Ron Goudswaard, of Wellington, asks :-

How did dinosaurs get so big?

Richard Holdaway, a palaeobiologist with Palaecol Research Ltd, responded.

Starting as they did with small (c. 1 m long) body sizes, diversification of dinosaurs into new niches could go mostly in one direction, towards larger bodies. Where larger bodies can be achieved structurally, through evolutionary history, they have been. If a group (i.e. dinosaurs) is successful, with an increasing number of species, and even if there is no tendency for increasing size – with as many species smaller than their ancestors as larger- the mean size among species must increase even though most new species are still small. That is just because there is more “space� to be bigger than to be smaller: there is definitely a lower limit to body size as well as an upper.

For herbivores, such as the sauropods, stegosaurs, hadrosaurs, and ceratopsians, large body size was an advantage because, first, metabolic rates are lower per gram of tissue in larger animals than in smaller, and second, large stomachs and long intestines can process and ferment more vegetation, especially tough vegetation. Up to the limits imposed by the strength of bones and connective tissue, then, larger herbivores are more efficient. Large size can also assist in maintaining a higher body temperature, as body surface area increases more slower (by the square of length) than does volume (length cubed), which would allow individuals to be more active. Long legs kept the big body off the ground and long necks allowed higher foraging but also were necessary for the animal to drink. As more large herbivores became available, the average size of predators increased too, exploiting the new range of large prey, and of large supplies of carrion. Diet was again a driver, but because of availability of unexploited prey and not to foster digestion.

Although we tend to associate large size with dinosaurs, some lineages of mammals achieved similar sizes after the large dinosaurs went extinct (small dinosaurs – the birds – are still with us). For example, some early rhinoceroses weighed 20 tonnes. We live at present at a time of dwarfs, with on land only the severely threatened elephants and rhinos to remind us that until 10,000 years ago, giant mammals, which achieved their size for the same reasons as the dinosaurs, were diverse and abundant on all continents. And whales, whose bodies are supported by water and which therefore do not have the same problems of support as land vertebrates, can reach lengths of 30 metres and body masses of more than 100 tonnes.

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