Title page illustration: Archaeopteryx lithographica by Charles Knight (courtesy Los Angeles County Museum of Natural History).

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Library of Congress Cataloging-in-Publication Data

Mesozoic birds : above the heads of dinosaurs / edited by Luis M. Chiappe and Lawrence M. Witmer. p. cm. Includes bibliographical references and index. ISBN 0-520-20094-2 (alk. paper) 1. Birds, Fossil. 2. Paleontology—Mesozoic. 3. Birds—Evolution. I. Chiappe, Luis M. II. Witmer, Lawrence M.

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The Debate on Avian Ancestry Phylogeny, Function, and Fossils

LAWRENCE M. WITMER

Ever since the time of Huxley (), the Avian Paleontology and Evolution in Frankfurt (Peters, debate on the place of birds within vertebrate ), Washington (Olson, ), and Beijing (Shi and phylogeny has been one of the highest-profile Zhang, ), respectively. The late s saw the publica- and hotly contested of all evolutionary de- tion of two important and high-profile reviews of early bates. What makes the origin of birds so contentious? Why avian evolution in the science weeklies (Chiappe, ; can we not agree and then move on to other issues? Will we Feduccia, ), a seminal and wide-ranging review (Padian ever regard the problem as solved? It would seem that and Chiappe, b), a major symposium with subsequent establishing the ancestry of birds would be a relatively sim- volume (Gauthier and Gall, ), and a host of books di- ple, empirical task, but, despite abundant data, the debate rected toward a broader audience (Feduccia, , b; rages like never before. I previously published a fairly ex- Chatterjee, a; Dingus and Rowe, ; Shipman, a; haustive review of the history of the controversy (Witmer, Paul, ). ), which has been updated and expanded by Feduccia Most important, however, are the many newly discovered (, b) and Padian and Chiappe (b). Rather than testaments of the avian transition coming from the fossil repeat the historical chronicle, this chapter seeks to explore record. For example, as documented in this volume, many the impact of more recent developments on the debate and new fossil birds relevant to avian origins have recently been also to touch on the allied debate on the origin of avian described, primarily from Spain, China, Argentina, Mada- flight. At the time of completing my previous review, it is gascar, and Mongolia, and, from Germany, even new speci- fair to say that the theropod hypothesis was the most widely mens of Archaeopteryx. Likewise, a number of new birdlike held notion, and other ideas (e.g., basal archosaurs, croco- theropods have come to light, such as Sinornithoides youngi dylomorphs) were on the decline. There simply was little (Russell and Dong, ), Unenlagia comahuensis (Novas substantial opposition to birds being dinosaurs. and Puerta, ), Sinornithosaurus millenii (Xu et al., In the intervening years, however, a great deal has hap- b), Caudipteryx zoui (Ji et al., ), Bambiraptor fein- pened. Protoavis texensis moved from the popular arena to bergi (Burnham et al., ), Nomingia gobiensis (Barsbold the scientific arena with the publication of Sankar Chatter- et al., a,b), Sinovenator changii (Xu et al., ), and jee’s  monograph (see also Chatterjee, , a, Microraptor zhaoianus (Xu et al., ), as well as important b, ;Witmer,b). Alick Walker () published new specimens of previously known taxa such as Troodon a large monograph on Sphenosuchus acutus in which he re- formosus (Currie and Zhao, ), Deinonychus antirrhopus newed his support for the relationships of birds to croco- (Witmer and Maxwell, ; Brinkman et al., ), Veloci- dylomorphs. Relationships of the basal archosauriform raptor mongoliensis (Norell and Makovicky, , ; Euparkeria capensis, previously little more than a historical Norell et al., ), oviraptorids (Clark et al., ), and or- footnote, resurfaced in a paper by Johann Welman (; nithomimosaurs (Pérez-Moreno et al., ; Makovicky and but see Gower and Weber, ). Various conferences pro- Norell, ; Norell et al., ), among others. vided a forum for discussion of new hypotheses and per- The debate heated up considerably with the publication spectives, such as the  meeting of the International in  of Alan Feduccia’s The Origin and Evolution of Birds. Ornithological Congress in Vienna (Bock and Bühler, ) In this book, Feduccia launched a vehement attack on the and the , , and  meetings of the Society of prevailing consensus that birds are nested within Thero-

3 poda, opening the door to other studies purporting to cast biology of their antecedents. For example, if birds are de- doubt on the theropod relationships of birds (e.g., Burke scended from theropod dinosaurs, then we might feel justi- and Feduccia, ; Ruben et al., ; Feduccia and Martin, fied in reconstructing nonavian theropods with a whole ; Martin, ; see also Thomas and Garner, ). suite of avian attributes, from feathers and endothermy Some of the scientific points discussed by Feduccia and his (Paul, ) to reproductive biology (Varricchio et al., ; colleagues will be taken up in this chapter, but it is worth- Clark et al., ) and locomotor attributes (Gatesy, ; while to examine briefly the tone the debate has taken and Gatesy and Dial, a). Indeed, there may be compelling direction to which it has turned because they cannot help reasons to do so, and, according to the inferential hierarchy affecting the science. The initial response to the volume of Witmer (a), these might be reasonable level II infer- (e.g., Norell and Chiappe, ; Padian, ) was highly ences. If the prevailing orthodoxy is correct, then all di- critical of Feduccia’s apparent disregard for the recent nosaurs are not extinct, and we thus have the potential to say cladistic analyses that argue for the theropod relationships a lot about even the extinct clades of dinosaurs. If, however, of birds, although several later reviews were either mixed birds have no close relationship with dinosaurs, then all of (Sereno, a; Witmer, b; Steadman, ) or highly a sudden dinosaurs seem more remote, less familiar, and favorable (Bock, ;Mayr,;Ruben,). The popu- perhaps even less interesting. Feduccia correctly noted that lar press was quick to provide an opportunity for the players the theropod hypothesis provides “a mechanism by which to voice strong opinions. Among the many barbs slung back you can vicariously study dinosaurs by stepping into your and forth to reporters were “paleobabble,” “total garbage,” backyard. There’s a real emotional investment here” “pure Fantasyland,”“the greatest embarrassment of palaeon- (quoted in Zalewski, :). In fact, for studies that em- tology of the th century,”“absurdity,”“poisoning his own ploy the extant phylogenetic bracket approach to make in- discipline,”“as impervious to evidence as the fundamental- ferences about extinct archosaurs (e.g., Witmer, a,b, ists,”“beyond ridiculous,”“just hot air,”“bombastic rhetoric a, a; Rowe, ; Hutchinson, a,b), it is partic- and armwaving,”“like taking candy from a baby,”and “that’s ularly fortunate and useful for birds to be dinosaurs because always the problem, these paleontologists just don’t know then extant birds and crocodilians together bracket a huge birds.” In order not to further the ad hominem tone of diversity of archosaurs. If birds are instead more closely re- the discourse, these remarks will remain unattributed here, lated to, say, crocodylomorphs, then our inferential base but the reader may consult McDonald (), Zalewski with regard to other archosaurs is weakened significantly. (), DiSilvestro (), Morell (), and Shipman Thus, resolution of the problem is critical for studies not (b).Clearly,the vituperative and combative turn that the just of birds and theropods but really of all archosaurs. debate has taken is likely to do little to advance the science— This chapter examines a variety of issues surrounding bombast is a poor substitute for evidence. the debate. In all cases, the goal is to discover how that par- Perhaps a more important question is, Why is the origin ticular issue relates and contributes to the resolution of the of birds so important that professional scientists would question of avian origins. The next section takes up the cen- make such strong public statements? In other words, what tral role that Archaeopteryx has played in the debate, and the is the larger meaning of the debate? It is impossible to es- following section briefly examines the controversial taxon cape the fact that, as with any form of human endeavor, per- Protoavis. The discovery of an incredible fossil deposit in sonalities may collide (and the media will be there with a China holds great importance for understanding avian ori- microphone). But the scientific stakes really are quite high, gins and is discussed here. The relationship of the origin of and for at least two reasons: crownward inferences and flight to the origin of birds, which has reemerged as a criti- stemward inferences. First, the origin of birds is critical if we cal topic with the publication of Feduccia’s  book, will are to gain a deeper understanding of birds themselves. then be taken up. The question of alternatives to theropod Establishing the phylogenetic relationships of Aves is the dinosaurs will then be explored, followed by an assessment logical first step in a wide variety of inferences such as the of the status of the theropod hypothesis itself. evolution of flight, feathers, metabolism, and various eco- logical and physiological parameters. All these attributes The Centrality of Archaeopteryx in the Debate have a phylogenetic history, and thus we must know what came before birds in order to truly comprehend birds. If we The first sentence of the abstract of John H. Ostrom’s  are interested in, say, tracing the evolution of avian cranio- landmark paper states: “The question of the origin of birds facial kinesis, it makes a great difference whether birds are can be equated with the origin of Archaeopteryx, the oldest viewed as being derived from theropod dinosaurs (Chatter- known bird” (Ostrom, :). This statement reflects a jee, ) or from crocodylomorphs (Walker, ). very common sentiment, with, for example, Martin (: Second, there is a sense that if we can sort out the origin ) regarding Archaeopteryx as occupying “the center of birds, we will automatically know a great deal about the stage” and Feduccia (:) calling it an “avian Rosetta

4 LAWRENCE M. WITMER Stone.” Certainly, virtually all the ancillary issues surround- like characters (e.g., the boomerang-shaped element could ing avian origins—from the origin of flight (Padian, ; not be a furcula because Hesperornis, Palaelodus, and em- Rayner, ; Feduccia, , ;Herzog,; Gatesy and bryonic birds have unfused clavicles). Lowe’s views on avian Dial, b) to feathers (Parkes, ;Dyck,;Griffiths, origins were complex and somewhat idiosyncratic (Witmer, ), endothermy (Ruben, , ), and others—either ) and were refuted to most people’s satisfaction by take Archaeopteryx as their starting point or use it as the Simpson () and de Beer (, ). ruler against which particular scenarios are measured. A Although it is tempting to marginalize Lowe’s views be- high-profile international meeting was devoted entirely to cause they were expressed so long ago, a number of modern Archaeopteryx, and, as the editors of the subsequent volume workers have questioned the avian status of Archaeopteryx. noted (Hecht et al.,:),it probably was “the first time that For example, in a series of papers from  to ,R.A. a scientific conference was devoted to a single fossil species.” Thulborn moved progressively closer to the conclusion that Martin (:) is no doubt correct in noting that “there are Archaeopteryx was a theropod dinosaur of no particularly probably more individuals who have worked on Archaeop- close relationship to birds (Thulborn, , ; Thulborn teryx than all other palaeoornithologists put together.” and Hamley, ); his  paper presented a cladogram in But Archaeopteryx is more than a series of scientific which tyrannosaurids, an ornithomimosaur-troodontid specimens (see Elzanowski, Chapter  in this volume). clade,and Avimimus were closer to birds than was Archaeop- Archaeopteryx has reached iconic status, partly because of teryx. He denied virtually all characters that unite Archaeop- the beauty of the specimens and partly because they have teryx with true birds to the exclusion of other theropod been important and prominent documents in establishing groups, noting that furculae were present in a variety of the fact of organic evolution; certainly creationists have re- theropods and that the presence of feathers in Archaeop- garded Archaeopteryx as a serious challenge (e.g., Cousins, teryx “probably signifies nothing more than a rare circum- ). Archaeopteryx is a celebrity. A respected scientific stance of preservation” (Thulborn, :). Thus, for periodical bears its name. It is the logo for museums and Thulborn (:),“Archaeopteryx is not a convincing ‘in- graces the covers of numerous books and magazines. As a termediate’ between reptiles and birds, nor is it an ancestral fossil celebrity, Archaeopteryx is more similar to Australo- bird” because there are other theropods even more birdlike pithecus in inspiring a sense of respectful awe and reverence than Archaeopteryx. Likewise, Kurzanov (, ) was so than to “pop favorites”like Tyrannosaurus. It is indisputable struck by the birdlike features of the Cretaceous theropod that Archaeopteryx has historically been the central focus of Avimimus portentosus (see also Vickers-Rich, Chiappe, and virtually all studies on the origin and early evolution of Kurzanov, Chapter  in this volume)—not least of which birds,and this is likely to continue for the foreseeable future. was the inference of feathered forelimbs—that he regarded The obvious next question is: Is this reliance—perhaps the supposed avian features of Archaeopteryx as insufficient even overreliance—on Archaeopteryx justified and wise? evidence to establish it as a true bird. The avian status of Archaeopteryx is often an unquestioned A similar theme was elaborated somewhat earlier by the assumption of many analyses, and in some phylogenetic Mongolian paleontologist Barsbold Rinschen, who articu- studies (e.g., Padian, ; Holtz, ; Weishampel and lated a notion that has major implications for the interpre- Jianu, ), Archaeopteryx alone stands as a proxy, in a tation of the avian attributes of not just Archaeopteryx sense, for all birds. The concern obviously is that if we have but really all theropods. Barsbold’s () concept of “or- hung all our conclusions about avian evolution on a fossil nithization”in theropod evolution suggests that various lin- that is peripheral to avian origins, then much of the research eages of theropods independently evolved birdlike attrib- for the past century may have been misguided (Witmer, utes but with no clade possessing the entire suite of avian ). The stakes could not be higher. Looking carefully apomorphies.Thus,Archaeopteryx is seen by Barsbold () back over the history of this debate reveals a small but con- as potentially just one of several parallel, “ornithized” line- tinuous thread of dissent (see Witmer, ). P. R. Lowe ages, “aberrant” in and of itself yet otherwise showing the (, ), for example, considered Archaeopteryx to be underlying affinity of birds and theropods. In fact, it was too specialized to be ancestral to later birds. Moreover, he conceivable to Barsbold () that perhaps more than one went so far as to claim that Archaeopteryx “represented group of “ornithized” theropods crossed the line into the culminating attempt of the reptiles toward flight, that is “birds” and that neornithine birds may actually be di- to say, it was a flying dinosaur. This, of course, implies a phyletic (reminiscent of Lowe) or even polyphyletic. It is belief in the diphyletic origin of feathers—a zoological not clear what kind of evolutionary process Barsbold envi- transgression for which I expect no mercy” (Lowe, : sioned that would produce the recurrent evolution of avian –). Lowe (:) denied any features that were features, but the fact that avian features have arisen repeat- “definitely avian as opposed to dinosaurian,” and he spent edly and independently in theropod evolution now seems much of the  paper attempting to refute the most bird- to be an inescapable conclusion.

