© 2000 by The Society for Integrative and Comparative Biology
Origin of Birds: The Final Solution?1
1 Department of Animal Biology and Department of Earth and Environmental Science, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce St., Philadelphia, Pennsylvania 19104-6045
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The origin of birds has been discussed since the discovery and description of Archaeopteryx in Bavaria in 1861. By 1868, Thomas Henry Huxley realized its significance as a connecting form, which illustrated how birds might have evolved from dinosaurs. A century later John Ostrom articulated a convincing modern case for the origin of birds from theropod dinosaurs. Recent cladistic analyses of theropod, bird and bird-like fossils seem to confirm this scenario of bird origins. The purpose of this paper is to examine both the philosophic principles and the practice of cladistic analysis upon which the dinosaur-bird link is currently based. Cladistics is based on a Popperian philosophy that emphasizes the hypothetical nature of all knowledge. Such a philosophy seems more suitable for analyzing idealized characters unrooted in time or space rather than real objects. A philosophy of critical realism seems more congenial for analysis of evolutionary biological individuals having a real history. Cladistics uses parsimony as a first principle, which may be rejected on the grounds that nature is prodigal in every regard. Parsimony based on morphology suffices only when there are no other data sets to consider. Cladistics systematically excludes data from stratigraphy, embryology, ecology, and biogeography that could otherwise be employed to bring maximum evolutionary coherence to biological data. Darwin would have convinced no one if he had been so restrictive in his theory of evolution. The current cladistic analysis of bird origins posits a series of outgroups to birds that postdate the earliest bird by up to 80 million years. This diverts attention from the search for real bird ancestors. A more coherent analysis would concentrate the search for real avian ancestors in the Late Jurassic.
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INTRODUCTION |
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The problem of the origin of birds has been one of the great challenges of evolutionary biology for more than a century. For most of this time, Archaeopteryx stood almost alone and unchallenged as THE avian ancestor. Recently there has been an explosive growth in the Mesozoic fossil record of birds, with new discoveries in the U.S., Argentina, Spain, Madagascar, Mongolia, and above all China (Chiappe, 1995
; Chatterjee, 1997). Moreover, China has also produced several striking theropods that had not crossed the avian level, even though some are argued to have carried feathers or "proto-feathers" (Chen et al., 1998; Ji et al., 1998; Xu et al., 1999). Interpretations of these new specimens from China have recently created cover stories in the scientific and semi-popular literature (e.g., Padian and Chiappe, 1998a; Padian, 1998; Ji et al., 1998; Ackerman, 1998). Cogent scientific attempts to understand the origin of birds began with the discovery of Archaeopteryx.
Archaeopteryx lithographica, revered today as the oldest-known bird, was recovered from the Late Jurassic Solnhofen Limestone of Bavaria in 1861. The specimen was sold to the British Museum and described in detail by Richard Owen (1863); Owen saw in it support for his theory of archetypes, which he advanced as an alternative to Darwin's evolutionary theory. Charles Darwin, whose Origin of Species was published in 1859, never embraced Archaeopteryx as a transitional form predicted by his theory of evolution by natural selection. Very early on, Thomas Henry Huxley (1868, 1870) located the origin of birds within the reptiles, with dinosaurs held to be highly probable sources. Huxley regarded Archaeopteryx as a very dinosaur-like bird, and held Compsognathus to be an even more bird-like transitional reptile, although he was not certain that it was a dinosaur, possibly due to its very small size. He believed that in the Triassic more nearly bird-like dinosaurs and dinosaur-like birds would be found, but that the actual divergence of birds and dinosaurs took place during the Paleozoic (Witmer, 1991). Huxley (1870, p. 31) offered a memorable assessment:
If the whole hind quarters, from the ilium to the toes, of a half-hatched chicken could be suddenly enlarged, ossified, and fossilized as they are, they would furnish us with the last step of the transition between Birds and Reptiles; for there would be nothing in their characters to prevent us from referring them to the Dinosauria.
