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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This specimen was collected from the Hettangian-Sinomurian Formation (Early Jurassic) in the Lufeng Basin of Junan, P.R. China. It was made available to the University of Texas High-Resolution X-ray CT Facility for scanning by Dr. A. W. Crompton of the Museum of Comparative Zoology, Harvard University, and Dr. Zhe-Xi Luo of the Carnegie Museum of Natural History.

This specimen was scanned on by Richard Ketcham and Matthew Colbert 21 April 1999. It was scanned along the coronal axis for a total of 446 slices. Slice thickness is 0.096 mm with an interslice spacing of 0.072 mm (for a slice overlap of 0.024 mm).

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