Under the Microscope: Dinosaurs were named by Sir Richard Owen in 1842. The name derives from the Greek words deinos meaning "terrible" and saura meaning "lizard", writes Prof William Reville
It was long assumed that dinosaurs had a reptilian physiology, that is a "cold blooded" slow body chemistry (metabolism) and that, since modern lizards are relatively slow growing, the dinosaurs grew slowly to their great adult size. It was assumed that large dinosaurs must have reached very old ages indeed. However, results of recent research is changing this familiar picture. This work is described by John Horner and others in Scientific American, July 2005.
Dinosaurs were very varied - some were herbivores, some carnivores, some were bipeds, some quadrupeds and nearly all were well adapted to terrestrial life. As a group, dinosaurs were large. The largest was an order of magnitude bigger than any animal that since trod the Earth. The biggest dinosaur known from a complete skeleton is a Brachiosaurus specimen found in Tanzania. It was 12m tall and weighed up to 60,000kg. The more modestly sized Tyrannosaurus rex was as big as the largest modern African elephant.
Today, only the blue whale, which grows up to 190,000kg in weight and 33.5m in length, can rival the largest dinosaurs in size. Most dinosaurs were much smaller than the giants - the smallest were about the size of a crow. A recent survey of 63 types of dinosaur indicated an average weight of about 900kg - about the weight of a bear.
Until relatively recently, scientists didn't know how to measure the age of dinosaurs or to calculate how fast they grew to adult size. This has now become possible by studying dinosaur bones. Cross-sections of these bones show concentric rings somewhat like the familiar rings seen in trees. As we know, you can tell the age of a tree by cutting through its trunk and counting the number of rings from the centre to the bark. The spacing between two successive rings represents one year's growth. The lines in dinosaur bones also occur annually and, although more difficult to accurately interpret than tree rings, they can be counted to calculate the age of the animal and also to calculate now long it took to reach adulthood.
Using this method, Horner and colleagues calculated that Tyrannosaurus rex grew to full size in 15-18 years. This is very rapid growth for a reptile and dinosaurs grew much faster than any other living or extinct reptile. Using the same technique other workers have calculated the rate of growth of the giant crocodile Deinosuchus which lived 75 to 80 million years ago and grew to 11m in length. This crocodile, an unambiguous reptile, took 50 years, three times longer than Tyrannosaurus rex, to grow to full size. A modern African elephant, closer in size to Tyrannosaurus rex, takes 25-35 years to grow to full size. Work on many other dinosaurs has also shown that they all grew to maturity relatively quickly.
The rapid growth rate of dinosaurs implies a high rate of metabolism. Dinosaurs did not grow like reptiles, but rather like birds and mammals and their basic metabolic rates were probably more like the metabolic rates of today's birds and mammals than those of today's reptiles. Dinosaurs were therefore much more likely to have been warm-blooded than cold-blooded. Also, a detailed examination of the structure of dinosaur bones shows that they look much the same as the bones of large mammals and birds and not like the typical bone of a reptile.
Modern birds are probably descended from dinosaurs. Many palaeontologists, but not all, agree that Archaeopterx was the ancestor of all living birds. It was found in limestone quarries in Germany in 1860. It is a 145 million-year-old crow-sized skeleton with wings covered in feathers, its only bird-like feature. The skeleton is non bird-like, with a long bony tail, teeth, and claws on its wings. Archaeopterx was almost certainly able to fly but the weak trunk-skeleton indicates that it was not a strong flier. In recent years many fossils of feathered dinosaurs have been found in China. Some of these clearly could not fly, but would have been good at climbing trees. This suggests that flight gradually evolved as tree-climbing dinosaurs slowly became more efficient leapers.
The awesome size and, probably, the ferocious nature of some dinosaurs exerts a strong grip on public imagination, particularly on the imagination of children. This is reflected in exhibitions and parks around the world devoted to dinosaurs, notable books of fiction (eg The Lost World by Arthur Conan Doyle) and popular films featuring dinosaurs (eg King Kong, One Million Years BC and Jurassic Park). Cartoons regularly portray dinosaurs - the best known is probably The Flintstones featuring a stone-age family living in a dinosaur world. This cartoon is so popular it may mislead people into thinking that dinosaurs and humans once co-existed. In reality humans didn't appear on Earth until many millions of years after dinosaurs became extinct.
The film Jurassic Park raises the fascinating possibility of recreating dinosaurs from dinosaur DNA recovered from a source such as mosquito blood fossilised in amber. It is not all clear that useful DNA could be recovered from such a source and, even if it could, current technology probably could not support the "growing" of a dinosaur. For example there is no current species related closely enough to dinosaurs to allow a dinosaur embryo to develop in its womb.
• William Reville is associate professor of biochemistry and public awareness of science officer at UCC - http://understandingscience.ucc.ie