His observations were cogent but possibly overstated. H.G. Seeley (in Huxley, 1870, p. 31) immediately objected that the similarities of the hind limbs to which Huxley alluded were owing "to the functions they performed rather than to any actual affinity with birds." Owen (1875) rebutted Huxley's case bone by bone and concluded "But, to the Biologist who rejects the principle of adaptation of structure to function, the foregoing facts and conclusions will have no significance" (Owen, 1875, p. 86). It is fair to say that functional convergence remains a significant problem today, one that current cladistic methods of phylogenetic analysis have not eliminated. As Feduccia (1996) notes, the choice of the chick itself is a loaded one, as gallinaceous birds are far from typical birds, being themselves poor fliers and adept on the ground, and thus most likely purely on adaptive grounds to resemble dinosaurs.
Support for the dinosaurian ancestry of birds was undermined by Gerhard Heilmann, a Dane who published a superb and authoritative anatomical and osteological analysis of birds and their possible reptilian precursors (Heilmann, 1926). Although Heilmann saw many similarities between birds and dinosaurs, he found the apparent absence of clavicles in dinosaurs, the structural precursor of the avian furcula, an insuperable barrier and instead postulated "thecodonts" as the ancestors of birds, a position supported by Tarsitano (1985, 1991). An alternative scenario invokes a crocodylomorph ancestry for birds, articulated by Walker (1972) but later withdrawn (Walker 1985); it was supported by Whetstone and Martin (1979) and still viewed with sympathy by Martin (1991).
In 1969 John Ostrom concluded a study on the dromaeosaurid Deinonychus antirrhopus (Ostrom, 1969) and then was able to recognize the first "new" specimen of Archaeopteryx since 1877 (it had been described as Pterodactylus crassipes by Meyer in 1857, four years before Archaeopteryx was known) (Ostrom, 1970). This discovery opened a floodgate of new Archaeopteryx discoveries, and seven specimens are now known (reviewed in Feduccia, 1996, and Chatterjee, 1997). Moreover, Ostrom (1973, 1975, 1976), in a series of brilliant and persuasive papers, made an extremely convincing case for the bird-theropod link. Subsequently, numerous examples of clavicles have been recognized among dinosaurs (Bryant and Russell, 1993; Chure and Madsen, 1996; Makovicky and Currie, 1998), and Heilmann's objection ceased to be relevant. With the advent of rigorous cladistic analysis (e.g., Gauthier, 1986), the theropod-dinosaur link has held up extremely well. It is probably fair to state that the bird-theropod link is as firmly established as any phylogenetic link could be (Padian and Chiappe, 1998a, b), and the position that was heretical for much of the 20th century undeniably has become the current orthodoxy. My own position is that there is every reason to believe that the ancestor of birds was a small coelurosaurian dinosaur. I hasten to add that none of the known small theropods, including Deinonychus, Dromaeosaurus, Velociraptor, Unenlagia, nor Sinosauropteryx, Protarchaeopteryx, nor Caudipteryx, is itself relevant to the origin of birds; these are all Cretaceous fossils (e.g., Smith et al., 1998; Swisher et al., 1999) and as such can at best represent only structural stages through which an avian ancestor may be hypothesized to have passed. The phylogeny so vigorously defended by Padian and Chiappe (1998a, b) and Ji et al. (1998) is not about the real ancestry of birds at all. Assuming that Late Jurassic (Tithonian) Archaeopteryx is the first bird, actual ancestors of birds should probably be sought no later than the Kimmeridgian stage of the Late Jurassic. Whether actual ancestors, if found, will resemble in any way the hypothesized stages is unknown, but it is actual observation of real fossils that separates hypothetico-deductive science from mere Scholasticism.
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The status of cladistics has changed greatly in the years since Hennig's Phylogenetic Systematics was published in 1966. What was once viewed as radical is now most emphatically the orthodox, even commonplace, approach to questions of phylogenetic reconstruction, such as the origin of birds. Dissenters from current orthodoxy are few, and how they have been dealt with provides another instructive case study in the sociology of science (cf. Hull, 1988
). Feduccia (1996) wrote a book entitled The Origin and Evolution of Birds, which presents the broadest and most comprehensive book-length examination of the problem of bird origins since Heilmann's. Although some have admired the book for its breadth of biological perspective (e.g., Dodson, 1997a; Mayr, 1997), other authors committed to an exclusively cladistic perspective have dealt with it both swiftly and harshly. A review entitled "Flight from Reason" was published in Nature, a leading scientific journal (Norell and Chiappe, 1996). Such a sulfurous heading poisons the well of scientific discourse and seems unworthy of an otherwise respected and responsible journal. Another review in a similar vein blasts Feduccia as follows: "Science depends not on authoritarianism, not on hegemony of single fields, and not on majority votes, but on method. Progress is made not only as more evidence accumulates, but also as methods of analyzing evidence are developed, tested, and standardized (emphasis mine). The difficulty of the present debate is that the methods used by the rest of the community are not used by a few who still object not only to those methods but to their results" (Padian, 1997). Such methods are "now generally preferred by the systematic community, including reviewers for the systematic division of the National Science Foundation" (Padian, ibid.). Such statements could be interpreted as promoting hegemony of ideas for systematic analysis and using the ultimate threat in science, the withholding of funding, for dissenters. Furthermore, statements such as "The work of Ji et al. (1998) should lay to rest any remaining doubts that birds evolved from small coelurosaurian dinosaurs" (Padian, 1998, p. 730) appear designed to stifle scientific debate. The events described in this paragraph smack more of politics than of science (Hull, 1988), and dispel any naïve notions that science is above the fray of human affairs (Lewontin, 1991).
