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.

*Click the Additional Imagery tab for endocast animations.

This specimen (IVPP 8275), the holotype, was collected from the Lower Lufeng Formation of Yunnan, China. The specimen was made available to The University of Texas High-Resolution X-ray CT Facility for scanning by Dr. Zhe-Xi Luo of the Carnegie Museum of Natural History. Funding for image processing was provided by a National Science Foundation Digital Libraries Initiative grant to Dr. Timothy Rowe of The University of Texas at Austin.

The specimen was scanned by Richard Ketcham on 6 February 2003 along the coronal axis for a total of 735 1024x1024 pixel slices. Each slice is 0.0191 mm thick, with an interslice spacing of 0.0191 mm and a field of reconstruction of 9 mm.

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Click on the thumbnail to the left for a pitch animation (1.8 mb) of the isolated Hadrocodium cranial endocast.

Click on the thumbnail to the left for a roll animation (1.8 mb) of the isolated Hadrocodium cranial endocast.