footprints. Four trackways of dinosaurs moving to the upper left, and at least one other dinosaur coming from the upper right and one from the lower left.James Steinberg/Science Photo Library
Fossilized dinosaur footprints have been helpful to palaeontologists in determining what kinds of dinosaurs lived in certain areas. Though they can only rarely identify the species of the animal through footprints, palaeontologists can tell the general type of dinosaur that made the prints. Footprints are particularly revealing in situations where there are consecutive footprints that continue in one direction. These are called trackways, and they have enabled palaeontologists to draw both physical and behavioural conclusions about dinosaurs. Trackways reveal that most dinosaurs walked upright and did not drag their tails. They also show evidence of which dinosaurs were living together. And in certain situations, the stride lengths in the trackways can be measured and used to estimate speed. Without fossilization, this would not be possible.
There are long-simmering controversies among palaeontologists about the details of how dinosaurs were born, how they lived and how they died. Although there is no exact way to determine how long each species of dinosaur lived, scientists estimate that most species lasted between 2 and 5 million years. Their life spans varied by species and size. Some of the larger dinosaurs, such as Allosaurus, are believed to have lived for around 50 or 60 years, while smaller ones like Compsognathus may have lived for only 5 to 10 years. Velociraptor is estimated to have lived for about 20 years, and Tyrannosaurus and Tarbosaurus about 30.
One of the most hotly debated questions was whether or not dinosaurs were warm-blooded. The advances in that area changed dinosaur science and offered a prelude to more complex theories.
In the late 1960s, palaeontologist John H. Ostrom led the way in arguing that dinosaurs were warm-blooded. Ostrom was a professor at Yale University and in his later years served as Curator Emeritus of Vertebrate Paleontology at the Peabody Museum of Natural History. This was a radical idea at that time. Ostrom’s 1964 discovery and subsequent study of Deinonychus led him to conclude that the animal’s horizontal posture and sleek body, combined with the sickle-shaped claw on each foot, dubbed the ‘terrible claw’, offered convincing evidence that it was an active predator with a high metabolism. He could also see that it looked very much like Archaeopteryx, the first bird. He had also found multiple specimens of Deinonychus in the same quarry in Montana. This caused him to assert that small, meat-eating dinosaurs were behaviourally complex and may have lived in packs. Ostrom’s student, Robert Bakker, further argued these characteristics meant that dinosaurs were, in fact, warm-blooded.
Velociraptor.
Joe Tucciarone/Science Photo Library
These revolutionary theories changed the way dinosaurs were shown and started what Bakker later dubbed the ‘dinosaur renaissance’, a period of study that eventually would double our recorded knowledge of dinosaurs. ‘All of those ideas were coming out of that one find,’ Phil Currie says.
There are now many lines of argument that dinosaurs were warm-blooded. One is bone histology, or microscopic anatomy. ‘The structure of dinosaur bones is very much like the structure of mammal or bird bones, and it’s quite different than what we see in reptiles,’ Currie says. However, he cautions that, by itself, bone histology doesn’t prove warm-bloodedness, because it may just indicate very active growth. ‘Then you argue you don’t have active growth unless you can sustain it, and you don’t sustain it unless you have warm-bloodedness,’ he adds.
The predator–prey ratio is another line of evidence. Dinosaur finds indicate a 5 per cent predator-to-prey ratio. This indicates that the predators were very active and had to maintain a low ratio of predators to prey otherwise they would consume all the available food. The opposite is seen in cold-blooded animals, where the predator-to-prey ratio can be as high as 50 per cent. Cold-blooded animals like snakes can survive a month on one meal, meaning that their environment can sustain far more predators.
Dinosaur-egg finds, coupled with nesting habits, have added another layer of evidence to the argument that dinosaurs were warm-blooded, and they have also helped palaeontologists study parental behaviour.
The very first dinosaur eggs reported scientifically were found in southern France and England back in the 1800s. However, at the time they were believed to be bird eggs, and therefore they were not properly categorized, despite the fact that they were discovered with dinosaur bones. The first identified dinosaur eggs were discovered by explorer Roy Chapman Andrews in Mongolia in 1923. At that point, scientists re-examined the eggs found in France and England and determined they were dinosaur eggs as well.
The early finds of dinosaur eggs were confused with bird eggs because the knowledge base and number of finds were both limited. Now scientists can easily tell a bird egg from a dinosaur egg by viewing a slice of the egg under a microscope and studying its anatomy. ‘If you look at an eggshell closely, you can see that there are pores going through it and in fact there is a crystalline structure to it,’ Currie explains. ‘Every species of animal has a different crystalline structure to its eggshell. Birds have eggs that are very close to dinosaur eggs, but they are a little bit different in their layering. Basically, dinosaur eggs have an extra layer in the crystalline structure – dinosaur eggs have two layers and bird eggs have a third layer on the outside, which generally isn’t found in most dinosaur eggs.’
Preparing a nest of fossilized dinosaur eggs, originally discovered in Mongolia.
Ria Novosti/Science Photo Library
Scientists have looked to dinosaur eggs for clues not only about how they reproduced but how they lived. Noted palaeontologist Jack Horner, who has published numerous scientific papers on dinosaurs and served as technical advisor to the movie Jurassic Park, and his colleague Bob Makela found some of the first evidence that dinosaurs cared for their young. Horner and Makela studied nests of dinosaur eggs discovered in the 1970s in Montana and dated to the Late Cretaceous. The large quantity of nests and the spacing between the nests led them to conclude that these dinosaurs nurtured their babies by protecting the nests and bringing them food. Horner and Makela named the new species of dinosaur Maiasaura, which means ‘caring mother lizards’. In 1983 in the same area, which was nicknamed ‘Egg Mountain’, they found similar egg sites for the carnivorous dinosaur Troodon.
‘What we learned from Egg Mountain, since we had Maiasaura, which are duckbill dinosaurs, and Troodon, which are carnivorous, was that these dinosaurs were doing basically the same thing,’ Horner says. ‘While there was no evidence that Troodon kept babies in their nests, they certainly kept them on their nesting horizon. These nesting grounds suggest that both groups of dinosaurs were social. We haven’t really had evidence for any dinosaurs of any kind since that would suggest they weren’t social. Basically, every group of dinosaur has been found in accumulations together, suggesting that they lived in some kind of aggregation.’
Another egg breakthrough occurred in 1987 when Currie and his team found the first hadrosaur eggs containing embryos in southern Alberta. ‘Identifying dinosaur eggs has only [come about] in the last 30 years because before that there was no association between eggs, embryos and adults,’ Currie explains.
The examination of eggs has also heightened the public’s interest in dinosaurs by making them seem more accessible. Of course, part of the fascination with dinosaurs is that they are big so many people are surprised that dinosaur eggs are relatively small. Still, the realization that dinosaurs had babies is compelling.
‘For a long time, people had just thought