THE DEBATE ON AVIAN ANCESTRY 5 The preceding discussion, of course, begs the question of bird, because we have defined it as such, as a member of the just what is a bird: How do we recognize birds? How do we clade Aves. define them in both a scientific and a colloquial sense? The Nevertheless, nomenclature aside, given the number of foregoing has presumed a more colloquial sense of birds, birdlike theropods and theropodlike birds, what is it about that is, a sense of “birds” being feathered vertebrates that fly Archaeopteryx that leads us to conclude that it is truly a bird or had flight in their ancestry. This idea generally works in (i.e., by definition part of Aves) and thus worthy of all the the modern time plane but breaks down when evaluated attention? Long before the recent flurry of discoveries, over the fossil record of theropods because the attributes of workers recognized that Archaeopteryx possessed few extant birds were acquired sequentially. Thus, there is no uniquely avian characters that, in modern parlance, would sharp line demarcating bird and nonbird—the distinction contribute to a diagnosis of Aves (see Owen, ;Heil- has become entirely arbitrary. Defining taxa has emerged as mann, ). For example, de Beer (:) listed only four a major focus for phylogenetic taxonomists, and Archaeop- “features which differ completely from the condition in rep- teryx in relation to the definition of “Aves” and “birds” has tiles and agree with that of modern birds”: () a retroverted likewise become a central test case (see Gauthier, ;de pubis, () a furcula, () an opposable hallux, and () feath- Queiroz and Gauthier, , ; Padian and Chiappe ers. Of these, a pubis with at least some measure of apo- b; Sereno, , b; Padian et al., ; see also Clark, morphic retroversion is now well documented for dro- Norell, and Makovicky, Chapter  in this volume). The is- maeosaurids, basal troodontids, and therizinosauroids sues surrounding this matter are lengthy and complex, and (Barsbold, ; Barsbold and Perle, , ; Norell and they have been well discussed in the references just cited. Makovicky, , ; Rasskin-Gutman, ; Xu et al., Traditionally, Archaeopteryx has been regarded as both a ), and furculae are turning out to be very widely dis- bird and a member of Aves. For example, Richard Owen, in tributed indeed among theropods (Barsbold, ; Bryant his  description of the London specimen, “declare[d] it and Russell, ; Chure and Madsen, ; Norell et al., unequivocally to be a Bird, with rare peculiarities indicative ; Dal Sasso and Signore, ; Ji et al., ;Makovicky of a distinct order in that class” (Owen, :). Likewise, and Currie, ; Norell and Makovicky, ; Xu et al., Haeckel (, ), Huxley (), and other early taxon- a). Although the opposable hallux may still stand as omists referred Archaeopteryx and other Mesozoic birds to an avian apomorphy (Gauthier, ; Chiappe, ;Feduc- Aves. All modern ornithology texts regard Archaeopteryx as cia, ; Sereno, b; Forster et al., ), it is, of course, within their purview. This traditional sense can be captured the presence of feathers that, since its discovery in , has by modern phylogenetic taxonomy by defining the name garnered for Archaeopteryx a seemingly unshakable posi- “Aves” using a node-based definition: Archaeopteryx, Neor- tion within Aves. However, even the uniqueness of feathers nithes (“modern birds”), and all descendants of their most to birds was cast into doubt in  with reports of non- recent common ancestor (see Chiappe, , ; Padian avian theropod dinosaurs with feathers or featherlike fila- and Chiappe, b; Sereno, , b; Padian et al., ). mentous structures from Liaoning, China (Ji and Ji, , The colloquial term “birds” is usually applied to this same  [see also Gibbons, , a, reporting on the work group. These definitions of Aves and birds are the ones of Ji and Ji]; Currie, ; Chen et al., ; Ji et al., ; adopted generally for this volume. However, an alternative Xu et al., a,b). Although the true identity of the fila- nomenclature pioneered by Gauthier () advocates a mentous structures has been called into question (Brush et crown-group definition for Aves that encompasses just the al., ; Geist et al., ; Gibbons, b) and has been clades of living birds: ratites, tinamous, neognaths, and all highly controversial, the reports fueled the persistent spec- descendants of their common ancestor. Gauthier () ulation, if not expectation, that feathers might have been defined the term “Avialae” as more or less equivalent to the present in taxa outside birds (Gauthier, ; Paul, ). traditionally conceived Aves, yet he applied the colloquial The startling announcement in  (Ackerman, ;Cur- term “birds” to Avialae. The Avialae convention has seen a rie, ; Ji et al., ) of nonavian dinosaurs with indis- fair amount of use, and with good reason (see Clark, Norell, putable feathers truly has shaken the foundations of just and Makovicky, Chapter  in this volume). Nevertheless, at what it takes to recognize something as a bird (Padian, this writing, preferences seem to be tending toward the Aves ). The importance of feathered dinosaurs is taken up in convention adopted here (see Padian and Chiappe, b; a later section. Sereno, ; and Padian et al., , for justification). In- Without the uniqueness of opisthopuby, furcula, and terestingly, Padian et al. () retained the term “Avialae” even feathers, is there any basis for uniting Archaeopteryx but redefined it as a stem-based taxon comprising Neor- with birds to the exclusion of other archosaurian taxa? nithes and all taxa closer to them than to the dromaeosaurid Definitions can be constructed to suit personal tastes, and, Deinonychus. Thus,if we regard the terms “Aves”and “birds” regardless of whether one defines Aves to exclude (Gauthier, as being more or less synonymous, then Archaeopteryx is a ) or include (Chiappe, ) Archaeopteryx, it is rele-

6 LAWRENCE M. WITMER vant to ask just what features suggest that Archaeopteryx is sumably predate Archaeopteryx by perhaps millions of the outgroup to less controversial birds. As it turns out, years. For example, whether or not there were trees in the there are many such features. In addition to the reflexed hal- Solnhofen landscape suitable for Archaeopteryx to perch lux, other oft cited synapomorphies (see also Elzanowski, upon (Peters and Görgner, ; Feduccia, , ; Chapter  in this volume) include () an elongate prenarial Padian and Chiappe, a,b) bears little relevance for the portion of the premaxilla (Gauthier, ; Sereno, b); arboreal versus cursorial origin of flight. On the one hand, () the breaking down of the postorbital bar (Chatterjee, the presence of Solnhofen trees would not automatically , a; Sanz et al., ; Sereno, b); () the absence mean that Archaeopteryx either used them or evolved from of dental serrations and the presence in the teeth of a char- an arboreal ancestor. On the other hand, the absence of acteristic constriction (Martin et al., ; Gauthier, ; Solnhofen trees would not dictate that flight arose in a ter- Chiappe, ; Chiappe et al., ); () the enlargement of restrial context, because Archaeopteryx could well have the cranial cavity (Gauthier, ; Chatterjee, a); () the evolved from fully arboreal ancestors and apomorphically caudal tympanic recess opening within the columellar re- became terrestrial or just visited Solnhofen seasonally. We cess (Witmer and Weishampel, ; Chiappe et al., ); can never know these things. The point here is that we () the presence of a caudal maxillary sinus (Witmer, ; have tended to run all hypotheses through the filter of Chiappe et al., ; Chatterjee, a); () fewer tail verte- Archaeopteryx, almost as if we believed that it was truly the brae, with the prezygapophyses reduced distally (Gauthier, first bird, not just the oldest known or most basal bird. ; Chiappe, ; Chiappe et al., ; Chatterjee, a; The historical centrality of Archaeopteryx in the debate is Sereno, b; Forster et al., ); and () various modifi- quite understandable given that, until very recently, it was cations of the shoulder girdle (Gauthier, ; Feduccia, all we had—that is, it provided the only solid evidential ; Chatterjee, a), although some of the shoulder basis for hypothesis testing. Although most analyses con- characters may have a broader distribution (Novas and tinue to support the basalmost avian position for Archaeop- Puerta, ; Forster et al., ; Norell and Makovicky, ; teryx, recent discoveries of Cretaceous birds and bird- Xu et al., b, ). like theropods have considerably lightened the load that Some of the foregoing characters may seem a bit subtle, Archaeopteryx must bear in teasing apart the details of early even trifling, but such is the nature of any phylogenetic tran- avian evolution (see discussion in Witmer, ). From Un- sition as it becomes better and better known. In fact, we enlagia, Rahonavis, and Microraptor to Confuciusornis, should predict that the number of characters per node Sinornis, and Concornis, new finds are documenting the de- should decrease (and that the characters themselves may tails of both the phylogenetic and functional transition to well seem more trivial) as sampling of the fossil record im- birds. Archaeopteryx will always merit a special place in the proves. What is remarkable is that so many characters do minds (and hearts) of scientists and the public in general. affirm the avian status of Archaeopteryx, thus providing Only recently, however, could Archaeopteryx assume its ample justification for the sharp focus placed on Archaeo- proper role in the drama of the avian transition as one of a pteryx. Although many of the characters listed previously are number of important players in an ensemble cast. far from “clean,” with homoplasy and missing data compli-  cating the picture such that Chiappe (Chapter in this vol- The Significance of Protoavis ume) found only three unambiguous synapomorphies for the node Aves, Archaeopteryx is indeed avian. Moreover, it A particularly difficult question is whether this ensemble has very few autapomorphies (Gauthier, ; Chatterjee, cast should rightly include the Texas fossils known as P. tex- ;Sereno,b) and thus,in a strictly phylogenetic sense, ensis. These fossils, whose discovery was announced only in may legitimately serve as a model for an avian ancestor. ,have had a troubled and controversial history.In a long However, there is a danger here. Even if Archaeopteryx is series of published works, Sankar Chatterjee (, , the best available model for an ancestral bird, the worry is , , a, b, ) argued that the Protoavis fos- that we might come to regard it as truly the first bird. Even sils are not only those of a bird but from a bird that lived  the German common name for Archaeopteryx—Urvogel— million years before Archaeopteryx! The fossils are generally carries this sense of being the very first bird. But Archaeop- attributed to two individuals excavated from the Early teryx had an evolutionary history, and, as with any organ- Norian Cooper Member of the Dockum Group of western ism, its phylogenetic heritage has an impact on not only Texas (Chatterjee, ), although other material from a dif- its form and function but also our interpretation of its ferent formation and county also was later referred to P. tex- form and function. Thus, although Archaeopteryx may be ensis (Chatterjee, , a, ; justification for this re- our best and oldest evidence for, say, feathers, there is no ferral has not been presented, and I will not consider that guarantee that Archaeopteryx gives us any direct glimpse material here). Although the report of any new Mesozoic into the origin of feathers and flight because both pre- bird is greeted with great interest, the reception of Protoavis

