Thursday, December 01, 2005

Archaeopteryx and the Raptors

National Geographic News and New Scientist are announcing the publication of a new archaeopteryx fossil - pictured below.

Actually, it's not so much new as previously unavailable:

Nine previously known specimens of archaeopteryx have led palaeontologists to conclude that birds probably evolved from small meat-eating dinosaurs, and are closely related to the dromeosaurs, a group that includes the velociraptor. Yet precisely how archaeopteryx is related to the raptors has remained unclear – key pieces of these previous specimens are missing.

But the newly revealed fossil appears to fill in many of the gaps. The specimen comes from the private collection of a worker at the Solnhofen limestone quarries in Germany, where the first archaeopteryx fossil was discovered. It has remained unknown to science until its owner's death, when the new owner made it available to scientists at the Wyoming Dinosaur Center in the US.

The interesting thing about the fossil is that, unlike previos specimens, this one has feet.

What makes this interesting is that:

Archaeopteryx, the fossil shows, had a hyperextendible second toe. Until now, the feature was thought to belong only to the species' close relatives, the deinonychosaurs.


Contrary to all previous reconstructions of Archaeopteryx, the hind toe of the new specimen is not completely reversed to form a "perching" foot as it is in modern birds.

The researchers believe that the fully reversed hind toe in other Archaeopteryx fossils shifted during preservation and never existed in the live animal.

In the new fossil, the foot looks more like that of the four-toed foot of Velociraptor and its other nonwinged theropod relatives. The specimen clearly lacks a reversed toe.

Because Archaeopteryx lacked this stabilizing toe, it almost certainly did not habitually perch in trees.


The shape and articulation of other bones of the new specimen also help tie Archaeopteryx to the theropods.

The bones of its hind legs, for example, have played an important role in the dispute about bird ancestry. The new Archaeopteryx specimen shows a clearly visible hind leg bone structure that is identical to that of theropod dinosaurs.
Archaeopteryx, therefore, is closely related to the theropods. This in turn means that theropod dinosaurs are the ancestors of the modern birds that followed Archaeopteryx.

The find, according to Mayr, "not only provides further evidence for the theropod ancestry of birds, but it blurs the distinction between basal [the earliest] birds and basal deinonychosaurs," their fearsome-clawed ancestors.

"I do think that the question of a theropod ancestry of birds can now be considered settled once and forever," Mayr said.

There is a dissenting view however:

Mayr told New Scientist that there are no unique traits shared by archaeopteryx and other early bird-like fossils that are not present in dinosaurs. This would either mean that archaeopteryx cannot be classed within the same evolutionary group as birds or that this group needs to be redefined.

But Peter Makovicky of the Field Museum of Natural History in Chicago, US, says those results are shaky because Mayr's group considered only three bird-like creatures; archaeopteryx, confuciusornis and a primitive bird called Rahonavis, that lived much later.

In October 2005, Makovicky carried out a separate study that links Rahonavis directly to the dromeosaurs and suggests this species may have evolved flight separately from archaeopteryx and other birds. Makovicky told New Scientist he found no change in the shape of his evolutionary tree when he added the new traits found for archaeopteryx.

Mayr's study will be published in tomorrow's edition of Science... (sigh)

Dear Kitty has a post on the subject as well (and I'm sure The Hairy Museum of Natural History will be doing a post on it!)