Trying to Understand the Other Side: An Attempt to Explain Systematic Ornithology's Take on the Origins of Birds
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Trying to Understand the Other Side: An Attempt to Explain Systematic Ornithology's Take on the Origins of Birds
The long history of this debate has been repeatedly characterized as a fundamental schism between ornithologists and paleornithologists on the one hand, and dinosaur paleontologists on the other. Though this is not entirely correct, it is true that most ornithologists sympathize more with the "thecodont" hypothesis in some guise, than they do the theropod/bird link. Being one who has for many years loved and studied ornithology, I find myself compelled to try and explain this other side. Perhaps one is driven to defend the reputation of the discipline from the accusations and scorn heaped upon it in response to the acts and works of some of its practitioners (e.g., Alan Feduccia).
The "thecodont" hypothesis has the weight of historical precedent behind it. It was for nearly three quarters of a century paleornithological canon, thanks to the masterful work of Gerhard Heilmann and Robert Broom. In and of itself makes this postulate one that will not die easily. Great names of significant renown in ornithological science supported the "thecodont" origin of birds, as did some of the most legendary evolutionary biologists (e.g., Alfred Sherwood Romer, and G. G. Simpson), and some still do (e.g., Ernst Mayr). And again we have the instinctive appeal to the authority of these iconic individuals, some, such as the late Dr. Alexander Wetmore being very much loved by anyone who has studied our feathered friends and their evolution. But there are subtler and yet more sensible reasons at the root of the long-lived opposition in the face of great evidence to the contrary.
Larry Martin commented on it once, I believe for Shipman's popular 1998 book on the matter of bird origins. Ornithologists have quite naturally come to the question of bird origins from Neornithes down, whereas paleontologists have generally approached it in just the opposite manner. When viewed through this lens, many of the central tenets of the "thecodont" camp are suddenly thrown into sharp relief: the emphasis on arboreality and small size, the early and precocial development of the flight apparatus, e.g. This outlook has only been bolstered by the past twenty years of paleornithological research in which an astounding archaic aviary from the Mesozoic has been revealed which is largely congruent with many of the long-held suppositions about patterns in early avian evolution. From this viewpoint, the idea that cursorial, terrestrial dinosaurs could give rise to birds seems counterintuitive.
We know that this is not the case, although not for the efforts of the theropod camp. Often their greatest enemies, the singular worst thing that theropod/bird advocates have ever done is make the tiresome and zealous assertion that as birds are derived from dinosaurs, flight must clearly have evolved from the ground-up. As a former proponent of the "thecodont" hypothesis, I can say that I long rejected the theropod origin of birds precisely because it seemed contingent upon the terrestrial origin of flight, which is without question a biophysical impossibility as outlined (e.g., Caple et al. 1983). So vociferous is the advocacy of this model, however, that far more plausible scenarios which blend the classic arboreal theory for flight origin (e.g., Bock 1965) and the dinosaurian origin of birds, are only of late receiving their due attention. It would seem in light of this, that there has been a fundamental breakdown in communication between the ornithologists on the one hand, and the paleontologists on the other.
And what of convergence, that ever-ready alibi of "thecodont" hypotheses great and small? The reliance on homoplasy exhibited by the research of Feduccia et al., has been derided as a rhetorical sham (e.g. Prum 2003), which is only partly correct. While I agree that the intellectual structure of the "thecodont" hypothesis is today riddled with humiliating errors in logic, subjectivity, and dogmatic soundbites, this assertion that convergence might explain the similarity between dinosaurs and birds is not without foundation. Convergence, reversal, and parallelism are astonishingly rampant in the evolutionary history of birds, perhaps more so than in any other vertebrate clade. This fact is rarely appreciated by those who are not familiar with ornithological science and yet it is one of the most striking aspects of avian biology and phylogeny. I do not intend to defend the use of convergence to explain away any theropod/bird synapomorphy, I merely wish to illustrate that at one time it was not unreasonable.
