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Birds Are Dinosaurs: Simple Answer to a Complex Problem

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Birds Are Dinosaurs: Simple Answer to a Complex Problem Commentary The Auk 119(4):1187±1201, 2002 Birds are Dinosaurs: Simple Answer to a Complex Problem ALAN FEDUCCIA1 Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280, USA Richard Prum's (2002) rancorous, unreviewed es- sported a vertical pubis, with fully developed pubic say on the theropod origin of birds is a one-sided foot and a predatory hand. Often a hypertrophied view of a dif®cult problem, full of anatomical mis- second foot sickle claw, like that of Deinonychus,was conceptions that are highly misleading, and advo- on display. Now, with the discovery of early Creta- cates that (p. 13), ``it is time to abandon debate on the ceous dromaeosaurs with somewhat retroverted pu- theropod origin of birds.'' His article is essentially a bes, Archaeopteryx has gradually had its pubis restatement and defense of a current dogma of pa- pushed back to the opisthopubic position to conform leontologyÐthat birds are living dinosaurs, directly to the most current view of dromaeosaurs and is of- descended from, or having shared common ancestry ten depicted as a terrestrial predator, with a sickle with, one of the most highly derived and specialized claw (Paul 2002), despite evidence that Archaeopteryx groups of Cretaceous theropods, the dromaeosaurs was arboreal (Feduccia 1993), and clearly did not (and Cretaceous troodontids), that are presumed to possess such a claw. Because all the known thero- have had ghost lineages going back into the Jurassic pods were terrestrial predators, Ostrom (1979) sug- Period. Advocates on both sides of the debate agree gested that the ¯ight feathers must have elongated in that birds are related to dinosaurs, but opponents of the context of insect traps and were later preadapted the birds-are-dinosaurs movement, including my- for ¯ight. The dinosaurian origin of birds thus orig- self, advocate a common shared ancestry of birds inated as a strange amalgam of overall similarity and and dinosaurs from basal archosaurs, with less spe- evolutionary scenarios involving endothermy and cialized anatomical baggage, at a much earlier time. ground-up ¯ight. Prum's (2002) assertion that I have The ``birds are living dinosaurs'' hypothesis dates linked the dinosaurian origin of birds to ground-up back almost three decades to when John Ostrom (see ¯ight is a misstatement; my exposition was a reac- Ostrom 1976), combining studies of his earlier dis- tion to the then current paleontological dogma. It covery of the late, early Cretaceous dromaeosaur was Ostrom who argued that the discovery of Dei- Deinonychus with his speculations on hot-blooded nonychus provided evidence for a ground-up origin (endothermic) dinosaurs, presented his new dino- of ¯ight (Dingus and Rowe 1998). Later, Padian (be- sarurian origin of birds theory. At its inception, all ginning 1983) spent decades trying unsuccessfully to theropods were highly energized, endothermic rep- make a convincing argument for ground-up ¯ight tiles (endothermic homeotherms), and the smaller origin in birds and pterosaurs, which he considered theropods had acquired feathers for insulation. Ar- a corollary of a theropod±pterosaur sister-group chaeopteryx was an earthbound feathered theropod hypothesis. that could not ¯y (Bakker 1975) but later learned to Paleontologists Dingus and Rowe (1998) linked the ¯y from the ground up (Ostrom 1979). At that time, dinosaur ancestry of birds with the origin of ¯ight the dinosaurian origin of birds had, of course, noth- ing to do with cladistic theory, but was based on the from the ground up, and the thecodont (basal archo- overall similarity of Deinonychus to Archaeopteryx.By saur) hypothesis with the origin of ¯ight from the 1978, Archaeopteryx was said to support ``two theo- trees down. ``Our map [of avian relationships] sug- ries: warm-bloodedness in dinosaurs and dinosau- gests that ¯ight evolved from the ground up, but ex- rian ancestry of birds'' (Ostrom 1978:168). I wrote actly how this happened is another question alto- the ®rst rebuttal to hot-blooded dinosaurs (Feduccia gether.'' As Bock (1999) noted, ``If the origin of birds 1973), and a mountain of evidence has been mar- and the origin of ¯ight are tightly linked in this fash- shaled against endothermy in dinosaurs during the ion, then the available discussion of all specialists in last three decades (Morell 1996). vertebrate ¯ight is that the origin of avian ¯ight from Nevertheless, Ostrom (1976) reconstructed the Ar- the ground up is exceedingly improbable, which chaeopteryx skeleton to closely resemble that of the would fatally weaken the dinosaur ancestry of known theropods; it was a terrestrial predator, and birds.'' Prum (2002) has now established an even more elaborate evolutionary scenario for the evolution of 1 E-mail: [email protected] ¯ight and feathers in birds, involving the origin of 1187 1188 Commentary [Auk, Vol. 119 feathers from ``dino-fuzz'' ®laments (Feduccia 1999) ery of feathered dinosaurs in