Wednesday, May 16, 2007
Birds--Fliers Not All
I probably should have blogged about this last year, but I honestly forgot about it until I was leafing through my papers and found Phil Senter's 2006 discussion about primitive birds and their flying abilities. Basically, Senter looks at the orientation of the scapula in relation to the ribcage in the fossil animals, kind of like in this story, and discovered that fossilization pretty well demonstrates real-life scapular orientation. In crown-group and enantiornithine fossils, the scapula sits atop the ribcage. This position, of course, lifts the glenoid up and out, so that the bird's arms "face" outward--thus allowing a flight stroke. However, fossils like Archaeopteryx, Jeholornis, and Confuciusornis have scapular blades that lie against the ribcage, angled upward, like the case in every other theropod dinosaur, including (sniff) the deinonychosaurs. The possibility of post-mortem skeletal movement is real, but it's interesting that in obviously avian fossils the scapulae are oriented in the modern manner consistently. Why would non-avian dinosaurs be more inclined to have loose scapulae?
This is fairly important, because all previous discussions of how Archaeopteryx are now moot (assuming that Senter is right). Instead, we're forced to confront the idea that Archaeopteryx, the first bird, could not actually fly. Rather, because it could not life its arms above the horizontal, it could not complete a recovery stroke and could not generate lift. Senter says that in the Ornithothoraces clade, which includes enantiornithines and neornithes, the scapula migrated up the ribcage to lay at the top of the ribs, on either side of the spinal column, allowing birds to complete a flight stroke.
Senter posits that Archaeopteryx, Confuciusornis, and Jeholornis were all gliders. And gliding does not equal flying. And yet all three of these "birds" were blessed with asymmetrical flight feathers. In a previous blog, I correlated asymmetrical feathers with flight, and flight with Aves. In this way, I was able to show that Microraptor is a bird, given its asymmetrical feathers which apparently equate flying ability. According to Phil Senter, this is not the case, and Aves may need to be rethought once again. If our definition of Aves includes the ability to achieve a recovery stroke, then a bunch of primitive forms have been kicked out. Apparently, asymmetrical feathers predated flight.
Of course, it asymmetrical feathers constitute the golden rule for defining Aves, than Archaeopteryx, Confuciusornis, Jeholornis, and the deinonychosaurs are still in the club.
P.S. I suppose the possibility exists that Archaeopteryx, Confuciusornis, Jeholornis, and deinonychosaurs constitute a sister group to the Ornithothoraces, whereupon the common ancestor of this largest group is defined as having asymmetrical feathers, but one branch (Archaeopteryx & Co.) were content to glide while the ornithothoraces developed powered flight. Exactly how close Archaeopteryx and Confusiusornis are is a matter of some debate.