The purpose of this review is not to discredit the use of cladistics altogether. Rather I wish to examine cladistic philosophy to see whether its vaunted claims are justified. I also wish to distinguish between the theory and the practice of cladistics. What I question is whether cladistics is the only way to draw valid phylogenetic conclusions. Is cladistic analysis the end of systematic thought, or is it only the beginning? As a tool in systematic analysis it has a certain heuristic value. An early example of its effectiveness is the recognition of a monophyletic Dinosauria (Bakker and Galton, 1974), a position corroborated by virtually all subsequent studies (e.g., Gauthier, 1986; Sereno, 1997). Thus was a century-old problem solved. But is cladistic analysis really necessary? Does it merely confirm the obvious? Both Huxley (1868, 1870) and Ostrom (1973, 1976) were able to draw cogent conclusions about the origin of birds from theropods without recourse to cladistics. Dodson (1993) compared the results of a formal cladistic analysis of the Ceratopsia with the clustered output of a morphometric analysis of the same group, and found that the purely phenetic technique, which lacked the elaborate set of assumptions of a cladistic analysis, captured most of the same phylogenetic information. It is thus not self evident that the human intellect, unaided by computers and algorithms, is incapable of objectively assessing characters and drawing correct inferences about phylogeny.
The great appeal of cladistics follows from its representation of characters as a binary code of 1s and 0s, and the ease by which the resulting matrix can be processed by a computer according to the prescribed algorithm (Wiley, 1981; Harvey and Pagel, 1991). It is, however, severely reductionistic, and a legitimate question to raise is whether the rich tapestry of biological phenomena can adequately be represented by a bleak landscape of binary digits based primarily on morphological characters? What price is paid for such a wholesale loss of information? Lewontin (1991) regards exclusive focus on reductionism as "pure ideology." The method is aesthetically unsettling and certainly lacks the quality of elegance so highly prized by mathematicians and physicists. The obsession with morphology to the exclusion of other biological attributes leads to lengthy lists of characters, often numbering in the hundreds. Rarely, however, are these characters described to the satisfaction of the critical reader, such that it is clear precisely what the supposed character is in order to recognize it unambiguously on another specimen (e.g., Gauthier, 1986; Sereno, 1986; Holtz, 1994; Novas, 1996). The assumption always seems to be that more characters are better, with little or no attention paid to the quality and biological relevance of the individual characters, and to the problem of redundancy versus independence of characters. Often analyses with truly large data sets lead to disappointing resolution with rampant uninformative polytomies. For example, cladistics fails to provide a useful resolution of the higher level phylogeny of mammals (Novacek, 1992, 1993); or of primates (Fig. 1A of Bloch et al., 1997). Cladistic analyses often fail when confronted with extensive parallelism. For example, Cracraft (1981) used cladistic analysis of birds to reunite hawks (Falconiformes) with owls (Strigiformes) and subsequently (Cracraft, 1982), reunited loons (Gaviformes) and grebes (Podicipediformes), groups that have long been regarded as distinct. These groupings have generally been rejected by ornithologists (e.g., Mayr 1981; Sibley and Ahlquist, 1990; Gill, 1995; Feduccia, 1996) on the grounds that the similarities are due to convergence.
