Read more about dinosaurs in our special issue
TIMOTHY ROWE walks into his windowless, basement X-ray lab and plonks a fist-sized hunk of bronze on the table. He looks at me, smiling, waiting for me to tell him what it is. The amorphous blob looks like some sort of mangled body part, with vestiges of chambers, squashed and asymmetrical. But what? The ensuing silence is filled with a menacing buzz from Rowe鈥檚 state-of-the-art CT scanner. After what seems like an age, he finally cracks and gives me a clue. 鈥淭he question,鈥 he says, 鈥渋s did it sing like a bird or roar like a dinosaur?鈥 Bingo! This misshapen lump must be the metal cast of an archaeopteryx head. And that persistently buzzing scanner holds the key to its secrets.
Rowe and his colleagues at the University of Texas at Austin have pioneered the use of high-powered computed tomography scans to image the brains of fossilised animals. Between their two machines they have up to five times the power of an average medical scanner, and the resolution to pick out details on the 5-micrometre scale. This technology has allowed them to probe the senses of some infamous dinosaurs, to get new insights into intimate details of their lives such as their hearing, sense of smell and balance. Last year, they used their scanner to address the delicate question of whether archaeopteryx, the original missing link between modern birds and their extinct ancestors, had dinosaur minds or bird brains.
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Rowe is just one of a growing group of biologically minded palaeontologists who are taking a new tack on studying dinosaur behaviour. Recreating the daily lives of these bewildering beasts has always been a bit of a guessing game when all that that you have to go on are fossilised bones, teeth and excrement. But in the last few years things have changed. Much of the new knowledge is coming from the spectacular fossil beds of Yixian in the Liaoning province of China where, by some miracle, dinosaurs have been caught napping, sitting on their nest, or frozen in time with a stomach full of dinner. The vast increase in fossilised material has resulted in some impressive advances in our understanding of dinosaur lifestyles, how they moved, ate, grew, migrated and even nurtured their offspring.
For Rowe, though, the major breakthrough has been technological: CT scanners powerful enough to sear through fossilised bone, and enough cheap computer power to analyse massive digital images. It was this promise that convinced palaeontologist Angela Milner to carry her precious archaeopteryx head all the way from its home in the Natural History Museum in London to his lab. Specimen BMNH 37001 is the only one of seven archaeopteryx fossils whose skull had not been completely flattened. 鈥淓verybody knew that archaeopteryx was a sort of halfway house,鈥 says Milner. 鈥淢ost of its skeleton suggested it was a small meat-eating dinosaur. But it also has modern feathers and a modern wing configuration. We had no idea how far its brain and senses had moved towards the birds.鈥
During 6 hours of cooking in the scanner, X-rays cut through the head in slices 23 micrometres thick, distinguishing between fossilised bone and rocky infilling. Then lead researcher Patricio Dominguez Alonso digitally picked away the brain case, leaving behind an endocast 鈥 a 3D representation of the inside of the head. In higher vertebrates, including archaeopteryx, the brain fills the skull almost completely and presses upon the interior bones, so an endocast is a good proxy for brain size and shape. What these scans revealed was that, beneath the skull, archaeopteryx had much in common with a modern bird. 鈥淲e thought that we knew as much as we could know about archaeopteryx until CT came along,鈥 says Milner. 鈥淚t definitely had a flight-ready brain.鈥
One revelation was the size and shape of the delicate semicircular canals in the inner ear, which are crucial for balance. They were highly arced like those of modern birds, a trait associated with an acrobatic or aerobatic lifestyle. As for the brain itself, archaeopteryx had massive bird-like visual centres jutting out from either side of the brain, and the apparatus for a superb sense of hearing. But perhaps the most notable feature was a hefty cerebellum 鈥 the brain鈥檚 鈥渁utopilot鈥, where sensory information is coordinated and integrated. Its relative size was far larger than even the birdiest dinosaurs, which include velociraptor. That is the real neural advance, says Rowe. 鈥淚ntegration is ultimately what it is all about 鈥 how you put senses together and make decisions. I would guess by looking at the brain that this animal had complex behaviours 鈥 one of those would have been flight, but they also might have included elaborate care of the young and broader territoriality.鈥
鈥淚 never thought we would find a fossil like this. When I first saw it I thought it was a fake鈥
According to Larry Witmer from Ohio University in Athens these findings make archaeopteryx an interesting contrast to the world鈥檚 first vertebrate flyers, the pterosaurs. In 2003 Witmer stuck a couple of their skulls under Rowe鈥檚 X-ray scanner to see how their brains had evolved for soaring. The endocasts showed that, like birds, pterosaurs devoted a considerable portion of their brain power to coordination and vision. But instead of ballooning out their semicircular canals, pterosaurs opted for a grossly inflated flocculus, which sits at the back of each side of the brain. In modern animals, this brain region receives and processes information about movement from the eye, neck and body, helping high-speed hunters such as hawks maintain a rock-steady gaze on their fleeing prey. The pterosaur鈥檚 ability to lock onto targets would have been vital if it preyed on fish, swooping from the air like a modern gannet.
