How did dinosaurs get so big?

Two key reasons are their bone structure and bad eating habits
Dinosaur Diplodocus carnegii
Diplodocus carnegii, which at up to 30 metres from head to tail is one of the largest known dinosaurs. Illustration by J. Smit in Hutchinson's Extinct Monsters, 1910

During the Late Triassic epoch, dinosaurs grew no bigger than today's largest mammals. However, during the Jurassic period, which began 200m years ago, they developed into giants.

One reason is that, like modern birds, many dinosaur bones were hollowed out by air sacs extending from their lungs, meaning that a dinosaur would have weighed significantly less than a solid-boned mammal of similar size. It follows that dinosaurs could support a much larger body with their four legs - up to 80 tonnes in the case of the largest plant-eating sauropods (in comparison, today's largest African elephants reach about six tonnes).

Feeding, too, would have been a serious challenge - how did these dinosaurs eat enough to support their size? According to recent research, one secret may have been that they did not need to chew their food as much as mammals today, but rather cropped branches, leaves and twigs, which they then swallowed whole, meaning they could take in a huge amount of food very quickly. And without the need for lots of bulky teeth for chewing, their heads were lighter so their necks could be longer - meaning they could reach a wider range of plants from one feeding spot.

Among the most famous long-necked dinosaurs is Diplodocus, one of the earliest giants to emerge in the Late Jurassic epoch and, at up to 30 metres from head to tail, one of the longest ever discovered. A cast of a Diplodocus skeleton has stood in the Natural History Museum's central hall for more than a century, forming one of the museum's most famous displays. This dinosaur possessed a hollow backbone, and its neck alone may have extended more than six metres - although, unlike in earlier depictions, it is now thought unlikely to have been able to bend its head upwards very far, so modern depictions show the head held horizontally out in front of the body.

The very largest sauropods evolved in the Cretaceous period, from 145m to 65m years ago. Their size offered two more benefits: a defence mechanism against large predators of the time, and also a means of retaining body heat (owing to their skin's small surface area relative to their enormous body volume - useful among colder blooded creatures; problematic for warm-blooded mammals because of the danger of overheating in warmer temperatures).

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