Restored endocast of the brain of BMNH 37001 rendered as a shell. Elements from the left side are reversed. Scale bar, 10 mm. a, Right lateral view. b, Dorsal view. c, Posterior view. Abbreviations: c, cerebrum (telencephalon); cb, cerebellum; fl, floccular lobe of the cerebellum; fm, foramen magnum; mo, medulla oblongata (rhombencephalon); o, olfactory lobe; ol, optic lobes (metencephalic tectum); ot, olfactory tract; pc, cerebellar prominence; pg, pineal gland (epiphysis); ss, semicircular sulcus. Cranial nerves: II, optic; III?, possible oculomotor; V, trigeminal; VII, facial; VIII?, auditory (or endolymphatic duct, see Fig. 5); IX?, glossopharyngeal; X, vagus; XII, hypoglossal.

You gotta love modern technology. Using computed tomography, investigators have done high resolution imaging of the fossilized cranium of Archaeopteryx lithographica, plucking out a virtual reconstruction of the creature's brain. This allows them to do some detailed comparative gross neuroanatomy, looking to see whether there are signs here of the transition from terrestrial crawler to aerial flyer.

"Bird-brained" might be an insult, but modern birds actually have brains of a reasonable size: smaller relative to their body size than the brains of mammals, but relatively larger than the brains of their reptilian ancestors. Their brains are also specialized in particular ways, with large optic lobes to manage the increased emphasis on the visual sense, and also as you might guess, the vestibular senses (the ones that help you determine your orientation in space) are particularly important.

Alonso et al. have examined the Archaeopteryx brain, and what they've found is that it falls well on the avian side of the organizational chart. It's got the same large optic lobes as birds, but the cerebral and cerebellar spheres are somewhat smaller and a few other anatomical features suggest that the animal's brain was "more primitive than any modern bird." They've also looked at the proportions of the semicircular canals and the cochlea, and they lie within the range of modern birds.

The reconstructed pictures are gorgeous, but if you really want to see something, take a look at the online digital data. There are more photos there, and some very nifty QuickTime movies that let you roll and yaw the scanned structure around, or look at slices through it.

One other piece of useful information: with the detailed scan, they've been able to more accurately measure the size of the Archaeopteryx brain, and see how it compares to birds and reptiles.

Encephalization index for birds, reptiles and Archaeopteryx.

The logarithmic scale of the graph can be a little misleading, but basically it shows that the brain is somewhat intermediate in size—it falls within the low end of the range for modern birds, but far above the relative size of reptiles.

It looks like we've got more details of transitional changes in the avian lineage, and a better picture of what it takes to evolve flight.

The remodelling of the brain towards the avian condition must have begun well before the appearance of Archaeopteryx 147 million years ago in the latest Jurassic. The convergent increase in visual and vestibular regions in pterosaurs 19 is further evidence that both an aerodynamic wing and a powerful central nervous system are integral to powered flight.

Alonso PD, Milner AC, Ketcham RA, Cookson MJ, Rowe TB (2004) The avian nature of the brain and inner ear of Archaeopteryx. Nature 430: 666-669.