英語語言學(xué)習(xí)：飛龍
Australia has some of the finest living dinosaurs in the world. Yes, I mean birds. Our birds were among the first purveyors of birdsong in ancient history, so the science suggested earlier this year, despite what David Attenborough has aid about our main tendency to squawk. We have the lyrebird, that great mimic, we have the most intelligent birds, the crows, magpies and parrots, and we have some of the most abundant populations of birds in every category and place. So why then don’t we have a matching range of real fossil dinosaurs? Are they still hidden? Well, John Pickrell has a new book to explore some of these questions and more. It is called Flying Dinosaurs – How fearsome reptiles became birds. He is the editor of Australian Geographic.
John Pickrell: Imagine, if you will, a world filled with billions of dinosaurs. A world where they can be found in thousands of shapes, sizes, colours and classes in every habitable pocket of the planet. Imagine them from the desert dunes of the Sahara to the frozen rim of the Antarctic Circle – and from the balmy islands of the South Pacific to the high peaks of the Himalayas.
The thing is, you don’t have to imagine very hard. In fact, wherever you live, you can probably step outside and look up into the trees and skies to find them, for birds are dinosaurs and they are all around you. Dinosaurs didn’t die out when an asteroid hit the Earth 66 million years ago.
Everything I was told as a child was wrong.
The idea takes some getting used to. On the face of it, birds don’t seem that similar to dinosaurs – they’re small, bright, quick and covered with feathers, whereas the dinosaurs I was told about as a kid were hefty, lumbering beasts, scaly and reptilian, much more like a crocodile than a bird. But the clues were there all along if only we knew what we were looking for. Theropod dinosaurs (the bipedal, carnivorous variety) share more than 80 small features of their skeletons with birds – far more than either share with any other group of animals.
An early clue to the link between theropods and birds came with the discovery of the first fossil of Archaeopteryx in a Bavarian quarry in 1861. It has been called the most important fossil ever found, not least because of what it tells us about dinosaurs.
Labelled the ‘first bird’ this prehistoric animal had wings and feathers, but also the long bony tail and teeth of a reptile. Its similarity to Compsognathus, a small dinosaur found in the same German limestones, was striking, and was even remarked upon at the time by British evolutionary biologist Thomas Henry Huxley.
But at this time it was only two years since Charles Darwin had unveiled his theory of evolution by natural selection in On the Origin of Species, and it seems the world wasn’t yet ready for the revelation of the link between dinosaurs and birds.
That connection would remain largely obscured until 1964, when Yale University palaeontologist John Ostrom stumbled upon the fossils of several lithe, athletic and deadly-looking dinosaurs called Deinonychus in the badlands of Montana. Ostrom resurrected the idea that Archaeopteryx was closely related to theropod dinosaurs such as Deinonychus, and so began the ‘dinosaur renaissance’ of the 1970s, which saw leading experts redefine dinosaurs as intelligent, speedy, warm-blooded creatures that were similar to birds.
The idea that birds were the direct descendants of dinosaurs still had its detractors, but much of the opposition fell away in 1996, when the fossil of a little dinosaur from China shook the very foundations of palaeontology.
Sinosauropteryx was undoubtedly a dinosaur, but the fossil clearly showed that it was covered in a fuzzy down of protofeathers that later studies revealed would have been ginger-coloured. This was the first of the feathered dinosaurs to be discovered, but whole flocks of feathered dinosaurs have since burst onto the scene, and we now have evidence for feathers of some kind in more than 40 species. Most hail from the 100–145-million-year-old Early Cretaceous rocks of China’s north-eastern Liaoning Province, which preserve fossils in remarkable detail.
Discoveries of feathered dinosaurs are coming thick and fast these days. Just in the past few weeks alone, there have been announcements about a new 'four-winged' species Changyuraptor yangi a new fossil specimen of the Archaeopteryx, and a feathered Siberian species, Kulindadromeus zabaikalicus.
Every new fossil is a small pebble of proof in an avalanche of evidence confirming that birds are the descendants of the theropod dinosaurs and that these animals were incredibly bird-like.
Beyond confirming the dinosaur–bird link, the fossils have offered clues about how feathers evolved in the first place, and how they might have been used for flashy display purposes and insulation long before they ever helped any creature become airborne.
There is now good evidence that most carnivorous theropod dinosaurs, even fearsome and well-known types — such as Allosaurus and Tyrannosaurus — had feathers, and that they used them for a variety of functions.
Feathers are so intimately entwined in our minds with flight that this idea takes some getting used to. Nevertheless, animals with flight feathers can't have appeared from nowhere, so it makes sense that the earliest feathers had another purpose entirely.
The majority of the new feathered dinosaurs have been discovered in north eastern China and Mongolia, but a smattering of finds come from elsewhere in the world, from places such as Germany, Madagascar and North America.