And as for the vitriol and ad hominem (best characterized, e.g., by Feduccia 1999a, 2002, see Discussion of Feduccia (1999))? That can only be explained by psychology and the inscrutable vagaries of human behavior. It is not scientific, and it is beyond the ability to account for in any way known to me. In my lengthy discourse over the years with Alan Feduccia, covering all manner of topics from the phylogeny of neornithines to the origin of birds, the question of why he so vigorously objects to the theropod hypothesis always distills to the reception of the theropod hypothesis, and the consequent reaction to those hypotheses which dispute it. Whatever the flaws in his research on avian origins, Feduccia is right in asserting that the concept that dinosaurs are still alive, clothed in feathers and eating suet at our feeders, exerts a profound influence on people. It is an inherently romantic idea, that these exotic monsters we all love as children never in fact died out. Who does not get a thrill from saying of the returning geese that the dinosaurs are back, and thus spring must be soon on its way? This thrall has led to much abuse by the popular press and the public, and in discussing dinosaurs and their extinction with either it is virtually impossible to find a scrap of factuality. In a similar but more alarming vein there has been excess and distortion amongst dinosaur paleontologists (e.g., the sometimes overzealous Robert Bakker and Greg Paul). These types of excess are without question fuel to the objections of the "thecodont" camp, and they are to be lamented regardless of where in this fray one stands. When asked of the reaction to his popular and generally well-received book The Age of Birds (1980), Feduccia has repeatedly commented that he was assailed as a heretic for coming to the very tentative conclusion that the theropod origin of birds was inferior to a generalized "thecodont" hypothesis. Herein we see one of the fundamental problems in this debate: both of the opposing sides have been equally militant and at times equally irrational in their arguments (e.g., Ostrom claiming that there was aboslutely no evidence Archaeopteryx was arboreal).
Yet there is more at work in this vitriolic response on the part of most paleornithologists. Though it was not born of cladistics, the theropod origin of birds has found within cladistic analysis its single greatest ally. Few concepts have torn the scientific world asunder in the manner of cladistic analysis and the debate regarding its utility, contrary to popular impression, is still being bitterly waged in the halls of academia and on the pages of peer-reviewed literature. Ornithologists have historically had a difficult time isolating explicit lists of unambiguous synapomorphies in their phylogenies, as often birds are bereft of such diagnostic characters, for a multitude of reasons (the uniformity imposed on the avian bauplan by flight being but one). Given this state of affairs, it was a commendable effort put forth by intrepid ornithological systematists who sought to use cladistics to sort out the phylogenetic disasters which awaited any student of avian evolution. Unfortunately, the first, most prominent, and most public attempts to utilize cladistics within an ornithological context, were undertaken by Joel Cracraft. No more catastrophic and absurdly idiotic cladistic work has ever been carried out in the long history of phylogenetic analysis, than that done by Cracraft, and one need only look at a sampling of his research to drive the point home (e.g., see Olson 1985a for a scathing review of Cracraft's work on "Gaviomorphae," and The Neornithine 'Big Bang' for a broader critique of Cracraft's work). Such farcical analyses as presented by Cracraft, did little to instill confidence in cladistic analysis in the ornithological community at large. More competent cladistic analyses (e.g., those of Raikow in which myology was emphasized), were generally ignored. And thus it is not difficult to conceive how the ornithological community would greet a postulate robustly supported by cladistic analysis with a modicum of skepticism, and find accepting this particular cladogram where many others had been pure fiction, any more palatable. The situation has only degraded, and reading a paper in which Feduccia or his colleagues discuss cladistics is akin to reading an article in a conspiracy theory circular (as noted by Prum 2003).
There are other implications of using cladistics in unravelling the origin of birds. Cladistic analysis has revealed that as birds are nested within Theropoda and thus we cannot accurately say that all of Dinosauria suffered extinction at the KT Boundary. Some endured, and thus birds are literally living maniraptoran theropods. This constitutes a fundamental redefinition of the very term "dinosaur" in all its meanings. People inherently resist such gestalt reorganizations, for whatever inconceivable reason, and it is a lamentable fact that the human mind is at times very much fond of typological ordering. Thus, changes in meaning such as this are bound to be opposed vigorously (e.g., Charig 1985). This is perhaps a factor in the vitriol displayed by the adherents of the "thecodont" camp. Birds are after all, as the remarkable Roger Tory Peterson put it, "the most splendid example of creation." To equate them to merely living dinosaurs, no more than a phenomenonally successful theropod clade, is perhaps in the minds and hearts of some ornithologists, to denude birds of some of their glory.
