For the Rock Record: Geologists on Intelligent Design

four · Pigeonholing the “Dino-birds”

59 f o u r · Pigeonholing the “Dino-birds” allison r. tumarkin-deratzian INTRODUCTION One of the central claims of the intelligent design movement is that certain features of biological organisms are “irreducibly complex,” with such tightly integrated components that removal of any one part renders the system incapable of functioning (Behe 1996). The existence of irreducibly complex structures has been repeatedly put forth as evidence against evolution via natural selection, on the grounds that such a system could not be assembled incrementally over time. Although most irreducible complexity arguments deal with the molecular and cellular levels, such as the vertebrate blood-clotting response or the bacterial flagellum (Behe 1996), a common organismal-level target has been the evolution of feathers and flight in birds. An in-depth refutation of the supposed irreducible complexity of the avian flight apparatus has been outlined by Gishlick (2004), on the grounds that the fossil record preserves evidence of multiple stages in the evolution of feathered flying birds from nonfeathered, earthbound dinosaurs. The purpose of this essay is not to repeat those arguments. Gishlick’s case is eloquently presented, but he is largely preaching to the choir. Those who accept the existence of “dino-birds” need little convincing that the avian flight apparatus is not irreducibly complex. On the other hand, those who doubt the very existence of a dinosaur-bird transition will see no logic in the argument against irreducible complexity. At the most basic level, a key issue in the public mind has remained 60 · a l l i s o n r . t u m a r k i n - d e r a t z i a n the same since the mid-nineteenth century, when Archaeopteryx from the Late Jurassic of Germany was first described (Meyer 1861). Are there convincing intermediate forms that link reptiles and birds? From the time of its initial discovery, Archaeopteryx has been regarded as an important player in the story of bird evolution, because it possesses features originally believed unique to birds (e.g., feathers) and other features more characteristic of traditional reptiles (e.g., a long bony tail, teeth, clawed fingers). So similar is Archaeopteryx to certain small theropod dinosaurs, that one specimen lacking the famous feather impressions was originally misidentified as the dinosaur Compsognathus (Wellnhofer 1974). As fossils of other extinct birds have been discovered, it has become clear that “reptilian” features are not unique to Archaeopteryx. All modern birds lack teeth, but several lineages of fossil birds possessed toothed jaws. Confuciusornis from the Early Cretaceous of China retains claws on its wings. Until recently, however, there remained one feature that reliably separated Archaeopteryx and other early birds from traditional dinosaurs—feathers. Beginning in the mid-1990s, the dividing line between theropod dinosaur and bird became noticeably blurry. Spectacularly preserved fossils of small theropods from Cretaceous rocks in Liaoning, China, showed traces of feathers or featherlike features. The first “feathered dinosaur” to be described (somewhat ironically, as a bird) was Sinosauropteryx (Ji and Ji 1996). Although Sinosauropteryx lacks true feathers, it exhibits a filamentous body covering comprising structures that have been widely interpreted as protofeathers. Since then, true feathers similar to those seen in Archaeopteryx and modern birds have been described from several other small theropod dinosaurs, such as Caudipteryx (Ji et al. 1998) and Microraptor (Xu et al. 2003). It is now apparent that feathers can no longer be considered features unique to birds. Microraptor even has asymmetrical flight feathers identical to those of modern birds, suggesting that the capacity for flight may have evolved in nonavian theropod dinosaurs. As discoveries of additional feathered dinosaurs further blur the traditional distinction between small theropods and birds, one might expect increased public acknowledgment of an evolutionary connection between dinosaurs and birds. This, however, has not necessarily been the case. Even as more and more feathered theropods are reported, antievolutionary arguments have continued to focus primarily on Archaeopteryx, and the supposed absence of intermediate or transitional forms in the fossil record. Until recently, the feathered theropods of China have been largely discounted as irrelevant to the discussion. This is mostly a result of the age of the Chinese fossils in relationship to the age of Archaeopteryx, combined p i g e o n h o l i n g t h e “ d i n o - b i r d s ” · 61 with a widespread misunderstanding of the way in which...