A new, slightly younger, and more birdlike species of Archaeopteryx has been identified. Where other Archaeopteryxes look like a midpoint between birds and therapod dinosaurs, this one has more in common with the creatures we see today. It cements the case that these are the true ancestors of birds, and allows us to see the order in which some modern features made their appearance.
In the Late Jurassic, some 150 million years ago, southern Germany was a shallow sea, dotted with tropical islands. Like an ancient Galapagos, the diversity of these islands apparently encouraged rapid evolution. Some bird-like features of the small dinosaurs on these islands may have been even more ancient, but the 12 specimens we have found represent the first evidence of an animal that combined them into something resembling modern avians.
Nevertheless, paleontologists have continued to debate whether today's birds descend from this small corner of the world. Dr Benjamin Kear of Uppsala University believes the identification of Archaeopteryx albersdoerferi will settle that.
A. albersdoerferi had air-filled bones, like modern birds, the team report in Historical Biology. The wishbone had more room for flight muscles to attach, the skull was fused, and there were fewer heavy teeth than other Archaeopteryx. All of these are consistent with a lifestyle dependent on flight. Meanwhile, debate remains as to whether other Archaeopteryx could truly fly short distances or whether they merely glided.
A. albersdoerferi lived approximately 400,000 years later than any of the other Archaeopteryx specimens we have found. It was encased in the Mörnsheim Formation, a distinctly younger deposit than the strata in which all other known Archaeopteryx occur.
"Significantly, however, when we examined the evolutionary relationships of various species of Archaeopteryx we found that its flight-related characteristics had appeared separately from those of more advanced bird-line dinosaurs, implying that flying lifestyles have developed more than once,” Kear said in a statement.
The new species was identified using the Daiting Specimen, found in 1990. The specimen was owned privately until 2009, when palaeontologist Raimun Albersdoerfer bought it, making it available for study.
The 12 Archaeopteryx fossils are sufficiently diverse that some have proposed each is a different species. However, a more favored categorization puts the other 11 into two species: A. lithographica and A. siemensii. Aside from one particularly incomplete fossil and another that has been stolen, the Daiting Archaeopteryx has been the most poorly understood of the specimens. Not only was it unavailable for study for so long but many of its bones and teeth remain trapped in the stone in which it was found.
Kear and co-authors addressed this using synchrotron microtomography, a sort of X-ray analysis that produces 3D images, revealing even the unextracted parts in great detail. Among other things, this showed that A. albersdoerferi acquired a number of adult features while still a juvenile, something that does not seem to be the case for the earlier members of the genus.
After the initial shock with which the first Archaeopteryx was received, just two years after the publication of the Origin of the Species, doubts have set in as to whether this truly is the link between dinosaurs and birds.
Not only has recent work shown birds are living dinosaurs, but numerous other species that combine wings and feathers with teeth and claws have been found living in other parts of the world at a similar time. This has raised questions as to whether Archaeopteryx really are a transitional species or if they evolved along a similar path to the true ancestors of birds, before coming to an evolutionary dead end.
