Brain & behaviour of dinosaurs

Bones have been big news recently, following the publication of two papers which document remarkable fossil finds. First, a group of palaeontologists led by Phil Gingerich of the University of Michigan described Maiacetus inuus, a primitive whale which lived in the water but gave birth on land, and which marks the transition between modern whales and their terrestrial ancestors. This was quickly followed by the report, from Jason Head's group at the University of Toronto, of Titanoboa cerrejonesis, a prehistoric snake which is estimated to have grown to 13 metres and to weigh more than a tonne.

Such spectacular discoveries always grab the headlines, and rightly so. There are, however, other recent developments in palaeontology, which have been largely overlooked, but are nevertheless equally interesting. The new findings come from Lawrence Witmer's lab at Ohio University's College of Osteopathic Medicine, where the main focus of research is the structure, function and biomechanics of the heads of vertebrates, both living and extinct.

Using sophisticated imaging techniques, Witmer's group scan the skulls of dinosaurs and compare their anatomical structure to those of modern animals. They then use these data to generate detailed three-dimensional digital reconstructions of the soft tissues inside the dinosaur skulls - the muscles, blood vessels, sinuses and brain. Analysis of the reconstructed brains enables them to make inferences about how the prehistoric beasts may have behaved. The Witmer lab is, in effect, bringing dinosaurs back to life.

How might computer simulations of the brains of organisms that became extinct some 65 million years ago provide insights into how those organisms behaved? This is possible because we already have some understanding of the relationship between structure and function in the brains of living animals, and of how different regions of the skull are associated with the senses. Hence, if a reconstruction shows well developed olfactory lobes, we could infer that the organism had a keen sense of sense of smell, and so on.  

More insights into dinosaur behaviour can be gained from the structure of the inner ear, which contains the cochlea and semi-circular canals - the former contains hair cells, which are sensitive to specific frequencies of soundwaves, while the latter are crucial for balance. The structure of the inner ear can therefore provide some information about an organism's hearing capabilities, auditory-related behaviours and, perhaps, how it may have moved. Until recently, the inner ear could only be observed in damaged fossils, where it appears as a series of tubes on the inside of the skull. However,  micro-computed tomography now enables small structures such as the cochlea from intact fossils to be visualized fully and in high resolution.

One group of dinosaurs to which Witmer has applied these methods and principles is the lambeosaurs, one of two subfamilies of the so-called duck-billed dinosaurs. Lambeosaurs had hollow bony crests on their heads, the function of which was a subject of debate. Some researchers speculated these large adornments enhanced the sense of smell by increasing the surface area of the olfactory epithelium, while others argued that they act as resonators for the sounds produced in communication.

When Witmer and colleague Ryan Ridgely scanned the skulls of four different lambeosaurs, they found a large discrepancy between the external shape of the crests and the shape of the nasal passages inside them - the airways were  much larger and more elaborate than they were thought to be. Reconstruction of the brain revealed that the olfactory lobe was very small, suggesting that the sense of smell was not important. By contrast, the parts that were involved in higher cognitive function were surprisingly large, and the analysis of the cochlear structure showed that  the lambeosaurs were probably sensitive to low frequency sounds.

These findings, which were presented at the annual meeting of the Society of Vertebrate Palaeontology in Cleveland, Ohio last October, suggest that lambeosaurs were capable of complex social behaviours and sophisticated modes of communication. They support the earlier claims of other researchers that these dinosaurs could produce low frequency bellowing calls, with which they may have communicated the presence of a predator to one another.  Rather than enhance the sense of smell, the large nasal passages may have acted to resonate these vocalizations, while the embellished crest may have evolved, like the peacock's tail  feathers, as a decoration for visual displays that were performed to attract potential mates. 

In a paper published last November in The Anatomical Record, Witmer and Ridegly reported that the heads of four different predatory dinosaurs, including Tyrannosaurus rex, also contained an extensive and complex system of nasal passages. From their 3D visualizations, they calculated the relative volumes of the brain, bone, air spaces and muscle in the T. rex head, and found that the brain cavity was tiny in comparison to the volume of air spaces. Although small, the brain had a large olfactory lobe, indicating a good sense of smell. The inner ear was also well developed, so T. rex probably also had good hearing, and individuals may even have been capable of recognizing one another from subtle differences in the vocalizations they made.      

Witmer and Ridegly also estimated that the head of T. rex weighed a massive 515 kg, and that the air spaces saved about 18% of the skull mass. This would have reduced the amount of bone, which is energetically expensive to maintain, and increased the space available for muscle, giving the "terrible lizard" powerful jaws with which it could tackle large prey. The well developed semicircular canals would have been beneficial for hunting, giving T. rex a good sense of balance, and enabling it to fix its gaze on fast-moving prey, while the large sinuses may have served as a ventilation system, with the jaw muscles acting like bellows pumps to move the air within it.

Most recently, Witmer and his colleagues reconstructed the cochlea of Archaeopteryx, the early prehistoric bird which lived some 150 million years ago and is an intermediate between modern birds and the Dromaesaurs (which were popularized as "raptors" in the Jurassic Park films). Last month, they reported that the duct surrounding the Archaeopteryx cochlea was of a similar length to that of modern birds. The length of the cochlea is closely related to hearing sensitivity in modern birds, suggesting that it had a similar hearing range too. It was estimated to be 600-3,400 Hz, which places it at the lower end of hearing sensitivity of modern birds, close to the emu. The structure of the cochlea provides clues about how and where the protoypical bird may have lived - good hearing capabilities are required for life in a large social group in which individuals communicate vocally, and in closed environments such as forests, where vision is limited.

Witmer, L. M. & Ridgely, R. C. (2008). The Paranasal Air Sinuses of Predatory and Armored Dinosaurs (Archosauria: Theropoda and Ankylosauria) and Their Contribution to Cephalic Structure. Anat. Rec. 291: 1362-1388. DOI: 10.1002/ar.20794.

Walsh, S. A. et al (2009). Inner ear anatomy is a proxy for deducing auditory capability and behaviour in reptiles and birds. Proc. R. Soc. B. DOI: 10.1098/rspb.2008.1390.