THE DEBATE ON AVIAN ANCESTRY 7 was unique, largely because of the implications that a Trias- words, a major concern has been that the Protoavis speci- sic bird holds. Perhaps surprisingly, despite the great age of mens are simply too poorly preserved, too scrappy, to be di- the fossils, Chatterjee has never argued for any major agnostic. Unlike Archaeopteryx and the Cretaceous birds changes in the general notion of avian ancestry from dro- from Spain and China, Protoavis is not a “slab animal”; that maeosaurlike coelurosaurian dinosaurs; in other words, is, it is not preserved in situ, but rather all the bones have Archaeopteryx remains the basal bird, and the Ostrom/ been prepared free of the matrix. Thus, without the aid of Gauthier hypothesis of theropod relationships is not chal- the positional information that slab animals preserve, the lenged. The irony that emerges is that Protoavis perhaps identification of isolated elements is difficult and can lead should have relatively little relevance for the origin of birds to widely different interpretations. The other side of this in that, according to Chatterjee’s cladograms, Protoavis is coin is that all sides of the elements are available for study nested well within Aves. rather than being half entombed in stone, as is the case for Skepticism about the avian status of Protoavis was im- slab birds such as Archaeopteryx. Nevertheless, it has been mediate and did not necessarily follow along lines of alle- difficult to confirm not only many of the structures but even giance to any particular theory of avian origins. For exam- some of the bone identifications made by Chatterjee (Wit- ple, Feduccia () and Martin (), on the one hand, mer, b). Perhaps Padian and Chiappe (b:) best and Ostrom (, , ), Wellnhofer (, ), characterized the situation by noting that the “material has Chiappe (, ), and Sereno (b, a), on the become a paleontological Rorschach test of one’s training, other hand, have all expressed doubt that the fossils of Pro- theoretical bias, and predisposition.” Coupled with this is toavis are adequate to substantiate the claim of a Triassic the problem that the specimens are extensively recon- bird. At the same time, Chatterjee has had some supporters, structed with plaster and epoxy, and it often seems that the including Peters (), Kurochkin (), and Bock (, published descriptions are of these reconstructed compos- ). Why the controversy? A fuller critical appraisal of ites rather than of the fossils themselves. the status of Protoavis is presented elsewhere (Witmer, But ultimately—even given the gravity of these and ), but a brief analysis is presented here. other concerns—it comes down to the fossils and their Detractors of Protoavis have raised a variety of com- structures. Are there clearly interpretable anatomical clues plaints, the most important of which relate to the taphon- revealing the phylogenetic relationships of the beast? Again, omy of the specimens and their preservation and prepara- a more comprehensive skeletal analysis is presented else- tion. With regard to the taphonomy, there is a widespread where (Witmer, b), but it is worthwhile to examine here concern that P. texensis is a chimera, that is, a mixture of a few of the more important anatomical systems. For Chat- more than one species. Indeed, the quarry from which the terjee (, , a, b), the skull, particularly the specimens derive is a multispecific bonebed that has temporal region, is the most critical, because he regarded recorded many taxa (Chatterjee, ), and thus mixing is a Protoavis as possessing the ornithurine condition: that is, possibility, as has already been suggested for other taxa from loss of the postorbital bone, leading to confluence of the or- the same quarry (e.g., Postosuchus kirkpatricki; Long and bit, dorsotemporal fenestra, and laterotemporal fenestra. Murry, ). Chatterjee (, b) has steadfastly main- Given that Archaeopteryx, Confuciusornis sanctus (Peters tained the association of the holotype and paratype skele- and Ji, ; Chiappe et al., ; Hou et al., ), and at tons of Protoavis, and Kurochkin () offered his support least some enantiornithines (e.g., the Catalan hatchling, based on his study of the original material. Nevertheless, the Sanz et al., ; Protopteryx fengningensis, Zhang and Zhou, specimens were collected inadvertently while removing ) retain both the postorbital bone and its contact with overburden with a jackhammer, and hence we can never be the squamosal, presence of the advanced ornithurine con- completely sure of the taphonomic setting. The possibility dition in Protoavis indeed would be highly significant and that Protoavis is a composite of several species is commonly would, in fact, argue for a higher phylogenetic position voiced but, even if true, does not rule out the chance that within birds than that advocated by Chatterjee (b). Un- some of the included bones are avian (Witmer, c). How- fortunately, the temporal region of the holotype skull has ever, as Chiappe () correctly pointed out, the chimera been assembled from disarticulated pieces, and thus the problem presents itself most insidiously during phylogenetic identity and positions of elements are not certain. The most analysis,the ultimate arbiter of avian origins,in that the mix- telling clues are found in the squamosal and quadrate; the ture of taxa means a mixture of characters, all of which leads absence of the postorbital is negative evidence and hence to phylogenetic nonsense. Thus, the taphonomic question— hard to evaluate. The squamosal identification is key, be- Is Protoavis a species or a fauna?—is a critical one, and one cause the element would lack an articular surface for the that is not likely to go away until new material is discovered. postorbital, which implies absence of the postorbital bone And, according to many of those who have studied the itself. Similarly, the putative quadrates are important, be- specimens, new material is needed desperately. In other cause they would be drastically modified along the lines of

8 LAWRENCE M. WITMER birds, presumably, according to Chatterjee, in association naive at best. Critical study of the original material is ab- with avian craniofacial kinesis. The squamosal identifica- solutely necessary, but even here, in the absence of newly tion is defensible in that the element in question has a cotyla collected material of known association, firm conclusions that could receive a quadrate, but other interpretations will likely be elusive. are possible. The quadrate identifications are less certain It would thus seem that Protoavis bears little significance (see Witmer, b). My general impression of the recon- for solving the riddle of the origin of birds. The specimens structed temporal region is that Chatterjee’s view is under- themselves are problematic, and so we rightly should be standable and justifiable but is not sufficiently clear to merit skeptical. But even assuming that Chatterjee has interpreted drawing firm phylogenetic conclusions. them % correctly, Protoavis would have little impact on Without question, the braincase is the most easily inter- the phylogenetic pattern of avian origins (Witmer, c, preted part of the skull, at least with respect to bone identi- b; Dyke and Thorley, ). Then why the often vitri- fications. It is doubtful that there are any indisputably avian olic controversy? The reasons are complex, but probably apomorphies in the braincase. However, it is indeed the a major component is that Chatterjee’s acceptance of the braincase of a coelurosaur in that it possesses cranial pneu- Ostrom/Gauthier orthodoxy would require that virtually all matic recesses (including the caudal tympanic recess, which theropod cladogenesis had taken place well back in the Tri- thus far is not known outside Coelurosauria), a large cere- assic, at least in the Norian, if not earlier—that is, right at bellar auricular fossa, a metotic strut, and a vagal canal the very dawning of the dinosaurs. I have elsewhere (Wit- opening onto the occiput (Chatterjee, , b; Witmer, mer, b) referred to this (somewhat whimsically) as the d, b), and thus the braincase may pertain to the “Norian Explosion.” The problem is that we have no real oldest known coelurosaur. evidence of such an explosion: no Norian tyrannosaurids, Postcranially, little is unambiguously avian. Exceptions no Norian oviraptorosaurs, etc. Thus, many people simply are the cervical vertebrae, which are truly heterocoelous, if have not accepted this proposition.This incongruity has not only incipiently so; have prominent ventral processes (hy- gone unnoticed and has been exploited by opponents of the papophyses); and have large vertebral foramina. Of course, idea of theropod relationships. For example, Martin (), heterocoely has a fairly homoplastic distribution within Tarsitano (), and Bock () were receptive to the avian birds (Martin, ; Chiappe, ), and Protoavis is not as status of Protoavis and pointed out that a Triassic bird heterocoelous as Hesperornis or most neornithines, but the would essentially disprove the prevailing notion of thero- vertebral structure nevertheless represents one of the few pod relationships. bona fide avian suites of Protoavis. There are many prob- But a long view is appropriate. The origins of many lems in the thoracic appendage (Witmer, b), not least theropod groups are constantly being pushed back further of which is the coracoid, which, although having a generally in time. Therizinosauroids have been reported from the advanced avian shape, seems positively minuscule in com- Early Jurassic of China (Zhao and Xu, ; Xu et al., a), parison with the rest of the skeleton. I cannot confirm the and Chatterjee () reported an ornithomimosaur from remigial papillae on the ulna or manus. In fact, the identifi- the Late Triassic of Texas, although such claims generally are cation of the four-digit manus itself has been called into controversial (Rauhut, ). As mentioned, Protoavis itself question, with Sereno (b) regarding it as the foot of an represents a temporal range extension for Coelurosauria. archosaur. The pelvic appendage likewise is not particularly Whether the idea of a Triassic bird will ever be more palat- birdlike. Perhaps the most avian feature is a medial fossa able is hard to predict, but stranger things have happened in within the os coxae regarded by Chatterjee (, b) as the history of science, and Protoavis may yet prove to be a a renal fossa; however, no other Mesozoic bird has a renal key player. For the present, however, it is probably both pru- fossa, and the structure in Protoavis differs somewhat from dent and justifiable to minimize the role that Protoavis plays that in neornithines (Witmer, b). in any discussions of avian ancestry. It is probably fair to state that the case for the avian sta- tus of P. texensis is not as clear as generally portrayed by The Significance of the Feathered fi Chatterjee. The temporal con guration and vertebral mor- Chinese Dinosaurs phology might argue for a position near Ornithurae, yet the long tail, the archaic ankle, the four-digit manus, and other The s will go down in history as a time when one of the features would argue for a basal position, probably well out- most significant fossil deposits ever discovered was brought side Aves. The braincase is more or less coelurosaurian. The to light. The Lower Yixian Formation (Chaomidianzi For- taphonomic problems and the possibility that this is a mation of some) and allied rock units in western Liaoning chimera may make this an intractable problem. An option Province, People’s Republic of China, have yielded a wealth is simply to take Chatterjee’s analyses at face value and pro- of fossil vertebrates, preserving—in often astounding ceed (as done by Dyke and Thorley, ), but this seems abundance—an entire fauna in all its diversity (Luo, ;

THE DEBATE ON AVIAN ANCESTRY 9 Swisher et al., ). The basal birds from these deposits are parent midline distribution of the filaments is indeed fully discussed in this volume by Zhou and Hou (Chapter ). consistent with dermal frills, which are widely present in With regard to the origin of birds,several additional taxa are modern squamates and even well known in some dinosaur relevant, particularly because of the preservation of integu- groups (e.g., sauropods: Czerkas, ; hadrosaurids: Lull mentary remains interpreted to be feathers or featherlike and Wright, ), the filaments are not actually in the filaments. At this writing, six theropod taxa (other than the median plane in all regions but rather are in some places off- indisputable birds) have been reported to have “feathery” set, such as the head region, which is not preserved in a skin (with, no doubt, more taxa on the way): Sinosaurop- straight lateral view (Padian et al., ). In fact, a routine teryx prima (Ji and Ji, ; Chen et al., ), Protar- finding with the Liaoning birds and dinosaurs is that the chaeopteryx robusta (Ji and Ji, ; Ji et al., ), feathers or filaments are preserved as a halo around the Caudipteryx spp.(Ji et al.,; Zhou and Wang,; Zhou skeletal remains. Thus, the “midline frill” is perhaps more et al., ), Beipiaosaurus inexpectus (Xu et al., a), safely interpreted as an artifact resulting from the animals Sinornithosaurus millenii (Xu et al., b; Ji et al., ,if being preserved lying more or less on their sides, such the juvenile dromaeosaurid pertains to this species), and that the halo would roughly correspond to the median Microraptor zhaoianus (Xu et al., ). The obvious signi- plane. Geist et al. () suggested that another problem ficance for the debate on avian origins is that if feathers are with the feather interpretation in Sinosauropteryx is that truly present in nonavian theropod dinosaurs, then this although some specimens may show a ruffle of fibers ex- should effectively close the door to any opposition to the tending along the tail, another specimen shows a smooth theropod hypothesis. The debate, for all intents and pur- outline along the tail. poses, will be over. It is valid to question whether these shortcomings falsify Thus, it is necessary to assess these claims carefully. See the feather hypothesis or may simply be ascribed to va- also the chapters in this volume by Clark, Norell, and garies of preservation. Nevertheless, the inference of feath- Makovicky (Chapter ) and Zhou and Hou (Chapter ) for ers in Sinosauropteryx has such profound implications— their assessments. The controversy began when Ji and Ji not only for the origin of birds but also for the origin of (: translation courtesy of Chen P.-J. and P. J. Currie) feathers and endothermy—that we should be compelled by identified feathers in Sinosauropteryx and argued that () the weight of evidence before accepting such momentous they were similar to modern down in lacking rachis and claims. In the acknowledged absence of calamus, rachis, barbs, and () they were restricted to a median frill running and barbs, the identification of these structures as “true” from the head to the tip of the tail dorsally and onto the ven- feathers in Sinosauropteryx is clearly unjustified.Also prob- tromedian surface of the tail. For Ji and Ji (), the pres- lematic is the inference of “protofeathers.” Although true ence of feathers required the referral of Sinosauropteryx feathers certainly had epidermal precursors that lacked to Aves. In the subsequent furor, there was a retreat from such definitive attributes as rachis and barbs, how would their interpretation as true feathers, being instead “proto- we recognize them? Chemical analysis showing unique feathers” (e.g., Brush et al., ). Moreover, the status of feather proteins might provide valid evidence, but again Sinosauropteryx as a bird was questioned, as it clearly had such studies have not been performed. Significantly, ac- the skeletal anatomy of a small theropod dinosaur. Indeed, cording to Prum’s (, ) developmental model of Chen et al. (), based on additional specimens, formally feather evolution, the filaments of Sinosauropteryx are en- referred Sinosauropteryx to Compsognathidae, a clade of tirely consistent with an early stage of feather evolution. relatively basal coelurosaurs. These authors also presented Moreover, Padian et al. () argued that these filaments the first in-depth morphological analysis of the integumen- have enough morphological attributes in common with tary structures, describing them as coarse, probably hollow, feathers that it is fair to accept that the filaments pass the filaments up to  mm in length; a chemical or elemental similarity test of homology with avian feathers. Finally, the analysis has not been published. The ultimate question, notion of feather precursors in Sinosauropteryx is signifi- of course, is, What makes these feathers or even “proto- cantly enhanced by the feathered theropods from the Yix- feathers” (Unwin, )? ian discussed later, leading Padian (:) to state that Indeed, Geist et al. () and Feduccia (b) sug- “doubts [raised by Geist et al. ()] can now be put to gested that the structures in Sinosauropteryx were in fact not rest.”Thus, in effect, the filaments of Sinosauropteryx might feathers at all and, moreover, that they were not external, be regarded as passing the congruence test of homology, as epidermal appendages of any kind. Rather, they argued that, well (Padian et al., ). Nevertheless, the evidence in based on comparative anatomy, the fossil structures more Sinosauropteryx obviously should be judged on its own closely resembled collagenous fibers supporting a midsagit- merits, and stemward inferences based on crownward ob- tal dermal frill, that is, internal structures that became servations require considerable justification (i.e., level II frayed in the process of decomposition. Although the ap- inference; Witmer a).

