That bird really IS a dinosaur, says paleontologist
|July 10, 2014||Posted by News under Evolution, News|
I’ve read through the paper by Czerkas and Feduccia, and it really isn’t very good. To argue that Scansoriopteryx is a bird, they also argue that all dromaeosaurids, troodontids, and oviraptorids are bird, not dinosaurs. And this isn’t a question of whether they are birds or dinos on a dinos-evolved-into-birds scale, it’s the idea that these groups started off from a completely different, non-dinosaurian group of archosaurs, and independently/convergently evolved a wide variety of dinosaurian traits. These include a fully upright stance, ossified tendons on the tail, a similar skull bone architecture, vertebral counts, etc.
The specimen itself is quite problematic, because it appears to be a juvenile. Thus many of the features that Czerkas and Feduccia argue are absolutely, definitively un-dinosaurian in Scansoriopteryx (such as the incompletely perforated acetabulum, the lack of a strongly inturned femoral head, the extent of the deltopectoral crest on the humerus, etc.) are much more easily explained in terms of ontogeny, rather than their very forced phylogenetic hypothesis. The elongate 3rd digit is quite odd, but it doesn’t remove Scansoriopteryx from the dinosauria any more than it could place it in aviale; it’s a one-off trait (and may also be some kind of ontogenetic stage).
Ok, that sound confusing, so here’s an example: Dinosaurs are defined by a series of anatomical traits. One is the possession of a perforated acetabulum (a hole through the hip socket), which allows the head of the femur to be oriented 90-degrees from the femur’s shaft (another dino trait), and results in the upright stance seen in dinosaurs. All dinos have a perforated acetabulum, and Czerkas and Feduccia even call this “the sine qua non of dinosaurian status” (unnumbered pages, but 6th page, bottom right). Since Scansoriopteryx has an incompletely perforated acetabulum, they conclude that it must not be a dino, and is also an early member of a (simplified here) Scansoriopteryx à Dromaeosaur à Archaeopteryx lineage. But a fully perforated acetabulum is seen in Deinonychus (a dromaeosaur, and in all known dromaeosaurs). What to do with this “sine qua non” in Deinonychus? It must be a convergence with dinos!
So using this singlar example of an incompletely perforated acetabulum and other questionable anatomical “traits” in a baby raptor-ish dino, Czerkas and Feduccia argue that all these things that we’ve called dinosaurs for decades are actually bird convergences on the dinosaurian body via a completely different ancestral archosaur stock. The authors provide no cladistic argument for their case, as would be expected in a paper making this kind of argument. Instead, they focus on a few traits they deem important, and in my opinion, badly misinterpret them.
In short, Feduccia is wedded to a particular hypothesis of HOW birds evolved (trees down): dinosaurs CAN’T be bird ancestors if for no other reason than that dinosaurs lived on the ground. So there, it’s settled. He has now convinced Czerkas that Scansoriopteryx isn’t a dino, even though Czerkas described it as such in 2002. In reading through their paper, I don’t see any compelling reason to move Scansoriopteryx (and the dromaeosaurs, troodontids, and oviraptors along with it) out of the Dinosauria. This critter is a maniraptoran theropod, and by all rights belongs in the Dinosauria.
So I would strenuously caution creationists and IDers not to jump on this as further proof of problems in the paleo realm of dinos and bird. This paper will be treated harshly not because its conclusions are unpalatable to the birds-are-dinos camp, but because its arguments and interpretation of the evidence are selective and strained.
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