Birds are dinosaurs: Simple answer to a complex problem

Auk, The, Oct 2002 by Feduccia, Alan

Commentary

Richard Prum's (2002) rancorous, unreviewed essay on the theropod origin of birds is a one-sided view of a difficult problem, full of anatomical misconceptions that are highly misleading, and advocates that (p. 13), "it is time to abandon debate on the theropod origin of birds." His article is essentially a restatement and defense of a current dogma of paleontology-that birds are living dinosaurs, directly descended from, or having shared common ancestry with, one of the most highly derived and specialized groups of Cretaceous theropods, the dromaeosaurs (and Cretaceous troodontids), that are presumed to have had ghost lineages going back into the Jurassic Period. Advocates on both sides of the debate agree that birds are related to dinosaurs, but opponents of the birds-are-dinosaurs movement, including myself, advocate a common shared ancestry of birds and dinosaurs from basal archosaurs, with less specialized anatomical baggage, at a much earlier time.

The "birds are living dinosaurs" hypothesis dates back almost three decades to when John Ostrom (see Ostrom 1976), combining studies of his earlier discovery of the late, early Cretaceous dromaeosaur Deinonychus with his speculations on hot-blooded (endothermic) dinosaurs, presented his new dinosarurian origin of birds theory. At its inception, all theropods were highly energized, endothermic reptiles (endothermic homeotherms), and the smaller theropods had acquired feathers for insulation. Archaeopteryx was an earthbound feathered theropod that could not fly (Bakker 1975) but later learned to fly from the ground up (Ostrom 1979). At that time, the dinosaurian origin of birds had, of course, nothing to do with cladistic theory, but was based on the overall similarity of Deinonychus to Archaeopteryx. By 1978, Archaeopteryx was said to support "two theories: warm-bloodedness in dinosaurs and dinosaurian ancestry of birds" (Ostrom 1978:168). 1 wrote the first rebuttal to hot-blooded dinosaurs (Feduccia 1973), and a mountain of evidence has been marshaled against endothermy in dinosaurs during the last three decades (Morell 1996).

Nevertheless, Ostrom (1976) reconstructed the Archaeopteryx skeleton to closely resemble that of the known theropods; it was a terrestrial predator, and sported a vertical pubis, with fully developed pubic foot and a predatory hand. Often a hypertrophied second foot sickle claw, like that of Deinonychus, was on display. Now, with the discovery of early Cretaceous dromaeosaurs with somewhat retroverted pubes, Archaeopteryx has gradually had its pubis pushed back to the opisthopubic position to conform to the most current view of dromaeosaurs and is often depicted as a terrestrial predator, with a sickle claw (Paul 2002), despite evidence that Archaeopteryx was arboreal (Feduccia 1993), and clearly did not possess such a claw. Because all the known theropods were terrestrial predators, Ostrom (1979) suggested that the flight feathers must have elongated in the context of insect traps and were later preadapted for flight. The dinosaurian origin of birds thus originated as a strange amalgam of overall similarity and evolutionary scenarios involving endothermy and ground-up flight. Prum's (2002) assertion that I have linked the dinosaurian origin of birds to ground-up flight is a misstatement; my exposition was a reaction to the then current paleontological dogma. It was Ostrom who argued that the discovery of Deinonychus provided evidence for a ground-up origin of flight (Dingus and Rowe 1998). Later, Padian (beginning 1983) spent decades trying unsuccessfully to make a convincing argument for ground-up flight origin in birds and pterosaurs, which he considered a corollary of a theropod-pterosaur sister-group hypothesis.

Paleontologists Dingus and Rowe (1998) linked the dinosaur ancestry of birds with the origin of flight from the ground up, and the thecodont (basal archosaur) hypothesis with the origin of flight from the trees down. "Our map [of avian relationships] suggests that flight evolved from the ground up, but exactly how this happened is another question altogether" As Bock (1999) noted, "If the origin of birds and the origin of flight are tightly linked in this fashion, then the available discussion of all specialists in vertebrate flight is that the origin of avian flight from the ground up is exceedingly improbable, which would fatally weaken the dinosaur ancestry of birds."

Prum (2002) has now established an even more elaborate evolutionary scenario for the evolution of flight and feathers in birds, involving the origin of feathers from "dino-fuzz" filaments (Feduccia 1999) as an insulatory mechanism (but with no evidence for endothermy in dromaeosaurs), the origin of a flight morphology in a terrestrial setting, and then the final achievement of "flight" from the trees down. That complex scenario, involving endothermy and preadaptations, has no name as yet. The last attempt to explain flight in dinosaurs was the "ground down" theory, a version of trees down, but involving jumping from rocks or leaping from cliffs, which was, in essence, a version of the trees down theory, biophysically. An arboreal flight origin from theropods was previously suggested by Chatterjee (1997) and Xu et al. (2000).