10 LAWRENCE M. WITMER Shortly after the announcement of Sinosauropteryx, Ji preservation of filamentous structures in the body regions, and Ji () announced the discovery of another feathered but the real question is whether the distribution of true creature, Protarchaeopteryx robusta. They regarded it as the feathers was more extensive in life or actually restricted to sister group of Archaeopteryx, even placing it within Ar- the tips of the hands and tail. chaeopterygidae, but Ji et al. () removed it from a posi- The association of true feathers with the skeletons of tion within Aves. Unlike the case of Sinosauropteryx, the Caudipteryx is beyond any doubt, which is important be- feathers attributed to Protarchaeopteryx are absolutely in- cause the skeleton is decidedly nonavian—that is, this is disputable, with clear rachis and barbs. Thus, if a phyloge- no chimeric association. The following discussion is not netic placement outside birds is justified, then a feathered intended to be a description of the bony anatomy of nonavian theropod would be at hand. Unfortunately, Caudipteryx but rather a tabulation of its primitive, non- the unique specimen of Protarchaeopteryx (NGMC ) avian attributes (see also Zhou and Wang, ). Although is quite poorly preserved, and many attributes either are it is more customary in this cladistic age to enumerate de- open to interpretation or beyond reliable observation (e.g., rived characters, documentation of the primitive characters about % missing data according to Ji et al., ). I was of this feathered creature is necessary to counter claims that unable to confirm the two plesiomorphies identified by Caudipteryx is in fact “a secondarily flightless bird, a Meso- Ji et al. () that would deny Protarchaeopteryx a higher zoic kiwi” (Feduccia, a:; b; see also Jones et al., position—a short frontal process of the premaxilla and ser- b). In addition to feathers, another significant avian rated teeth. The premaxilla is badly damaged, and the teeth apomorphy would be the shortened tail. In Caudipteryx seemed to lack clear serrations; Ji et al. () regarded the there are only  caudal vertebrae, the same number as in serrations as so small (–/mm) that they were not visible Archaeopteryx and fewer than in any other known nonavian even with my hand lens, but one then wonders if something theropod (Ji et al., ). Moreover, the distal portion is so small can be regarded as truly a “serration.”It may lack a clearly very stiff, although, as correctly noted by Ji et al. reversed hallux, which would be an important plesiomor- (; see also Zhou et al., ), definitely not fused into a phy, but neither foot of the holotype is well preserved. Given pygostyle (or a “protopygostyle,” as Feduccia [a] called the current state of our knowledge of Protarchaeopteryx, it it). Other than its short length and distal stiffening, nothing is difficult to predict whether better specimens will show it about the tail is particularly birdlike. Its distal caudal verte- to be outside or within Aves. It is even conceivable that a sis- brae have very short centra (Ji et al., ), rather than the ter group relationship with Archaeopteryx, as originally sug- elongate distal centra observed in Archaeopteryx and Ra- gested by Ji and Ji (), will be borne out (see Elzanowski, honavis (Forster et al., ). Moreover, the proximal caudal Chapter  in this volume). Certainly, Protarchaeopteryx is haemal arches (chevrons) are very long and spatulate, again very close to the transition to birds, which makes its state of unlike those of basal birds and unlike those of even most de- preservation all the more frustrating. rived nonavian coelurosaurs. The closest match to the tail of Much better preserved, however, is the material of Caudipteryx may well be among oviraptorosaurs. As partic- Caudipteryx (Ji et al., ; Zhou and Wang, ; Zhou et ularly well demonstrated by Nomingia (Barsbold et al., al., ). Caudipteryx in many ways seems to be the perfect a,b), oviraptorosaurs display the following derived “feathered dinosaur.” It possesses clearly “avian” feathers characters: a reduced number of caudal vertebrae ( in (i.e., with calamus, rachis, and barbs), yet, unlike those of Nomingia—only  more than in Caudipteryx), rigid distal Archaeopteryx, these feathers are not part of a flight appa- tail with short centra, relatively long transverse processes on ratus, and hence Caudipteryx obviously did not fly. More- the proximal caudals (Sereno, a), and elongate and over, Caudipteryx lacks many of the derived bony features spatulate haemal arches. Although Barsbold et al. (a,b) unique to “proper” birds and hence has justifiably been regarded the tail of Nomingia as bearing a “pygostyle,” it is hailed as the first animal to be discovered that is both indis- certainly not homologous (or even that similar) to the avian putably feathered and indisputably not a bird. Indeed, structure, and I would tend to reserve that name for Caudipteryx truly begs the question of just what may be pygostylian birds (see Chiappe, Chapter  in this volume). called a “bird” in the colloquial sense of the word. In any event, the shortened tail of Caudipteryx is not par- As mentioned, feathers are known for the two widely ticularly birdlike and is basically matched by the tails of studied specimens described by Ji et al. (); the several oviraptorosaurs. new specimens reported by Zhou and Wang () and Zhou et al. () advanced a few additional birdlike Zhou et al. () confirm a consistent pattern. In their pre- characters that, although they still regarded Caudipteryx as served state, well-developed feathers are largely restricted to a nonavian dinosaur, “indicate that its phylogenetic posi- the manus and distal portion of the tail. As far as can be dis- tion remains a debatable issue.”Not having examined their cerned, the inner and outer vanes are symmetrical about the new specimens firsthand, I cannot comment in detail on the rachis. Ji et al. () and Zhou and Wang () reported birdlike attributes, but a few points are pertinent. Of the

THE DEBATE ON AVIAN ANCESTRY 11 birdlike characters that they advance, some are clearly of Caudipteryx are clearly visible in the new material de- homoplasies (e.g., manual phalangeal formula of --), scribed by Zhou and Wang (; see also Zhou et al., ). some are more widely distributed in maniraptorans (e.g., Both ischia are well preserved and show only a single process tooth form, uncinate processes), and some are open to in- that is large and triangular. This shape is exactly like that of terpretation (e.g., the “partially reversed” hallux). They also the obturator process of, say, dromaeosaurids and ovirap- pointed to the remarkably short trunk (only nine thoracic torosaurs. Another primitive character thus would be the ab- vertebrae) and elongate hindlimbs, birdlike features that sence of the proximodorsal process. had earlier attracted the attention of Jones et al. (b). A number of other primitive characters of Caudipteryx Jones et al. (b) argued that Caudipteryx possessed a can be added to those discussed by Ji et al. (; see also strikingly birdlike attribute relating to the location of the Zhou and Wang, , and Zhou et al., ). For example, center of mass and the proportions of the trunk and the jugal is a typically nonavian theropodan jugal with a hindlimb; these parameters were entirely unlike those of very large postorbital process.Birds,on the other hand,have any known nonavian theropods but indistinguishable from lost the postorbital process of the jugal or, at most, have re- those of cursorial birds. They provided three alternatives to duced it to a small process. Even taxa that retain a post- explain these data. First, perhaps simply Caudipteryx apo- orbital bone and a dorsotemporal arch (e.g., Archaeopteryx, morphically and convergently developed a locomotor style the Catalan enantiornithine nestling, alvarezsaurids) lack a similar to that of cursorial birds. Second, perhaps Caudip- large postorbital process of the jugal and basically have a ju- teryx was a nonavian theropod that had flight in its ances- gal bar. The postorbital bone of the enantiornithine Proto- try. And third, perhaps Caudipteryx was in fact “a secondar- pteryx has a long jugal process that might reach the jugal, ily, flightless, post-Archaeopteryx, cursorial bird” (Jones et but such a contact is not clear on the specimens (Zhang and al., b). The authors clearly favor this third hypothesis. Zhou, ). The only certain exception is Confuciusornis, Testing all three hypotheses is firmly within the realm of which curiously possesses a complete postorbital bar phylogenetic analysis, and the paper of Jones et al. (b) formed by contact of the postorbital and jugal bones. But was a functional analysis, not a comprehensive phylogenetic even in Confuciusornis most specimens have a relatively study. small postorbital process of the jugal (in some cases, little Nevertheless, despite the presence of true feathers, bird- more than a bump), and the postorbital bone makes up al- like hindlimb proportions, and perhaps other, less certain most all of the bar (Martin et al., ; Peters and Ji, ; features, Caudipteryx displays a variety of plesiomorphic Chiappe et al., ; Hou et al., ; Zhou and Hou, Chap- characters throughout the skeleton that, when taken to- ter  in this volume). It may be noted here that the Eichstätt gether, clearly place it outside Aves. Ji et al. () listed three specimen of Archaeopteryx displays a somewhat bifid cau- such characters. Their first two characters are very similar dal extremity to the jugal. The dorsal prong of this bone and relate to the quadratojugal and its contact with the could be interpreted as a postorbital process (e.g., Paul, quadrate and squamosal. I concur that the quadratojugal of ), but it seems to be situated too far caudally to reach NGMC --A bears the primitive character of a relatively the ventral ramus of the postorbital as preserved in the long dorsal (squamosal) process that probably is sutured to Berlin specimen (see also Chiappe et al., ); hence, I tend the quadrate, and the new specimens reported by Zhou et al. to agree more with the restoration of Archaeopteryx pro- () confirm this arrangement. Caudipteryx clearly lacks duced by Chatterjee (). the small quadratojugal of birds, including such basal birds Another primitive character of Caudipteryx is the rela- as Archaeopteryx, Confuciusornis, and enantiornithines. tively very deep mandibular fenestra, as evidenced by the The other plesiomorphic trait cited by Ji et al. () in- deep caudal embayment of the dentary of the paratype volves the retention of a prominent, triangular obturator skull. Absence of a mandibular fenestra had been thought process of the ischium. Again, I fully agree, and I regard the to characterize Aves because such an opening is absent in shape of the ischium as one of the clearest manifestations of Archaeopteryx, hesperornithids, Ichthyornis, and neor- the position of Caudipteryx outside Aves. Basal birds have nithines, but the discovery of mandibular fenestrae in Con- complex ischia (Forster et al., ) that generally are charac- fuciusornis (Martin et al., ; Chiappe et al., ; Zhou terized by a small (or even absent) obturator process and and Hou, Chapter  in this volume) makes this assessment instead a large, tablike (i.e., rectangular) proximodorsal a bit problematic. Nevertheless, the fenestra in Confuciusor- process extending up toward the ilium. This is the condition nis is not nearly as deep as in Caudipteryx and has an un- in, for example, Archaeopteryx, Confuciusornis, and enantior- usual form and thus may well be a reversal. The mandibu- nithines. The very birdlike theropod Unenlagia comahuensis lar fenestra of Caudipteryx, on the other hand, is very (Novas and Puerta,) presents the intermediate condition comparable to that of dromaeosaurids, oviraptorosaurs, of possessing both a large obturator process and a proxi- and other nonavian coelurosaurs and thus represents the modorsal process. The shape and orientation of the ischium primitive condition.

