Comparison of Thrinaxodon, Kryptobataarand Monodelphis braincase and ear cochlea (3D recon)

Comparison of the braincase and cochlea of Thrinaxodon, Kryptobataar and Monodelphis. CT reconstructions through the braincase of Thrinaxodon (top) show a small passageway that housed a simple, tubular brain. In an extinct Cretaceous mammal, the multituberculate Kryptobataar (middle), the brain is greatly inflated compared to Thrinaxodon, equaling or exceeding the level reached by modern opossums (the least-encephalized of living mammals), illustrated here by Monodelphis (bottom). The shape of the endocranial cavity indicates that these mammals possess the neocortex - one of the most distinctive of mammalian brain attributes. Also visible in the reconstructions is the basic shape of the cochlea - the special sensory organ of the inner ear that converts mechanical energy into the neural stimuli that the brain perceives as sound. Thrinaxodon has simple sac-like cochlea. In contrast, modern therian mammals like the opossum have a cochlea that is spiraled 360° or more. Compared to Thrinaxodon, the mammalian ear is sensitive to a much wider range of frequencies, particularly high frequencies. The Cretaceous multituberculate Kryptobataar shows an intermediate condition. (Thrinaxodon and Monodelphis courtesy of Dr. Tim Rowe, University of Texas at Austin, Kryptobataar courtesy of Dr. Guillermo Rougier, American Museum of Natural History)

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