Tuesday, December 15, 2015

Pyramid reduction hypothesis

The pyramid reduction hypothesis is a credible explanation of the origin of bird fingers. It is "developmentally plausible, and is also consistent with the phalangeal reduction pattern seen in basal birds."
It is consistent with a pterosaur to bird theory.
It is inconsistent with a dinosaur to bird theory.

http://www.ivpp.ac.cn/qt/papers/201403/P020140314320497349455.pdf
(Xu and Mackem 2013)
The ‘pyramid reduction hypothesis’ assumes II-III-IV identities
for neornithine manual digits and postulates the existence
of a conservative five-digit pattern with a gradual,
bilateral reduction of phalanges and metacarpals in avian
evolution [9]. One proposed mechanism postulates that an
elevation in peripheral BMPs, signaling factors that modulate
cell survival and proliferation [60,61], drove bilateral medial
and lateral digital reduction [9]. This hypothesis is developmentally
plausible, and is also consistent with the phalangeal
reduction pattern seen in basal birds [9,23]. However, it predicts
that the direct avian ancestor had a five-fingered hand
with dominant digits II, III, and IV [9] which is inconsistent
with the digital reduction data from basal theropods (e.g.,
all known basal theropods, including ceratosaurs, have a
vestigial digit IV) [5,62–64].
In fact, the pyramid reduction hypothesis implies that either birds are not descended from theropod dinosaurs, or that some as yet to be discovered
basal theropods were five-fingered with dominant digits II, III, and IV.

And concerning the alternative homeotic shift (frameshift):
Also, it is difficult to identify plausible selective pressures that would drive this type of homeotic shift, considering that the post-frameshift adult hand would be morphologically identical to the pre-frameshift condition [69].

https://www.academia.edu/11078361/Pentadactyl_pattern_of_the_avian_wing_autopodium_and_pyramid_reduction_hypothesis
ABSTRACT
We report herein that a pentadactyl developmental pattern is evident in early wing morphogenesis of Gallus (chicken) and Struthio (ostrich). Five avascular zones (spatially predestined locations of contiguous metacarpal and phalangeal aggregation) and four interdigital vascular spaces are established by the regression patterns of autopodial vasculature. Transient vestiges of the first and fifth metacarpals are confirmed histologically and histochemically. They lie within the preaxial-most and postaxial-most avascular zones, respectively. These observations reveal conservative patterning of the avian hand and corroborate a II-III-IV metacarpal interpretation, argue for II-III-IV identity of ossified digits in birds, and favour a simple reduction rather than a homeotic shift in terms of the phenotype expressed by Hox genes in the phylogeny of the avian manus. We suggest that gradual, bilateral reduction of phalanges and metacarpals, via apoptosis mediated by BMP, occurred during the evolution of birds (Pyramid Reduction Hypothesis). This is congruent with the establishment of a central wing axis that became co-opted for coordinated movements. On the basis of evidence presented here, the direct avian ancestor is predicted to have been five-fingered with dominant digits (+ metacarpals) as follow: II, III, IV.

Note the following from Xu and Mackem 2013:
"However, it predicts that the direct avian ancestor had a five-fingered hand with dominant digits II, III, and IV [9], which is inconsistent with the digital reduction data from basal theropods (e.g., all known basal theropods, including ceratosaurs, have a vestigial digit IV) [5,62–64]."

Xu and Mackem note that the Pyramid Reduction Hypothesis "is inconsistent with the digital reduction data from basal theropods". But it is only inconsistent if birds evolved from dinosaurs. If birds did not evolve from dinosaurs, then there is no inconsistency. 
In fact, as Xu and Mackem acknowledge, one possible implication of the Pyramid Reduction Hypothesis is that "birds are not descended from theropod dinosaurs".

And here is an even more subtle point:
Xu and Machem conclude that for the Pyramid Reduction Hypothesis to be correct:
"the direct avian ancestor had a five-fingered hand with dominant digits II, III, and IV".
They have overlooked the possibility that the ancestor had digits I-II-III-IV and lost digit I in the transition to primitive bird. Which is what I suggest occurred in the transition from pterosaur to primitive bird.


SUMMARY:
For years, people have been striving to reconcile the I-II-III digits of dinosaurs with the II-III-IV digits of birds.
Why?
Because they believe that birds evolved from dinosaurs.
Why do they believe that?
Because the cladistic analyses seem to support that idea.
How so?
Because they show oviraptors and alvarezsaurs as intermediates (outgroups) between dinosaurs and primitive birds (Euparaves).
BUT recent studies show that oviraptors and alvarezsaurs are WITHIN Euparaves.
So there is no cladistic analysis evidence to support the dino to bird theory.
Therefore there is no reason to strive to reconcile the I-II-III digits of dinosaurs with the II-III-IV digits of birds.
Which is good, because there is no credible way to reconcile them in the first place.


For reference:
http://precedings.nature.com/documents/6375/version/1
Reply to “Limusaurus and bird digit identity”
Xing Xu1, James Clark2, Jonah Choiniere2, David Hone1 & Corwin Sullivan1
Morphological data from extinct theropods, even without considering Limusaurus and ceratosaurs, clearly contains two contradictory signals for the identification of tetanuran manual digits. Thus, neither our hypothesis nor the frameshift hypothesis is able to avoid a substantial number of homoplasies.