12 LAWRENCE M. WITMER The thoracic girdle of Caudipteryx is also quite primitive semblance to oviraptorosaurs, but even more striking is the and has none of the avian apomorphies seen in members of conformation of the jaws. In both groups, the dentary is Aves. For example, the scapula has a relatively broad blade very deep between the mandibular fenestra and the sym- with a pronounced distal expansion, indicating the reten- physeal portion, which is deflected ventrally; moreover, the tion of a broad suprascapular cartilage. The blade clearly is symphyseal portion is medially inflected, and the caudal not the slender and elongate structure seen in all basal birds. processes of the dentary diverge widely around the The shape of the coracoid is more or less that of a conven- mandibular fenestra, both of which are attributes of ovi- tional nonavian coelurosaur coracoid, with a quadrilateral raptorosaurs (Makovicky and Sues, ). The premaxilla shape, proximal supracoracoidal nerve foramen, and mod- has an extensive prenarial portion (which is also an avian erate biceps tubercle. The coracoid certainly is not the elon- apomorphy), and the naris itself is retracted (extensively in gate “straplike” bone seen in ornithothoracine birds. An- oviraptorosaurs). Finally, the maxilla is very characteristic, other primitive trait here relates to the orientation of the being a relatively small, rostrally displaced triangular ele- girdle in that it is located on the lateral aspect of the thorax ment. Many of the postcranial elements compare well with with the scapula at an angle to the axial column rather than oviraptorosaurs but also with other clades of coelurosaurs. on the dorsal aspect of the thorax with the scapula parallel However, the tail of Caudipteryx, as detailed previously, is to the column. The former condition is the primitive con- quite similar to that of oviraptorosaurs in that both are dition, whereas the latter condition is observed in all birds short and proximally very thick.Given that my observations (Jenkins, ), including Archaeopteryx, Confuciusornis, were not part of a comprehensive phylogenetic analysis, it and other basal birds. One hesitates to make too much of was gratifying to see these impressions of an ovirap- the orientation of elements in two-dimensional specimens, torosaurian Caudipteryx borne out by numerous cladistic but, taken at face value (and all specimens agree on this analyses presented at the Ostrom Symposium at Yale Uni- point), Caudipteryx again displays the primitive condition. versity in  (e.g., by P.C. Sereno, T.R. Holtz, M.A. Norell, The pelvic girdle of Caudipteryx presents primitive, non- and P.J. Currie), suggesting broad independent discovery of avian characters beyond the ischiadic shape noted earlier. these relationships (and a heartening affirmation of phylo- For example, the ilium is relatively very tall directly above genetic systematics). Barsbold et al. (a,b) also regarded the acetabulum, and its preacetabular portion is not ex- Caudipteryx as a basal oviraptorosaur. More significant, panded cranially; this is the typical condition for most non- Caudipteryx was included in the very extensive phylogenetic avian coelurosaurs. In birds, on the other hand, the ilium is analysis of Sereno (a). Sereno scored Caudipteryx for relatively low, with a greatly elongate preacetabular portion  characters (only % missing data) and found not only (see Elzanowski, Chapter  in this volume; Zhou and Hou, that Caudipteryx is well outside Aves but also that it is in- Chapter  in this volume). The pubic apron is extensive in deed a basal oviraptorosaur, sharing a dozen characters with Caudipteryx, measuring about % of total pubic length in oviraptoroids. the holotype. This is considerably more than the % meas- Thus, feathers of essentially modern structure do indeed ured in the London Archaeopteryx, the bird with the longest predate the group conventionally known as “birds.” This known pubic apron, and may even exceed that of some finding may seem shocking, but it is to be expected. This dromaeosaurids (Norell and Makovicky, , ). surprise again may relate to the pervasive sense of Archaeop- Finally, Zhou and Wang () and Zhou et al. () teryx as truly the Urvogel, or “first bird.”Common sense, of noted that the pubis is not retroverted (as argued by Feduc- course, dictates that the elaborate feathers of Archaeopteryx, cia, b) but rather is directed cranially, as in most non- arranged as they are in their “modern” array of primaries avian theropods. and secondaries, must have had predecessors. However, The picture that emerges from this brief survey of Caudipteryx will likely remain difficult for some to accept, Caudipteryx is of a feathered theropod dinosaur that is perhaps because it is such a dramatic repudiation of oppo- probably well outside the avian lineage. I have not per- sition to the theropod origin of birds. formed a more extensive formal analysis, but it seems read- In fact, more “feathered dinosaurs” are likely to come to ily apparent that it would be much less parsimonious to in- light. For example, Xu et al. (a) described Beipiao- clude Caudipteryx within Aves. And this point leads to the saurus, a new therizinosauroid theropod from the Lower question of the phylogenetic position of Caudipteryx. The Yixian Formation that bears filamentous dermal structures analysis of Ji et al. () was not very inclusive, using only that are perhaps similar to those of Sinosauropteryx. These Velociraptor as an outgroup to the Chinese taxa and birds. structures lack the unambiguous feather structure seen in My initial study of the specimens suggested a number of de- Caudipteryx (i.e., they lack calamus, rachis, and barbs), but rived features pointing to oviraptorosaur relationships for they are clearly present on areas of the body that cannot Caudipteryx, including the following. The jaws are almost be explained away as remnants of a median frill. In Bei- completely edentulous in Caudipteryx, which is indeed a re- piaosaurus, filamentous structures are associated with ele-

THE DEBATE ON AVIAN ANCESTRY 13 ments of both fore- and hindlimbs. The best-preserved fila- in this animal, although this finding awaits confirmation ments are attached to the ulna, where some approach  with better-preserved material. mm in length. Xu et al. (a) describe some filaments as It is also relevant at this point to mention the findings of distally branched and with hollow cores. These filaments Schweitzer et al. () on the biochemistry and morphol- have the same “protofeather” problems as did those of ogy of fibrous integumentary structures recovered from the Sinosauropteryx (i.e., Are filaments truly the evolutionary head region of the alvarezsaurid Shuvuuia from Mongolia precursors of feathers?), but their association with the limbs (see also Chiappe, Norell, and Clark, Chapter  in this vol- and their considerable length clearly indicate that they are ume). Schweitzer et al. () reported two important ob- some kind of epidermal appendage rather than an artifact servations about these structures. First, biochemical studies of desiccating dermal collagen (see also Prum, ). are consistent with their being composed of beta keratin, a Another nonavian theropod with preserved integumen- protein found in the feathers and scales of sauropsids. Sec- tary filaments is the Yixian dromaeosaurid Sinornithosaurus ond, the structures were apparently hollow. At present, the millenii (Xu et al., b). Unfortunately, the filaments are only structures known to be both hollow and composed of not in their natural positions, and thus, for example, the beta keratin are avian feathers. These findings are more cluster of filaments adjacent to the skull cannot be reliably provocative than conclusive, and, given the controversial attributed to the head region. A very significant finding of phylogenetic position of alvarezsaurids (see Novas and Sinornithosaurus is a negative one, and that is the absence of Pol, Chapter  in this volume), one should be hesitant to hand and tail feathers. No true feathers (i.e., with rachis and make too much of these findings. Nevertheless, they may be barbs) of the sort seen in birds and Caudipteryx have been legitimate evidence for feather or featherlike structures out- recovered with Sinornithosaurus. This is a bit troubling be- side Aves. cause the phylogenetic hypothesis of Sereno (a) pre- In sum, the significance of these Chinese (and Mongo- dicts that,minimally,hand and tail feathers should be found lian) fossils for the debate on avian origins, in one sense, in dromaeosaurids. However, given that the integumentary should be minimal. That is, we should not be surprised at structures are not in life position and that the Sinor- the identification of feathers in a group of animals that a nithosaurus specimen is generally jumbled somewhat on the broad consensus had always thought was close to avian an- slab, it is probably best not to make too much of this absence cestry. The discovery of feathers in, say, Caudipteryx simply and assume that it is preservational. Close examination re- adds one more apomorphy to the long list of derived char- veals some details suggesting that the filaments of Sinor- acters linking birds with theropod dinosaurs. The disproof nithosaurus are more structured than those of Sinosaurop- of feathers in any of these Chinese forms would simply re- teryx and Beipiaosaurus and hence more similar to avian move one character; all the others would remain. Likewise, feathers. Xu et al. (b) documented branching of some forcing Caudipteryx to be within Aves because of its posses- filaments, the compound construction of filamentous bun- sion of true feathers (Cai and Zhao, ) would not auto- dles, and even the occurrence of basal tufts of filaments, all matically strip it of its clear theropod heritage. Thus, in this features indicative of a more structurally complex integu- context, feathered dinosaurs are not that important. But, mentary covering. This report was followed shortly by the of course, in this high-profile, high-energy debate, rhetoric, announcement by Ji et al.(; see also Norell,) of a new regrettably, sometimes seems paramount to evidence. specimen of a juvenile dromaeosaurid that is very similar to Feathers—that quintessentially avian trait—have always Sinornithosaurus and may even be the same species. The been the great definer of birds. The presence of unambigu- specimen preserves the integument in place and affirms ous feathers in an unambiguously nonavian theropod has the complex nature of the integument in dromaeosaurids. the rhetorical impact of an atomic bomb, rendering any Not only does the juvenile specimen show branching and doubt about the theropod relationships of birds ludicrous. tufted filaments, but it also shows fibers branching off of a fi central axial lament—that is, it shows structure that could The Relationship of the Origin be interpreted as being the rachis and barbs of a “true” of Flight to the Origin Of Birds feather. Moreover, Ji et al. () argued that the structures were so well ordered that birdlike barbules almost certainly The origin of birds is, at its core, a matter of genealogy. That had to have been present.As in Caudipteryx, the tail and fore- is, regardless of your systematic philosophy—whether it be limbs have the best-organized integumentary structures. cladistic, phenetic, or eclectic—avian ancestry is a question Microraptor, the tiny dromaeosaurid reported by Xu et of phylogeny, or, more precisely, phylogenetic reconstruc- al. (), lacks hand and tail feathers but has the now typ- tion. For most biologists, phylogenetic reconstruction has ical filamentous coat. As in the juvenile dromaeosaurid, become more or less synonymous with phylogenetic sys- some integumentary impressions bear a rachislike struc- tematics or cladistics, whereby the distribution of attributes ture, suggesting that true feathers might have been present among taxa forms the primary raw data used to develop hy-

14 LAWRENCE M. WITMER potheses of relationship. The debate on the origin of birds, attributes not typical of theropod dinosaurs (Martin, , however, has been unusual in that a different approach has , ; Tarsitano, , ; Feduccia and Wild, ; been applied by a minority of workers for many years (Wit- Feduccia, , a). mer, , b, ). This approach is () to create the Again, my intent is not to explore this model or its the- most likely scenario for the origin of avian flight, () to de- oretical premises but rather to evaluate the validity of the duce from this scenario the morphological features likely to approach. We are faced with two interesting and obviously be present in the hypothetical “proavis,” and then (), as I related issues: the genealogical ancestry of birds, and the have said before (Witmer, b:),“to search the animal evolution of flight in birds. The question then becomes, kingdom for a match.” Thus, functional hypotheses on the Is resolution of one issue logically prior to resolution of origin of flight are being used to test phylogenetic hypothe- the other? The answer is yes, and most theorists would ar- ses on the origin of birds. This distinction between the func- gue that workers such as Martin, Feduccia, and Tarsitano tional and phylogenetic approaches to avian origins has not have the logical order reversed. In other words, the phylo- been widely appreciated. genetic question of avian ancestry must precede the func- For decades, of course, discussions on the origin of flight tional question of how flight arose. There is a fairly exten- in birds have been dominated and dichotomized by the sive literature on the relationship of functional inference to arboreal hypothesis (a.k.a. the “trees down” theory) and phylogenetic inference, most prominently discussed by the cursorial hypothesis (a.k.a. the “ground up”theory). The Lauder (, , ; Lauder and Liem, ), although cursorial hypothesis has been closely associated with the others have commented on the issue (e.g.,Padian,,, notion of relationships to theropod dinosaurs, whereas ; Liem, ; Bryant and Russell, ; Weishampel, the arboreal hypothesis has been tied to the “alternative an- ;Witmer,a). These authors all agree that functional cestry”hypothesis (that is, the origin of birds from a usually hypotheses and scenarios are best tested within the context poorly defined group other than theropods, most often of a strict hypothesis of phylogenetic relationships, prima- basal archosaurs). It is my intention in this section neither rily for the simple reason that evolutionary history con- to evaluate these ideas nor to provide a historical account; strains functional systems and their evolution. The evolu- these are beyond the scope of this chapter (see Hecht et al., tionary “starting point” for any functional transition is ; Feduccia, ; Shipman, a). Rather, my goal is to absolutely critical. The evolutionary trajectory from a examine how these two functional hypotheses relate to the Megalancosaurus-like form to a flying bird will be much dif- phylogenetic question of avian origins. Moreover, it is ferent from the trajectory from a Deinonychus-like form to worthwhile to question the strict coupling of the cursorial a flying bird, regardless of whether these “starting points” hypothesis with theropods, on the one hand, and the arbo- are arboreal, terrestrial, aquatic, or whatever. Determining real hypothesis with alternative ancestors,on the other hand this evolutionary “starting point” is a matter of genealogy, (Witmer, ). For example, is the arboreal hypothesis that is, of phylogenetic inference, not functional inference. truly inconsistent with theropod relationships? Thus, the details of any functional transition, such as the Opponents of theropod relationships have argued origin and refinement of flight, can be best dissected with strongly for a tight linkage between these functional and the tool of a well-resolved phylogenetic hypothesis (see phylogenetic issues. For example, Feduccia (:viii) stated Cracraft, ; Chiappe, ; Sereno, b, a). that “a dinosaurian origin of birds is inextricably linked Bock (, , ), on the other hand, has argued with the cursorial, or ground-up origin, of avian flight, vigorously and persuasively for the validity of what which is a biophysical impossibility.”Martin (:) char- amounts to a “function-first,” scenario-based kind of ap- acterized the debate on avian origins exclusively in func- proach, couched in the philosophical terms of historical- tional terms, claiming that “in the great bird-dinosaur de- narrative explanations. Nevertheless, most current opinion bate, the participants huddle in two camps, which has found such scenario building in the absence of a strict paleontologists have nicknamed ‘ground up’ and ‘trees phylogenetic hypothesis to fall short on the grounds of down.’” Bock so intertwined the functional and phylo- testability. More to the point, it seems unjustified to believe genetic questions that he entitled a paper “The Arboreal that such scenarios—no matter how intuitively appealing, Theory for the Origin of Birds” and used “origin of birds” such as is the case with the arboreal theory—can overturn and “origin of flight” almost interchangeably (Bock, ). as well substantiated a phylogenetic hypothesis as is the Tarsitano (, ) also strongly advocated this approach. theropod hypothesis. The hypothesized steps in the func- The basic premise here is that flight began in animals that tional transition from an arboreal proavis to a flying bird are lived in high places (trees, in most formulations) and made generally tested by only plausibility or modeling rather than use of gravity and expanded body surface area to slow de- hard data. On the other hand, phylogenetic hypotheses are scent during falls and leaps; hence,these animals should have much better grounded in tangible evidence—in this case, been small, quadrupedal, and with arboreal adaptations— actual objects (bones) that can be observed, measured, and

