A series celebrating 200 years of dinosaurs continues as palaeontology undergraduate Harry Bridger shares his experience investigating dinosaur eye size.
Students measuring the orbit length of dinosaurs (Triceratops horridus and Struthiomimus sedens) mounted at the Oxford Museum of Natural History. Although measurements used in the study were taken digitally, the use of rulers on actual specimens or casts demonstrates the methodology. Image by Harry Bridger.
From beetles to birds and everything in between, vision is one of the most important senses in an animal’s toolkit, so understanding how they use it can tell us a great deal about their biology, behaviour and role in their ecosystem. Unfortunately, such an understanding becomes obscured for extinct animals like dinosaurs, as the fossil record simply doesn’t preserve most of the soft tissues associated with vision.
However, a recent study published in the Journal of Vertebrate Palaeontology has shed new light on how dinosaurs and their relatives saw the world. In living animals, a sense of sight is often linked to how big the eye is - a larger eye equals better vision - so untangling the visual capabilities of dinosaurs becomes a matter of determining eye size. In birds and crocodiles, the closest living relatives of the dinosaurs, the ratio between the size of the orbit (eye socket) and the size of the eye increases at a reliable rate. Therefore, by measuring the length of the orbit in dinosaurs, it is possible to predict how large their eyes may have been.
Using existing data from the scientific literature, skull length and orbit diameter were measured in 400 species of archosauromorphs, the major group of reptiles that contains dinosaurs, pterosaurs, crocodiles and their closest relatives. This was carried out by a cohort of 22 palaeontology undergraduates from the University of Birmingham, providing students with an opportunity to contribute to scientific research they might not otherwise get until later in their career. Each student was assigned a group or clade of around 20-35 species to examine. They then used these measurements to calculate how big the eye may have been in each species. As well as feeding into this study, each student’s collected data was also used for a personal research assignment as part of their undergraduate course.
The giant hadrosaur Shantungosaurus giganteus was found to have the largest eye of any dinosaur, while the tiny pterosaur Nemicolopterus crypticus had the smallest eye of any animal in this study. Image by Harry Bridger.
For the majority of groups, including crocodiles, the orbit was found to make up roughly 20% of the total skull length. The exception to this trend is the dinosaurs, most of which had either a larger or smaller orbit than predicted. This likely stems from how diverse dinosaurs were - as they filled every available ecological niche, so they obtained an incredible array of shapes and sizes. There is a preference among smaller dinosaurs for larger orbits, which would have granted them better visual acuity while remaining small. Likewise, relatively large orbits were simply not needed in giant dinosaurs because their eyes were already so large. The biggest eye of all was found to belong to the giant hadrosaur Shantungosaurus giganteus, measuring between 125 and 141 millimetres in diameter. Though it also possessed the longest skull and widest orbit diameter overall, its orbit was relatively small given the animal’s size.
These results show that the largest eyes, and therefore the greatest visual acuity, belonged to the largest dinosaurs despite their relatively small orbits. In fact, their large eyes likely would have given them greater visual capabilities than comparably-sized mammals like elephants or rhinoceros, improving the hunting capabilities of giant predators like Tyrannosaurus rex, and allowing herbivorous dinosaurs to better discern between different sources of food. Additionally, the large hadrosaurs such as Shantungosaurus and horned dinosaurs like Triceratops possessed large eyes in conjunction with an enlarged olfactory bulb, the part of the brain responsible for smell, greatly enhancing their overall sensory capabilities. This may have enabled them to better locate potential sources of food than other herbivorous dinosaurs, giving them an evolutionary leg-up and allowing them to dominate ecosystems in the Late Cretaceous.
The relative orbit sizes of archosauromorphs projected onto a phylogenetic tree. Most groups tend not to deviate too much from the average relative orbit size of around 0.20. Dinosaurs often deviate from this trend, for instance most Maniraptoriformes have a relatively large orbit, and many members of Ornithopoda and Marginocephalia have a relatively small orbit. Image from Lautenschlager et al. 2023.
By applying principles learned from modern animals to the fossil record, this study has given a rare glimpse into how the senses of dinosaurs and their relatives may have guided their evolution. Even so, questions remain as to how the visual capabilities of dinosaurs would have affected their behaviour, and how big of a component sight may have been in their overall sensory toolkit.
Associate Professor in Palaeobiology
Staff profile for Dr Stephan Lautenschlager, Lecturer in Palaeobiology in the School of Geography, Earth and Environmental Sciences at the University of Birmingham.
