‘When you know they have babies and are very much like modern animals in so many ways. Even within the eggs – we could see that the babies had been moving their jaws so they could grind down their teeth – it says something about the biology of the animals, which brings it home big time.’
The study of the eggs has revealed how some dinosaurs nested and protected their eggs. Oviraptorids, which were small, roughly human-sized dinosaurs, were originally known as ‘egg snatchers’. In the 1920s, the plant-eating Protoceratops was the most common kind of dinosaur at the site where the eggs were found, so the conclusion was that the nest of eggs found with the oviraptorid skeletons belonged to Protoceratops. But as palaeontologists made more oviraptorid egg finds, they realized that the dinosaurs were, in fact, protecting their own eggs, not stealing those of other dinosaurs.
‘We now know that in the case of oviraptorids that they stayed in one spot when laying eggs. First, they laid a pair of eggs, then they turned a little and laid another pair next to the first pair. And then they turned to lay another pair, and so on. So basically they stood on one spot and laid a circle of eggs around their feet,’ Currie explains.
‘Egg finds offer compelling evidence that dinosaurs were warm-blooded.’
He stands in the middle of his office, expands his arms like wings, and excitedly acts out the scene. ‘At the same time they are doing that, they took their hands and scooped sand onto the eggs. Once the first layer was down, they lay a second layer and then sometimes a third layer, all of which were buried in sand. This process created a trench around the nest,’ he continues, mimicking how the ancient animal might have done it. ‘In some cases, we know precisely how they were laying their eggs. In the case of oviraptorids, it was in 1990 when we found our first nest with a mother on top; since then at least four more nests have been found with the mothers sitting on top.’
Currie believes that in addition to egg finds showing how the mother protected her eggs, they offer compelling evidence that dinosaurs were warm-blooded. ‘The study of eggs can even tell you something about physiology. If the mother is lying on the nest – birds lie on their nests to keep the eggs at a constant temperature – it may well tell us that these dinosaurs were warm-blooded and in fact were brooding their eggs and keeping their eggs warm.’
The evidence that supports this theory comes from the layout of the eggs in the nest. The eggs formed a circle around the outside of the nest like a doughnut. The mother was standing in the middle, which allowed her chest to cover the eggs in the front and her tail to cover those in the rear. With her arms outstretched around the eggs to the sides, she could also protect those eggs.
‘If you look at the feathered dinosaurs where the feathers are behind the arms, the feathers would cover those eggs on top,’ Currie says. ‘Large dinosaurs couldn’t do that – they didn’t have large enough wingspans. In the case of these dinosaurs that have been found brooding, it has been suggested that there were several reasons that these long feathers developed, including as a mechanism for shading the nest, for protecting the eggs and/or for keeping them warm. Brooding doesn’t work if you are cold-blooded.’
What have not been found are Tarbosaurus eggs – yet. ‘It’s probably because of the fact that they would have been in an environment with acidic groundwater so the eggs were destroyed,’ Currie says. ‘Of course, there are other possible explanations, one being that they would have nested farther inland in a place where things weren’t getting fossilized. Still, we may eventually find a Tarbosaurus egg in the right place, where it did have a chance to fossilize. If we are lucky, there will be an embryo inside and we’ll be able to put the whole story together.’
Because of their nesting habits and feathers, it has been established that oviraptorids and dromaesaurids like Velociraptor were probably warm-blooded. That has led Currie and several other palaeontologists to argue that it therefore makes sense that the closely related Tyrannosaurus would have been warm-blooded, though this is still considered somewhat forward thinking.
Currie lays out the argument. ‘Velociraptor and oviraptorids were warm-blooded in all likelihood,’ he says. ‘They are feathered dinosaurs; it doesn’t make any sense to have feathers on your body as insulation unless you are warm-blooded. The advantage [cold-blooded] lizards have is that when they get cold, they just move into the sun and they warm up pretty quick. But if you put feathers on them, it would be like taking an ice cube and wrapping a blanket around it and sticking it outside; it doesn’t work. So there are a lot of reasons to think that the little guys are warm-blooded; notably these include the fact that they are so close to the ancestry of birds, and that they have bone histology like modern mammals and birds.’
Tyrannosaurus, like Tarbosaurus and Albertosaurus, was relatively closely related to Velociraptor and oviraptorids, so Currie concludes that because of the fact that they are closely related to these warm-blooded creatures, they were almost certainly warm-blooded, too. ‘It doesn’t make sense that you have guys in your ancestry who are warm blooded, that all your close relatives are warm-blooded and that your descendants are warm-blooded, but you are not,’ he says. ‘So it makes sense that tyrannosaurs were warm-blooded.’
As radical as it was to accept that dinosaurs were warm-blooded and laid eggs, it was even more revolutionary to see them as big, non-flying birds rather than just scary, oversized lizards. In 1974, John Ostrom, the palaeontologist who found Deinonychus, revised his description after the discovery of a more complete specimen and championed the idea that birds were descended from dinosaurs. Robert Bakker backed it up with additional research, and their work provided the dinosaur renaissance with another major development.
Ostrom’s scientific paper reiterated the idea that had been presented a hundred years earlier by British scientist Thomas Henry Huxley, a defender of Darwin’s theory of evolution, who had proposed that birds were descended from dinosaurs. Huxley came to this conclusion after studying Archaeopteryx, the oldest known fossilized bird. Archaeopteryx lived during the Late Jurassic period, and its features suggested that it was a transitional fossil between dinosaurs and birds. Since Archaeopteryx was discovered in 1861, only 10 specimens have been found, despite Herculean efforts to find more.
Additional evidence for this theory came in 1986 when Jacques Gauthier, a scientist who is now based at Yale University, published a list of more than 125 characteristics shared uniquely by birds and dinosaurs. Currie calls this very, very powerful evidence under any kind of modern scientific analysis. ‘We don’t even have such strong evidence for other transitions such as reptiles into mammals,’ he points out. ‘A lot of palaeontologists by the mid-1980s already believed that birds came from dinosaurs. I got into researching it because of the fact that our Late Cretaceous dinosaurs from Alberta were very bird-like in a lot of ways, and it started me thinking about it and publishing on the subject.’
The theory was slow to make its way into the mainstream. Currie believes this is because most ornithologists don’t work on fossils, and only work on modern birds. ‘Maybe 50 per cent of the scientists working in palaeontology believed birds came from dinosaurs, maybe 5 per cent of ornithologists believed it, and very few people in the public believed it because they had never heard of it.’
In 1996, Currie himself came face-to-face with the evidence from one of the very first specimens showing the transition in what turned out to be a complicated situation where cultures, science and publishing collided.
The story began in 1994 when a farmer in north-eastern China found a fossil of what was thought to be a species related to Archaeopteryx and sold it to a local museum. The farmer made some money on the transaction, so he went out and dug some more and found another fossil, which he sold to a different Chinese museum. Scientists were aware of those finds and figured there were more where those had come from. In 1996, at the Tucson Rock and Mineral Show, there were hundreds of specimens resembling this bird, complete with feathers. Excitement began to grow because this was a bird
