The Cretaceous Short Armed Alvarezsauridae

NORELL, AND JAMES M. CLARK The discovery of Mononychus olecranus (Perle et al., 1993a later emended to Mononykus olecranus, see Perle et al., 1993b) from the Late Cretaceous of the Gobi Desert stimulated a fruitful new discussion about the origin and early diversification of birds and the evolution of their flight. Mononykus not only led to the recognition of a previously unknown clade of theropod dinosaurs but also suggested, through its phylogenetic placement as the...

The Significance of the Feathered Chinese Dinosaurs

The 1990s will go down in history as a time when one of the most significant fossil deposits ever discovered was brought to light. The Lower Yixian Formation (Chaomidianzi Formation of some) and allied rock units in western Liaoning Province, People's Republic of China, have yielded a wealth of fossil vertebrates, preserving in often astounding abundance an entire fauna in all its diversity (Luo, 1999 Swisher et al., 1999). The basal birds from these deposits are discussed in this volume by...

Lawrence M Witmer

Ever since the time of Huxley (1868), the Ijgjjlllj debate on the place of birds within vertebrate Ipigfes phylogeny has been one of the highest-profile and hotly contested of all evolutionary debates. What makes the origin of birds so contentious Why can we not agree and then move on to other issues Will we ever regard the problem as solved It would seem that establishing the ancestry of birds would be a relatively simple, empirical task, but, despite abundant data, the debate rages like never...

The Status of the Theropod Hypothesis

The only explicit hypothesis for the phylogenetic relationships of birds states that avian ancestry is fully embedded somewhere within the nexus of maniraptoran theropod dinosaurs, probably nearest to Dromaeosauridae and or Troodontidae among known groups. As such, it is the only game in town. As mentioned previously, opponents of theropod relationships have regarded the hypothesis as basically an unfortunate outcome of sloppy application of phylogenetic systematics. However, the idea was...

The Significance of Protoavis

A particularly difficult question is whether this ensemble cast should rightly include the Texas fossils known as P.tex-ensis. These fossils, whose discovery was announced only in 1986, have had a troubled and controversial history. In a long series of published works, Sankar Chatterjee (1987, 1988, 1991,1995,1997a, 1998b, 1999) argued that the Protoavis fossils are not only those of a bird but from a bird that lived 75 million years before Archaeopteryx The fossils are generally attributed to...

Unenlagia

One of the more newsworthy discoveries of 1997 was the announcement by Novas and Puerta of a theropod from Patagonia, Unenlagia comahuensis, that exhibits several features of the shoulder and pelvis that are otherwise known only in Avialae among theropods. Unfortunately, Unenlagia is known only from a single partial skeleton. These features include the presence of a hypopubic cup on the caudal surface of the distal end of the pubis, the presence of a pubic apron, and a laterally oriented...

Cladistics and the Fossil Record of Theropods

Critics of the hypothesis that birds are theropod dinosaurs have claimed that the stratigraphic record supports their view, by indicating that the relevant theropod taxa occur too late in time. Feduccia 1996 90 , for example, states, Birds are supposed to be derived from the coelurosaurian dinosaurs, and the form initially used to relate dinosaurs to birds, Deinonychus, is from the early Cretaceous, some 40 million years after Archaeopteryx, and the most birdlike dinosaurs are from the late...

Cladistic Approaches to the Relationships of Birds to Other Theropod Dinosaurs

NORELL, AND PETER J. MAKOVICKY 3pa For many years the study of bird origins , focused on Archaeopteryx, with its feathers is taken as conclusive proof of its avian status, its long, bony tail and toothed jaws pro claiming its intermediate position between birds and their reptilian ancestors, and its geological antiquity sufficient to qualify for ancestral status. Its evolutionary origin thereby became equated with the origin of all birds. This simplistic view was...

Cladistic Analyses of Coelurosaurian Theropods

Cladistics was developed as a formal analysis only within the past three decades, so the relationships of extant birds to fossil archosaurs were not subjected to cladistic analysis until the mid-1980s e.g., Thulborn, 1984 Gauthier, 1986 . But the groundwork of comparisons was laid by Ostrom 1976 , who cataloged the many similarities of Archaeop-teryx to theropod dinosaurs, dromaeosaurids in particular, and explicitly interpreted them in terms of synapomorphies shared derived characters . The...

The Enigmatic Birdlike Dinosaur Avimimus portentosus Comments and a Pictorial Atlas

Avimimus Anatomy

CHIAPPE, AND SERGEI KURZANOV 3pa The phylogenetic position of Avimimus is a Siijfc - puzzle that is yet to be solved. Since its an- is nouncement by Kurzanov in 1981, Avimimus ' f has been alternatively regarded as a nonavian theropod close to the ancestry of birds e.g., Thulborn, 1984 Norman, 1990 Holtz, 1994 Currie, 2000 or as a flightless, basal avian e.g., Chatterjee, 1991, 1995, 1999 . It is hoped that the comments and the high-quality stereophotographs...

Systematic Paleontology

Diagnosis Laterally compressed synsacral vertebrae concave and convex cranial and caudal articular surfaces of synsacrum, respectively short coracoid, wider than it is tall, lacking bicipital tubercle sternum stout and subtriangular in section humerus with prominent, proximally projected ventral tubercle and a single condyle ulna with a hyper-trophied olecranon process and a single cotyla large and proximally expanded radiocarpal facet of radius hyper-trophied and strongly depressed metacarpal...

The Relationship of the Origin of Flight to the Origin Of Birds

The origin of birds is, at its core, a matter of genealogy. That is, regardless of your systematic philosophy whether it be cladistic, phenetic, or eclectic avian ancestry is a question of phylogeny, or, more precisely, phylogenetic reconstruction. For most biologists, phylogenetic reconstruction has become more or less synonymous with phylogenetic sys-tematics or cladistics, whereby the distribution of attributes among taxa forms the primary raw data used to develop hy potheses of...

The Status of Alternatives to the Theropod Hypothesis

There is no question that the theropod origin of birds is by far the most popular hypothesis on avian ancestry. The question then arises,Are there credible alternatives In most previous reviews e.g., Ostrom, 1976 Gauthier, 1986 Wit-mer, 1991 Feduccia, 1996 Padian and Chiappe, 1998b , the debate on avian origins was divided into three competing hypotheses 1 the theropod hypothesis, 2 the crocodylo-morph hypothesis, and 3 the basal archosauriform or thecodont hypothesis. However, in recent years...

Preface

Less than two years after the 1859 publication of Charles Darwin's Origin of Species, the first skeletal specimen of Archae-opteryx was discovered a nearly perfect evolutionary missing link and since that time, the origin and early history of birds have been prime topics of debate among natural historians and evolutionary biologists De Beer, 1954 Desmond, 1982 Elzanowski, Chapter 6 in this volume . Not much later, the paleontological exploration of the American West resulted in the discovery of...