Yet are ornithologists really at fault in this reaction? Are birds just a rather successful theropod clade? If so why not expunge the term "Aves" from the record and be done with it. We can refer to the feathered creatures as dinosaurs and be rid of the term "birds." Indeed, as some systematists seem hell-bent on defining Aves out of existence, this might very well come to pass. The question must be, is this accurate or at all desirable? Birds, though descended from dinosaurs, have enjoyed 65 million years of evolution that have rendered extant avians among the most unique and spectacularly divergent vertebrates in the history of life. Modern birds bear little resemblence to their dinosaurian forebears, just as humans bear little resemblence to our sarcopterygian ancestors. To eliminate the class Aves and expunge the term "bird" from our scientific and colloquial lexicon is akin to insisting that we call humans sarcopterygian fish, since that is in fact what humans are. While technically correct, the absurdity of such a stance is self-evident. There must be a limit between the concept of phylogeny and classification if either is to retain its usefulness.
Most alarming of all though, is the way in which some researchers have embraced cladistics with a religious zeal, as profound as any that of any fundamentalist. Olson's (2002) otherwise bitter epilogue in the pages of The Auk describes this phenomenon accurately as the congregations at the "Kingdom Hall of Hennig's Witnesses." So radical have these cladists become that they have derided anything which does not produce a cladogram, as unscientific. For them, cladistics is more than just a tool for deciphering evolutionary history, it has become the One True Way, the only valid way of doing science. In a similar vein, researchers who did not use cladistics in their systematic analyses, from A. S. Romer to Alexander Wetmore and John H. Ostrom, have been implicitly labelled as pseudoscientists. Central to this argument against other methods of phylogenetic analysis is that cladistics has a monopoly on objectivity, and yet this is pure fantasy. Cladistics is as subjective as any other method of phylogenetic reconstruction, and the attempt to claim that via some magical formula phylogenetic reconstruction can be rendered as objective as the so-called "hard-sciences", has no basis in reality. The search for pure objectivity in phylogenetic reconstruction will prove as elusive as the search for the fabled Sangreal.
In the defense of cladistics (and it should be noted that the author is not anti-cladistics, though he is anti-cladist), it is in fact a tremendously useful methodology for phylogenetic reconstruction, for reasons outlined elsewhere (e.g., Cladistics), and most practitioners of cladistic analysis do not elevate it dogmatic religion.
Whatever the reasoning, there is or at least was a logical basis for the objections raised by the "thecodont" camp, and their criticisms of the theropod origin of birds are not entirely derived from hollow bombast and rhetoric. It is only of late that they have degenerated into as much, and in the process tarnished the reputation of ornithological systematics as a whole. And concomitantly the theropod hypothesis advocates have in often cases grown equally arcimonious in their replies. Such as it is, this is perhaps inevitable. Nevertheless, one must wonder: what would Dr. Wetmore have thought?
And what of the claim, rising from Prum's (2003) scathing reply to Feduccia (2002), that current critiques of the theropod origin of birds are not science? While the structure of Feduccia et al's. arguments leaves much to be desired, the claim that the "thecodont" origin hypothesis is not scientific, is most alarming. Opponents of this hypothesis have charged that as it fails to make explicit predictions, it cannot be falsified and is thus by default not scientific. Yet is this actually the case? Being a former proponent of this theory, for a very long time, let us examine the allegation by concentrating on what I feel from study and personal experience, are the salient points of the "thecodont" hypothesis, regardless of the details.
This hypothesis is fairly simple in its assertions:
a) Birds are derived from basal archosaurs, between the Euparkeria node and the Dinosauria node
b) The origin of birds was in the early Jurassic or late Triassic.
c) Similarities between dinosaurs and birds are non-homologous and thus convergent
Thus, one should expect to see the following:
a) Avimorph forms amongst the basal archosaurian assemblage
b) Evidence for an evolutionary history of birds before the Archaeopteryx node.
c) Demonstrable lack of homology between elements advanced as synapomorphies between Theropoda and Aves.
