How the avian brain evolved

It has been the belief of many palaentologists so far that Archaeopterix Lithographica , the first flying dinosaur, had a brain size that was intermediate between that of non-avian dinosaurs and modern birds. However, a new study by Ms. Amy Balanoff and her team indicates that the brain size of A. Lithographica was smaller than that of many earlier dinosaur species. Ms. Balanoff is Instructor of Research Anatomical Sciences, Department of Anatomical Sciences, Stony Brook University School of Medicine, Stony Brook, New York, U.S. The study is published in a recent issue of the journal Nature .

Brain size is an important determinant of the ability to fly and the greater the relative brain size (relative to body mass) the greater the neurological capabilities required of flight. Birds are distinct in this aspect as their brains, particularly fore brains, are expanded relative to body size.

The expansion by volume of the avian endocranium (the space inside the skull housing the brain) began early in theropod (carnivorous dinosaur) evolution and recent studies unlike previous ones suggest that avialans, the family to which

A. Lithographica belongs, are not unique in their brain-readiness for flying. In fact many earlier dinosaurs had larger endocranial space than

A. Lithographica .

This fact was borne out of the study which involved Computed tomography analyses of fossil skulls of the phylogeny of dinosaurs including modern birds. Previous studies have established that

A. Lithographica was not the basal bird. Is this finding of brain size further proof that A. Lithographica is not a basal bird? “ In a way that is true,” Ms. Balnoff noted in an email. “Our study shows that it is becoming harder to define what a bird is. Features that we thought were characteristic of birdsshow up much earlier in evolutionary history than we previously were aware of (within non-avian dinosaurs).”

If non-avian earlier dinosaurs had flight-capable brains, why did they not evolve into birds?

Ms. Balanoff notes: “The story with the flight ready brain is that it evolved before flight did. Like other flight-related features of birds the large brain evolved early in the history of non-avian dinosaurs — well before the evolution of powered flight. Only later in their history did dinosaurs evolve the capability to fly. It’s a good illustration of mosaic evolution in which disparate, seemingly unrelated pieces were put together for another use.”

The study considered the relationship between total endocranial volume and body size of theropod lineages most loosely related to avialans. The researchers also divided the endocasts into volumetric partitions to estimate the major neuro-anatomical regions such as olfactory bulbs, cerebrum, optic lobes, cerebellum and brain stem.

This was done to find out how the various endocranial regions of the avian brain evolved in relation to the body size and to one another. The relatively high correlations between the total volume and changes in size of different brain regions, indicate that each region with the exception of the olfactory bulbs, expanded along the avian stem.

There was no structural constraint on the growth of endocranial volume. The fact that the regions expanded together shows that the surrounding skeleton of the tissues was highly responsive to evolutionary growth of the various regions of the brain. Encephalisation (the excess growth of brain tissue in relation to body size) also took place due to gross expansion of the tissue or decrease in body size or a combination of the two.