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A Word on ABSRD (long)





I hate to beat my own drum too much, but I?ve been asked (off-list) to please-explain what exactly I mean by ABSRD (Anything But a Small Running Dinosaurs). How, for example, does it differ from BAND (Birds Are Not Dinosaurs). Well, as advocated by Feduccia and others, it doesn?t. At least not in my not-so-humble opinion.

There are two schools of thought on the origin of birds. The first view is that birds evolved from small, bipedal theropods. This is the view favored by most paleontologists who have worked with theropods and Mesozoic birds. The skeletons of such theropods suggest they were cursorial and ground-dwelling animals, although the possibility that some smaller theropods spent at least part of their lives in trees cannot be excluded. Among non-avian theropods, _Microraptor_ in particular shows good arboreal adaptations in its pes.

The other view, supported by Feduccia, Martin, Olson, and Ruben proposes that birds evolved from small arboreal diapsids in the Triassic. On the identity of these ancestors, Feduccia and others are a little vague. What is obvious is that Feduccia and others will consider almost any Triassic diapsid as possibly ancestral to birds, as long as it is (a) small in size, (b) shows arboreal adaptations, and (c) isn?t a dinosaur. In the absence of an explicit proposed ancestor or sister taxon, I have termed this theory on bird origins ?Anything But a Small Running Dinosaur? (ABSRD).

If it isn?t clear by now ;-) , I favor the theory that birds evolved from dinosaurs. I give BAND/ABSRD short shrift. If anyone is interested, here?s why.

As expressed by Feduccia, ?Anything But a Small Running Dinosaur? is poor science. As a professional biologist who dabbles in evolutionary theory, I have a lot of problems with the methodology behind ABSRD/BAND. You see, Feduccia have already made up their mind on how birds evolved: Birds evolved in trees and their ancestor had feathers, and any similarities between theropods and Mesozoic birds is due to convergence. With this in mind, Feduccia and co then examine the fossil record and assess each fossil on the basis of how it ?fits? with their theory.

_Caudipteryx_ is a good example of this thinking. According to recent cladistic analyses, _Caudipteryx_ is a basal oviraptorosaur and is positioned outside the Aves. In other words, according to these analyses, _Caudipteryx_ is not a bird. The existence of a feathered, ground-dwelling nonavian theropod is the kiss of death for Feduccia?s ABSRD model. However, one DML list member, who happens to agree with Feduccia, told me (off list) that Caudipteryx is a bird because Feduccia has shown that the skull of _Caudipteryx_ has features in common with the skull of early enantiornithines. Thus, _Caudipteryx_ is a secondarily flightless offshoot of volant enantiornithine stock. Make sense?

Not to me it doesn?t. After looking at the sum of _Caudipteryx_?s skeletal characters, Feduccia has chosen a certain subset of characters that are congruent with his theory on bird origins. These are ?good? characters because they fit with his theory. The other characters (such as the oviraptorid and nonavian characters of Caudipteryx) are ?bad? characters because they DO NOT fit with his model; they must be ?primitive? or convergent characters. He has assessed characters as ?good? and ?bad? based on a preconceived theory and not based on anything resembling an objective analysis. In assessing data, you have to look at ALL the data ? and take the good with the bad.

This is where cladistics comes in. Admittedly, cladistics is not perfect. Every time you crunch a matrix through PAUP (or whatever program you use) there?s no guarantee that it will spit out The One True Tree. The fossil record has enormous gaps, so the data set is going to be very incomplete. And, of course, cladistics relies upon the principle of parsimony: the shortest route between two character states is assumed to be the most likely. Nevertheless, it?s a good start. Cladistics is a method by which we can attempt to determine which characters shared by a given group of taxa are due to shared ancestry and which are not (i.e. due to convergence). It?s a way of examining ALL the available data, and has as its basic principle the not unreasonable assumption that the more closely related two taxa are, the more characters they will have in common. Assessing which features are primitive and which are derived, and which shared characters are due to shared ancestry and which are convergent, comes after the analysis has been carried out.

Proponents of ABRSD/BAND do not like cladistics. ?Garbage in, garbage out? as Feduccia has described it. But if a method gives you a result that conflicts with your theory, it may just be that the theory is flawed, not the method. In the 25 years since Jacques Gauthier carried out a cladistic analysis of the Saurischia, cladograms of the Theropoda have not changed all that much. There are a few differences of opinion among current theropod phylogenies (the position of the tyrannosaurids, alvarezsaurids, etc), but the overall topology of the Theropoda is still very much the same. And how many new genera of theropods and Mesozoic birds have been described during that time??!!

Most published cladograms, such as those of Holtz, Sereno, Chiappe, Norell (and many others), have dromaeosaurids as closest to the origin of birds (Aves). But, if a new specimen is discovered which results in, say, oviraptorosaurs being placed closer to the origin of birds than dromaeosaurids, then so be it. Holtz and Sereno will not commit hari kari in shame nor be fired from their jobs (or at least I hope not, on both accounts). It is just that an expanded data set, upon analysis, is producing a different signal. Theropod paleontologists will not say ?We KNOW dromaeosaurids are closer to birds than oviraptorosaurs, so we can ignore characters in the oviraptorosaurs that show otherwise.?

Then there are the purely intuitive phylogenies, like the model proposed by Feduccia and others for the origin of birds. The facts must fit the theory, not contrariwise as it should be. This is why Feduccia has had so much difficulty finding a group that might be ancestral to birds. In the quest for Anything But a Small Running Dinosaur as the ancestor of birds, some pretty unlikely critters have been proposed as ?relevant? to the ancestry of birds. _Longisquama_ has recently been associated with bird origins on account of its putative furcula and superficially feather-like dorsal appendages (which are more likely to be unusual scales or skin membranes). This is despite the fact that there is no good evidence that _Longisquama_ is even an archosaur. _Megalancosaurus_ was similarly proposed as a taxon close to the origin of birds because of its bird-like head, strap-like scapula and other superficially bird-like features. But, again, this ?analysis? of _Megalancosaurus_ simply plucked the superficially bird-like characters out of this taxon and ignored all the rest. Feduccia?s ABSRD theory expects to find a small, arboreal non-dinosaurian proavian in the Triassic, so fossils of small, arboreal, non-dinosaurian animals from the Triassic are viewed accordingly to whether they might possibly be bird ancestors. To me, this is like ramming square pegs into round sockets.

More seriously, ABSRD reminds me of those awful 18th and 19th century pseudosciences. Phrenology is a perfect example. As John van Wyhe recently said of phrenology ?? like so many popular sciences, Gall [Franz Joseph Gall, father of the ?science? of phrenology] and the phrenologists sought only confirmations for their preconceived hypotheses and did not apply the same standard to contradictory evidence. Any evidence or anecdote which seemed to confirm the science was readily and vociferously accepted as "proof" of the "truth" of phrenology. At the same time, contradictory findings, such as a very selfish and disagreeable person having a well-developed organ of Benevolence were always explained away.?

Pseudoscience, alas, is still with us. The ABSRD origin of the birds is just another manifestation, one that has trickled into the realms of paleontology.

Thanks for tuning in.


Tim



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Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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