Though most of the dinosaur species for which we have confirmed evidence of feathers hail from China, we can actually be pretty sure that dinosaurs all over the world had feathers.
This is because the species from China with feathers come from all over the family tree of carnivorous dinosaurs and some of them are early members of lineages that had descendents all over the world.
The reason we can see feathers on many fossils from the north-eastern Chinese province of Liaoning is because there is a very unusual level of preservation there. More than 90 per cent of fossils in the rest of the world preserve just hard tissues, such as teeth and bones – but those in Liaoning often have soft tissues such as internal organs as well as skin impressions and delicate and beautiful traces of feathers surrounding them. The unusual level of preservation in fossils in Liaoning is to do with the very fine-grained volcanic ash that preserves the animals.
Perhaps some of the most exciting new discoveries, however, are those that hint at how dinosaurs did eventually take to the skies. We now know that the dinosaurs most closely related to birds were small predatory species, a number of which, such as Microraptor, Anchiornis and Xiaotingia – quite incredibly – had four wings and a long feathery tail. Their hind limbs and tails had flight feathers of the kind we see only on the forelimbs of modern birds, so it’s likely they used them to glide between the trees of China’s swampy Cretaceous forests.
We also now know that dinosaurs were bird-like in many other aspects of their physiology and behaviour too. From nesting, brooding and sex, to metabolism, development and even the diseases that afflicted them, many of the traits found in birds today were inherited from the dinosaurs. The boundary between dinosaurs and birds has become utterly blurred.
Along with the new fossils, and renewed interest in dinosaurs, have come fresh interpretations of how dinosaurs lived their lives. In 1993 when the film Jurassic Park hit cinemas, nobody could have predicted that we might know something about the sounds that dinosaurs made and the colours they were decked out in, but clever new methods have begun to probe these kinds of details too.
Starting in 2010 a whole series of studies have been able to make educated guesses about the colour of dinosaur feathers by looking at tiny structural details of fossilised feathers under the microscope. In the feathers of living birds, pigments are packaged up in tough little parcels which vary in shape depending on the colour – and it turns out the same was true in prehistoric feathered animals too. In terms of vocalisations, we can be pretty sure that dinosaurs didn’t make the kind of mammal-style throaty growls and roars which Hollywood typically depicts them as having had. This is because neither birds, nor crocodiles – which are the closest living relatives to dinosaurs today – have vocal cords as mammals do. The true sounds that dinosaurs made may be much more like the sounds of birds, but slowed down and deepened to approximate the sounds coming from much larger animals.
It may surprise you to hear, that we have learnt more about dinosaurs in the two decades since Jurassic Park than during the whole of history up to that point.
The 1990s seemed like a golden age of dinosaur discovery, but fossil finds since then have dwarfed it. Around one new species is currently discovered every week, many in China, but others in South America, Mongolia and Africa. There’s so much new knowledge it’s hard to keep up, but one thing’s for certain – if you love dinosaurs, this a great time to be alive.
This may lead you to wonder how Australia fits into this picture of new dinosaur discovery and could our continent ever yield feathered dinosaurs? The answer is that perhaps it already has. A fossil site near the town of Koonwarra in Victoria’s Gippsland has rocks made of fine-grained sediments, and a very unusual level of preservation for Australia.
Starting in 1961, a series of 12 fossilised feathers were found here all dating to between 115 and 118 million years ago, during the Early Cretaceous period. It was assumed these were bird feathers, but in light of all the new discoveries from China it is now just as likely that they are dinosaur feathers.
Researchers led by Dr Tom Rich at Museum Victoria in Melbourne now have plans to return to the site to search for the remains of entire feathered dinosaurs in 2017.
Australia and Antarctica are the continents in the world with the least dinosaur fossils. Australia has only around 17 known species and many of these were described from single fragments of bone. More species of dinosaurs have been found in single quarries in China than have been found on our entire continent.
Australia’s landmass is geologically ancient and it’s also flat and heavily weathered, which means it’s very difficult to find dinosaurs here. The best places to find fossils are in mountain ranges and hills were freshly exposed rocks are eroding away, but these fossil hunting sweet spots are few and far between Down Under.
There’s no doubt that part of the reason we have so few dinosaurs is because of Australia’s unusual geology, but limited funding for museum scientists and palaeontologists is also part of the reason too. As a nation we have no more than a handful of vertebrate palaeontologist spread very thinly over a vast area.
In comparison China has an army of diggers out there right now and that is why their stunning fossils have fuelled the current golden age of dinosaur discovery.
If Tom Rich is able to secure the funding for his Gippsland expedition in 2017 then there’s a chance of finding incredible feathered dinosaur fossil in Australia too – and what a coup for scientific discovery that would be.
Robyn Williams: It certainly would, but could we wait that long, or continue to depend on so few palaeontologists? I think I know the answer to that. John Pickrell edits Australian Geographic magazine and his new book is called Flying Dinosaurs – How fearsome reptiles became birds.
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