Birds are dinosaurs: Simple answer to a complex problem

Auk, The, Oct 2002 by Feduccia, Alan

According to Sereno (1997), the most notable cranial character (synapomorphy) present throughout the Dinosauria is the loss of the postfrontal bone, but that feature is also found in the Crocodilia. Likewise, the Saurischia is equally difficult to define, especially given the large suites of theropod characters in such "thecodonts" as Postosuchus (Chaterjee 1985). The theropod-like Postosuchus, if included in a theropod cladistic analysis would probably crank up the phylogenetic ladder to about the ceratosaur level. Definitions of dinosaurian subgroups are equally elusive, and most workers have abandoned trying to define coelurosaur, the group for years said to include avian ancestors. Illustrating the difficulty of defining dinosaur groups, Carroll (1988:290) pointed out, "The 'canosaur' families may each have evolved separately from different groups that have been classified as coelurosaurs."

If, as proposed by Paul (2002) and Czerkas et al. (2002), dromaeosaurs are actually birds, either flightless or becoming flightless, then the question of bird origins is again completely reopened. But aside from the obvious problems, that proposal has major implications because both camps in the debate would have portrayed dromaeosaurs incorrectly, and as Czerkas et al. (2002:120) note, "cladistics has presented a highly misleading interpretation of the evidence," and (p. 122) "The origin of birds stems further back to a common ancestor of pre-theropod status." If correct, whatever the case, the presence of Cryptovolans as a dromaeosaur with fully developed flight feathers, an avian style hand and sternum, dromaeosaur teeth, sickle claw, and a stiffened dromaeosaur (rhamphorhyncoid) tail, should send all those involved in the debate on bird origins back to the drawing board. Conclusion.-The origin of birds, like the origin of all the groups of Mesozoic archosaurs, is a difficult problem to resolve, with all major groups diverging from basal archosaurs in the Triassic. Like the intractable problem of pterosaur origins, the precise nature of bird origins from archosaurs, despite current dogma, still remains elusive. Much more attention should be given the small basal archosaurs of the Triassic, following the excellent study by Benton (1999) of the small archosaur Scleromochlus, variously thought to position phylogenetically as either a basal sister group of Pterosauria or of Dinosauromorpha. This small archosaur shows many birdlike features, as does Megalancosaurus, Longisquama, and Coesasaurus, and Benton (1999) established a new group name, the Avemetatarsalia ("bird feet") for the clade consisting of Scleromochlus and Ornithodira.

In the Encyclopedia of Dinosaurs (Currie and Padian 1997), Padian (p. 178) notes that he and May (1993) proposed "to define Ornithischia as all those dinosaurs closer to Triceratops than to birds, and Saurischia as all dinosaurs closer to birds than to Ornithischia. Dinosauria is defined as all descendants of the most recent common ancestor of birds and Triceratops;" and (p. 494), "Ornithischia and Saurischia are sister taxa, stem groups of Dinosauria; hence any member of Dinosauria must belong to one of these two groups ." What about the basal archosaurs Postosuchus (nearly a theropod), Ornithosuchus, or Lagosuchus and Lagerpeton (sometimes included in Dinosauria, but not belonging to either the Ornithischia or Saurischia)? And, if those forms are included, it means that the vast majority of characters used in theropod-bird cladograms are primitive. Most workers are reasonably happy with the validity of the lineage leading to theropods, with basal forms characterized by such late Triassic forms as Eoraptor and Herrerasaurus, but our knowledge of the relationships of sauropods and ornithischian dinosaurs is strictly limited. So, too, is our knowledge of basal archosaurs (thecodonts). If the thecodonts can be termed a "garbage bag assemblage" (Prum 2002:1), then why not the Dinosauria?