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Re: vertebrate numbers of digits
Since no one else has given this audience the background for
understanding this problem, I shall regurgitate here a tidbit I wrote for
another place, and hope it helps some in this forum.
First, let me say that it's not just one or two errant
embryologists who believe that birds have digits II, III, and IV in the
manus...it's apparently nearly _all_ embryologists. I can only speculate
that there are roughly equal numbers of embryological scientists as there
are paleontological scientists, so we are dealing here with the clash of
two scientific dogmas. I will simply present here the evidence as I've
read it, and leave it for all of you to individually decide which side has
more convincing evidence.
Here is a report on the past reasoning for embryologists to say
birds and theropods have different hands, and therefore can't be closely
related. The two main articles I'm using to concoct this report are:
Hinchliffe, J.R., and Hecht, M.K. (1984). "Homology of the bird wing
skeleton: embryological versus paleontological evidence." _Evolutionary
Biology_ 18: 21-39.
Hinchliffe, J.R. (1985). "'One, two, three' or 'two, three, four': an
embryologist's view of the homologies of the digits and carpus of modern
birds," pp. 141-160 in Hecht, M.K., Ostrom, J.H., Viohl, G., and
Wellnhofer, P. (eds.) _The Beginnings of Birds: Proceedings of the
International _Archaeopteryx_ Conference, Eichstätt_. Willibaldsburg:
Freunde des Jura-Museums Eichstätt.
I've simplified things here a bit for the non-scientists in the crowd.
OK! First thing: some history. Sir Richard Owen, waaaaaay back
in 1836, described the elements in the hand of a modern bird as digits II,
III, and IV (the Roman numerals are convention in enumerating digits; this
becomes helpful later when Arabic numerals are used to enumerate the
phalanges in each digit). Owen's hypothesis was later contradicted by
Parker, who said they were I, II, and III; Abel later supported this based
on _Archaeopteryx_, and it became virtual gospel amongst paleontologists
thereafter. Why? Well, Romer, in his classic _Vertebrate Paleontology_
and _Osteology of the Reptiles) textbooks, outlined the logic behind it.
Romer described the basic, ancestral, 5-fingered reptile manus (based on
numerous different fossil taxa). This ancestral hand had a phalangeal
formula of 2, 3, 4, 5, 3 -- that is, digit I (the thumb) has 2 phalanges in
it (including the claw), digit II has 3, etc. Now, by way of comparison,
the three fingers of _Archaeopteryx_ have the formula 2, 3, 4 -- as you can
see, this corresponds very well to digits I, II, and III of the ancestral
reptile hand. (By examining your own hand, you can see that humans have a
formula of 2, 3, 3, 3, 3.)
Embryologists, on the other hand, have a different point of view
for birds -- they find modern birds to have digits II, III, and IV. In
comparison to _Archaeopteryx_, this would mean that modern birds had to
lose a single phalanx from each digit (which, given the formula in human
hands, clearly isn't impossible). The basis for this, acc'd to Hinchliffe,
probably is in a couple of embryological studies done in the 1940's and
'50's, where embryologists, specifically one named Montagna, claimed to
have been able to observe in bird embryos the centers of ossification for
all 13 of the primitive reptilian carpal elements. Hinchliffe points out
that no one else since has been able to see all 13, and thinks Montagna was
probably in error. He also points out that birds are so derived,
embryonically speaking, that the standard dying of sectioned embryos won't
necessarily reveal the carpal homologies because birds simply don't
"recapitulate phylogeny" and develop and subsequently lose all 13 of the
primitive carpal elements embyogenically.
Developmentally, lizards and mammals that reduce or lose digits on
the hand (or foot) both lose digits from the outside inwards -- that is,
digits I and V are the first to be reduced and then lost, then II and IV,
etc.. This is fairly well supported based on paleontological evidence.
The assumption has always been that this is the easiest way to lose digits,
and that, because it's the most parsimonious way, birds must have done it,
Hinchliffe has done his own embryological work. He begins by
explaining how the bones in the embryo form. Bones begin to form initially
by the condensation of precartilaginous mesenchyme; once formed, it become
chondrogenic -- it's made of cartilage, and forms a framework on which
actual bone can be built (actually, the bone sequentially replaces it).
Hinchliffe has foregone the traditional "stain and look" methods, and
instead, uses a radio isotope of sulfate (SO4) which attach only to very
specific sites in the cartilaginous framework, thus eliminating the messy
"smearing" that staining can produce. This technique indicates that only
the very largest cartilaginous centers that had been identified by previous
embryologists are actually present.
