Bird metacarpal homology

Jonathon R. Wagner

Something George Olshevsky said about a month ago got me to lookin' up stuff, and I came across:

Hinchliffe 1985 "One, two, three" or "Two, three, four": An Embryologist's View of the Homologies of the Digits and Carpus of Birds, in The Beginnings of Birds (Bronner and Drentler, Eichstatt, 1985).

Hinchliffe attempts to demonstrate, using two different lines of embryological evidence, that the digits of the avain carpometacarpus are II-III-IV, then proceeds to use this as evidence that birds are not derived dinosaurs.

The latter point strikes me as weak, since he is using embryological evidence to dispell the homologies posited by workers who are looking only at osteological evidence, while a priori accepting the homolgies these workers postulate for another group. On the other hand, part of the point of the paper is that homologies established on osteological evidence may be weaker than is often thought. In any case, if one were to accept Dr. Henchliffe's findings at face value, most parsimoniously it would simply cause us to reconsider the homologies of the theropod manus (translation: if birds' digits are II-III-IV, given the evidence, isn't it just a simpler conclusion that dinosaurs' digits were II-III-IV?).

The case in point is the ulnare, which was previously thought to make up part of carpometacarpus. Hinchcliffe demonstrates that an element, which he considers the ulnare (he's got me convinced), actually disappears during ontogeny, and an element, which he considers the pisiform, fuses with the carpometacarpus.

For those of you out there who really groove on this stuff, there's also a bone identified as X for which the homologies are unclear. This bone fuses to the carpometacarpus in the place where the ulna was thought to attach.

Hinchliffe's first major point involves the location of the bone he identifies as the pisiform. He identifies it based on it's location with respect to the radius, ulna and the "postaxial" border. Based on it's location between a small splint he identifies as a vestigial metacarpal (which he assigns as V), he declares that the minor metacarpal of birds is homologous to the IV digit of other tetrapods.

Which leads to the questions

1. Is there evidence that the bone Hinchliffe identifies as the pisiform is in fact homologous to that bone in other animals? Either Paul or Bakker states that dinosaurs lack a pisiform. Of course, given Hinchliffe's statements on osteologically determined homologies, can we be sure of anything?

2. If it is the pisiform, is it wholly improbable that this bone might migrate to support the carpometacarpus if digits IV and V were lost? In the paper, Hinchliffe does not consider the possible effects reduction of digit IV/V/whatever would have on the pisiform.

Hinchliffe's second point, which I will admit is way, way over my head (my apologies to all if I mangle the terms), involves a region of mesenchymal cell death on the anterior (thumbward/alular,etc) and distal ends of the wing bud. He interprets this anterior cell death as possibly being the "mechanism for eliminating the tissue which ... formed digit I." He cites evidence (Ede 1971, Summerbell 1981, refs below) that the width of the mesenchyme is related to the number of digits formed. He interprets this anterior cell death as possibly being the "mechanism for eliminating the tissue which ... formed digit I." He does not say whether this necrotic zone is present in other tetrapods (except the mouse), but points out that mice (no species given), which have an antero-posteriorly longer digital plate, do not have necrotic zones, and grow five digits. Not really negative evidence, but perhaps questionable.

This second point is more interesting, but leads to the questions

1. Is there evidence for such a necrotic zone in any non-avian tetrapods, especially animals which lose digits in ontogeny?

2. Is his association of the necrotic region with the loss of digit I consistent, considering that while there is a short-lived posterior (pinkie-ward) necrotic zone in the species he was studying, this is not present in other avian species, while all extant avians have experienced the loss of two digits? How certain can we be of the theory that the anterior zone is associated with digit loss when the posterior zone, in an area which certainly loses at least one digit (so we think) has no necrotic zone in other avian embryos. Could this be related to the fact that there seems to be a vestigial metacarpal still present on the posterior (pinkie) side?

3. Conversely, in this one species where there is a posterior necrotic zone, there is in fact a vestigial metacarpal, interpreted in the article as V. This seems to muddy the association of a mesnchyme with digit loss. Is this not as contradictory as it seems?

4. Does anyone know the argument made in the references cited above (full refs below) for the width of the limb bud and digit reduction? Is the evidence only from birds?

I should add that, to the untrained eye, in the pictures presented, it certainly looks like the alular digit is digit II.

I must say that, for all that, this paper is very readable for someone with little experience in embryology, although the terminology can be a bit confusing. Anyone with any knowledge of osteology or embryology, please feel free to help me better understand this paper.

Refs cited by Henchliffe:

• Ede, D.A. 1971. Control of form and pattern in the vertebrate limb - In: Control mechanisms of growth and differentiation (Davies, D.D. and Balls, M. eds.) pp 235-254 Symposium 25 of the Society of Experimental Biology. Cambridge University Press.
• Summerbell, D. 1981. The control of growth and the development of pattern across the antero-posterior axis of the chick limb bud - J. Embryol. exp. Morph. 63:161-180. Cambridge.