Dinosaurs were, we probably all agree, rather magnificent animals. We can get a great sense of their power and form from fossil skeletons, even when only a few bones have been preserved. From the towering titanosaurs to brutally efficient velociraptor hunters – we all have ideas about dinosaurs that would fit easily into movies and glossy posters.
How close is the Hollywood image to the reality of dinosaur life in the Mesozoic?
But how close is the Hollywood image to the reality of dinosaur life in the Mesozoic? What do we know about how they really lived, what they looked like and what they actually got up to? As we'll see, researchers have learned a lot about all of these aspects of dinosaur biology, but not necessarily from studying the grandest and largest fossils so far discovered.
In fact, some of the most interesting insights have come from poo.
Karen Chin, from the University of Colorado at Boulder, has been studying fossilised dinosaur faeces – called coprolites – for 25 years. "Bona fide dinosaur faeces are pretty rare but we do find them in some places," she says. "If they're well preserved they can tell us some of the things that the dinosaur had been eating."
No surprises here, surely, you might think. Dinosaurs either ate leaves – or each other, right? Although Chin does cite one coprolite which shows exceptionally well preserved muscle tissue – meat – she has found evidence of unexpected meals, too. In some fossil faeces belonging to plant-eating dinosaurs, Chin has found evidence that they had been eating wood.
One of those had a stomach full of pine needles
There was so much of it that Chin concluded the dinosaurs hadn't simply ingested a little wood when stripping leaves from trees. In fact, as more of these wood-containing coprolites are found, preserved from different time periods, it's beginning to look like eating wood may have been a seasonal habit for these creatures, though Chin can't yet be sure.
The dinosaurs might have eaten this wood, which appears to have been covered in fungus, as a way of getting extra resources during an era when large plant-eaters didn't have much else to graze on – grasses didn't become common on Earth until the final stages of the dinosaur era.
More clues to dinosaur diets come from exceptionally rare fossils that preserve the actual gut content.
"I wouldn't like to give you an exact number but I would guess for herbivorous dinosaurs there are probably fewer than five examples known with the gut content," says Paul Barrett, at London's Natural History Museum.
"One of those had a stomach full of pine needles, the other one that we know about had a stomach full of fruits."
You suddenly had this snapshot of what it was like to hatch, be born and grow up as a dinosaur
The field in which people study things like dinosaur guts and faeces is known by some as "palaeobiology". It's a discipline which essentially asks how ancient plants and animals actually lived and functioned millions of years ago.
For decades there was little evidence we could use to understand dinosaur palaeobiology. But in the late 1970s, palaeontologist Jack Horner discovered fossilised eggs, embryos and baby skeletons. This revolutionised our understanding of what dinosaurs were like as juveniles. Horner's findings were made at a famous site in the US state of Montana – Egg Mountain.
"All at once you had thousands and thousands of eggs, embryos and then skeletons of babies, teenagers and adults," says Matt Carrano at the Smithsonian Museum of Natural History. "You suddenly had this kind of snapshot of what it was like to hatch, be born and grow up as a dinosaur."
This, and discoveries of other nesting sites, has taught us that many dinosaurs engaged in pre-natal care of their eggs and may well have stayed with their young for long periods of time after they hatched. Both behaviours made dinosaurs seem a lot like today's birds.
We have also got better at guessing the age of dinosaurs from their remains thanks to a fuller understanding of what they looked like when they were very young. In the past, Carrano notes, with some species it wasn't always clear whether a fossil showed a baby dinosaur or just a small adult one. Having a specimen in an egg, of course, proves that it only began life at that size.
Eggs, interestingly, also offer another clue to something that we – perhaps surprisingly – find very difficult to tell about dinosaur fossils: is it a male or female?
Feathered fossils emphasise the similarities between dinosaurs and birds
Barrett says working this out can be "almost impossible" but that, sometimes, the very nature of the fossil gives it away.
"That's because egg-laying females have a different kind of bone to non-egg-laying females and to males," explains Barrett. "They produce this kind of bone basically as a store of calcium when they're laying eggs."
It's called "medullary bone" and it is found in egg-laying birds today. In 2005, the discovery of fossilised medullary bone prompted scientists to revise the gender of a well-studied T. rex fossil from male to female.
One of the big revelations in dinosaur palaeobiology during the past 30 years has been the discovery of many feathered fossils – further emphasising the similarities between dinosaurs and birds. Few historical depictions of dinosaurs – if any – showed them with feathers, but it is now clear to scientists that several species did have some, even if they didn't use them for flight like birds do.
Over 150 years ago, we first learned that one species of dinosaur – Archaeopteryx – was feathered, but there was little evidence that other species were similarly adorned. All that changed when a deposit of feathered dinosaur fossils was discovered in China during the 1990s.
Their feathers could well have been used for communication
"It's turned out to just be unbelievably rich," says Carrano. "In terms of the physical, external appearance of dinosaurs that's helped us almost more than anything else to really move ahead … and in doing so it's opened that door to ask questions like, why did they look like this?"