Given that these are three explicit predictions which can be generated from the basic architecture of the "thecodont" hypothesis, it is not entirely clear how the actual argument (and not the way in which it is being presented), is unfalsifiable. We have no need to examine whether or not the evidence corroborates or refutes any of these predictions, as this has been done elsewhere (e.g., Avian Phylogeny and Origins, A reply to Ashby Camp and TrueOrigins on Avian Phylogeny), but this exercise does demonstrate that fundamentally, the "thecodont" hypothesis is perfectly scientific, and the flaws in its presentation are just that.
An unceasing criticism of the "thecodont" hypothesis, is that it cannot produce a specific outgroup or ancestral taxon to Aves. This allegation while accurate, has long troubled the author, for a variety of reasons. First and foremost it is rather misleading, as the proponents of the "thecodont" origin of birds have advanced several taxa as being similar to their hypothesized avian ancestor (e.g., Megalancosaurus preonensis, Longisquama insignis). More important though is the nature of the criticism itself. Is it a valid point to say that if one cannot produce the exact ancestor of a specific clade, in an alternative hypothesis, that therefore this hypothesis is unscientific? I can find no readily apparent reason for this to be so, as there are quite simply times as Feduccia et al. have tirelessly noted, that precise resolution of a probolem is not possible. Therefore, I think it wise for adherents of the theropod origin of birds to avoid using this as a principal critique of the actual concept of a "thecodont" origin hypothesis.
Given the fact that the actual concept of a "thecodont" ancestry for birds is perfectly scientific and subject to empirical test, one must wonder if the charge to the contrary (which Prum carefully avoids in his 2003 article, but other observers are less careful), is in fact based more on the idea that cladistics is the universal panacea for all phylogenetic reconstruction. Needless to say, if cladists will categorically reject any hypothesis which does not utilize cladistics, it becomes apparent that they are interested in defining nature, not describing it. To do so is to render cladistics philosophy, not science.
In the opinion of the author, current presentation of the "thecodont" hypothesis is rendered unscientific because Feduccia and his colleagues have made their presentation so malleable, that it can conform to any new data as its authors please, but I cannot reiterate enough my opinion that the actual concept of a "thecodont" origin of birds, is not in and of itself unscientific (though it is an inferior hypothesis as it has less data to support it than do the alternatives).
In his review of Feduccia (2002), Prum makes an appeal to the ornithological community to let sleeping "thecodonts" lie, and retire a hypothesis which can no longer accomodate the available data, no matter how dear historically it has been to this discipline. I cannot stress the need to move beyond the current debate architecture any more so than Prum has, but I feel his warning is as equally applied to the community of dinosaur paleontologists. Their sanctimonious pontification, rancorous allegations, and blatant excess in some instances is as harmful to their own hypothesis and the public impression of the science of avian origins, as is the fundamentalist attitude of some of the "thecodont" camp. It is time to move beyond the current debate architecture, but this applies as equally to each side as it does either in isolate.
References:
Charig, A. 1985. Analysis of the several problems associated with Archaeopteryx. In: Hecht et al. (eds.), The Beginnings of Birds, Proceedings of the International Archaeopteryx Conference Eichstatt Germany 1984, 21-29.
Feduccia, A. 1999a. 1,2,3=2,3,4: Accommodating the cladogram. Proceedings of the National Academy of Sciences 96: 4740-4742.
Feduccia, A. 2002. Birds are dinosaurs: Simple anser to a complex problem. The Auk 119(4): 1187-1201.
Prum, R. 2002. Why ornithologists should care about the theropod origin of birds. The Auk 119(1): 1-17.
Prum, R. 2003. Are current critiques of the theropod origin of birds science? Rebuttal to Feduccia (2002). The Auk 120(2): 550-561.
Olson, S. L. 1985a. The fossil record of birds. In: Farner, D.S., King, J. R. & Parkes, K. C., Avian Biology, Volume VIII, 79-252.
Olson, S. L. 2002. Review of New Perspectives on the Origin and Early Evolution of Birds. Proceedings of the International Symposium in Honor of John H. Ostrom, Gauthier, J. & Gall, L. The Auk 119(4): 1202-1205.
JGK
