Abstract from Rowe, T., Macrini, T.E., and Luo, Z.-X. 2011. Fossil evidence on origin of the mammalian brain (Science, 332, 955-957):
Many hypotheses have been postulated regarding the early evolution of the mammalian brain.
Here, X-ray tomography of the Early Jurassic mammaliaforms Morganucodon and Hadrocodium
sheds light on this history. We found that relative brain size expanded to mammalian levels, with
enlarged olfactory bulbs, neocortex, pyriform cortex, and cerebellum, in two evolutionary pulses.
The initial pulse was probably driven by increased resolution in olfaction and improvements in
tactile sensitivity (from body hair) and neuromuscular coordination. A second pulse of olfactory
enhancement then enlarged the brain to mammalian levels. The origin of crown Mammalia saw a third
pulse of olfactory enhancement, with ossified ethmoid turbinals supporting an expansive olfactory
epithelium in the nasal cavity, allowing full expression of a huge odorant receptor genome.
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