Mighty bite
Witmer has his own scanner these days, and palaeontologists now send their fossils to him. One specimen in the queue is the near metre-long head of a Baryonyx walkeri, a crocodile-like spinosaur uncovered in Surrey, UK. The completed CT scans will be delivered to Emily Rayfield at the Natural History Museum in London, where she will break down the digital images, subject them to computerised stress and strain, and figure out just what kind of bite the spinosaur packed. The original baryonyx fossil was found with remnants of fish scales in its stomach, along with the broken bones of a juvenile iguanadon. But what else did this gargantuan predator eat? Rayfield鈥檚 analysis may help determine whether it could have mowed down a pterosaur on the wing 鈥 as hinted by one pterosaur fossil with a baryonyx tooth embedded in its spine.
Rayfield was the first person to explore animal mechanics using a method called finite element analysis (FEA), more usually used by industrial engineers to test the strength of buildings. Her first subject, back in 2001, was the 12-metre-long carnivorous demon Allosaurus fragilis. She found that its maximum bite force was a modest 3500 newtons compared to the 13,000-newton chomp of a Tyrannosaurus rex. And the FEA spat out another interesting titbit 鈥 the animal鈥檚 skull seemed to be massively 鈥渙ver engineered鈥. It could withstand up to 23 times the force of the bite before breaking, suggesting that allosaurus employed a hatchet-like slash and tear method of feeding, using its razor-sharp teeth and violent jerks of the head to slice apart prey.
Just last summer Rayfield unleashed FEA on T. rex. Its feeding habits have long been debated. Jack Horner from the Montana State University in Bozeman is among those who have argued that the lumbering, tiny-armed monster was a mere scavenger, not an active predator. Rayfield found that the sturdy skull 鈥 which can reach over 1.5 metres in length and is constructed with large openings and loosely connected facial bones 鈥 is designed to withstand frenzied feeding forces. 鈥淭he skull does seem to have some shock absorbing ability, which hints at being predatory,鈥 she says.
Rayfield鈥檚 model supported the 鈥減uncture-pull鈥 method of feeding 鈥 akin to the ploy used by great white sharks. In 1996, Greg Erickson from Florida State University in Tallahassee was the first to suggest that T. rex favoured this technique. Erickson also uses FEA to get an insight into form and function, but he still believes the best way to understand dinosaur behaviour is to look at the evidence on the ground. 鈥淭he bridge to behaviour has to be done with trace evidence 鈥 like bite marks and coprolites,鈥 he says. His ideas about T. rex鈥榮 table manners were based on a giant hip of a triceratops, riddled with bite marks and overlapping tooth scrapes that perfectly matched the bone-crunching maw of a T. rex. And, in 1998, he was among a team who analysed a 鈥渒ing-sized coprolite鈥. This fossilised turd, a staggering 44 centimetres long and with a volume of .2.4 litres, could only have come from T. rex, according to the researchers. Inside were the battered and shredded bones of a young herbivore 鈥 the type of prey a predator would probably pick.
But Erickson isn鈥檛 just interested in T. rex鈥榮 eating habits. Last summer he sliced open drawerfuls of cast-off bone fragments, ribs and pelvises, and used the growth rings to measure their age. Many animal bones show annual growth lines, probably reflecting slower growth during the winter. He and his team were able to estimate weight from the diameter of the thigh bone. This led to the discovery that the king reached its massive 6000 kilogram adult size through an impressive teenage growth spurt. Between the ages of 14 and 18 it put on at least 2 kilograms a day. This speedy growth suggests that dinosaurs had a souped-up metabolism, far more advanced than your average reptile. 鈥淚t just shows that the Atkins diet doesn鈥檛 work,鈥 quips Erickson. The weighty beast may have been far from speedy at this stage, though. The biomechanical models of John Hutchinson, at the Royal Veterinary College in Hatfield, UK, indicate that after reaching about 1000 kilograms or 12 years old, it probably could not move at more than about 40 kilometres per hour.
Erickson is now focusing on the horned or ceratopsian dinosaurs, which include such celebrities as triceratops. He is hoping to figure out just what their massive horns were for. The assumption was they used them for defence against predators, a fair guess given the number of scars and bite marks on ceratopsian fossils. But Erickson and his colleagues are exploring other explanations. By putting together a growth series, all the way from embryo to adult, they鈥檒l be able to see at what age the horns appear, how they develop, and maybe even tease apart the different sexes. If full horn development coincided with maturity, say, it is unlikely they were primarily for defence. If only male ceratopsians had horns, they were probably used in male-on-male status contests or as ornaments to impress the females. If both males and females had horns, they might have helped the dinosaurs recognise their own species and avoid accidentally breeding with similar-looking neighbours in a crowded ecosystem. The fact that the horns are weak, yet few of the fossil horns are broken lends weight to the latter idea.
