dentary-squamosal jaw joint, among other traits. These traits likely evolved outside of Mammalia, in Mammaliaformes. Recent fossil discoveries from South America elucidate the evolution of the brain in taxa outside of Mammaliaformes. A new skull of a close mammal relative, Pseudotherium argentinus, from the Late Triassic Ischigualasto Formation in Argentina was scanned at The University of Texas High-Resolution X-ray Computed Tomography (CT) Facility. CT data reveal a unique combination of ancestral and derived characters. Phylogenetic analysis supports a sister-taxon relationship between Pseudotherium and the derived Tritylodontidae. An endocast reconstruction further supports a derived phylogenetic position among cynodonts. A comparison of the Pseudotherium endocast with other cynodont endocasts suggests that the cerebral hemispheres enlarged in probainognathian cynodonts, while the total endocranial volume remained relatively constant until the origin of Mammaliaformes. This expansion is also present in the foramen magnum. Encephalization volume and skull length were compared with foramen magnum size in extinct and extant cynodonts, birds, and lizards. Endocranial volume and foramen magnum size, and foramen magnum size and skull length, are correlated following a 3/2 power law. Therefore, foramen magnum size can be used to predict endocranial volume for fossils. Regression analyses show foramen magnum size is significantly increased in Mammaliaformes. This change reflects an increase in medulla oblongata size and supports a concerted model of brain evolution outside of Mammalia. Because the increase in medullary size coincides with the origin of the neocortex, I hypothesize that the mammalian pyramidal tract originated in Mammaliaformes. The hypotheses from this dissertation are testable given recent fossil discoveries. However, morphological data of those specimens are not readily available. Morphological descriptions in this dissertation are accompanied by detailed figures, and the data will be digitally archived and made publically available on DigiMorph.org. The intent is to model a communication and data-sharing standard to be implemented in all future fossil discoveries.