New Archaeopteryx Fossil Grounds Evolutionary Ideas About Flight
The 11th fossil of Archaeopteryx, found in German limestone, has dished up some surprises that finish off the old evolutionary icon.
It’s big news when a new exquisitely-preserved specimen of the world’s most famous (and beautiful) fossil—Archaeopteryx—is discovered.
The famous Archaeopteryx fossils were listed as No. 5 in Jonathan Wells’ 2000 critique of Darwin evidences, Icons of Evolution. At the time, eight specimens of Archaeopteryx were known; one was just a feather, and one had been lost. The Berlin specimen was the best; the “London” specimen (because it ended up in London’s Natural History Museum) was the next best. All had been found in Solnhofen limestone in Germany. While Archaeopteryx had modern-looking flight feathers, it also possessed unique traits, like a scaly head, teeth, wing claws, and a long bony tail. Wells argued in 2000 that Archaeopteryx could not be an ancestor to modern birds, because it appears too early in the fossil record. It also provided no help to evolutionists about the origin of flight. It must have been, therefore, a member of an extinct lineage of birds. The missing link that was once Archaeopteryx, therefore, was still missing, he ended.
Not until more recent years were so-called “feathered dinosaurs” found in China (dated before and after Archaeopteryx) to expand and complicate the ecology of extinct birds and their alleged evolutionary ancestry. The story of Archaeopteryx itself has evolved in the interim. Now it is no longer considered a transitional form (1/16/13). Some suggest it devolved from a flying bird (11/12/13).
Analysis of specimen #11 was reported by Nature today (the authors are not aware of the provenance, [date and location], of the specimen, since it came from a private collection). So what does the new specimen show about the old icon? One new finding involves feathers on the legs that they dub “feather trousers.” Asymmetric pennaceous feathers are usually diagnostic of flight; strangely, this specimen’s tail feathers are asymmetric, but the wing feathers and “feather trousers” are symmetric (whereas on Microraptor gui, the leg feathers are asymmetric). The plumage dismisses an evolutionary speculation that flight evolved first in bi-plane fashion, on a glider such as Microraptor gui, then as powered flight. Here’s the new interpretation from the abstract:
Here we describe a new specimen of Archaeopteryx with extensive feather preservation, not only on the wings and tail, but also on the body and legs. The new specimen shows that the entire body was covered in pennaceous feathers, and that the hindlimbs had long, symmetrical feathers along the tibiotarsus but short feathers on the tarsometatarsus. Furthermore, the wing plumage demonstrates that several recent interpretations are problematic. An analysis of the phylogenetic distribution of pennaceous feathers on the tail, hindlimb and arms of advanced maniraptorans and basal avialans strongly indicates that these structures evolved in a functional context other than flight, most probably in relation to display, as suggested by some previous studies. Pennaceous feathers thus represented an exaptation and were later, in several lineages and following different patterns, recruited for aerodynamic functions. This indicates that the origin of flight in avialans was more complex than previously thought and might have involved several convergent achievements of aerial abilities.
The authors’ cladogram shows a confused mess. There is no longer a single line of feathers leading to flight; instead, there are mosaics of traits appearing here and there but not over there. Microraptor gui is shown as the first creature with asymmetrical feathers (indicative of flight) and an alula (helpful in low-speed flight), but Archaeopteryx lacks the alula. Because the story is “more complex” now, the new interpretation is that pennaceous feathers and flight must have evolved several times by “convergent evolution.”
Pennaceous feathers appear in no particular evolutionary order. “Based on these results,” the authors state, “the evolution of pennaceous feathers is generally decoupled from the origin of flight and might be related to other biological roles.” Were they for insulation? camouflage? sexual display? brooding? balance? Whatever happened, the authors reject the idea that four-legged gliding preceded two-winged flight:
Although some taxa possess prominent feather trousers, an initial aerodynamic function, as previously hypothesized, can be rejected because the presence of this character is extremely variable within Paraves, and elongated hindlimb feathers were usually restricted to the tibia and are symmetrical in shape. Thus, probably non-volant taxa such as Anchiornis or Xiaotingia might have used these feathers for display, breeding or other functions. The display function might be supported by the finding of complex colour or iridescent patterns in hindlimb feathers. Archaeopteryx might have used its ‘trouser’ additionally as a vertical plane during landing, similar to recent raptors. Only for Microraptor does an improved aerodynamic adaptation of the trousers seem plausible, as indicated by the triangular shape of the trousers and the asymmetrical shape of the feathers. These results contradict the hypothesis that the flapping flight of modern birds was preceded by a four-winged gliding stage, and indicates that flight ability in Avialae [the lineage containing Archaeopteryx] and Microraptor evolved convergently and was functionally different.
The best tale they can invent is that feathers evolved early, and were put to use in flight and non-flight uses by different lineages via “convergent evolution.” In Darwin jargon, the feathers were “repeatedly, and probably convergently, recruited for aerodynamic functions.” That makes it sound so easy. What about all the other physiological changes for flight? (See documentary Flight: The Genius of Birds for details.)
This indicates that the origin and evolution of flight in theropod dinosaurs were more complex than previously thought, and (as already shown for other anatomical complexes, for example forelimb myology and breathing apparatus) could draw on structures that evolved in different functional contexts.
In short, “flight” was able to “draw on” structures that had already “evolved in other functional contexts.” They seem to be suggesting that feathers are just one of many structures that first had some other function that was “recruited for aerodynamic functions” by “convergent evolution” in unrelated lineages. This is “co-option” on steroids. What would be the alternate functions, though, for a flow-through lung system, redesigned muscles, redesigned digestive and excretory systems, redesigned brain, new navigation abilities, and all the other bird-specific anatomical traits, if not for flight?
In short, it’s going to be a lot harder to illustrate the origin of flight in textbooks from now on.
How are the news media spinning this re-interpretation of Archaeopteryx?
- Dinosaur-Era Fossil Shows Birds’ Feathers Evolved Before Flight (National Geographic); An ancient bird ancestor from the dinosaur era sported feathers, but couldn’t fly.” Mark Norell is quoted: “It’s very hard to say feathers evolved for any one reason.” Senior author Oliver Rauhut told NG, “Once pennaceous feathers had evolved, early feathered dinosaurs could have relied on them to eventually fly.”
- Flight may have evolved multiple times in birds (Science Magazine). “The discovery raises the intriguing prospect that flight may have developed multiple times in the ancestors of birds.”
- Early bird Archaeopteryx ‘wore feather trousers’ for display (BBC News). Dr. Rauhut contradicts National Geographic; he told the BBC News, “I’m pretty sure it could fly. Though of course there is still a debate about how well it could fly.”
National Geographic‘s artistic reconstruction of the animal looks pretty birdy: something like a roadrunner without a beak. Mark Norell thinks it flew, but poorly, like a turkey or something similar. Even so, nobody calls turkeys or roadrunners non-birds or transitional forms. Archaeopteryx was a bird, albeit a strange one.
Does anybody care at this point that the authors prefer the “ground-up” (cursorial) theory of flight instead of the “tree-down” (arboreal) theory? Where’s WAIR? Calling all storytellers! Dial Ken pronto (6/25/14).
We like it when more data come in. They usually have the effect of toppling icons of evolution. Archaeopteryx was a good example. The first specimen was discovered two years after Darwin published the Origin of Species, and was widely hailed as a transitional form confirming Darwin’s theory. It would be nice to see the expression on his face if he saw the icon topple 150 years later. It would be like getting two turkeys for the price of one.