Cladistics or phylogenetic systematics (Hennig, 1966) is a highly formalistic and precise system of thought, with a well defined internal system of logic whose axioms are neither intuitive nor neutral in describing biological reality. It creates its own reality. A much bruited deduction of the system is that birds, by virtue of holophyly (a monophyletic taxon must include the ancestor plus all of its descendants), are not merely descendants of dinosaurs, they are dinosaurs (e.g., Padian and Chiappe, 1998a, b; Dingus and Rowe, 1998). Such a conclusion is not an objective fact of nature but a consequence of the cladistic system that one is free to accept or reject. It makes no difference to a chicken or an ostrich or Triceratops whether it be labeled as a dinosaur or not; the terms dinosaur and bird are human constructs, and as such are subject to definition and the criterion of usefulness especially in facilitating communication both among scientific workers and between a scientist and the public. The term dinosaur has been subject to considerable discussion recently. Bakker (1986) suggested that membership be extended to include the Pterosauria, generally regarded as a near outgroup of the Dinosauria (Novas, 1996). Though this suggestion has not been embraced, the logic is defensible, necessitating only the transfer of a label several nodes down the cladogram. Several of these nodes have already been labeled Dinosauromorpha and Dinosauriformes in recognition of the near-dinosaur status of Lagerpeton, Marasuchus, and Pseudolagosuchus (Novas, 1996). Patterson (1993) has, presumably facetiously, advocated removal of the name Dinosauria altogether and replacing it with Aves on the grounds of priority. In any case, the point is clear that the conclusion that a bird is a dinosaur is not a fact of nature but literally an artifact of the cladistic system.
Given that an important function of classification is communication, a major shortcoming of the practice of cladistics is communication. For example, the word dinosaur was not previously problematic—it was universally understood. Within cladistics it has now been redefined to include birds (holophyly), and then a new and cumbersome phrase, non-avian dinosaur, has been substituted. This is not progress; this is semantic obfuscation not enlightened communication. The first editor who permitted such a tortured expression should have forfeited his editorship for failure to uphold minimal standards of English. In no other realm of human discourse is it permissible to state what an entity is not rather than what it is. Another importunate malapropism is the statement "birds are avialan maniraptoran coelurosaurian tetanurine theropod saurischian dinosaurs" (Dingus and Rowe, 1998, p. 205).
An additional peculiarity of the cladistic system related to holophyly is that each taxon is a subset of its parent. The system refuses to acknowledge the origin of qualitatively new levels of organization, such as those traditionally recognized as Linnean classes (e.g., Aves, Reptilia, Amphibia, all of which are suspect under cladistics). There is an undeniable logic to this carried to a certain degree. For example, a monophyletic Reptilia may be preserved by including birds and excluding Synapsida (otherwise known as mammal-like reptiles!) and basal forms such as the captorhinomorphs, henceforth known as basal amniotes. By similar logic, the term Amphibia may be restricted to frogs, salamanders, caecilians and their common ancestor (otherwise known as Lissamphibia), thereby excluding Paleozoic types henceforth known as basal tetrapods. Beyond this, common sense balks. For example, all tetrapods including humans are logically a subset of osteichthyan fishes, and by regress, ultimately of blue-green bacteria. At what point does the rigidity of the cladistic system become logically absurd? Perhaps sooner rather than later.
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At the very heart of the cladistic method is the principle of parsimony, the concept that the phylogeny requiring the fewest steps of character transformation is the preferred explanation. Rieppel (2000) is refreshingly candid in admitting that parsimony is not a testable hypothesis, not something that is correct or incorrect, but it is a first principle, which must be either accepted or rejected a priori. It is my thesis that parsimony, at least the naïve form practiced and defended by cladists, should enjoy no special status. As a first principle, I reject it. Parsimony makes sense only if there is some reason to believe that nature is parsimonious. Is there any reason to believe that a phylogeny requiring 200 steps is more likely to reflect the actual course of evolutionary history than one with 201 steps (or one with 97,977 steps over one with, say, 97,990 steps—as per Bloch et al., 1997
)? Indeed, the idea that nature is parsimonious is a rather peculiar one, indeed an austere, possibly Calvinistic one. It flies in the face of a lengthy tradition in Western thought that speaks of the plenitude of nature, the fullness of creation, or to use Darwin's term, the prodigality of nature (Mayr, 1982). The evolutionary phenomena of homoplasy, character-state reversal and back mutation are not "parsimonious" but are expressions of the prodigality of nature. Is a method predicated on the rarity of such phenomena a reliable tool for the reconstruction of phylogeny?
