The fascination of the ‘Urvogel’
Some 150 million years ago Bavaria lay at the bottom of a shallow sea, which was part of one of the northern arms of the precursor of what we now know as the Mediterranean. This was the habitat of Archaeopteryx. All eleven specimens of the iconic primitive bird so far discovered were found in Northern Bavaria and, not only are they among the most widely known fossil, they also represent some of the best preserved skeletons from the Mesozoic era. Dr. Oliver Rauhut, a paleontologist in the Department of Earth and Environmental Sciences at LMU, who is also affiliated with the Bavarian State Collection for Paleontology and Geology in Munich, is currently engaged on an intensive study of the Archaeopteryx specimen that has the best preserved plumage: It is the eleventh example of the species to come to light.
Archaeopteryx represents a transitional form between reptiles and birds. It is possibly the earliest, and certainly the best-known, bird fossil. It proves that modern birds are directly descended from predatory dinosaurs, and so are themselves essentially modern-day dinosaurs. All 11 examples of the genus were found in the valley of the Altmühl in northern Bavaria, an area known to paleontologists as the Solnhofen Archipelago. “It was a habitat dominated by islands and reefs, and Archaeopteryx probably lived on the small offshore islands,” says Oliver Rauhut. The waters surrounding the islands were dotted with reefs, and between the reefs were lagoons, which were shallow enough to be subject to periodic evaporation. Salt accumulated, and fine-grained, chalky sediments were deposited in the lagoons. In this hypersaline environment, no subsurface organisms could survive that might break down the carcasses of dead animals which were washed into these depressions. This accounts for the generally excellent state of preservation of the fossils that have been found in the limestone strata around Solnhofen.
The dinosaur-feather connection
Rauhut and his colleagues have now examined the traces of plumage that are discernible in the eleventh Archaeopteryx under the microscope with the aid of both visible and ultraviolet light. The use of different wavelengths of light for illumination enables one clearly to differentiate preserved soft body parts from the skeletal elements, which fluoresce under UV light. The results of the investigation have now appeared in the journal Nature. “For the first time, we were able to analyze details of the feathers on the body, the tail, and especially on the legs,” says Rauhut. This has also allowed the researchers to carry out a more comprehensive comparison of the distribution of plumage in Archaeopteryx and other species of feathered dinosaur than had been possible up to now. “Comparisons with other feathered predatory dinosaurs indicate that the plumage in the different regions of the body varied widely between species. That in turn suggests that primordial feathers did not evolve in connection with flight-related roles, but originated in other functional contexts,” says Dr. Christian Foth of LMU and the Bavarian State Collection for Paleontology and Geology in Munich, first author on the new paper.
Many examples of previously unknown feathered predatory dinosaurs (theropods) have been recovered in recent years, primarily from geological strata in China. Some of these finds are older than Archaeopteryx, but are also related to true birds, and thus help to situate the Solnhofen specimens in a wider evolutionary context. Only a few years ago, the remains of a 150 million-year-old theropod were discovered in the Franconian Jura nearby. Subsequent studies by Rauhut and his colleagues revealed that this megalosaur species was also endowed with a dense covering of hair-like primitive feathers. “This may indicate that all theropods bore feathers,” he says. “If so, we would finally have to discard the old image of dinosaurs as giant reptiles encased in a carapace of thick scales.”
The versatility of feathers
The plumage of Archaeopteryx No. 11 is so well preserved as impressions in the rock matrix that its distribution allows one to make plausible inferences with regard to its function. The whole body was covered in feathers. On the hindlimbs, this basal bird had long, symmetrically shaped feathers, which get shorter as one approaches the ankle joint. Its tail feathers were also highly elongated.
These features are compatible with the idea that feathers initially functioned primarily as an insulating layer. Advanced species of predatory dinosaurs and primitive birds with feathered forelimbs, like Archaeopteryx, may have used them as balance organs when running, like ostriches do today. Moreover, feathers could have served useful functions in brooding, camouflage and display. Thus, the feathers on the tail, wings and hind-limbs were most probably employed for display purposes.
The long leg feathers could have had a protective function, as in modern birds of prey, where they mitigate the effects of biting by their prey species, or might have acted as brakes, like miniature parachutes, when landing. However, the new findings do not support the theory that the capacity for powered flight first emerged in four-winged species which had acquired the ability to glide.
Archaeopteryx itself was probably able to fly. “Interestingly, the lateral feathers in the tail of Archaeopteryx had an aerodynamic form, and most probably played an important role in conferring the capability to fly,” says Foth.
Many questions remain
On the basis of their investigation of the plumage of the new fossil, the researchers have been able to clarify the taxonomical relationship between Archaeopteryx and other species of feathered dinosaur. Here, the diversity in form and distribution of the feather tracts is particularly striking. For instance, among dinosaurs that had feathers on their legs, many had long feathers extending to the toes, while others had shorter down-like plumage. “If feathers had evolved originally for flight, functional constraints should have restricted their range of variation. And in primitive birds we do see less variation in wing feathers than in those on the hind-limbs or the tail,” Foth explains.
These observations support the notion that feathers acquired their aerodynamic functions secondarily: Once feathers had been invented, they could be co-opted for flight. “It is even possible that the ability to fly evolved more than once within the theropods,” says Rauhut. “Since the feathers came first, different groups of predatory dinosaurs and their descendants, the birds, could have exploited these structures in different ways.”
Overall, therefore, the eleventh Archaeopteryx fossil argues that the evolution of feathers and the origins of powered flight in vertebrates followed a much more complex trajectory than previously assumed. Like all the other specimens of the genus, it thus makes a highly significant contribution to our understanding of the evolution of the biosphere.
The new fossil was made available for study by a private individual in 2011. “The collector also had the specimen registered on the list of protected German Cultural Heritage to ensure that it would remain accessible to science. This is a very good example of successful cooperation between private collectors and academic paleontologists,” says Rauhut.
LMU paleontologists will continue to explore the riches of their fossil collections for insights into evolutionary history, in particular with a view to understanding what made the dinosaurs so successful for so long. But they are not about to lay the new Archaeopteryx aside. They have now embarked on a detailed anatomical description of the material and hope to clarify “a number of unsolved problems”, as Rauhut puts it. One of these concerns the relationship between the different Archaeopteryx fossils now known. Do they perhaps represent more than one species? “It is quite possible that different species evolved on the different islands. But so far, we know nothing about the evolutionary dynamics in the Solnhofen Archipelago,” he says.
- To learn more about Oliver Rauhut’s research, see the following article in LMU’s research newsletter “insightLMU”: “The Patagonian puzzle”