THE DEBATE ON AVIAN ANCESTRY 15 compared—and hence cladograms are subject to more rig- As outlined previously, the theropod school is on much orous tests. firmer theoretical ground in placing phylogeny logically But, in many respects, it is the role that cladistic analysis prior to function. In fact, it was this kind of reasoning that has played in the theropod hypothesis that has elicited the helped bolster the cursorial origin of avian flight. That is, opposition. Opponents of theropod relationships have si- since the theropod outgroups of Archaeopteryx and other multaneously waged a war against phylogenetic systemat- birds were more or less large animals, the origin of flight ics, because, for authors like Feduccia (), Martin (), clearly is best understood in this terrestrial or cursorial con- and Bock (),“cladistic analysis . . . lies at the core of the text (Padian, ; Gauthier and Padian, ; Padian and debate concerning bird origins” (Feduccia, :). Such Chiappe, a,b). However, as noted by Sereno and Rao statements are a little difficult to reconcile with the fact that, (), arboreality was apparently an early adaptation for say, John Ostrom (, ), who is not a cladist, formu- birds. Hence, the debates about the arboreality versus ter- lated the theropod hypothesis using precisely the systematic restriality of Archaeopteryx have always been seen as critical. methodology these authors advocate. Nevertheless, more Arguments on both sides have been presented for decades recent authors have indeed employed phylogenetic system- (see Hecht et al., ; Paul, ; Feduccia, ; Padian and atics, and some reviewers of Feduccia’s  book agreed Chiappe, a,b), and, once partisanship is eliminated, no that differing systematic philosophies are part of the source clear consensus emerges. Interestingly, independent studies of the conflict (Norell and Chiappe, ; Sereno, a; on pedal proportions in Archaeopteryx and other taxa Witmer, b; see, in particular, Padian, , for an analy- (Hopson and Chiappe, ; Zhou, ) have agreed in sis of this issue). What is pertinent here for the “function showing that the feet of Archaeopteryx are basically inter- versus phylogeny” debate is that, given the role of cladistic mediate between those of a terrestrial cursor and those of analysis in () modern functional inference in general and an arboreal bird. Thus, Archaeopteryx itself is inconclusive () the theropod hypothesis for avian ancestry in particular, in establishing the phylogenetic level at which arboreality it seems unlikely that those in the Feduccia/Martin school occurred (assuming for the sake of argument that it oc- will adopt the “phylogeny-first, function-second” approach curred only once). to understanding the origin of flight advocated here. The question ultimately comes down to the actual thero- Perhaps the greatest irony for this whole issue is that pod ancestor of birds: what it looked like and how it lived there probably is no adequate justification for tightly cou- its life. Although virtually all recent analyses put Dro- pling the cursorial theory with theropod relationships and maeosauridae or Troodontidae (or the two together as the arboreal theory with alternative ancestry. It is conceiv- Deinonychosauria) as the sister group of Aves, neither is able that the Feduccia/Martin school is correct that the ar- truly the ancestor, and hence known forms like Deinonychus boreal model for the origin of flight is the superior model— or Troodon can only go so far as models for the true avian but the evolutionary starting point may in fact be a small ancestor (see Gatesy, Chapter  in this volume, for an in- theropod dinosaur. Why must these functional and phylo- sightful discussion). Virtually all early birds are small ani- genetic models be coupled (Witmer, )? The coupling mals, so, at some point in the transition to birds, miniatur- of these models has more to do with the tactics of the de- ization took place. Small size has many virtues. That is, in bate than the debate itself. Advocates of the alternative- the absence of the constraints imposed by large mass, small ancestry/arboreal pairing have pointed to the large size of animals can exploit a broad behavioral repertoire without such obviously terrestrial theropods as Tyrannosaurus or necessarily having to develop novel morphological adapta- even Deinonychus in the hope of illustrating how ludicrous tions. This line of reasoning is obviously leading toward the the notion of an arboreal/theropod origin of birds is (e.g., possibility that the miniaturization took place within a lin- Tarsitano, ; Martin, , ; Feduccia, , b). eage that we would probably recognize as “nonavian,” that Advocates of the theropod/cursorial pair called attention to is, a lineage of little dinosaurs. If such a tiny theropod ha- the same terrestrial attributes of theropods in arguing for bitually used trees or other high places (and one can easily their position (e.g., Ostrom, ). In general, each camp envision many sound reasons for doing so), then perhaps has chosen a single point on which to be immovable and the arboreal model propounded by opponents of theropod hence forces the functional or phylogenetic issue to fall in relationships would apply equally well to theropods. This line with that point. For the Feduccia/Martin school, the ar- notion is not new, and a number of workers have argued for boreal theory is unshakable, and hence all phylogenetic an arboreal origin of avian flight from tiny, dromaeosaur- possibilities must be concordant—and theropods, they ar- like ancestors (Abel, ; Paul, , , ;Witmer, gue, are the height of discord. On the other hand, the thero- c; Chatterjee, a,b; Xu et al., ; Zhou and Wang, pod school has remained intransigent on the phylogenetic ). In particular, Chatterjee and Paul have developed issue, and hence the functional transition to flight has been fairly elaborate models and have identified a number of fea- constrained. tures of dromaeosaurlike theropods that may indicate ar-

16 LAWRENCE M. WITMER boreal capabilities. Significantly, a variety of tiny theropods, potheses: () the theropod hypothesis and () the “not- such as Bambiraptor (Burnham et al., ) and Micro- theropod” or, as I have termed it previously,“the alternative raptor (Xu et al., ), have begun to turn up in the fossil ancestry hypothesis.” Relationship to crocodylomorphs, record. originally proposed by Walker (), seems to have simply It is not my aim here to evaluate models for an arboreal faded away in that earlier advocates, such as Martin (, origin of avian flight from theropod dinosaurs. Although ),Walker (), and Tarsitano (), have not renewed the idea has a lot of merit, virtually all models on the origin their support. The basal archosauriform hypothesis re- of avian flight are so speculative and so data-poor that any ceived a significant boost from Welman (), who sug- satisfactory resolution is unlikely any time soon. In fact, gested that Euparkeria shares with Archaeopteryx to the ex- there are serious testability problems for all these models. clusion of theropods and crocodylomorphs a large suite of For example, mathematical models for the origin of avian derived characters in the cranial base. This new “thecodont” flight abound (e.g., Caple et al., ; Balda et al., ;Nor- hypothesis has received to date no additional adherents and berg, ; Rayner, ; Pennycuick, ;Herzog,; was severely challenged by the detailed analysis of Gower Ebel, ; Burgers and Chiappe, ), but they all suffer to and Weber (). Thus, for the present, the crocodylo- varying extents from testability problems—and this prob- morph and basal archosauriform hypotheses no longer ap- lem pertains to all models, regardless of the phylogenetic pear to merit serious consideration. starting point. The fact is that we simply have paltry data on Indeed, opposition to the theropod origin of birds has the functional capabilities of any of the principal taxa (e.g., become almost exclusively just that, an argument of op- dromaeosaurids, troodontids, Triassic archosauromorphs position rather than an argument of advocacy. Criticism is like Megalancosaurus or Longisquama). Despite numerous a necessary and appropriate part of the scientific process, studies, even the basic lifestyle of Archaeopteryx is disputed. and opponents have published a number of papers taking It is conceivable that the origin of flight—as a matter of sci- issue with certain of the characters (Martin et al., ; entific discourse—is out of reach. We may simply never Martin, , ; Tarsitano, ; Feduccia, ). It is not have the appropriate data to adequately test any models. In my goal here to analyze these criticisms or to provide re- fact, this is probably the reason that the debate on the ori- sponses, although a few will be touched on in the next sec- gin of flight has raged uncontrolled for a century with no tion. My main point here is that opponents have sought to sign of resolution in sight. All ideas remain active because destroy but not build in that they have lost sight of the goal almost none can be falsified. of phylogenetically linking birds to actual taxa. The cladis- It is fair to regard the foregoing as overly pessimistic, but tic approach is more constructive in that a particular phy- one thing that must be true is that modeling the origin of logenetic hypothesis is refuted not simply by criticizing avian flight is a very poor research strategy for discovering the characters but rather by offering an alternative that the origin of birds. The origin of flight is logically and better accounts for the available data, that is, an hypothe- methodologically secondary to the phylogenetic origin of sis that is more parsimonious, a shorter tree. In ,I birds. There is currently no good reason to rigidly couple stated: “At present, supporters of relationships of birds models of the origin of flight with particular phylogenetic with crocodylomorphs, ‘thecodonts,’ or mammals have clades.And, perhaps most troubling, the details (or even the failed to produce a competing cladogram, and in this broader pattern) of the functional transition to powered respect the coelurosaurian hypothesis is uncontested” flight may be lost in time and virtually unrecoverable in any (Witmer, :). rigorous scientific sense. That statement still stands today, largely because there are no serious alternative phylogenetic hypotheses. For The Status of Alternatives some time, opponents have offered a variety of small, gen- to the Theropod Hypothesis erally poorly preserved Triassic forms as being relevant to the debate (Martin, , , , ; Tarsitano, , There is no question that the theropod origin of birds is by ; Feduccia and Wild, ; Feduccia, ). These Trias- far the most popular hypothesis on avian ancestry. The sic taxa include Megalancosaurus, Cosesaurus, Sclero- question then arises,Are there credible alternatives? In most mochlus, and Longisquama. These forms do not constitute previous reviews (e.g., Ostrom, ; Gauthier, ;Wit- a clade but are a hodgepodge of basal archosaurs or basal ar- mer, ; Feduccia, ; Padian and Chiappe, b), the chosauromorphs. Megalancosaurus and Cosesaurus both debate on avian origins was divided into three competing pertain to the archosauromorph clade Prolacertiformes hypotheses: () the theropod hypothesis, () the crocodylo- (Sanz and Lopez-Martinez, ; Renesto, ). Sclero- morph hypothesis, and () the basal archosauriform or mochlus has been thought to be related to a variety of taxa, “thecodont” hypothesis. However, in recent years it has be- most commonly pterosaurs and dinosaurs (Padian, ; come apparent that there really are just two major hy- Gauthier, ;Benton,). Longisquama has never been