Cladistic analysis is based on the primacy of morphology as data. It systematically excludes or suppresses other kinds of data, such as may derive from stratigraphy, biogeography, ecology, physiology, or DNA hybridization (Ghiselin, 1984). The justification appears to be that to admit data from fields other than morphology is to multiply ad hoc hypotheses. Rieppel, for example, notes that evaluation of the fossil record requires theory, for example, the principle of superposition. This seems to imply that morphology requires no theory, which is patent nonsense. The vivid and on-going discussion about the homology of the avian digits (e.g., Burke and Feduccia, 1997; Chatterjee, 1998; Wagner and Gauthier, 1999) illustrates that the fundamental process of identifying homology, the very basis of comparative biology, is richly laden with theory. Furthermore, there is something objectionable about the way that cladists use morphology—it is highly typological to the point of being essentialistic or idealistic, and thus pre-Darwinian in nature. Ideal morphology and cladistics have more to do with characters floating in the ether than with real organisms rooted in time and space. The biological world is populated by organisms not characters.
All sciences aspire to the claim of Popperian rigor. By explicit invocation of the hypothetico-deductive method and the principle of falsifiability, cladists claim to be strictly Popperian. But, as Ghiselin (1984, p. 219) pointed out, "by insulating their so-called theories from any but a very limited range of data, they do the precise opposite of what Popper had in mind.... Popper clearly distinguishes between ad hoc hypotheses, intended to preclude refutation, from auxiliary hypotheses, which enrich the system and narrow down the range of acceptable possibilities. There need be nothing ad hoc in phylogenetics about invoking stratigraphy, biogeography, genetics, embryology, or ecology. Popper calls not for naïve parsimony, but for stringency."
As evolutionary biologists our principal interest lies in recovering information about real organisms in real time; we should have limited tolerance for mere hypothetical schemes. Popperian philosophy emphasizes the hypothetical nature of all knowledge, and that objective knowledge and truth are unobtainable. Thus cladistics seems more relevant to a hypothetical world than to the real one. I reject the proposition that all knowledge is hypothetical—this is simply false. I know how I will feel when I drop a hammer on my foot, and I know that I shall die; these things do not require the consensus of observers. Any Popperian foolish enough not to know the consequences of stepping in front of an on-coming bus or standing beneath a falling piano will have his or her genes removed from the gene pool in a decidedly Darwinian manner! What is required is a philosophy that is more congenial to the real world that we inhabit. Critical realism with its axiom "epistemology models ontology" or "what we can know reflects the way things really are" (e.g., Polkinghorne, 1994, 1998) comes to mind. We accept the existence of a real world, and our ability to know something about it, subject always to correction.
I am prepared to accept evolution as a first principle. There is no reason why we should labor under a pre-Darwinian paradigm. Of course, it is possible that the world is really only 6,000 yr old, and the Creator has merely antiqued the world to give it the appearance of having evolved, but I reject this position (Dodson 1997b, 1998). Another artifice of cladistics is the denial of the possibility of recognizing ancestors, even though estimates of the frequency of ancestors in the fossil record range as high as 10% (Foote, 1996). Only a system that affirms the hypothetical character of all knowledge could posit a series of Early and Late Cretaceous theropods (maniraptorans) as successive sister-groups (closest relatives, potential ancestors) of a bird (Archaeopteryx) that lived in the Late Jurassic. Rational analysis (critical realism) affirms that, as the real ancestors of birds lived prior to the appearance of Archaeopteryx, these fossils remain completely unknown and their characters cannot be inferred; that is, there is no reason to expect they will look just like Velociraptor, Protarchaeopteryx, Sinosauropteryx or any other putative structural prototype or cladistic outgroup of birds. Having eschewed scholasticism and its debates about the number of angels on the head of a pin, we must observe nature not theorize about it.