Many feathered dinosaurs clearly wouldn't have been able to fly. But their feathers could well have been used for communication – perhaps with one dinosaur displaying visual signals to another during fights or courtship. The feathers might even have been useful during nesting.
"If you tuck your arms in by your side and you have feathers, the feathers complete the coverage of the nest, that's another possibility," explains Carrano. "It compels us to think about these things. Things that you would never have thought about before."
Besides feathers, we know a little bit about what dinosaur skin was like from some rare fossils that show it. Barrett says these suggest scaly skin – a bit like a crocodile's – covered horned dinosaurs like triceratops.
And what about dinosaur colour?
Very recently, researchers uncovered and analysed rare evidence of pigment in the fossils of marine reptiles that shared the world with dinosaurs. The researchers found evidence of melanin in an ichthyosaur – a dolphin-like species – suggesting it had a very dark skin colour. Another paper described similar dark pigments hinting at stripes on the feathers that covered the tail of Sinosauropteryx, a small meat-eating dinosaur. The researchers say these stripes "can reasonably be inferred to have exhibited chestnut to reddish-brown tones".
Sinosauropteryx, a small meat-eating dinosaur, had a reddish-brown striped tail
Barrett notes, though, that such studies are really at the cutting edge of palaeobiology.
"It's an interesting idea but we're still a long way off from knowing what the majority of dinosaurs would have looked like," he says. "So all the reconstructions you see are basically poetic license."
Another area of research that is very new, but which also promises to challenge previous assumptions, is the reconstruction of dinosaur vocalisations. We might think we know what a dinosaur roar was like, but until recently our ideas about this have been pure conjecture. In Jurassic Park for instance, the sound of the Tyrannosaurus rex was achieved by slowing down a baby elephant's roar and mixing it with the noises of other, growling, animals.
Although we still don't know for sure what T. rex sounded like, researchers have been able to take digital scans of fossilised skulls belonging to other dinosaurs in an effort to understand the noises that might have emanated from them.
"We can build models of those skulls in the computer and, if you like, blow virtual air through those skulls," explains Barrett. "We can reconstruct some of the soft tissue and actually come up with some ideas about what sort of sounds those noses and throats may have made."
The frills on Protoceratops skulls might have been used to attract a mate
Helpfully, dinosaurs also left many markings in the earth that they walked on which tells us about how they moved. Fossilised tracks of footprints can be matched to species with the right shape of foot and, as a result, scientists can come up with pretty good estimates for how quickly they walked or ran. This is all done by taking into account the distance between strides, the shape of the foot and the assumed weight of the animal based on what we know about its size from fossilised skeletons.
Recently, large scratches in the ground also suggested to some researchers, led by Martin Lockley from the University of Colorado at Denver, that dinosaurs engaged in a kind of "prehistoric foreplay" – in which they might have been displaying their nest building abilities to try and impress potential mates.
Some birds do this today – but they usually only do so near the sites where they actually nest. No evidence of nests was found near the fossilised dinosaur markings. As a result, Barrett for one says he is "quite sceptical" of the idea that this was why they were produced. Still, at least we know that dinosaurs engaged in this earth-scratching behaviour – even if we're not yet sure why.
We are getting better at depicting dinosaurs accurately
Another interesting finding relating to sexual interaction between dinosaurs was made last year by researchers at Queen Mary, University of London. A survey of the frills on the heads of a small dinosaur called Protoceratops showed that they grew disproportionately in adults. It could be an indication that the frills were used for attracting mates because they were less prominent in young, non-reproducing specimens.
The last 30 to 40 years of dinosaur research have been rich with discoveries that tell us far more about how these creatures lived than we thought we would ever know. Indeed, as Carrano explains, for a long time palaeontology tended to dismiss the idea that finding out about dinosaur life was even possible.
"You'd be gradually learning as you became a palaeontologist that you couldn't really figure any of that out," he comments. "It's a totally different world now."
And Barrett says that we are getting better at depicting dinosaurs accurately as a result. He jokes that he would prefer to see fewer volcanoes in images of dinosaurs, since volcanoes aren't as common as some artists would have you believe, but the materials for better representation are there. For example, studies of fossilised vegetation suggest that the environment would have been a lot drier than some people used to think was appropriate for certain species of dinosaur.
"They used to show these animals lounging around in swamps," he says. "We now know a lot of dinosaurs actually lived in very dry environments that could sometimes be quite sparsely vegetated."
Dinosaurs were truly remarkable creatures that lived many millions of years before humans ever walked the earth. Through careful study of preserved remains, computer reconstructions and informed inference, however, we are getting better at thinking about them as living, breathing species. What was it like to be one, and fight for survival in that ancient world? We're getting a better idea all the time
There are many secrets that dinosaurs took with them to the grave. But the more we know about the things they left behind – from fossilised poo to fossilised feathers – the better we get at knowing what they were really like, and how they really lived.