Birds are dinosaurs: Simple answer to a complex problem

Auk, The, Oct 2002 by Feduccia, Alan

According to Prum (2002) the temporal paradox is reduced to some 24 my. However, he assumes that Archaeopteryx is the first bird; but contrarily, Archaeopteryx is a well-developed avian in many anatomical features as well as in wing and feather structure. But, for discussion let us assume that there is some 24 my separating the two groups, with Archaeopteryx as the first bird. Such a gap would be close to half the time period of the age of mammals, and when we consider that whales evolved from land ungulates in a time frame of some 5 to 8 my, the time disparity becomes even more astounding.

As for the Chinese fossils, there has been attempt after attempt to make the fossil deposits older and older, but they still date, at the earliest, to an average of about 120-124 Ma, still some 26 to 30 my after Archaeopteryx. It is certainly possible that dromaeosaurs date earlier than that time, but given the advanced avian state of Archaeopteryx, it is also equally probable that early birds are much earlier than Archaeopteryx.

Of greater interest, however, is the fact that there are many dinosaurs from the Jurassic and Triassic periods, but none of the Triassic theropods (or protheropods such as Herrerasaurus, Eoraptor) have any major birdlike features. For example, the late Triassic Coelophysis has the typical theropod, laterally flattened serrated teeth, greatly elongated cervical ribs, five sacral vertebrae, an astragalocalcaneum with no ascending process of any development (although considered a synapomorphy of Theropoda), no pubic boot, and a carpal region with seven carpals (no semilunate bone), as in Herrerasaurus.

The pleas by paleontologists and now Prum (2002) for ghost lineages of unknown dinosaurs is possible, but the dinosaurs are among the best known Mesozoic vertebrates. For example, there are 173 dinosaur discoveries (some duplication) from the Late Triassic, 130, 128, and 318, from the Early, Middle, and Late Jurassic, respectively; and 399 from the Early Cretaceous (information from DinoData, see Acknowledgments). This is not a meager vertebrate fossil record! From the famous Late Triassic Ghost Ranch in New Mexico, full or partial skeletons of some 1,000 individuals of the basal theropod Coelophysis have been recovered and a similar concentration of the related Syntarsus were discovered in Zimbabwe. Prum (2002) accuses those who advocate an earlier common ancestry of birds and dinosaurs from early archosaurs of dreaming up hypothetical ancestors, but the hypothetical "ghost" lineages of earlier dromaeosaurs that would qualify as avian ancestors are equally imaginary, and by whatever time span those unknown lineages are pushed back, one must equivalently push back the forebears of Archaeopteryx.

Finally, I feel it necessary to comment on the "thecodonts," which Prum (2002) characterizes as "a polyphyletic garbage bag assemblage of early archosaurs." Contra Prum (2002), it was in fact South African Robert Broom, not Heilmann, who established the idea that birds and dinosaurs shared a common ancestor among the Thecodontia. As for the thecodonts (basal archosaurs with a antorbital fenestra), if they are poorly defined, then certainly the same could be said of the Dinosauria and its various subgroups. There has been a recent revival of interest in the famous Early Triassic thecodont Euparkeria, and Welman (1995) has discovered a suite of avian-like anatomical features in the basicranium. Paul (2002: 179), an ardent advocate of the "birds-are-dinosaurs," and more recently, "dinosaurs-are-birds" school, admits that, "Euparkeria is a suitable ancestral type for birds ... and ... Euparkeria is a good ancestral type for all archosaurs." I would suggest that taxa most suitable for avian ancestry are likely to fall morphologically beyond Euparkeria, but prior to the appearance of the earliest and most primitive theropods. However, I do not expect some massive cladistic analysis to resolve this problem. Carroll (1988: 8) emphasized that, "When attempting to establish relationships of any group within the fossil record, we must emphasize the earliest known members, because they have had the shortest amount of time to evolve new characters since their initial divergence. Hence, they should provide us with the best opportunity to identify the derived features that they share with their closest sister group."