Wednesday, 31 October 2012

Bone Tooth Skin Feather Frozen in Stone

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Ask any kid, they’ll tell you the coolest thing in the world is dinosaurs. The fossil record is continually throwing up evidence of new dinosaurs, each one even cooler than the last.

Dragons curl and glide through Chinese mythology: ideal, noble, fearsome creatures; the essence of Chinese culture. Where does this notion of giant flying reptiles come from? The ancient Chinese book of divination, the I Ching, hints at it. The I Ching is a divination tool, a collection of many old traditions, stories, omens, hexagrams. One of these omens reads “dragons encountered in the fields.” In places named for dragons — rivers, fords, mountain fossilized bones of ancient and extinct animals have been discovered. Now, in modern day China, astounding deposits of beautifully preserved fossils are upending and transforming our knowledge of dinosaurs.

In Europe, it was believed that fossilized bones of mammals, and possibly dinosaurs, gave rise to the myths of dragons and griffins. The skeleton of the Protoceratops is a good fit for a griffin — a mythical creature with an eagle’s head and lion’s body. Elsewhere, fossilized dinosaur footprints have been marked with symbols, and bits of bone have been drilled and used for jewellery. Our history with dinosaurs goes back for millennia. Human beings have given their bones names and made up stories and explanations for these giant creatures, long gone from the earth.

The earth is 4.8 billions years old, and the first life appeared around 3.8 billions years ago, evolving into more varied, sometimes more complex organisms. Animals and plants died, were buried by silt, sediment and sand and over time, their soft tissues and the organic molecules that made them were replaced by minerals, forming fossils. In the mid-to-late 1800s, early palaeontologists and explorers began unearthing fossils everywhere — in fields, buried in the sides of cliffs, in caves. Evidence began to mount that the Earth had an ancient, archean past — more ancient than the Bible, far older than was realized, older, stranger, unfamiliar.

Where did life come from? Did life evolve? Where did humans come from? Where did reptiles come from? And where did birds come from? Fossils began to provide evidence for the evolution and origin of the animals that live today and of vanished life forms that once swam through the seas and walked the continents. The bones of the Iguanodon and Megalosaurus were the first giant reptilian bones to be discovered. Iguanodon’s fused vertebrae clearly belong to a giant reptile. The anatomists and scientific artists of the day came up with models of what these creatures might look like and began to develop our first ideas about how the ancient world looked. As more and more fossils were found, scientists realized that there was a distinct class of giant reptiles with unique characteristics; a whole new species. They were given a name — dinosaurs, or “terrible lizards.” The man who gave them that name was Richard Owen, one of the great anatomists of the time, who became a great rival of Charles Darwin. Since those early times, the remains of dinosaurs have been found on every continent.

When the first dinosaur bones were discovered, we thought they were large, flightless birds. Thomas Huxley, not Michael Crichton of Jurassic Park fame, was the first person to suggest that the bones belonged to giant birds, and that birds evolved from reptiles. This idea was rejected early on; for example, dinosaurs had no wishbone (like birds) and there was no evidence for air sacs in the part of the dinosaur where they were expected to be found. As more fossils of dinosaurs were found, the popular image of dinosaurs crystallised: we had evidence for their skin, teeth, bones, and imprints of the brain. The cranium — the space where the brain of an animal resides — of dinosaurs was tiny, compared to the size of the body. Giant reptiles were stupid, scientists decided; this must explain why they didn’t survive to the present day. As reptiles, they must have been cold-blooded, and as fearsome as Tyrannoaurus rex and other predators were, obviously they must have been lone hunters. So our image of dinosaurs became fixed: Stegosaurus, T. rex, Brontosaurus, Triceratops, were always drawn on a primeval earth, volcanoes belching lava and smoke against the background of some early form of palm tree. Fossil exotica.

Our ideas about dinosaurs remained fixed for a century, until about the 1960s, as new ideas about how they might have lived were suggested, even revived, and evidence for more and more species and how they lived was found all over the world. John Ostrom was the first to revive the idea that all dinosaurs didn’t die out, but that a branch of them evolved into birds. Evidence for this was not forthcoming, but the famous fossil of the bird-like reptile, (or reptile-like bird) Archaeopteryx, had feathers and features of modern birds. The fossil was so perfect that the imprint of feathers could be seen in the sandstone millions and millions of years after it died.

Dinosaur eggs have been found in nests, and groups of nests were found in one extraordinary site, suggesting that dinosaurs nested together. Then, the fossilized footprints of the tiny dinosaur Compsognathus, a species belonging to the clade called theropods — criss-crossing and moving together — were found, showing they flocked and lived together. So dinosaurs were social animals, and they took care of their young.

