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Abstract

Endocranial casts of Mesozoic mammals and of some cynodonts are reviewed. New tentative reconstructions of brains of Probainognathus and Therioherpeton are given. It is claimed that the endocast of Amblotherium is an artefact. Brains of Mesozoic mammals were lissencephalic, with no flexure, had very large olfactory bulbs, relatively extensive cerebral hemispheres diverging posteriorly, and large paraflocculi. Within this pattern two types are designated: the cryptomesencephalic type (large vermis, no dorsal midbrain exposure, and no cerebellar hemispheres) which occurs in Triconodonta and Multituberculata; and the eumesencephalic type (wide cerebellum, cerebellar hemispheres, and large dorsal midbrain exposure) which occurs in Cretaceous Tribosphenida. Overlap of the midbrain took place in individual lines of the Tribosphenida at different times during the Tertiary. If advanced cynodonts (e.g., Probainognathus and Therioherpeton) had narrow cerebellum and exposed midbrain, then both types could develop from them: the cryptomesencephalic by overlap of the midbrain by an enlarged vermis, and the eumesencephalic by acquisition of enlarged cerebellar hemispheres. If, however, the midbrain was overlapped in advanced cynodonts, then they belong to the cryptomesencephalic type. If so, the eumesencephalic type would have developed from cryptomesencephalic by secondary exposure of the midbrain and acquisition of enlarged cerebellar hemispheres. This latter is less likely, as it would involve the reduction of an already expanded vermis. The expansion of cerebral hemispheres suggests that neocortex was possibly present in all Mesozoic mammals and in some cynodonts.

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