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Is It a Bird or a Dinosaur?

Archaeopteryx

Is it a bird? Is it a dinosaur? The exact position of Archaeopteryx in the evolutionary tree remains debated. Main Illustration by Nobumichi Tamura. Inset photo by Michael Lee

By Michael Lee

As a new specimen of Archaeopteryx is unveiled, scientists argue whether this famous creature is a true bird or just another bird-like dinosaur.

As one of the most famous fossil animals, and an icon of evolution, Archaeopteryx has long attracted attention and controversy. The discovery in 1861 of a creature with the wings and feathers of a bird, but the tail and teeth of a reptile, could not have been more timely: only 2 years earlier, Charles Darwin’s theory of evolution had predicted such “missing links”. Archaeopteryx exemplified the evolutionary process caught in the act.

In the intervening years, a handful more specimens of this creature have come to light. As spectacular evidence of evolution, and a thorn in the side of creationists, the 11 known examples of Archaeopteryx have been subjected to perhaps more scrutiny than any other fossils. However, recent developments have highlighted that we still have a lot to learn about Archaeopteryx, and indeed the evolutionary origin of birds.

The status of Archaeopteryx as a primordial bird has been challenged repeatedly throughout its history. Creationists have regularly questioned the authenticity of the fossils, but their arguments have usually lacked scientific credibility.

However, in the 1980s a group of leading scientists claimed they had found tangible evidence of forgery. Significantly, none of the scientists were palaeontologists or biologists. They were led by the eminent astronomer Sir Fred Hoyle, whose only other foray into biology involved the controversial “panspermia” hypothesis, which proposed that life has spread throughout the universe via microbes hitching rides on meteorites.

Hoyle and colleagues proposed that the feathers in the first two Archaeopteryx fossils discovered were a hoax, and had been added by smearing a thin veneer of carefully matched cement around the bones and then imprinting this halo with the feathers of modern birds. Among other things, they argued that other specimens of Archaeopteryx lacked feathers, and that the two halves of each split slab did not fit together perfectly (due to the doctoring).

Their claims proved to be scientifically baseless. Indeed, they were published not in a peer-reviewed scientific journal but a photographic magazine, and the authors of a rebuttal published in Science admitted that their comprehensive reply was akin to “using a sledgehammer to crack a rather trivial nut”. They pointed out that some specimens of Archaeopteryx do preserve feathers – they are simply more faintly preserved. Also, the only mismatch between the two halves of each fossil slab was explained by the documented removal of small amounts of rock to expose more of each fossil (recent circular excavations around the fingers in the Berlin specimen can be seen clearly in the figure inset opposite). X-rays and detailed microscopic analysis predictably confirmed that the slabs were homogenous and untampered: most notably, ancient hairline cracks on each side matched up perfectly – which would be impossible if one or both sides was a recent addition.

In more recent times it has been palaeontologists and evolutionary biologists who have been challenging the importance of Archaeopteryx as a primitive bird. Ever since the 1970s, palaeontologists have identified numerous bird-like features in small, advanced meat-eating dinosaurs such as Velociraptor, including lightened skeletons and arms that folded like bird wings.

The resultant idea that dinosaurs evolved into birds was strikingly confirmed from the 1990s onwards by a series of stunning Chinese fossils that, like Archaeopteryx, have a mélange of reptile-like and bird-like features. For instance, Microraptor is one of the smallest known meat-eating dinosaurs and sports long feathers on its arms and, surprisingly, legs.

These discoveries have reduced the privileged position of Archaeopteryx, which now became just one of a series of intermediates between dinosaurs and birds. However, Archaeopteryx still occupied an important position in this evolutionary procession between the increasingly large flock of feathered dinosaurs and true birds.

Two studies in late 2011 have now pushed Archaeopteryx into the flock of bird-like dinosaurs, thus diminishing its importance in the evolutionary origin of birds. In one study led by Xing Xu of the Chinese Academy of Sciences, Archaeopteryx was placed at a similar level in bird ancestry as Velociraptor and related dinosaurs. Indeed, Archaeopteryx seems to possess a smaller version of Velociraptor’s sickle-shaped “killing claw”. Another study led by Darren Naish of the University of Portsmouth put Archaeopteryx in an even more remote position to birds – below Velociraptor.

These studies were impressively detailed. They analysed hundreds of anatomical features across birds and dinosaurs, and constructed evolutionary trees using a widely-employed technique called cladistics, which gives every trait an “equal say” in determining the branching pattern: the branching pattern supported by the most traits is the preferred evolutionary tree.

However, there are other methods for inferring evolutionary trees. One method, called maximum likelihood, places greater emphasis on traits that are evolving more slowly because rapidly evolving traits are more likely to evolve convergently. For example, hair colour in mammals evolves rapidly, so not all white mammals are closely related.

When we applied maximum likelihood methods to the anatomical data in Xu’s study, the resultant evolutionary tree pushed Archaeopteryx above the flock of bird-like theropods, restoring it to its privileged position between these feathered dinosaurs and birds.

However, the debate is far from resolved, and new discoveries might ironically make it even harder to pin down the precise position of Archaeopteryx. There was previously a huge evolutionary gap between dinosaurs and birds, plugged solely by Archaeopteryx. Now this gap is bridged by a range of feathered dinosaurs, making it difficult to nail down the position of Archaeopteryx at a precise point along this continuum.

This disagreement about the position of Archaeopteryx in an evolutionary tree (within, or above, the bird-like dinosaurs) has important implications for the origin of flight. Among dinosaurs, Archaeopteryx and birds share a unique form of locomotion: flapping flight powered only by the forelimbs. Although many feathered dinosaurs are known, none are known to have flown in a bird-like manner: the human-sized Deinonychus had arms rather than wings and was undoubtedly earth-borne, while the tiny Microraptor used all four limbs to either glide or fly.

If Archaeopteryx and birds are closest relatives, as we suggest, this would suggest that bird-like flight evolved only once, in their common ancestor. Alternatively, if Archaeopteryx and modern birds evolved from different dinosaur ancestors, then bird-like flight must have originated twice: a stunning example of convergent evolution.

The anatomical and energetic demands of powered flight are huge, and to have it evolve in parallel in closely related lineages would be extraordinary. True flight, as opposed to gliding, has evolved only three other times in the entire animal kingdom: in insects, bats, and pterosaurs.

Thus, 150 years after its discovery, the 150-million-year-old Urvogel (“original bird”) remains a central figure in debates about the evolution and origin of birds.

Fittingly, 2011 saw a new specimen of Archaeopteryx come to light. Only the eleventh specimen known, it was unveiled in Munich near the Solnhofen limestone quarries where all specimens have been found. The information in the Munich specimen, and application of new technologies to the other ten specimens (such as high-resolution CT scanning of the braincase), should unlock additional secrets but will likely raise new questions.

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Michael Lee is senior research scientist at the South Australian Museum and the University of Adelaide.