Weighing dinosaurs with Dr Roger Benson

Roger Benson and his colleagues have been ‘weighing’ dinosaurs all in the name of science. We find out more… Your recent research has seen you 'weighing' dinosaurs – that sounds incredibly difficult, how did you manage it? Weighing modern animals is relatively easy, all you need is a set of scales of the right size. One of my co-authors, Dr Nicolás Campione at Uppsala University, had spent a lot of time collecting measurements of the skeletons of living animals, for which live weights were known, from giant elephants to tiny lizards. He wanted to know which skeletal measurements provide the best indication of weight. He found that the circumferences of the shafts of some of the leg bones, the humerus and femur, were best. This makes a lot of sense. The legs are basically pillars whose job it is to hold the animal up, so the thickness of those pillars tells you about the animal's weight. From there, all we needed was to measure lots of dinosaur leg bones. More than 1000 dinosaur species are known, from all continents. So their fossil skeletons are kept in museums all over the world. Fortunately, between the members of the project team, we'd already collected a lot of appropriate measurements during other dinosaur research projects. There were just a few gaps to fill in, and some species were too fragmentary to weight. But in the end we had weights for 426 species. [caption id="attachment_38688" align="alignright" width="200"] Archaeopteryx, the first bird, was a small dinosaur weighing only 1 kg. Small size might have been key to the evolutionary success of birds. Image courtesy of Roger Benson[/caption] And what was the purpose of this dinosaur weigh-in? First off we wanted to know how much each dinosaur weighed, and to understand how the range of dinosaur weights changed through time. Body mass can tell you a lot about an organism. For example, bigger animals have more efficient metabolisms, so they can live on lower quality food in large quantities. In fact, almost all the basic properties of an animal change in a predictable way with mass. For this research, we were using evolutionary changes in body mass as a very simple way to capture evolutionary changes in ecology. It's true that animals of the same size could be very different in ecology. But the important point here is that if body mass changes significantly during evolution – for example, a 10-fold change from 100 kg to 1000 kg – then we can be pretty sure that ecology has changed. So we thought that by looking at rates of evolutionary size change in different dinosaur lineages we could tell something about rates of ecological change – the rates at which dinosaurs, and different groups of dinosaurs, diversified into new ecological niches throughout 170 million years of their evolution. Dinosaurs are a good group to investigate this in because scientists are confident now that birds are the living descendants of dinosaurs. So we were particularly interested to know if there was anything special about early birds, compared to the other dinosaurs that they lived alongside. So how did dinosaur size affect their evolution and survival? Well, we found that birds and their closest relatives (the maniraptoran dinosaurs – a group that includes Velociraptor) had fast rates of evolution compared to other dinosaurs. In these animals, even closely related species were sometimes very different in size. So it seems clear that they explored new ecological niches more rapidly. Indeed, birds and other maniraptorans diversified in to many different ecologies not seen in other dinosaurs. It was also striking that birds could be very small compared to other dinosaurs. So the smallest dinosaur in our study was a bird from 120 million years ago, in the Cretaceous, called Qiliania. The smallest non-bird dinosaurs weighed about 1 kilogram. But Qiliania weighed 15 grams, so it was the size of a sparrow, and 6 million times smaller than the biggest dinosaur, Argentinosaurus, which weighed 90 tonnes. That's an astonishing difference. Small animals see the world differently to large animals because they are small enough to exploit specialised ecologies in local microhabitats. So birds and other small dinosaurs in general might have been able to evolve rapidly because a much wider range of ecological niches was available to them. This means they achieved a much greater ecological diversity than big, slowly evolving dinosaurs. We think that having a diverse range of ecologies is important for surviving extinction. It increases the chances that a group includes one of the lucky few species with the right ecology to survive in the harsh extinction environment. A meteorite struck the Earth 66 million years ago and most dinosaurs went extinct. We don't know exactly which ecologies were likely to survive and why. But small mammals and small lizards survived the extinction well, so it’s not surprising that birds, which were by far and away the smallest dinosaurs, also survived. [caption id="attachment_38689" align="alignleft" width="200"] Tyrannosaurus rex weighed 7 tonnes and was the largest land predator of all time. Image courtesy of the Royal Ontario Museum.[/caption] Did the research throw up any surprising results? Yes. In addition to birds and their relatives, we found that dinosaurs of all types had fast rates of evolution early in their history, about 220 million years ago. This was a great for dinosaur evolution and new kinds of dinosaurs like giant quadrupedal sauropods, stegosaurs, and big carnivores all appeared then of shortly after. However, rates rapidly slowed down in most groups, and fundamentally 'new kinds' of dinosaur rarely appeared after that. So birds and other maniraptorans are the exception to this. They are the only group that managed to keep evolving new ecologies. And this might have played a big part in their continued evolutionary success. Could you tell us more about your role at the University of Oxford, and what's next for your research? I'm an Associate Professor of Palaeobiology. That means I teach students about fossils, and what they can tell us about evolution and Earth's history. But I have some spare time for research too. I'm really interested in finding out whether what we found for birds and dinosaurs also applies to other groups. Biologists are fascinated by the vast diversity of different types or organisms in the world around us, and the patterns of evolution that gave rise to it. I want to know if important groups like mammals and fish also result from a long evolutionary history of continuing ecological innovation, and whether the kind of evolutionary stagnation we see in extinct dinosaurs was common among groups that were once successful, but now extinct. What prompted your interest in palaeontology, and do you have any words of wisdom for budding scientists wanting to get into the field? I was interested in dinosaurs as a child. I think many people are. But I went to university to study physics. I took a geology course and quickly developed a fascination with Earth's history, especially fossils and evolution. Luckily I was on a flexible science course, so I choose options in biology and geology for future years. Science is exciting because it involves a lot of different ways of thinking about the world around us. Once you've studied for a while you start to realise there are lots of unanswered questions about evolution, ecosystems, and the universe. But they require a rigorous, often mathematical approach and serious training to get to grips with, which can be off-putting. You can't really know enough maths these days. But I'd say more people study fossils because they are passionate about ancient life and the thrill of discovery. You can make those discoveries in the field, or in the lab, and I think it's important to be enthusiastic about many different types of related fields – biology, geology, physics, maths.