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dinosaurs had been around for some 160 million years and were at the peak of diversity. The different groups were highly specialized in many ways. They lived everywhere on the planet, all the way from the Arctic to the Antarctic at that time, in environments that were quite diverse. In North America, there were coastal regions and areas that were closer to the mountains that were rising at the time, and there were also dry areas and deserts in some places. Each one of those habitats was home to different groups of dinosaurs because each dinosaur species was adapted for specific climates and environments.

      Size is often the first thing people think of when dinosaurs are mentioned. Though the dinosaurs portrayed in popular culture are usually shown to be huge, the fact is that most dinosaurs were the size of humans or even smaller. Because the fossil record is incomplete, scientists sometimes use educated guesswork to estimate the absolute size of the biggest and smallest dinosaurs.

      The longest dinosaur is believed to have been Seismosaurus, which lived during the Late Jurassic period. Seismosaurus was a sauropod, or plant-eater, with a long neck and swooping tail that from head to tail measured between 35 and 40 metres (115–135 feet) long. The length is estimated because only part of the best Seismosaurus skeleton was recovered from its northern New Mexico site and the specimen is still being prepared. Despite Seismosaurus’ impressive length, it weighed less than 30 tonnes – heavy but nowhere near the heaviest sauropods (found in Argentina) that are believed to have weighed as much as 100 tonnes. (The largest carnivores were Tyrannosaurus rex and Giganotosaurus carolini, both of which lived in the Cretaceous period, the former in North America and the latter in South America. They each measured about 13.5 metres (45 feet) long and weighed about 6 tonnes.) By comparison, a blue whale can grow to 30 metres (100 feet) in length and weigh a shocking 180 tonnes.

      On the opposite end of the scale, a herbivore find that was aptly named Mussaurus, or ‘mouse lizard’, would have fit in the palm of an adult’s hand. Compsognathus, or ‘elegant’ dinosaur, one of the smallest adult carnivores, was the size of a large chicken and weighed approximately 3 kilos (7 pounds). Microraptor was an even smaller carnivore find in China from the Early Cretaceous.

      Dinosaurs fall into two basic categories: herbivores (plant-eaters) and carnivores (meat-eaters). The plant-eaters greatly outnumbered the meat-eaters. Some of the best-known herbivores were all quadrupeds, including sauropods, anklyosaurs, stegosaurs, hadrosaurs and horned dinosaurs like Centrosaurus. Sauropods had long necks, tiny heads and massive bodies. Ankylosaurs were very wide-bodied with massive plates of bone covering them like armour. Hadrosaurs are commonly called duckbilled dinosaurs because their mouth looked very much like a modern duck’s bill. Stegosaurus was distinguished by rows of bones along its back that developed into plates. Centrosaurus is one of the most common ceratopsians, or horned dinosaurs, which can be identified by their unique skull features not found elsewhere in the animal kingdom. On the tip of the upper jaw is a rostral bone, which forms what looks like a parrot’s beak.

      ‘Fossils – objects that have gone through permineralization.’

      The best-known bipedal carnivores are Allosaurus, Velociraptor, Albertosaurus and Tyrannosaurus. Allosaurus, or ‘different lizard’, was a large predator in the Jurassic period with extremely sharp teeth and it often measured over 8.5 metres (28 feet) long. Velociraptor and its closest relatives (the ‘Raptors’) were feathered dinosaurs, most of which only grew to the size of large dogs. Each had four claws on each foot, one of which was adapted as a can opener and used to disembowel prey. Albertosaurus is noteworthy for its two-fingered hands and massive head containing dozens of large, sharp teeth. Despite being a top predator in its area and weighing more than 2 tonnes, Albertosaurus didn’t come close to measuring up to the monstrous Tyrannosaurus.

      Dinosaurs were given their names by the scientists who described them in scientific papers. Because all languages are different, the names are then translated into Latin or ancient Greek, the common languages of scientists the world over, despite being ‘dead’ tongues. There is no set way to name a dinosaur. Some of the names focus on a characteristic of the dinosaur and others on how it might have lived. For example Tyrannosaurus rex means ‘tyrant lizard king’, while Tarbosaurus translates as ‘terrifying lizard’. Other names refer to the locations where they were discovered; Albertosaurus was first found in Alberta, Canada. Dinosaurs have also been named as tributes to people, such as Othnielia, which was named in honour of palaeontologist Othniel C. Marsh.

      What scientists know about dinosaurs has come from fossils – objects that have gone through permineralization, the process by which minerals are deposited in the pores of bones and turn to stone. Fossilization is a very fickle process. The sediments and groundwater must be right for preservation. There must be an accumulation of sediments and no rain and wind washing them away over the millions of years. In the mountains, erosion prevents long-term accumulation of fossils, because if bones were buried, it wouldn’t be for very long because the sediments would be washed out of the mountains into the low lands.

      Fossil is derived from the Latin word meaning ‘dug up’, and this is truly the case for dinosaur fossils. Most evidence of dinosaurs comes from original bones infilled with minerals, rather than from imprints of them frozen in time or bone that has been replaced by stone. In some cases, dinosaur bones were encased in ironstone nodules after they were buried, and this protected the bones from water-carrying minerals in solution so that to this day the fossils look just like modern animal bones. Others lines of evidence come from footprints, eggs and even skins that have been fossilized and preserved in stone. Scientists are able to date many dinosaur fossils from the rocks they are found in, and this has enabled them to establish an accurate timeline for dinosaurs despite the fact that they lived millions and millions of years ago.

      The dinosaur timeline was established largely through a process called radiometric dating of fossils. This involves comparing radioactive isotopes to the decayed material found in the same rocks or the surrounding layers of rocks. In North America, significant volcanic activity created ash beds that contain radioactive material. In this case, when scientists find a layer of ash, then a layer of fossils, then another layer of ash, they know that the fossils between those two ash beds are bracketed by the two dates, the lower bed providing the older date and the higher bed the younger. They can then compare those fossils to similar ones found in Europe and conclude the Europeans ones are the same age. The radiometric dating techniques used provide dates of plus or minus 10 per cent accuracy, so 90 million years would have left a 9-million-year margin of error. But more sophisticated testing now provides dates to plus or minus a couple of hundred thousand years. In human age, that’s not very close, but in terms of dinosaurs, it is.

      Dinosaur 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

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