ONCE the world was stalked by giants, from the 6-tonne flesh-eating Tyrannosaurus rex to Brachiosaurus, a lumbering vegetarian sauropod that weighed in at an impressive 88 tonnes.
But 65 million years after the dinosaurs disappeared, we still know relatively little about how they came to be so big. Now, however, an ever-growing collection of specimens, computer models and new visualising technologies are helping to unravel the mystery.
Gregory Erickson, a palaeontologist at Florida State University in Tallahassee has examined a large number of small, discarded T. rex bones sitting in museum drawers, and found they contain a treasure trove of well-preserved growth rings. These cast-offs have allowed Erickson and colleagues to chart the first growth curve for T. rex.
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It turns out that T. rex only became a giant in adolescence, undergoing an exponential growth spurt for about four years (Nature, vol 430, p 772). It achieved its gigantic size not by growing for longer, as modern mammals and lizards do, but by growing drastically faster. An adolescent would have gained around 2 kilograms a day between the ages of 14 and 18, before its growth slowed down into adulthood.
By using polarising light microscopes, the team was able to age the skeletons of 20 specimens, including the famous Sue, on display at the Field Museum in Chicago. Sue, it emerges, is not only the largest T. rex known, but also the oldest. And even she died relatively young at 28. 鈥淭. rex lived fast and died young,鈥� says Erickson. 鈥淭hey were like the James Dean of dinosaurs.鈥� The youngest specimen, previously labelled as a dwarf Tyrannosaurus, was only 2 years old.
鈥淲e really have a new quantitative tool that will open up new avenues of research,鈥� says colleague Peter Makovicky, 鈥渂ecause now we can start asking questions about the biology of extinct creatures like we can for living ones.鈥�
How, for example, were such dinosaurs able to move their gigantic bodies? Some of the answers were presented to the International Congress of Vertebrate Morphology, held in Florida this month. For instance, John Hutchinson at the Royal Veterinary College in Hatfield, UK, described how interactive software, which models a creature鈥檚 mass, centre of mass, and inertia based on skeletal evidence, is revealing more about how T. rex walked.
Hutchinson has previously shown that T. rex鈥檚 size would limit it to running at no more than 25 miles per hour. To run any faster, the dinosaur鈥檚 skeleton would have had to support a biologically impossible amount of muscle (快猫短视频, 2 March 2002, p 6). However, the weight limit on running fast only kicks in for animals over about 1 tonne, which corresponds to a 12-year-old juvenile T. rex, suggesting young, smaller T. rex were quite mobile.
His new models, which were validated using ostrich skeletons, which move in a similar way to T. rex, show that the dinosaur鈥檚 posture was rather unsteady, hampered by a strong tendency for the head to pitch forward. But while the team originally thought that inertia would hinder movement, it turns out to have been a strong stabilising force for large dinosaurs like T. rex.
Sauropods had their own way of coping with being so large. Matthew Bonnan, at Western Illinois University at Macomb, has confirmed that the bones of giants such as Brachiosaurus also had a unique way of growing that allowed the animals to support their huge weight. Most vertebrates, including large modern mammals like rhinos and elephants, remodel the shape of their bones as they grow in order to cope with increasing weight.
But a survey, by Bonnan, of the major leg bones of three species of sauropod taken from 14 collections in North America, has confirmed that older sauropods have exactly the same shape leg bones as younger animals, confirming the idea that their bones grow isometrically throughout their life. The sauropods鈥� only concession to growing larger, Bonnan says, was to flex their limbs less. 鈥淭hey move with a pendulum motion, just enough for their feet to clear the ground. They鈥檙e not very athletic animals.鈥�
Both this growth pattern, and the morphological similarities between sauropods and their two-legged ancestors, has led some palaeontologists to suggest the largest dinosaurs known had a body design predisposed to gigantism.