What has been revealed by this is that, in an embryonic bird, the
following elements are present: WRIST: radiale, ulnare, pisiform, a distal
carpal at the base of the central metacarpal, and an element called "X"
because it's homology is unclear (it is located ventral to the ulnare);
HAND: metacarpals II, III, IV, and a tiny V. Element "X" is important
because, during embryogenesis, it replaces the ulnare. The radiale,
ulnare, and pisiform are easily identified because of their proximity to
such obvious and irrefutable elements as the ulna and radius. The
pisiform, a distal carpal (that means it's at the base of the metacarpus
instead of at the distal end of the ulna and radius), forms at the base of
metacarpal V in the embryos of all other tetrapods; thus, the tiny
chondrified center that appears distal to it must be metacarpal V, and
thus, the three other digits must be II, III, and IV. That the metacarpal
distal to the pisiform is digit IV "can be ruled out on the grounds that
number V would have to be lost in the evolutionary modification of the
developmental pattern [and IV] would have to be placed in a topographically
incorrect position posterior to the pisiform." As embryogenesis proceeds,
elements X and the distal carpal fuse with the metacarpals, leaving only
the pisiform and the radiale separate (presumably, the radiale cited by
Hinchliffe here is the "semilunate" carpal; it's certainly much larger than
the pisiform, and it correctly situated) -- he doesn't say so, but I
suspect that he belives that these two unfused elements then become the
unfused two carpals seen in _Archaeopteryx_. In modern birds, they also
fuse with the other wrist and hand elements to become the carpometacarpus
of modern birds (a peramorphic attainment).
Long before the embryonic chick forms clearly distinguishable,
separate chondrification centers in the hand and wrist, the whole arm is
designated by a limb bud on the embryo. Early on, this bud develops two
necrotic (dead cell) zones: one anterior (the Anterior Necrotic Zone, or
ANZ), and a posterior one (PNZ). Alterations of these zones in mutant
chicks produces interesting results: increasing them causes the loss of
digits; decreasing them causes polydactyly. The ANZ forms where digit I
would be and the PNZ where digit V would be. Studies of mouse embryos
(mice, of course, retain all 5 digits) develop neither an ANZ nor a PNZ.
In the chick, the PNZ does not persist (and never even forms in some bird
embryos), and, overall, the region in which the PNZ briefly appears closely
resembles this region in the mouse limb bud.
Hinchliffe acknowledges that the replacement of the ulnare with
element "X" in the avian embryo renders the traditional use of topographic
relationships of chondrification centers to each other to identify things
questionable at best, but that, in addition, it means that the assumption
of elements in adults -- fossils and otherwise -- as homologous to those of
other taxa are also questionable (e.g., the identification of the "ulnare"
in _Archaeopteryx_). However, he believes that, based partly on the
topography and partly on the development of the limb bud, that the evidence
leans more towards the interpretation of the avian manus as having digits
II, III, and IV.
Another couple of articles that are of interest in this matter are:
Thulborn, R.A. and Hamley, T. L. (1982). "The reptilian relationships of
_Archaeopteryx_." _Australian Journal of Zoology_ 30: 611-634.
Thulborn, R.A. (1993). "A tale of three fingers; ichnological evidence
revealing the homologies of manual digits in theropod dinosaurs," pp.
461-463 in Lucas, S.G. and Morales, M. (eds). _The Nonmarine Triassic:
New Mexico Museum of Natural History and Science Bulletin_ 3.
A part of the first article here includes Thulborn's resolution to
the problem of birds having digits II, III, and IV and theropod
traditionally being interpreted as having I, II, and III by simply saying
that tridactyl theropods have II, III, and IV (the pentadactyl manus of
primitive theropods, such as _Herrerasaurus_, had not yet been discovered).
The second article attempts to support the interpretation of the theropod
manus as having II, III, and IV via an interpretation of various
quadrupedal Middle and Late Triassic footprints from France and the eastern
U.S. as having been made by theropods. While the pes prints of most of
these are very theropod-like, they include hand-prints that have 4-5
fingers, in which digits I and V are much smaller than II, III, and IV.
The obvious problem, which Thulborn admits, is that it is very difficult to
determine which, if any, tracks may have been made by true theropods and
which by non-dinosaurian ornithosuchian archosaurs, which were more
frequently quadrupedal than any known theropod and which had theropod-like
Clearly, then, there are some significant barriers on both the
embryologic and paleontologic sides of this issue that must be hurdled.
Thulborn's solution -- saying that tridactyl theropod hands have digits II,
III, and IV would have been nice if it hadn't been for the interesting
hands of both _Herrerasaurus_ and _Eoraptor_, which have greatly reduced
digits IV and V and which imply that theropod hands really do have digits
I, II, and III. Before anyone just "pshaws" the embryological evidence,
please let me say that, because I'm no embryologist, I have probably not
presented the best-sounding view of the embryological evidence; indeed, the
Hinchliffe articles I cited cite many more technical articles which I have
not read (and which I don't know if I _could_ read and comprehend fully,
but I've ordered a batch of it from ILL to give it a shot) that,
presumably, present more compelling reasons for interpreting things the way
they have. I find it difficult to believe that the interpretation of the
embryological bird hands as having II, III, and IV is just blindly bought,
hook, line, and sinker, by so many embryologists (who probably number at
least as many as paleontologists) if it's so weakly supported. IOW,
there's probably something to it, and classically trained paleontologists
aren't able to effectively refute embryological evidence simply because
they don't understand it well enough; simultaneously, I would think that
embryologists suffer a similar handicap when it comes to the fossil
____/_\,) .. _
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Jerry D. Harris (214) 768-2750
Dept. of Geological Sciences FAX: 768-2701
Southern Methodist University
Box 750395 firstname.lastname@example.org
Dallas TX 75275-0395 (Compuserve: 102354,2222)
"Science _does_ have all the answers -- we just don't have all
-- James Morrow