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The mammalian nasal cavity is characterized by a unique anatomy with complex internal features. The evolution of turbinals was correlated with endothermic and macrosmatic adaptations in therapsids and in early mammals, which is still apparent in their twofold function (warming and moistening of air, olfaction). Fossil evidence for the transformation from the nonmammalian to the mammalian nasal cavity pattern has been poor and inadequate. Ossification of the cartilaginous nasal capsule and turbinals seems to be a feature that occurred only very late in synapsid evolution but delicate ethmoidal bones are rarely preserved. Here we provide the first mCT investigation of the nasal cavity of the advanced non-mammaliaform cynodont Brasilitherium riograndensis from the Late Triassic of Southern Brazil, a member of the sister-group of mammaliaforms, in order to elucidate a critical anatomical transition in early mammalian evolution. Brasilitherium riograndensis already had at least partially ossified turbinals as remnants of the nasoturbinal and the first ethmoturbinal are preserved. The posterior nasal septum is partly ossified and contributes to a mesethmoid. The nasal cavity is posteriorly expanded and forms a distinctive pars posterior (ethmoidal recess) that is ventrally separated from the nasopharyngeal duct by a distinct lamina terminalis. Thus, our observations clearly demonstrate that principal features of the This article includes AR WOW Videos, which can be viewed at

A digital cranial endocast of the specimen UFRGS-PV-596-T, Riograndia guaibensis, was obtained from µCT scan images. This is a small cynodont, closely related to mammaliaforms, from the Late Triassic of Brazil. Riograndia presents big olfactory bulb casts and the cerebral hemispheres region is relatively wider than in other nonmammaliaform cynodonts. Impressions of vessels were observed and a conspicuous mark on the dorsal surface was interpretated as the transverse sinus. The encephalization quotient calculated is greater than the range of the most nonmammaliaform cynodonts. The ratios between linear and area measurements of the dorsal surface suggests four evolutionary changes from a basal eucynodont morphology to mammaliaformes, evolving the increase of the relative size of the olfactory bulbs and the width of the cerebral hemispheres and cerebellum. The data supports the hypothesis of the neurological evolution of the mammlian lineage starting by a trend of the increase of the olfactory bulbs, which is associated to adaptations on the nasal cavity. This trend must be linked to the selective pressures for the small faunivorous, and probably nocturnal, animals, and represents an initial improvement of the sensory receptor system, leading to further development of the "superior" structures for sensorial processing and integration.