Study of a digital cranial endocast of the non-mammaliaform cynodont Brasilitherium riograndensis (Later Triassic, Brazil) and its relevance to the evolution of the mammalian brain

摘要

A digital cranial endocast of the specimen UFRGS-PV-1043-T, Brasilitherium riograndensis, was obtained from high-resolution computed tomography (lCT) scan images. This taxon is a small cynodont from the Late Triassic of Brazil, and has been used as the sister-group of the mammaliaforms in cladistic analyses. The digital endocast of UFRGS-PV-1043-T is mostly complete, allowing the description and collection of accurate linear and volumetric measurements, which were taken and compared with other non-mammaliaform cynodonts. Impressions of vessels were observed in the inner walls of the braincase. Despite the lack of a cribiform plate and the presence of a wide orbital vacuity, the endocast of Brasilitherium shows olfactory bulb casts that are relatively larger than in other non-mammaliaform cynodonts, suggesting a pattern of gradual increase in size and improvement of the olfactory sense for these structures toward the mammalian condition. The cerebral hemispheres are elongated and clearly divided by a median sulcus. The parafloccular casts are well defined, and their position corresponds to the maximum width of the endocast. In the ventral view, a large hypophyseal cast and a wide opening for the cavum epiptericum are evident. The encephalization quotient (EQ) calculated for Brasilitherium is greater than the range of EQs reported for most non-mammaliaform cynodonts (although it may be similar to that of some taxa, according to the equation used to estimate their body masses), but it is smaller than that of the mammaliaforms and mammals. A slighter increase in the brain size of Brasilitherium compared with other non-mammaliaform cynodonts was observed, along with a more significant increase in the size of the olfactory bulbs. This study supports the proposition of an early evolution of the mammalian brain associated with selective pressures for better sensorial acuity, especially regarding improved olfaction, which began with small Triassic mammaliamorphs.