Nature featuring a as an insulatory mechanism (but with no evidence painting of two feathered dinosaurs from China (Ji for endothermy in dromaeosaurs), the origin of a et al. 1998), named Caudipteryx and Protarchaeopte- ¯ight morphology in a terrestrial setting, and then ryx. The paper was followed quickly by the pro- the ®nal achievement of ``¯ight'' from the trees nouncement by Nature editor Gee (1998) that ``the de- down. That complex scenario, involving endothermy bate is over'', presaging the comment by Prum (2002: and preadaptations, has no name as yet. The last at- 13) that, ``it is time to abandon debate. ...''But,not tempt to explain ¯ight in dinosaurs was the ``ground so fast. That particular cladistic analysis was based down'' theory, a version of trees down, but involving on a sloppy analysis of some 90 characters, of which jumping from rocks or leaping from cliffs, which nearly half were primitive and nearly half were not was, in essence, a version of the trees down theory, present in the fossil taxa. The remaining characters biophysically. An arboreal ¯ight origin from thero- included sutured, rather than ligamentous, quadra- pods was previously suggested by Chatterjee (1997) tojugal-quadrate contact, quadratojugal contact with and Xu et al. (2000). the squamosal, and presence of obturator process of In 1986, Gauthier codi®ed the theropod origin of the ischium, and in Caudipteryx all of these features birds in a cladistic context and it has been the dogma are ambiguous. They claimed serrated teeth were of vertebrate paleontology and the popular press present in Protarchaeopteryx, but none of those ex- since that time. Although most ®elds of science are amining the actual specimen, including myself, struggling with methodology, systematics (and par- could see any serrations, and the photograph in Na- ticularly vertebrate paleontology) has adopted a sin- ture shows no serrations. And, in the original de- gle, inviolate approach to establishing phylogeny. scription (Ji and Ji 1997), a special point is made that Statements by Prum (2002:4) such as, ``it is univer- Protarchaeopteryx teeth are characterized by the lack sally agreed,'' and ``conclusive evidence of the stron- of serrations. Jones et al. (2000a) have subsequently gest possible,'' and ``wealth of and increasing shown conclusively that these taxa are secondarily strength of the evidence'' (p. 5) characterize the zeal ¯ightless birds, ``Mesozoic kiwis,'' and have nothing of the new school of cladism. to do with dinosaurs. The problem, of course, is that if the generated Although Prum (2002) makes it sound as though phylogeny is wrong (e.g. clades of hesperornithi- the debate largely centered on the origin of ¯ight forms, loons and grebes; clades of hawks and owls, coupled to bird origins, that is not the case, and I cer- Cracraft 1982, 1986; lung®sh and tetrapods, Rosen et tainly agree that the discovery of small theropods ca- al. 1981), then the cladistic inference can lead to di- pable of tree-climbing renders certain aspects of that sastrous effects. Classic examples include the argument moot. Yet, one must realize that some 30 ground-up origin of ¯ight (now on display in many years were spent by a large number of paleontolo- general biology texts), and its corollary, the ground- gists writing scores of papers trying to make a con- up origin of pterosaurs (Padian 1983), now complete- vincing argument for a ground-up origin of ¯ight ly rejected (Unwin and Henderson 2002). Philoso- (Padian and Chiappe 1998a, b). Most recently Padian pher David Hume urged that one should hold it more and Chiappe (1998a) erroneously portray the arbo- likely that one had been deceived than the laws of real and highly skilled ¯ier Confuciusornis as a ter- nature should stand suspended (Close 1993). In the restrial, feathered predator in a preposterous pos- ®nal analysis we must test hypotheses of homology ture which adorned the cover of Scienti®c American. by examining individual characters independently Olson (2000:839) set the record straight in a review of cladistic hypotheses to avoid the circular reason- of a monograph on the famous Chinese bird by ing engendered by that approach. Chiappe et al. (1999): The new version of cladistics can be termed a cla- do-phenetic approach; it involves coding vast num- The authors, steeped in cladistic fundamentalism, bers of primitive and derived characters, often to have been among the more insistent proponents of conform to the preconceived phylogeny advocated the origin of birds from theropod dinosaurs, with its attendant corollaries, such as the origin of ¯ight by the investigator. As James Clark (1992:533) noted, fromthegroundup....This paper will stand as an ``similarity lies in the eye of the beholder, and the exemplar of manipulation of information to conform particular hypothesis being advocated strongly col- to preconceived ideas, but it is otherwise insuf®- ors perceptions of morphological resemblance.'' ciently credible or comprehensible to constitute a Whatever the case, cladistics is incapable of recog- lasting addition to knowledge.
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