Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia (2002)

Auk, The, Apr 2003 by Prum, Richard O

Feduccia's (2002) conclusion that feathered theropods present a new challenge to all workers in the field is disingenuous, because advocates of the theropod origin have hypothesized that dromaeosaurs were feathered for several years (Ji et al. 1998, 2001; Padian 1998, 2001; Prum 1999; Sereno 1999; Xu et al. 2000, 2001; Norell et al. 2002; Prum and Brush 2002). Feduccia never entertains the possibility that new evidence fulfilling a major prediction of the theropod origin of birds could be interpreted as definitive support for this hypothesis of evolutionary history. There are now more than a dozen nonavian theropods with feathers, and they belong to a diversity of theropod groups including oviraptorisaurs, therizinosaurs, alvarezsaurids, and basal coelurosaurs (Prum and Brush 2002). If all those feathered theropods were included in birds, as Feduccia is apparently prepared to do, it would only contribute to the indisputable phylogenetic relationship between theropods and dinosaurs (Prum and Brush 2002). Admitting that dromaeosaurs are feathered and closely related to birds makes it impossible to argue that birds are not theropod dinosaurs.

Does the discovery of asymmetrical flight feathers on the forelimbs and hind limbs of dromaeosaurs affect the debate about the evolution of avian flight. Doubless the four-winged dromaeosaurs Microraptor gui (Xu et al. 2003) and Cryptovolans (Czerkas et al. 2002) may revolutionize our understanding of the origin of avian flight (Prum 2003). Specifically, wing morphology of the four-winged theropods fulfill Feduccia's (2002: fig. 6) ideal of a gliding progenitor for avian flight, and demonstrate that there is no necessary conflict between the theropod origin of birds and a gliding origin of flight (Chatterjee 1997, Prum 2003, Xu et al. 2003). It appears that the avian flight apparatus may have evolved from a combination of terrestrial-cursorial adaptations that were exapted into the flight stroke, and subsequent selection for aerial gliding locomotion (Prum 2003). Those specimens do not adversely affect the hypothesis of theropod phylogeny and avian origins and are perfectly good dromaeosaurs, complete with, for example, the "killer claw" on the second toe, serrate teeth, and bony supporting rods in the tails. Obviously, future phylogenetic analyses including new data and taxa may change the topology of the theropod tree, but none of those findings places any doubt on the placement of birds within the theropod dinosaurs. As I concluded in my Perspectives, the theropod hypothesis provides an increasingly detailed and coherent perspective on the origin of birds. Now, the theropod hypothesis provides new support for Feduccia's preferred gliding hypothesis for the origin of flight, rendering moot his frequent claim that theropods are functionally inappropriate to be avian ancestors.

DIGIT HOMOLOGY

As I discussed in my Perspectives, the phylogenetic hypothesis for dinosaurs leads unambiguously to the conclusion that the digits of the bird hand are digits 1-2-3. Feduccia and classical developmental biologists have long maintained that the digits of the hand of birds were digits 2-3-4 because they develop in positions distal to the metacarpals 2-3-4 (Burke and Feduccia 1997; Feduccia 1999a, 2002). Wagner and Gauthier (1999) proposed the frame-shift hypothesis as a solution to the apparent conflict. The hypothesis states that theropods, including birds, evolved to develop digits 1-2-3 distal to the metacarpals 2-3-4.