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The endocranial cavity of a nonmammalian eucynodont and its bearing on the origin of the mammalian brain
SVPCA Conference
Platform presentation (20 minutes)
*Tom S Kemp
Combination of complete preparation and CT scanning of a skull of the chiniquodontid eucynodont has revealed as much of the size and general morphology of the brain as is likely to be possible in cynodonts. The resulting reconstruction is partly speculative because of the failure of much of the brain, notably the sides and base of the cerebral hemispheres, to be impressed on bony surfaces. Features that can be reasonably well inferred include a relatively large olfactory bulb, very narrow cerebral hemispheres, and a well-developed, mammalian-sized cerebellum.
There is an anomaly in that the brain of the eucynodonts was very unlike that of mammals, whilst much of the rest of the anatomy points to a mammal-like biology, with fine neuromuscular control and at least partially endothermic physiology. On the basis of this evidence, the novel view is proposed that the mammalian brain evolved in two separate stages. The first consisted of elaboration of the cerebellum in association with increasingly precise proprioception and neuromuscular control, and is found in the cynodonts. The second consisted of the expansion of the cerebral cortex, leading to the characteristic mammalian neocortex, in association with elaboration of the olfactory and auditory sensory input. The latter coincided with the entry of the ancestral mammal into its small-bodied, insectivorous, nocturnal habitat.
Of the two conflicting current accounts of the origin of the mammalian brain, this conclusion supports the ‘outgroup’ hypothesis that the dorso-ventral ridge (DVR) of the cerebral hemispheres of living sauropsids is not the homologue of the mammalian neocortex, rather than the ‘recapitulation’ hypothesis, which posits that it is.
London 2020