How well did the dinosaurs perceive their environment? A new study of the brain anatomy of therizinosaurs, plant-eating dinosaurs that lived during the Cretaceous Period, has revealed interesting sensory links with their notorious meat-eating cousins Tyrannosaurus rex and Velociraptor. An international team of scientists, including PhD student Stephan Lautenschlager and Dr Emily Rayfield of the University of Bristol, found that the senses of smell, hearing and balance were well developed in therizinosaurs and might have affected or benefited from an enlarged forebrain. These findings came as a surprise to the researchers as exceptional sensory abilities would be expected from predatory and not necessarily from plant-eating animals.
Therizinosaurs had a very distinctive, often confusing set of characteristics. Their long necks, wide torsos, and hind feet with four toes used in walking resembled prosauropod dinosaurs. Their unique hip bones, which pointed backwards and were partially fused together. Among the most striking characteristics of therizinosaurs are the enormous claws on their hands, which reached lengths of three feet in Therizinosaurus. The unusual range of motion in therizinosaur forelimbs, which allowed them to reach forward to a degree other theropods could not achieve, also supports the idea that they were mainly herbivorous.
It was not until the mid-1990s, after Alxasaurus was discovered and shown to possess more typically theropod features, and Therizinosaurus was recognized as a member of the segnosaur group, that their true identity as herbivorous descendants of the carnivorous theropods became generally accepted. The scientists who described Falcarius noted that it seemed to represent an intermediate stage between carnivorous and herbivorous theropods, a sort of missing link between predatory maniraptorans and plant-eating therizinosaurs. Although they are now classified as theropods, therizinosaurs had skulls similar to those of sauropods and the shape of their teeth and jaws make it likely that they were herbivores.
Inspired by these paradoxes, the international team of palaeontologists decided to take the first close look inside the heads of these strange and enigmatic dinosaurs.
They studied the brain and inner ear anatomy of therizinosaurs using high-resolution CT scanning and 3D computer visualisation to find out more about their sensory and cognitive capabilities and how these had evolved with the transition from meat- to plant-eating.
The focus of the study was the skull of Erlikosaurus andrewsi — a 3-4m (10-13ft) therizinosaur, which lived more than 90 million years ago in what is now Mongolia.
Lead author, Stephan Lautenschlager of Bristol's School of Earth Sciences said: "Our results suggest that therizinosaurs would have used their well-developed sensory repertoire to their advantage which, for herbivorous animals, must have played an important role in foraging, in the evasion of predators or in social complexity."
"This study sheds a new light on the evolution of dinosaur senses and shows it is more complex than we thought."
Co-author, Professor Lindsay Zanno of the North Carolina Museum of Natural History and the North Carolina State University agrees: "Once you’ve evolved a good sensory toolkit, it’s probably worth hanging on to, whether you’re hunting or being hunted."
Fellow author Lawrence Witmer, Chang Professor of Paleontology at the Ohio University Heritage College of Osteopathic Medicine said: "Of course the actual brain tissue is long gone from the fossil skulls but we can use CT scanning to visualize the cavity that the brain once occupied and then generate 3D computer renderings of the olfactory bulbs and other brain parts."
This study has important ramifications for our understanding of how sensory function evolved in different dinosaur groups and whether it was developed as a response to their environment or simply inherited by their ancestors. In particular, in the light of the transition from dinosaurs to birds, these results should prove to be very interesting.
For further information see Therizinosaur.
Skeleton image via Wikipedia.