Dr Clare Sansom, Senior Associate Lecturer in Birkbeck’s Department of Biological Sciences reports on the 2019 Rosalind Franklin Memorial Lecture, delivered by Professor Emily Rayfield on how computational tools are reshaping our understanding of form and function in fossil animals.
Since 2016, the School of Science at Birkbeck has held an annual lecture named for one of its most distinguished alumni, Rosalind Franklin. This lecture, which is always given by a notable woman scientist, forms part of the school’s Athena SWAN programme. Each Rosalind Franklin lecturer’s research field is closely aligned to one of the three departments that make up the School, Biological Sciences, Earth and Planetary Sciences, and Psychological Sciences. This was the year of the earth sciences, and the lecturer, Emily Rayfield, was a professor of paleobiology at the University of Bristol. She gave an engaging talk about how computer modelling is helping us understand the biology and behaviour of fossil animals, beginning with the dinosaurs.
Emily drew a contrast between the techniques used to study living animals and prehistoric ones. With living animals, there is a chance, at least, that we will be able to observe their behaviour, but with prehistoric ones all we have to go on is the fossils they leave behind. So how can we approach a question such as, what – and how – did dinosaurs eat? We can begin to understand this problem by relating the skulls, and particularly the jawbones, of living animals to their diets. Plotting the measured bite force of reptiles’ jaws, including those of the closest living relatives of dinosaurs, the crocodiles and alligators, against the size of those jaws, and then scaling up to the size of dinosaur jaw bones has suggested that the largest would have had a bite force of over 10 tonnes.
It is possible to get some idea of what fossil animals ate by, literally, looking at their dung. Fossilised faeces, or coprolites, are frequently found, and geologists can estimate the size of the creatures that produced them, as well as finding clues to their diets. The largest that have been seen are likely to have come from Tyrannosaurus rex. This monster, one of the largest land-dwelling carnivores that ever lived, measured over 40’ from nose to tail and stood about 12’ tall at the hips. Skeletal fragments found in dinosaur coprolites include those from some of the first birds. Fossilised bite marks and teeth, which differ in shape and size between herbivorous and carnivorous animals, can also fill gaps in the picture of dinosaur diets.
Feeding is only one aspect of animal behaviour, although an important one. Emily opened out her question to ask what the shape of an extinct animal’s bones can tell scientists about its behaviour more generally. Bones all respond to externally applied forces of stress (load per unit area) and strain (stretch per unit length). Wolff’s Law, which dates from 1892, states that any change in the function of bones, and therefore in the stresses and strains that they are exposed to, is directly followed by changes in their shape. In human terms, if an individual overloads his or her bones by, for example, taking up a strenuous sport, the bones will gain mass, whereas disuse will cause bone loss in astronauts exposed to low gravity as well as the chronically sick. Mechanical loads experienced in utero affect the shape of the developing embryo and, going back to the example of feeding, animals that experience different diets develop different-shaped jaws. This can be observed in individuals of the same species, with mice raised on only soft food developing less efficient jaws than those raised on hard pellets. It is also reflected in species differences in both living and extinct animals: animals and their environments may have changed dramatically since ‘deep time’, but the laws of physics – and the basic structure of the cells and tissues they operate on – have not.
It is, of course, impossible to measure the stresses and strains that a dinosaur bone will have been subjected to, but it is not impossible to deduce them. This is where computers come in, via a mathematical technique known as finite element analysis. In this, a complex structure is broken down into a number of simple shapes. A force is applied to each element and a computer program is used to estimate how it moves and changes shape.
To apply this technique to a fossil, you need to start with a digital model of that fossil, and this can now be done quite easily using a CAT scanner similar to those used in medicine. The model is then completed by adding any bones missing from that specimen. The model is combined with information from living relatives to estimate the stresses and strains on the bones.
Professor Emily Rayfield
Armed with all this data on the forms of, and loads experienced by, dinosaur skulls, it is possible to ask complex questions about their mechanics and evolution. It is now quite well known that modern birds evolved from a group of dinosaurs, and this begs the question of how they evolved their characteristic, but extremely diverse, beaks. Some herbivorous dinosaurs in the group known as the theropods (three-toed) had beaks and comparing models of similar sized dinosaur skulls with and without beaks has suggested that a beak reduced stress and strain during feeding. Large theropods were found to have experienced proportionally lower stress during feeding than smaller ones, with the exception of Spinosaurus, which had much higher stress than expected for its size.
At the end of the lecture, Emily moved on from the largest land-based fossils to look at some of the smallest: a group of primitive shrew-like mammals known as the ‘Jurassic fissure mammals’ that lived in crevices between rocks some 200 million years ago. Working with Pamela Gill, an expert on the anatomy of these creatures, Emily examined the fossilised jaws and teeth of two species and predicted differences in the speed and strength of their bites. Comparing patterns of wear on the teeth of these mammals with modern bats suggested a similar range of insect diets. This implies that, even at the very beginning of the mammalian radiation, species that occupied similar niches were beginning to diversify their diets; and it provides another example of how studies of the mechanics of fossil bones can lead to insights into the lives of animals from hundreds of millions of years ago.