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The search for the ancestry of birds is a real evolutionary problem. In the 19th century, one could take the position that the fossil record was too incompletely known to constrain the time of origin of birds or of any major group (Huxley sought the origin of birds in the Triassic or earlier (Huxley, 1868
; indeed he sought the ancestors of mammals in the Silurian!—Desmond, 1982). Today the fossil record strongly indicates that there is no credible evidence of birds prior to the Late Jurassic, when Archaeopteryx appeared. There is no evidence of an evolutionary radiation until the Early Cretaceous (Chiappe, 1995; Padian and Chiappe, 1998b), which suggests that the true origin of birds is not to be sought much earlier than that time. In order to understand the ancestry of birds, we seek a method of phylogenetic analysis that can provide us the best possible insights into the real course of evolution. A method that denies in principle the prospect of determining the ancestor and provides us with "hypothetical" ancestors that lived 60 to 80 million years after the event took place does not on the face of it fit our reasonable requirements. It follows that the analysis of the origin of birds is not the closed topic that authors have claimed. How should phylogenetic analysis proceed?
As a method, cladistic analysis may be a defensible way to start provided the investigator realizes the limitations of the method and is prepared to be self-critical about the results. The most parsimonious trees, and the consensus tree may certainly be examined. But work should not cease. Cladistics is not the end of science but only its beginning. The output has to be examined critically. As McKenna (1987, p. 65) put it, "One should follow a computer only up to the edge of an intellectual cliff." What we seek is not a maximally parsimonious construct but a maximally coherent one. If no complementary data are available, the cladistic analysis of morphology must stand faute de mieux. But if other data are available, scientific rigor demands that they be examined; to fail to do so is to opt for incoherence, and to prefer the idealized and Platonic over the biologically real and evolutionary. The more independent data sets that are congruent, the more coherent and convincing the analysis is. Stratigraphy is one potential data set (Wagner, 1995; Clyde and Fisher, 1997). Darwin defined evolution as descent with modification. To the extent that a well sampled fossil record documents the temporal course of evolution through successive strata, stratigraphy should be a relevant consideration. Clyde and Fisher (1997) concluded from an analysis of 29 fossil data sets that a relaxation of morphological parsimony by 4% resulted in a 49% decrease in stratigraphic parsimony debt. Why should the unencumbered creation of ghost lineages (e.g., Norell, 1993) in order to preserve hypothetical (idealized) schemes based only on morphology be regarded as parsimonious? Not for Popper's sake! Similarly, biogeography may be a legitimate factor in the selection of a preferred phylogeny. A phylogeny that requires multiple dispersal events between continents is less rather than more parsimonious than one that posits evolution in situ.
In summary, the goal of systematic biology is to reconstruct the rich history of life. This is best accomplished by beginning with a philosophy that recognizes the possibility of real knowledge and that incorporates as much information as possible in reconstructing a coherent picture of life. Evolutionary systematic biology should not be held in thrall to a philosophy that promises only hypothetical knowledge and delivers purely ideal analyses that patently bear no relationship to evolutionary history. To maintain that the problem of the origin of birds has been solved when the fossil record of Middle or Late Jurassic bird ancestors is nearly a complete blank is absurd. The contemporary obsession with readily available computer-assisted algorithms that yield seemingly precise results that obviate the need for clear-headed analysis diverts attention away from the effort that is needed to discover the very fossils that may be true ancestors of birds. When such fossils are found, will cladistics be able to recognize them? Probably not, as recognition of ancestors is operationally excluded by the method (Foote, 1996). Again we must ask why we should be saddled with a method that cannot give the information we seek? Cladistic analysis of morphological data may be a useful starting point for this enterprise but there is no theoretical or practical justification for stopping there. A maximally coherent phylogeny will only be obtained when all available data are considered. Darwin made convincing the case for evolution because he demonstrated the coherence of independent data sets (morphology, embryology, physiology, biogeography, paleontology) rather than relying only on a single kind of data. Why should we accept as convincing a method of systematic analysis that for ideological reasons trivializes all data sets other than morphology? We can do better!
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ACKNOWLEDGMENTS |
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I am grateful to Dr. Paul F.A. Maderson and Dr. Dominique G. Homberger for the invitation to participate in this symposium, and to Allison Tumarkin, Joel K. Hammond, Matt Lamanna and Dominique Homberger for their careful reading of this MS; full agreement with the contents herein is neither indicated nor contraindicated. I am also grateful to Dr. Olivier Rieppel for sharing with me the MS version of his talk that was presented at the Society of Vertebrate Paleontology symposium in stratocladistics in October. 1997. This project was carried out with the aid of NSF EAR 95-06694.
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FOOTNOTES |
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1 From the Symposium Evolutionary Origin of Feathers presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 6–10 January 1999, at Denver, Colorado.
2 E-mail: dodsonp{at}vet.upenn.edu
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