On Sun, 04 Jan 98 05:38:56 +0800, Stephen Jones wrote:
SJ>...I was interested to read in New Scientist of an recent
>article in SCIENCE that there is even more evidence that birds did
>not come from dinosaurs, because the presumed nearest dinosaurs to
>birds, the theropods, are missing different digits ....
>
>"TRADITIONAL thinking about the ancestry of birds has been
>challenged by biologists in the US. They say that a comparison of
>dinosaur claws with bird wings and feet contradicts the widespread
>theory that birds evolved from small, flesh-eating or almost
>vanished, during evolution. But now Ann Burke and Alan
>Feduccia...have found that birds and theropods are missing different
>digits. "As a result, it's almost inconceivable that one group
>derived from the other," Feduccia says..." (Knight J., "Dinosaur
>theory put to flight," New Scientist, 1 November 1997, p20)
I have now read the SCIENCE article (there were two), and it confirms
the homological dilemma. Either: 1) birds are not closely related
to therapod dinoasaurs, but because of their close morphological
similarity, it must be due to convergence, in which case there must
be some doubt about other close fossil morphological similarities; or
2. if birds are closely related to therapod dinoasaurs, then normally
highly conserved homological features can change radically, which
would mean that other fossils thought to be distantly related may
actually be closely related. Either way, it would seem to render
homology, one of the mainstays of common descent, as a less reliable
guide to fossil relationships. Here is an extract from the first
article:
---------------------------------------------------------
The Forward March of the Bird-Dinosaurs Halted?
Richard Hinchliffe
Birds are dinosaurs." This is the provocative beginning to the
chapter on the origin of birds in a recent book on dinosaur
evolution. That it is by now the conventional view, repeated in
student text books of vertebrate zoology, is a tribute to the success
over the last 20 years of John Ostrom in persuading many biologists
that Archaeopteryx, the first bird, is a feathered dinosaur. But in
reality, the bird- dinosaur link remains contentious and a report by
Burke and Feduccia on page 666 of this issue reopens this question.
The first two Archaeopteryx fossils were found in the mid-19th
century in the 150 million-year-old Jurassic limestone of Bavaria,
both displaying remarkably preserved feathers and a reptilian
skeleton. Such a convincing mosaic of bird-reptile features was a gift
of the evolutionary gods to those such as Thomas Huxley who were
trying to establish the transformation of members of one vertebrate
class into another in evolution. Huxley saw a close dinosaur-
Archaeopteryx relation, but until the 1970s the prevailing view
was that Archaeopteryx (and therefore modern birds) derived from
the thecodonts, relatively unspecialized basal archosaurs from which
the dinosaurs probably radiated. A dinosaur origin was ruled out
because modern birds and Archaeopteryx were thought unlikely to
have regained the clavicle or wishbone that the thecodonts possessed
but the dinosaurs had apparently lost. When some dinosaurs were
then discovered with clavicles, the way was open for the return of the
"dinosaur origin" hypothesis. Ostrom proceeded to draw attention to
many similarities in the skeletons of Archaeopteryx with those of a
particular dinosaur group, the theropods. Later these
similarities were formalized into a list of synapomorphies, or shared
derived characters, by Gauthier in a Sadistic analysis supporting
the dinosaur origin hypothesis.
Particularly convincing was the evidence of similarity in limb
structure (see figure). Like Archaeopteryx, theropods have a wing/
fore-limb digit number that is reduced from five to three. Some
dromaeosaur theropods such as Deinonychus have the three main
digits showing similarity in claw structure and in the number of
elements (and their form) in each digit, while a wrist bone (the
semilunate) appeared similar to one in the same position in
Archaeopteryx. By the 1990s, the once heterodox "dinosaur origin"
theory had become orthodox.
[...]
This new work addresses a question that has been on the table for 150
years: the identity in modern birds and Archaeopteryx of the three
major digits of the wing, a highly specialized form of the
pentadactyl limb. Are the bird digits the first, second, and third
(I-II-III), or are they the second, third, and fourth (II-III-IV)?
How this question is answered affects in an important way the
interpretation of the evolution of birds. In dinosaurs there is good
evidence for the I-II-III formula since primitive theropods such as
Herrarasaurus have two posterior vestigial digits (x) and thus a
I-II-III-x-x structure. If the interpretation is that birds have
digits II-III,. IV, this presents a real problem for the theory of
their theropod dinosaur origin.
[...]
This convincing evidence of II-III-IV wing digit identity will not be to
the liking of one cladistic supporters of a dinosaur origin of birds. For
them, it introduces the possibility of convergence (rather than
common origin) as an explanation of the similarities between the
structure of the forelimb (and, indeed, of other structures) of
theropods and the wing of Archaeopteryx.
[...]
In reality, there is no easy solution to this question of bird
origins, and for the moment the theropod dinosaur origin holds sway.
Its supporters can point to some very striking theropod similarities
with Archaeopteryx. But many of these could be due to convergence,
with the birdlike dinosaurs appearing in the Cretaceous often some 75
million years after Archaeopteryx. Opponents of the orthodoxy are
less united, but the thecodont origin still has support (10, 11).
The problem for this view is the long evolutionary gap, with no
convincing intermediates. What we need is a proto-Archaeopteryx find
to complement the numerous post-Archaeopteryx finds that are now
being made. But for the time being this important developmental
evidence that birds have a II-III-IV digital formula, unlike the
dinosaur I-II-III, is the most important barrier to belief in the
dinosaur-origin orthodoxy.
[...]