A similar picture is emerging in the pachycephalosaurs or dome-headed dinosaurs. Their thick layer of spongy bone, perfect for shock absorption, was often thought to be used for head-butting, bighorn-sheep style. Yet there is no record of fractured skulls. What鈥檚 more, when Mark Goodwin from the University of California, Berkeley, and Horner pieced together a growth series they found that the spongy texture actually disappeared as the animals matured. Rather than being a shock absorber, it looks like just an artefact of the fast-growing bone.
So what was the dome for? Species recognition, perhaps, like triceratops, says Goodwin. Although there is no evidence of a difference between the sexes, or even a sure-fire way of sexing these fossils, the microstructure of dome bone suggests they were covered in keratin. Keratin, the protein that makes up feathers, readily assumes pigment, allowing modern male birds to use lurid colours as a sexual advertisement. Goodwin admits there is no knowing what the final, flamboyant frill of the pachycephalosaur dome looked like. Soft tissues are rarely preserved, so the chances of finding conclusive evidence are slim, but he is hopeful. 鈥淚 hope that some day we鈥檒l see a pachycephalosaur skull with the indication of what was covering their heads,鈥 he says. 鈥淎nything is possible.鈥
Too good to be true?
Such optimism reflects the current mood in palaeobiology. In March researchers reported the amazing discovery of remnants of still-stretchy soft tissue and even blood cells in a T. rex leg. Then there is the seemingly endless flow of extraordinary finds coming out of China, which have left many experts thinking that anything really is possible. In the 1990s fully feathered dinosaurs started emerging from the Yixian fossil beds. Discoveries there confirm that ancient feathers can be recovered with their colours intact. The haul to date includes the metre-long caudipteryx, complete with its fan of striped tail feathers, similar to the showy plume of some modern birds.
The fossils of Yixian also reveal more direct indications of ancient behavioural patterns. Last year, Mark Norell from the American Museum of Natural History in New York and his colleagues found a new dinosaur, which they called Mei long. The chicken-sized dinosaur belongs to the group of theropods most closely related to birds, and was found curled up with its legs folded under and head tucked down, as if to keep warm while sleeping as modern birds do. 鈥淚 never thought we would find a fossil like that,鈥 says Norell. 鈥淲hen I first saw it I thought it was a fake.鈥 In 1995, Norell had found a related dinosaur, an oviraptor, sitting on its eggs, seemingly incubating them. These poses suggest that, like birds, these dinosaurs were warm-blooded.
Palaeontologists working in China have found several specimens in 鈥渓ife postures鈥. Last year they turned up what looks like a dinosaur cr猫che 鈥 34 juveniles and a lone adult of the small, beaked herbivore psittacosaurus. The youngsters looked far too big to be hatchlings, leading researchers to conclude that some dinosaur parents provided continuing care for their offspring. Such protection may have been vital 鈥 in January, a team in Liaoning reported finding a juvenile psittacosaurus in the belly of a small dog-sized fossil mammal.
The question of parental care has been taxing the experts for some time. They have unearthed beds with tryannosaurids of different ages, suggesting some sort of family group or hunting pack, and found nests of the 鈥渄uck-billed鈥 herbivore hadrosaurus filled with tiny and weak babies, indicating that parents actively cared for their vulnerable young. But no find to date is as conclusive as the cr猫che. And Norell hints that there is more to come from Yixian. 鈥淭here鈥檚 a lot of evidence that these guys were hanging out together for a long time after they hatched,鈥 he says.
鈥淚t looks as if these guys were hanging out together for a long time after hatching鈥
Making tracks
This would certainly bolster conclusions about dinosaur social life drawn from their footprints. Densely overlapping tracks show clearly that large herbivores, such as brontosaurus, travelled in herds. Often their prints are accompanied by those of large carnivorous theropods, possibly stalking and hunting their prey. Many of the most impressive trackways are along the dinosaur freeway, which follows the coastal plains of North America. The density of tracks along this north-south stretch suggests that dinosaur herds migrated seasonally, and close inspection reveals some surprising aspects of their behaviour. Martin Lockley from the University of Colorado, Denver, studied one area in New Mexico and found that large, adult ornithomidis 鈥 bird-like theropods 鈥 tended to travel south, while smaller juveniles went north. 鈥淲e used to think that dinosaur trackways were rare and of very little value,鈥 he says. 鈥淏ut they are incredibly common and just overlooked. Tracks are like the nearest things we have to movies of living dinosaurs.鈥
Of course we will never be able to see dinosaurs walk, stalk their prey, defend themselves against attack, court their mates or rear their offspring, but Norell for one is confident that soon many of the burning questions about their lives will be answered. 鈥淲ith the combination of new discoveries and technology, we are really starting to understand some basic aspects of dinosaur biology,鈥 he says. 鈥淧reviously there wasn鈥檛 a lot of hard evidence. Now it鈥檚 like a game, just to see if we鈥檙e clever enough to figure it all out.鈥