THE DEBATE ON AVIAN ANCESTRY 17 subjected to adequate phylogenetic scrutiny; Sharov () Still, Jones et al. (a) regarded the structures as feath- placed it in “Pseudosuchia,” and Haubold and Buffetaut ers probably homologous to those of birds. The implica- () agreed, although Charig (:) argued that “the tions of such a hypothesis were not explored in the paper. justification for this assignation is obscure.” Tarsitano For example, if feathers are a very basal innovation among (:) and Feduccia (:) referred to these taxa as archosaurs, then this would constitute strong support for “avimorph thecodonts” in that they regarded them as basi- the interpretation of the filaments of, say, Sinosauropteryx as cally birdlike. The resemblances, however, have never been feathers (which would be ironic given that Jones and col- particularly strong or numerous. Authors such as Feduccia, leagues were such vocal opponents of feathered dinosaurs). Martin, and Tarsitano generally have not considered these But if feathers are a basal character evolving in the Triassic, taxa to be truly ancestral to birds but rather as merely rep- then where are all the Triassic and Jurassic fossil feathers? resentative of what their hypothesized arboreal proavis was There are abundant unequivocal fossil feathers in the Cre- like. In other words, these taxa show that there were small, taceous, but none prior to those of Archaeopteryx in the Late arboreal, quadrupedal animals running around before Jurassic (see Kellner, Chapter  in this volume). Archaeopteryx (although it should be pointed out that the The Jones et al. (a) paper carefully avoided any preferred habitat and mode of life of these animals are per- statement on the origin of birds, but the authors were very haps not as obvious as commonly portrayed). vocal in the associated media furor (e.g., see Stokstad, But even given that such animals existed and are con- ), arguing that the finding of feathers in Longisquama sistent with the arboreal theory for the origin of avian refuted the theropod hypothesis and that Longisquama it- flight, this does not constitute actual evidence relevant to self is “an ideal bird ancestor” (J. A. Ruben quoted in Stok- the ancestry of birds. For example, unless Megalancosaurus stad, :). This example of disparity between scien- is being considered as close to the ancestry of birds, tific and public statements is just the latest in the long whether or not it has a “straplike scapula”or a “birdlike or- history of the debate on avian origins, and it is best to focus bit” (Feduccia and Wild, ) is of questionable signifi- on the scientific evidence. In this case, the paper of Jones et cance. It is conceivable that viable candidates for avian an- al. (a) offered no scientific statement on the ancestry of cestry could emerge from such a nexus of “avimorph” birds. In fact, their claims of homology of the integumen- forms, but such hypotheses will continue to be relegated to tary appendages of Longisquama with avian feathers was an the fringe unless they are framed in explicit phylogenetic incidental point of the paper, based basically on their opin- terms and take head-on the theropod hypothesis on its ions and not on a careful phylogenetic treatment, which is own terms. the ultimate arbiter of homology. It is fair to say that Jones Nevertheless, one of these “avimorph” forms captured et al. (a) demonstrated that the integumentary ap- broad attention when Jones et al. (a:) pointed to a pendages of Longisquama are more interesting and unusual number of features of the integumentary appendages of than previously thought. Beyond that—and in the absence Longisquama that led them to conclude that these structures of a phylogenetic analysis—Longisquama and its appen- represent “nonavian feathers, probably homologous to dages are as irrelevant to the debate on the origin of birds as those in birds.” The most compelling resemblances center are the other “avimorph” forms. on the presence of a calamuslike base wrapped in a pre- In sum, at present there remains no credible alternative sumably epidermal sheath, indicating that the appendages to maniraptoran theropod dinosaurs for the origin of birds. probably developed in a follicle, as is characteristic of feath- Previous tangible alternatives (crocodylomorphs, basal ar- ers and unlike scales. However, their interpretation of the chosauriforms such as Euparkeria) have been refuted or structures coming off the central axis as separate “barbs” summarily dropped because of lack of interest.What has re- seems overly generous at best. These structures unite dis- placed these are intangible “models” that conform to pre- tally, forming a continuous ribbon around the periphery of conceived notions on how bird flight evolved, that is, taxa the appendage. This distal union is completely unlike the that, although not truly related to birds, are “much like what situation in avian feathers, and even if a few tolerably simi- we would expect” the true ancestors to be. In some ways, it lar examples—all of which are specialized feathers—can be seems as if the search for real avian relatives has been sup- found among birds, it is clearly not the primitive avian con- planted by the mission to discredit both the theropod hy- dition (Kellner, Chapter  in this volume). It seems more pothesis and the cladistic methodology that continues to likely that the “barbs” identified by Jones et al. (a) are corroborate the hypothesis. Fossils such as Longisquama in fact plications or corrugations in a continuous structure, may someday emerge as more relevant players in the debate, which would be more consistent with a modified scale than but if the media hype surrounding the Jones et al. (a) a feather. Reisz and Sues () also were critical of the hy- paper was any indication, even Longisquama will be just an- pothesis of Jones et al. (a), advancing many of the same other attempt to develop a rhetorical weapon to attack the arguments just articulated. theropod hypothesis and cladistics.

18 LAWRENCE M. WITMER The Status of the Theropod Hypothesis control” (as he later put it [Martin, :]) as inappropri- ate but rather as a normal part of the scientific process in The only explicit hypothesis for the phylogenetic relation- which new data or claims are evaluated. In this light, let us ships of birds states that avian ancestry is fully embedded briefly examine these three categories. somewhere within the nexus of maniraptoran theropod di- The “time problem”—or “temporal paradox,”as it is of- nosaurs, probably nearest to Dromaeosauridae and/or ten known—relates to the fact that the closest nonavian Troodontidae among known groups. As such, it is “the only theropod sister groups of birds are all Cretaceous in age and game in town.” As mentioned previously, opponents of hence younger than Archaeopteryx. The issue has been theropod relationships have regarded the hypothesis as raised many times over the years, but Feduccia has wielded basically an unfortunate outcome of sloppy application of it as a bludgeon. For example, he stated that “to such work- phylogenetic systematics. However, the idea was formulated ers [paleontologists] it is inconsequential that birdlike di- by Ostrom () and initially supported (e.g., Bakker and nosaurs occur some  million or more years after the ori- Galton, ; Thulborn, ; Thulborn and Hamley, ) gin of birds” (Feduccia, :vii). Elsewhere, Feduccia without cladistics. In large measure, the many cladistic (:, italics in original) painted an even worse picture, studies that have followed have served mostly to support, claiming that “most of the supposed similarities between clarify, and update Ostrom’s original work. More impor- the urvögel [meaning specifically Archaeopteryx] and dino- tant, they have repeatedly tested the hypothesis (although, saurs are seen in birdlike dinosaurs that lived  to  mil- to be fair, it must be pointed out that they rarely include lion years later.” The latter date in the second quote would nondinosaurian taxa in the analysis). Among the more im- actually put these “birdlike dinosaurs” in the Eocene (!) portant cladistic studies are those of Padian (), Thul- —perhaps a simple mistake on Feduccia’s part, but it re- born (), Gauthier (), Holtz (), Novas (), flects a consistent hyperbolic exaggeration of the time dis- Forster et al. (), Sereno (a), and Clark, Norell, and cordance (Witmer, b). Makovicky (Chapter  in this volume). It is true that, say, Velociraptor is  My younger than Until the reemergence of Alan Feduccia in the debate in Archaeopteryx, but other dromaeosaurids and troodontids the mid-s, opposition to the theropod hypothesis had are much closer in age to Archaeopteryx: Deinonychus is  become basically mute, and, in my opinion, theropod ad- My younger, Utahraptor is only  My younger, Sinor- vocates had become complacent (Witmer, b). But Fe- nithosaurus is only about  My younger, and Sinovenator is duccia reenergized the opposition, enlisting new recruits less than  My younger (Swisher et al., ; Xu et al., ). (e.g., J. A. Ruben) and strengthening former alliances (e.g., There are much greater time discordances in the dinosaur with L. D. Martin). The warfare metaphor is intended to be fossil record (Sereno, b, a) than this one. But, lighthearted, but there is clearly a sense that this group feels moreover, there are a variety of fragmentary specimens as if it is fighting a holy war against a great oppressor; (mostly teeth) of animals that closely resemble those of Feduccia (quoted in Shipman, b:) went so far as to re- dromaeosaurids and troodontids recovered from Middle gard Ruben as a “comrade in the war against hot-blooded Jurassic deposits that predate Archaeopteryx by  My dinos.” This group has offered criticisms on a number of (Evans and Milner, ; Metcalf and Walker, ). Simi- fronts. Much of the criticism has taken the form of com- larly, Zinke () reported on an extensive collection of ments to the media, book reviews (Martin, , ), pop- theropod teeth from deposits perhaps just slightly older ular articles or books (Feduccia, , , b), and than Archaeopteryx; Zinke made firm assignments of other outlets outside normal peer review (e.g.,Martin ). these teeth to Dromaeosauridae ( teeth), Troodontidae Chief among these criticisms is that relating to the func- ( teeth), and Tyrannosauridae ( teeth). Finally, Jensen tional problems of evolving flight in an arboreal context and Padian () described fragmentary but provocative when theropods seem to have been such obligate terrestrial skeletal material of maniraptoran theropods from the animals; having discussed the multiple fallacies in this gen- Late Jurassic Morrison Formation. Even if some of these eral approach earlier, I will turn to other issues. More precise taxonomic assignments do not stand scrutiny, specific challenges can be grouped into three categories: () they clearly indicate that there were nonavian manirap- the time problem, () problems of morphological interpre- torans that existed prior to Archaeopteryx. Moreover, tation or homology, and () single problems of such signi- Brochu and Norell () pursued the temporal paradox ficance that they alone would falsify the theropod hypothe- issue by comparing stratigraphic consistency indices and sis. Martin (:) regarded these criticisms as causing a other phylogenetic metrics among various hypotheses “collapse of various anatomical arguments for a bird- for avian origins, and they found that the theropod hy- dinosaur connection followed by determined efforts [by pothesis actually compares favorably to the alternatives advocates of theropod relationships] to bolster failing char- when considered globally across the cladogram. Thus, not acters.” However, I do not regard such efforts at “damage only is the time problem not particularly severe, it does

THE DEBATE ON AVIAN ANCESTRY 19 not even exist, and its perpetuation in the face of such data flict arises because embryologists have repeatedly come up is untenable. with the result that the avian hand skeleton has digits II–IV Opponents of theropod relationships have questioned (Hinchliffe and Hecht, ; Hinchliffe, ; Shubin and either the interpretation or the homology of a number of Alberch, ). Opponents of theropod relationships (Tar- characters (see Martin, , , , ; Tarsitano, ; sitano and Hecht, ; Martin, ; Tarsitano, ;Fe- Feduccia, , a). Only one of the higher-profile char- duccia, ) seized on this as a potentially fatal flaw: if acters will be discussed here as an example, and that is the birds truly evolved from dinosaurs, then birds would have semilunate carpal, one of the classic Ostrom characters. Os- had to have lost one finger (I) and gained another (IV)— trom () originally—and erroneously—regarded the an unlikely proposition. element in Deinonychus as a radiale (i.e., a proximal carpal For more than a decade this debate was basically a stale- element), even though it clearly was much more tightly ar- mate, until a paper published by Burke and Feduccia () ticulated to the metacarpus than to the antebrachium. Gau- reopened the issue. The paper offered few new data or in- thier (), without fanfare, corrected this error and re- sights; instead, it largely restated the conflict, updated the garded it as a distal carpal, which is the identity of the embryological component by integrating the primary-axis semilunate element in birds. Nevertheless, the homology paradigm of Shubin and Alberch (), and asserted that has been vigorously questioned (see Martin, , ; these data refute the theropod relationships of birds. There Feduccia, , and references therein). Until relatively re- are valid complaints that can be leveled at the Burke and Fe- cently, the complete carpal structure was understood for duccia study (e.g., see Chatterjee, a; Garner and relatively few theropods. However, the wrist is now known Thomas, ; Padian and Chiappe, a,b; Zweers and in many theropods, and the presence of a semilunate carpal Vanden Berge, ), and I could add some additional characterizes a broad taxon (Neotetanurae), where it can ones. But rather than expand this discussion with elaborate be seen to be a distal carpal element (Sereno, a). For counterpoints, a broader issue needs to be raised, and that example, within coelurosaurs there are specimens (e.g., is the relationship between different kinds of data, in this Scipionyx samniticus [Dal Sasso and Signore, ] and case paleontological and embryological. There is a sense Sinornithoides youngi [Russell and Dong, ; personal among some that the embryological signal must be correct observation of IVPP V]) that clearly show the semi- because it involves seemingly high-tech bench science and lunate carpal to be distal to another carpal element (the makes reference to hox genes, as opposed to the dust and radiale), clinching its identity as a distal carpal. Neither dirt of paleontology. The fact is that both disciplines require Feduccia () nor Martin () cited the new evidence a lot of interpretation of the data. The observed embryonic from Sinornithoides, despite the fact that Russell and Dong condensations and their pattern of connectivity do not (:) clearly identified both a radiale and a semilunate come with unequivocal labels but rather require much in- carpal. There seems to be little reason to doubt the homol- terpretation before elements can be identified. For instance, ogy of the carpal elements of nonavian maniraptorans, the structure that Burke and Feduccia () identified as a Archaeopteryx, and other birds. Some of the challenges to transient metacarpal V in chicken hand development could other characters have been addressed by other workers, for indeed be just that, but it is never part of the metacarpal ar- example, the furcula and sternum (Norell and Makovicky, cade and could just as easily be regarded as a bifurcation of ; Norell et al., ; Makovicky and Currie, ; Clark the adjacent carpal condensation. Given that these embry- et al., ; Sereno, a) and the pelvis (Norell and ological data are no “cleaner” than paleontological data, I Makovicky, , ). am reluctant to accept their implications, particularly when Two issues have been proposed as being so important studies by Hinchliffe (e.g.,) clearly document that avian that they alone would have the power to overthrow the en- hand development is a complicated and unusual system tire theropod hypothesis. These issues are the homology of (with the ulnare progressively disappearing and replaced by the manual digits and the evolution of the lung ventilatory a mysterious “X element” of uncertain origin). Thus, I do mechanism. The question of digital homologies has a fairly not find compelling the paper of Wagner and Gauthier long and extensive history (see Hinchliffe and Hecht, ) () that argued that, in effect, both embryologists and but basically involves a conflict between paleontology and paleontologists are correct. They suggested that there has embryology. There is almost unanimous agreement that been a “frame shift” in “developmental identities” over the the pattern of digit reduction observed throughout thero- course of theropod phylogeny such that embryonic con- pod phylogeny indicates that the digits of maniraptorans densations II–IV differentiate into definitive digits I–III are I-II-III (Ostrom, ; Gauthier, ; Tarsitano, ; somewhere near the origin of Tetanurae. It is a tidy and in- Feduccia, ; Sereno, b; Chatterjee, a; Padian triguing hypothesis, but it seems largely untestable (since and Chiappe, b). Thus, those who regard birds as di- the embryology of fossil taxa is unknowable) and probably nosaurs accept that birds also retain digits I–III. The con- is circular (they must “postulate a frame shift” and hence