(Hinchliffe R., "The Forward March of the Bird-Dinosaurs Halted?",
Science, Vol. 278, 24 October 1997, pp596-597)
---------------------------------------------------------
Colbert's authoritative "Evolution of the Vertebrates", claims that
Archaeopteryx is descended from "small theropod dinosaurs":
"Here was a truly intermediate form between the reptiles and the
birds. The skeleton alone was essentially reptilian, but with some
characters trending strongly toward the birds. The feathers, on the
other hand, were typical bird feathers and because of them
Archaeopteryx is classified as a bird-the earliest and most primitive
member of the class...It has long been evident that birds are
descended from archosaurian reptiles, and for many years it has been
thought that they had a thecodont ancestry Recent studies, however,
present convincing evidence that birds are the direct descendants of
small theropod dinosaurs." (Colbert E.H., "Evolution of the
Vertebrates", Third Edition, 1980, p183-184)
So it seems that yet another claimed transitional form, ie. theropod
dinosaur-Archaeopteryx bites the dust? Raup's admission that "more
than a hundred years after Darwin...We actually may have fewer
examples of smooth transition than we had in Darwin's time because
some of the old examples have turned out to be invalid when studied
in more detail" (Raup D.M., in Godfrey L.R., ed., "Scientists
Confront Creationism", 1983, p156), has even greater force.
Here is an extract from the second article:
---------------------------------------------------------
"Developmental Patterns and the Identification of Homologies in
the Avian Hand
Ann C. Burke and Alan Feduccia*
Homologies of digits in the avian hand have been debated for 150
years....Cladistic analyses nest birds within the theropod
dinosaurs..One key synapomorphy uniting theropods is a manus reduced
to three digits. These digits are identified as I-II-III because of
early theropods...show dramatic reduction of digits IV and V. A
theropod origin of birds implies that the digits of the avian manus
must also be I-II-III. However, neontologists have identified the
digits in the avian hand as II-III-IV in consideration of
developmental anatomy....
[...]
The developmental evidence of homology is problematic for the
hypothesized theropod origin of birds. This conflict pivots on the
significance awarded to different types of data in the identification of
homology. Comparative ontogenetic data suggest that a conserved
developmental program is causally involved in patterning the amniote
limb. The identification of early embryonic topographic landmarks
and the connectivity of cartilage precursors permits the identification
of specific digits as they develop in the pentadactyl hand.
It is parsimonious to assume that the theropod limb developed with a
typical primary axis through distal carpal IV followed by typical
development of the digital arch and digits III, II, and I, followed bat a
subsequent regression of the precursors of the fourth digit. Strong
reduction of digit IV after its precocious appearance is seen in some
lizards. The alternative would be an entirely new developmental
program. It is unlikely that a shift between the typical amniote mode
of development that generates digit IV through the primary axis, to a
limb that develops digit III through a convergent primary axis, would
maintain the pattern of cartilage condensation that is identical in
avian, crocodilian, chelonian, and mammalian limbs, and the
consistent patterns of gene expression between chicken and mouse
limbs.
As the primary axis invariably gives rise distally to digit IV in
amniotes, it serves as a consistent marker of digital identity and
assigns the homologies of the reduced bird hand as digits II-III-IV. A
variation of this pattern wherein the primary axis runs through digit
III, would eliminate any phylogenetic significance from the
morphological and molecular similarities in amniote limb
development. If such a condition could be demonstrated, patterns of
limb development would have to be decoupled from phylogeny, and
this stereotypic pattern of development accepted as convergence.
The discrepancies that arise between different methods draw attention
to a central problem of evolutionary biology, the distinction between
homoplasy and synapomorphy. It remains an open question how
heavily to weigh developmental characters in phylogenetic
reconstruction. The inclusion of fossil characters is essential to
our understanding of evolution. However, until we disqualify
developmental patterns as a means of establishing homologies, the
developmental patterns that identify avian digits as II-III-IV, cannot
be ignored."
(Burke A.C. & Feduccia A., "Developmental Patterns and the
Identification of Homologies in the Avian Hand", Science, Vol. 278,
24 October 1997, p666-668)
---------------------------------------------------------
This last paragraph states what is indeed "a central problem of
evolutionary biology": are morphological resemblances in any
particular instance due to homoplasy (convergence) or synapomorphy
(shared characters)? Indeed, because of convergence which is now
recognised to be more common than previously thought, Carroll
questions the degree to which shared characters are of value in
establishing phylogeny:
"All means of establishing homology other than those involving
developmental patterns and the fossil record are based on the
principle of parsimony-that it is more logical to accept a hypothesis
that depends on a small number of processes rather than one based on
a large number of independent processes. Cladists have applied this
principle broadly to judge alternative relationships on the basis of
the relative number of derived characters that they share. In
general, this principle is logical and to some degree underlies all
scientific thinking. However, we may question the degree to which it
is applicable to establishing phylogenies. In the case of
phylogenetic analysis, the use of parsimony is based on the
assumption that most characters evolved only once and that
convergence is rare. Surprisingly, supporters of this doctrine have
never tested this assumption. In contrast, biologists working with
both modern and extinct groups argue that convergence is very common
(Cain, 1982; Carroll 1982). Arguments for the close relationship of
groups based only on the common presence of derived features are of
little value, if convergence is equally or more common than the
unique origin of derived characters." (Carroll, R.L., "Vertebrate
Paleontology and Evolution", 1988, pp7-8)
Therefore it seems that common ancestry might be generally
true, but because of the high incidence of convergence it may be
difficult to establish in any particular case.
God bless.
Steve
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Stephen E (Steve) Jones ,--_|\ sejones@ibm.net
3 Hawker Avenue / Oz \ Steve.Jones@health.wa.gov.au
Warwick 6024 ->*_,--\_/ Phone +61 8 9448 7439
Perth, West Australia v "Test everything." (1Thess 5:21)
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