One major question about dinosaurs concerns how such large animals, purportedly reptiles, managed to maintain their body temperature. Since they are so reptilian, it was assumed that they would be cold-blooded, requiring external heat sources to give them energy. But this wasn’t really feasible, given the size of many dinosaurs, and the fact that many clearly lived in cold climates. So how did they maintain their body temperature? Scientists hadn’t yet found fossils that could answer this question.

Then, in 1996, Li Yumin, a farmer and part-time palaeontologist from Northeast China, discovered a feathered dinosaur.

Sinosauropteryx prima was the first dinosaur to be found with a downy fuzz covering its entire body. Until this fossil was discovered, the idea that dinosaurs were related to birds remained in the realm of theory. But here, in this exquisite fossil of a small dinosaur with a very long, curving tail, was evidence of primitive feather-like filaments, rather than the feathers of modern birds. The fossil was so well preserved that there was evidence of colour, and that the tail was striped. It was said that when John Ostrom saw the first photos of the fossil, he went into shock. Perhaps some dinosaurs did evolve into birds.

Upwards of 500 species of dinosaurs have been found: from small bird-sized creatures to the giant Tarbosaurus and Brachiosaurus. One fossil hunter found a single vertebra 2m in height. It suggested that a gargantuan animal, more than 30m in length, once walked the earth. But the bone, already so fragile and delicate, crumbled away into nothing, and no other bone of similar size has been found since.

While the vast majority of early dinosaurs were found in the Americas, China is now proving to be an important fossil hunting ground. Sinosauropteryx was found in the Yixiang Formation, a geological formation made up of volcanic ash and river sediments. One is tempted to imagine dinosaurs fleeing from volcanic eruptions en masse, only to be caught by the ash, to be preserved for millennia until curious humans went searching for their remains in hope. Astonishingly, many of the feathered dinosaurs found thus far come from this formation.

This year, the Yixiang Formation came up with another win. Xiu Xing, a famous Chinese palaeontologist who has described many new species of dinosaurs, reported the discovery of the largest feathered dinosaur found to date: Yutyrannus huali. Not only was this dinosaur larger than the feathered dinosaurs found thus far — Sinosaruopteryx, Caudipteryx and many others — but it is related to Tyrannosaurus rex, the most feared and beloved predator. T Rex also belongs to the theropod group, and the evidence for a feathered relative suggests the tantalising possibility that T Rex might have had feathers, even if only as juveniles. Y. huali (an elision-portmanteau of the Latin and Chinese meaning “beautiful feathered tyrant”) has a fine, downy fuzz — much finer than that found on Sinosauropteryx — all over its body. Its size suggests that larger dinosaurs could also have had feathers, that they had more complex ways of maintaining body temperature. But best of all, it tells us that a branch of the dinosaurs did evolve into birds. Suddenly, the traditional image of a T. rex, Stegosaurus, Triceratops and Brachiosaur in halcyon volcanic meadow was gone, changed forever. Some monstrous creature, half bird and half reptile must take its place.

Apart from bones and feathers that have turned to stone, scientists have other evidence that dinosaurs evolved into birds. One of the best ways we have of working out how different species are related is by comparing proteins and DNA of different species to see how closely they are related. So far, it’s proven difficult to obtain DNA or proteins from fossils; after all, the youngest fossils of dinosaurs are 65 million years old. The proteins and DNA would largely have broken down, but a few scientists claim to have obtained and observed proteins in T Rex bones, Dr Mary Schweitzer among them. When the proteins were analysed, they were found to contain collagen, an important, if common, structural protein. And these proteins were found to be most similar to collagen found in the common backyard-farmyard Gallus gallus — the chicken.

As more well-preserved fossils emerged and as our understanding of anatomy increased, scientists began to look at some of these fossils with fresh eyes. One of the key characteristics of birds is air sacs; a complex system by which birds breathe and retain oxygen. It’s now apparent that dinosaurs did have air sacs, they were just located in an unexpected part of the animal.

So dinosaurs, who seemed wondrous and strange animals to begin with, turned out to be more wondrous and strange than we could have imagined — bizarre bird-like reptiles, unlike anything we know today. Their descendants developed feathers, sometimes of gorgeous plumage, and their songs and calls have entranced us for centuries.

The dinosaurs are still with us.

Read 2412 times Last modified on Thursday, 01 November 2012
Upulie Divisekera

Upulie Divisekera is a cake-loving molecular biologist and science communicator with an evangelical interest in dinosaurs.

Follow her on Twitter @scienceupulie