20 LAWRENCE M. WITMER cannot then use these data to deduce a frame shift). In sum, pump hypothesis, Ruben et al. () made no mention of unless some new line of evidence arises, I continue to find any relevance of this research to the origin of birds, al- the I-II-III assessment to be the most conservative and the though Feduccia (a) continued to tout such evidence best supported. as damning to the theropod hypothesis. Whereas the discussion on digital homologies often Despite such external challenges to the theropod origin seems to be a tired topic, a fresh new challenge came from a of birds, this hypothesis has survived and continually gains paper by J. A. Ruben and colleagues (). In this paper, news adherents. This final section will examine briefly the they argued that theropod dinosaurs lacked an avian-style theropod hypothesis from within, particularly with regard flow-through lung (i.e., with abdominal air sacs, etc.) but to the diversity of opinion. In my  review, I was able to rather had a crocodilianlike “hepatic piston” whereby the report a fair amount of diversity within the general notion lungs were ventilated with the assistance of a hepatic- that birds were somehow closely related to theropods. At diaphragmatic complex that was retracted (like a piston) by that time, there were active hypotheses that suggested that diaphragmatic muscles attaching to the pubis. Despite ob- birds were closest to coelophysoids, troodontids, ovirap- vious significance for dinosaur physiology, what has engen- torosaurs, dromaeosaurids, and Avimimus (see Witmer, dered more controversy is a statement in the Ruben et al. , and references therein). At the dawn of the twenty- () paper suggesting that the presence of such a lung ven- first century, there is remarkably little diversity. Instead, the tilatory system in theropods would effectively deny them phylogenetic analyses from all the sources cited earlier seem the possibility of being the progenitors of birds. Their point to be converging on close relationships to Dromaeosauri- is that the hepatic piston is an evolutionarily canalized sys- dae, Troodontidae, or a Deinonychosauria clade (Dro- tem that could never evolve into an avian system. Thus, de- maeosauridae + Troodontidae). spite all the evidence from cladistics, birds could not have Still there is diversity. A recent development that has not had their origins among theropod dinosaurs. received wide attention derives from the collaboration of Evaluating this proposition involves two separate issues. G. A. Zweers and J. C. Vanden Berge (Zweers et al., ; First, did theropods actually have a crocodilianlike hepatic Zweers and Vanden Berge, ). These authors have de- piston? And second, even if they did, how do we know that vised an elaborate and intriguing scheme for the evolution such a system could not evolve into the avian system? The of the feeding or trophic apparatus in birds. Despite Feduc- first issue is really beyond the scope of this chapter. Never- cia’s consistent portrayal (e.g., Feduccia, , ) of the theless, Ruben et al. (:) stated that “the hepatic- origin debate as basically a dichotomy between ornitholo- piston diaphragm systems in crocodilians and theropods gists and paleontologists, Zweers and Vanden Berge, two of are convergently derived.”Thus, the theropod system would the leading anatomical ornithologists in Europe and North constitute, according to the inferential hierarchy of Witmer America, respectively, firmly embedded birds within Thero- (a), a level III inference, that is, a relatively weak soft- poda. In fact, they not only “embedded” birds within tissue inference requiring exceptionally compelling mor- Theropoda but actually “scattered” avian clades through- phological evidence. The second issue is more pertinent out Theropoda in that they, somewhat reminiscent of Lowe here in that it speaks directly to the origin of birds. The (, ), argued for the polyphyly of birds (Zweers problem with the claim of canalization by Ruben et al. and Vanden Berge, ). In their scheme, Archaeopteryx, () is that it is based not so much on evidence as on au- Alvarezsauridae, and Enantiornithes form a clade with thority. That is, canalization is asserted rather than demon- dromaeosaurids as the basal taxon; hoatzins, cranes, and strated. These authors have argued that, basically,“you can’t palaeognaths form a clade with Hesperornis, Ichthyornis, get there from here.”How could we test this hypothesis that and ornithomimids; and all other birds and Confuciusornis the avian system could not evolve from the presumed he- form a clade with troodontids as its basal taxon. It is im- patic piston of theropod? An obvious test is a phylogenetic possible to do justice here to the complexity of the func- one: integrate it with other data and let it play out on the tional arguments presented in these papers, although they cladogram. Given the considerable evidence that birds are are very enlightening and engaging whether or not one ac- embedded within Theropoda, it would seem that indeed cepts the authors’ phylogenetic scheme. In fact, throughout “you can get there from here,” even if the physiological or both papers, particularly Zweers et al. (), it is not that anatomical mechanism is at present obscure. clear whether the scheme is intended to reflect a hypothet- In sum, regardless of whether the inference of the ical functional framework or a true depiction of phylogeny. hepatic-pump ventilatory system in theropods is suffi- But in Zweers and Vanden Berge (:) it eventually be- ciently robust to be sustained, there seems to be little in the comes clear that they indeed regard birds as polyphyletic, Ruben et al. () paper that requires an overhaul of our describing “successive waves of avian radiation.” Neverthe- views on avian origins. Significantly perhaps, in a more re- less, they recognized that “at several points this scenario cent paper that sought to bolster the theropod hepatic- does not coincide with the most recent avian phylogeny,

THE DEBATE ON AVIAN ANCESTRY 21 which remains to be explained” (Zweers and Vanden Berge, similarities remain homologous. The “successive waves of :). avian radiation” scheme described by Zweers and Vanden Indeed, it is likely that their scheme would be found to Berge is not far removed from the successive waves of thero- be less parsimonious than a cladogram produced by, say, pod descent from an avian stem envisioned by Paul and Ol- Chiappe or Sereno. But their notion of “successive waves of shevsky. In all these formulations, the evolution of birds and avian radiation” is not entirely new and represents just the theropods is hopelessly intertwined. The current orthodoxy most recent version of the idea that birds and “conven- (Ostrom, ; Gauthier, ; Sereno, a) has produced tional” theropods are more intertwined than commonly a fairly tidy phylogenetic pattern. These nonstandard views thought. For example, G. S. Paul (, , ) suggested are decidedly untidy, yet they still should receive serious that perhaps the traditional ancestor-descendant relation- consideration, and this will happen only when they are ships have been interpreted backward: perhaps some “con- framed in explicit, phylogenetic terms. ventional” theropods (such as dromaeosaurids, troodon- fl tids, oviraptorosaurs) are in fact secondarily ightless Conclusions descendants of a persistent lineage of “protobirds.” This protobird lineage would have had its origins in the Jurassic The issues surrounding the origin of birds are wide rang- period with Archaeopteryx, becoming progressively more ing, and this chapter has attempted to capture this diver- birdlike in the Cretaceous. Paul () cited a variety of sity. As a result, it is a bit of a hodgepodge and has sought lines of evidence by which “neoflightless” taxa could be to touch on those issues that, in particular, have controlled identified, pointing in particular to the shoulder girdle and the debate. Throughout the chapter, I have focused on the thoracic appendage. In some ways, this hypothesis arises debate itself, because how things are discussed affects from the realization that, unlike in other groups of flying what is discussed. That is, rhetoric and science are, lamen- vertebrates (i.e., bats and pterosaurs), flightlessness has tably, inextricably linked. As I am neither a sociologist been a recurrent evolutionary theme of birds. Thus, what nor a psychologist, I have tried not to delve into matters would a flightless form look like at about the Archaeopteryx of motivation, politics, and ego. Nevertheless, in this stage? It might look very much like a small dromaeosaur. modern media age, it would be naive to think that these Paul () acknowledged that the cladistic representation factors have not helped shape the debate, and the sociol- and discovery of such a pattern are problematic. Paul is not ogy of the debate would be a very interesting study indeed. alone in deriving some “nonavian” theropods from birds. Science and the scientific method, however, will ultimately For example, Elzanowski (, ) and Lü () re- prevail and produce a broad consensus on at least the garded oviraptorosaurs as being not just very closely related major issues. to birds but potentially a clade of early flightless birds. If this For paleontologists faced with controversy, the tradi- is proven true, Feduccia (a), perhaps ironically, would tional appeal is for more and better fossils. In this case, then be correct that the basal oviraptorosaur Caudipteryx is however, methodology—not fossils—will have to be the just a Mesozoic kiwi after all! Finally, Olshevsky () pro- key to achieving agreement and converting dissenters. We posed the “Birds Came First” (BCF) theory, which main- already have abundant well-preserved fossils documenting tains that the evolution of archosaurs is characterized by an the transition to birds among theropod dinosaurs. Yet arboreal “central line” of “dino-birds” that sprouted terres- those opposed to theropod relationships remain unwilling trial branches, giving rise to the various clades of archo- to accept and apply the cladistic methodology that has elu- saurs. Throughout the Mesozoic, this central line would cidated this transition. If the theropod ancestry of birds is have gotten more and more birdlike, and thus their terres- the nonsense that some would have us believe, why is it that trial offshoots also became progressively birdlike. As in so many highly trained specialists seem to keep confusing Paul’s hypothesis, the Cretaceous coelurosaurs would be birds for dinosaurs or vice versa? The list of taxa that have secondarily flightless. bounced back and forth between birds and theropods is All these latter ideas are truly out of the mainstream of quite long: Alvarezsauridae, Archaeopteryx (Eichstätt spec- current thought and present some problems for testing by imen), Archaeornithoides, Avimimus, Avisaurus, Bradyc- phylogenetic analysis. In a sense, they are similar to the neme, Caenagnathus, Caudipteryx, Limnornis, Oviraptori- scenario-based, “function-first” methodology criticized in dae, Palaeocursornis, Protarchaeopteryx, Protoavis, Wyleyia. an earlier section. Nevertheless, they merit the scrutiny that It would seem to be simple common sense to think that they have never adequately received. As a class, they are all birds and dinosaurs must have some close relationship if we very similar in that they propose an iterative process to the have such trouble telling them apart. Of course, evolution- evolution of birds and theropods. For what it is worth, these ary convergence is the usual explanation invoked by oppo- proposals have the distinct advantage that all the supposed nents of theropod relationships to explain the resem- time discordances basically disappear yet all the anatomical blances. But this, too, seems to fly in the face of logic: on

22 LAWRENCE M. WITMER the one hand, we are told that the similarities have arisen S. Chatterjee, L. M. Chiappe, P. J. Currie, T. D. Jones, G. S. Paul, because of convergence—the independent acquisition of J. A. Ruben, Xu X., and Zhou Z. Funding was provided by the  similar attributes due to similar function and mode of National Science Foundation (IBN- ) and the Ohio Uni- life—but then, on the other hand, we are told that thero- versity College of Osteopathic Medicine. pods and the ancestors of birds had totally different body plans, body sizes, preferred habitats, and modes of life (i.e., Literature Cited large, bipedal, terrestrial theropods versus small, quadru- Abel, O. . Die Vorfahren der Vögel und ihre Lebensweise. pedal, and arboreal alternative ancestors). How can they Verhandlungen der Zoologischen-botanischen Gesellschaft, have it both ways? Cladistic methodology argues that con- Vienna :–. vergence and homology are not asserted or assumed but Ackerman, J. . Dinosaurs take wing. National Geographic     rather are hypothesized and subsequently tested within a ( ): – .  comprehensive phylogenetic analysis. I fully expect that, if Bakker, R. T., and P.M. Galton. . Dinosaur monophyly and a new class of vertebrates. Nature :–. we had the true and complete phylogenetic tree, we would Balda, R. P., G. Caple, and W. R. Willis. . Comparison of the fi indeed nd that some of the characters shared by birds and gliding to flapping sequence with the flapping to gliding se- maniraptoran theropods were convergently acquired—but quence; pp. – in M. K. Hecht, J. H. Ostrom, G. Viohl, certainly not all of them, considering that there is hardly a and P. Wellnhofer (eds.), The Beginnings of Birds. Freunde bone of the body that fails to show synapomorphies at des Jura-Museums, Eichstätt. some level. Barsbold R. . Opisthopubic pelvis in the carnivorous di-    The debate on avian ancestry has had a long and con- nosaurs. Nature : – . ———. . Carnivorous dinosaurs from the Cretaceous of tentious history, and this is likely to continue for some time. Mongolia. Transactions of the Joint Soviet-Mongolian It all started with Archaeopteryx, and Archaeopteryx remains Paleontological Expedition :–. [Russian, English sum- fi the central gure, although, if the situation is as compli- mary] cated as Paul () or Zweers and Vanden Berge () sug- Barsbold R. and Perle A. .Modification of saurischian pelvis gest, this may prove to have been folly. A remarkable devel- and parallel evolution of carnivorous dinosaurs. Transactions opment witnessed in recent years is that, as support for the of the Joint Soviet-Mongolian Paleontological Expedition    theropod origin of birds mounts and becomes more : – . 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