Knocked off its perch

Is it a bird? Is it a plane? Or might it be something else?
THE life-sized reconstruction of the Archaeopteryx fossil in Melbourne Museum’s walk-through gallery has visitors in thrall. Most museums and school textbooks describe the small, crow-sized creature as the world’s ‘‘earliest known bird’’.
But not for much longer. New and compelling evidence from China suggests that museum exhibits and the textbooks may need to be rewritten. That’s because Archaeopteryx, which has the wings of a bird but the teeth and tail of a dinosaur, may not have been a bird after all: it was, palaeontologists now believe, a type of feathered dinosaur that roamed the Earth, and sometimes flew, 150 million years ago.
The controversial revelation, which has rocked the global science community, means that another creature must now be found to explain how birds evolved.
‘‘It seems that the flight of Archaeopteryx as a bird has come crashing to an end,’’ says Melbourne Museum vertebrate palaeontologist Tim Holland. ‘‘The recent fossil find from China has led us to rethink its role in avian evolution.’’

For many years, Archaeopteryx has graced the pages of scientific journals and general textbooks as the earliest and original form of bird. Dating from the Jurassic Period, the landmark fossil has served as an emblem for the transition between theropods (mostly carnivorous dinosaurs that walked on two legs, had large jaws and short forelimbs) and winged birds.
Archaeopteryx fossils were first discovered in Solnhofen in Germany in 1861, two years after Charles Darwin’s On the Origin of Species was published. The find fascinated the 19th-century English who were trying to get their Victorian minds around the process of evolution through natural selection.
‘‘The discovery of Archaeopteryx occurred at just the right time to illustrate the theory of evolution,’’ Dr Holland explains. ‘‘Many scientists were searching for so-called ‘missing links’ in the fossil record, including noted biologist Thomas Huxley, who latched on to Archaeopteryx as a perfect example of evolution caught in action. Indeed, it seemed to show a mixture of primitive and advanced characteristics.’’
At a glance, the most obvious feature of Archaeopteryx was its feathers, formed by the outermost layer of skin. At the time of Huxley’s suggestions, feathers, which are made of the protein keratin that forms claws, hair, nails and scales, were found only in birds.
Yet Archaeopteryx also shared many features with small carnivorous theropod dinosaurs, including a mouth filled with numerous teeth, forearms with three clawed fingers and a long, bony tail.
‘‘The similarities were so many that some specimens of Archaeopteryx were accidentally misidentified as a type of small, flightless theropod known as Compsognathus,’’ Dr Holland says.
The presence of primitive ‘‘dinosaur’’ features, along with established ‘‘bird’’ features, resulted in Archaeopteryx being regarded for the past 150 years as the earliest and most primitive avian. During that time, though, features previously thought to be unique to either birds or dinosaurs were found in many fossil forms, Dr Holland says.

Searching for evidence

‘‘Numerous types of early birds from the time of dinosaurs are now known; some of them had teeth like Archaeopteryx, while others had toothless beaks,’’ he says. ‘‘Many theropod dinosaurs have also been found with light, hollow bones and wishbones like birds — including the most famous dinosaur of all, Tyrannosaurus rex.’’
The most striking discovery was that many groups of theropod dinosaurs were also covered in feathers. This includes members of the Therizinosauroidea, Oviraptorosauria, Tyrannosauroidea and Deinonychosauria families.
‘‘As such, many popular depictions of some theropods, like the featherless Utahraptor from the BBC TV series Walking with Dinosaurs, are probably incorrect — based on fossil remains from their relatives.’’
The evidence, Dr Holland explains, is to be found anywhere from the ulna in the forearm of Velociraptor, which is covered in small bumps for the attachment of feathers, to the excellently preserved feathered dinosaurs from China’s Liaoning province.
Among the fossil discoveries from Liaoning is the Late Jurassic deinonychosaur Xiaotingia, which has drawn attention as a result of its close resemblance to Archaeopteryx.
When details of the anatomy of Xiaotingia and Archaeopteryx, along with data from other animals, were subjected to complex computer analyses, the position of Archaeopteryx dramatically shifted within the dinosaur-bird family tree.
As a result, Archaeopteryx is no longer nested within the Avialae group containing birds and their close relatives. Instead, it appears as a member of the Deinonychosauria group, which includes such dinosaurs as Utahraptor, Velociraptor and Xiaotingia.

The contenders

‘‘This has resulted in a scramble to determine which other forms now represent the earliest known birds,’’ Dr Holland says. Among the contenders is a small group of climbing animals known as Scansoriopterygids, which includes creatures with very long, skinny fingers and elaborate ribbon-like tail feathers.
Hypotheses on the origin of early birds will probably change again, as future fossil discoveries add more information to the already detailed dinosaur-bird family tree, he surmises.
‘‘It may even be that the relationships surrounding Archaeopteryx will alter again, too. Regardless of the new title holder, birds are still the descendants of small theropod dinosaurs, and are regarded as living dinosaurs themselves. It is just a little more difficult to determine when this transition happened, and which animals really were birds beneath all those feathers.’’

More to unveil

Recent work conducted on Xiaotingia and Archaeopteryx has revealed something else. If Archaeopteryx does represent an early deinonychosaur, it’s likely that flight or at least gliding occurred throughout several lineages of theropod dinosaurs.
Small primitive deinonychosaurs probably lived in trees, with later forms coming down to the ground and evolving larger bodies.
Later deinonychosaurs, such as Deinonychus, also show evidence of fierce pack hunting, while creatures such as Troodon are regarded as the most intelligent of all known non-avian dinosaurs.
‘‘Aspects of this progression seem familiar, with some ancient primates also descending from the trees to develop increased intelligence and body size on the ground — culminating in the rise of man,’’ Dr Holland says. ‘‘It’s another fascinating instance of separate groups evolving along similar lines — at least for part of their journeys.’’

Controversy rages on

The received wisdom is that birds evolved from dinosaurs. But not all scientists are convinced of this. Oregon State University zoologist John Ruben, for one, claims that some species of dinosaurs evolved from birds.
His argument is supported by an article in the international journal Proceedings of the National Academy of Sciences that details research on a feathered fossil known as a Microraptor — a small deinonychosaur that flew on two sets of wings arranged like a biplane. The research paper suggests that it descended from earlier, bird-like creatures.
This, Dr Rubens says, is in line with the idea that birds evolved from an earlier group of reptiles that included crocodiles, alligators and flying pterosaurs. They lived before the dinosaurs appeared and rose swiftly to become the dominant land vertebrates during the early Triassic Period.
‘‘Recent heavily reviewed articles in very credible journals have raised serious doubts about the whole ‘birds-are-dinos’ scenario that has been so heavily pedalled to the public,’’ Dr Ruben says. ‘‘Most disturbing is that the dinosaur community has failed to deal with these new data or to even acknowledge their existence.’’

Reign of terror

The terrible lizards known as dinosaurs ruled the planet during the geological period known as the Mesozoic Era, between 250 and 65 million years ago.
The era is split into shorter time spans called the Triassic (dating from 250 to 200 million years ago), the Jurassic (200 to 145 million years ago) and the Cretaceous (145 to 65 million years ago).

Source: The Age