7 comments:

  1. Xu, Mackem 2013
    Some recently obtained fossil data are not supportive of
    the frameshift hypothesis. The frameshift hypothesis postulates
    a seamless change in the structure of the three functional
    digits, so that the digits that develop in positions 2–4
    in tetanuran theropods are morphologically identical to
    digits I–III of more basal theropods [12,13]. However, the
    hands of basal tetanurans possess a combination of features
    originally associated with digits I-II-III and II-III-IV,
    respectively, i.e., most metacarpal features supporting
    II-III-IV identities and most phalangeal features supporting
    I-II-III identities [10]. This demonstrates that any homeotic
    shift at the base of the Tetanurae must have been incomplete.
    Also, it is difficult to identify plausible selective
    pressures that would drive this type of homeotic
    shift, considering that the post-frameshift adult hand
    would be morphologically identical to the pre-frameshift
    condition [69].

    ReplyDelete
    Replies
    1. https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiLhNH7kd7JAhVMLB4KHUnkC4UQFgglMAE&url=http%3A%2F%2Fwww.ivpp.ac.cn%2Fqt%2Fpapers%2F201403%2FP020140314320497349455.pdf&usg=AFQjCNGUWv0kLHhNJ6kg9I5EYdMXImES2A&sig2=LPU0gPYq__pdmHFZouPwqQ&bvm=bv.110151844,d.dmo

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  2. http://precedings.nature.com/documents/6375/version/1

    Reply to “Limusaurus and bird digit identity”
    Xing Xu1, James Clark2, Jonah Choiniere2, David Hone1 & Corwin Sullivan1
    "Morphological data from extinct theropods, even without considering Limusaurus and ceratosaurs, clearly contains two contradictory signals for the identification of tetanuran manual digits. Thus, neither our hypothesis nor the frameshift hypothesis is able to avoid a substantial number of homoplasies."

    ReplyDelete
  3. https://www.google.ca/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0ahUKEwiLhNH7kd7JAhVMLB4KHUnkC4UQFgglMAE&url=http%3A%2F%2Fwww.ivpp.ac.cn%2Fqt%2Fpapers%2F201403%2FP020140314320497349455.pdf&usg=AFQjCNGUWv0kLHhNJ6kg9I5EYdMXImES2A&sig2=LPU0gPYq__pdmHFZouPwqQ&bvm=bv.110151844,d.dmo (Xu, Mackem 2013)

    "The ‘lateral shift hypothesis’ accepts II-III-IV homologies
    for tetanuran digits and suggests that the three functional
    manual digits took on features that primitively characterized
    more medial ones in theropod evolution via three partial homeotic changes [10,15,66]. It suggests a threestage
    scenario of manual evolution in theropods: first,
    reduction of digits IV and V near the base of the Theropoda;
    second, reduction of digit I by the point of divergence
    between ceratosaurs and tetanurans; third,
    complete loss of digit I and reappearance of a fully functional
    digit IV in early tetanuran evolution (Figures 2 and
    3). The core proposal of the lateral shift hypothesis is
    that, while a frameshift took place, it was incomplete and
    piecemeal in nature [13], so that the transformed digits retained
    vestiges of their original morphology (Figure 3). The
    lateral shift hypothesis has been considered to be a modified
    version of the pyramid reduction hypothesis [12,13],
    but this reflects a misunderstanding of both. The pyramid
    reduction hypothesis requires a gradual, bilateral reduction
    of phalanges and metacarpals from a condition of fivefingered
    hand that has dominant digits II, III, and IV [9],
    while the lateral shift hypothesis suggests re-evolution of
    a fully functional digit IV by enlarging the small metacarpal
    IV in basal theropods and adding more phalanges, rather
    than losing phalanges."

    ReplyDelete
  4. http://www.researchgate.net/publication/259253962_Thumbs_down_a_molecular-morphogenetic_approach_to_avian_digit_homology
    Thus the change of the phalangeal formula (as in the PRH [Pyramid Reduction Hypothesis]) is actually caused by the change of the transcriptome (as in the FSH [frame shift hypothesis]), which in turn is directly caused by the loss of digit I (probably shh and hoxD mediated).

    Avian forelimb digit homology remains one of the standard themes in comparative biology andEvoDevo research. In order to resolve the apparent contradictions between embryological andpaleontological evidence a variety of hypotheses have been presented in recent years. The proposalsrange from excluding birds from the dinosaur clade, to assignments of homology by differentcriteria, or even assuming a hexadactyl tetrapod limb ground state. At present two approaches prevail: the frame shift hypothesis and the pyramid reduction hypothesis. While the former postulates a homeotic shift of digit identities, the latter argues for a gradual bilateral reduction of phalanges and digits. Here we present a new model that integrates elements from both hypotheses with the existing experimental and fossil evidence. We start from the main feature common to bothearlier concepts, the initiating ontogenetic event: reduction and loss of the anterior‐most digit. It is proposed that a concerted mechanism of molecular regulation and developmental mechanics is capable of shifting the boundaries of hoxDexpression in embryonic forelimb buds as well as changing the digit phenotypes. Based on a distinction between positional (topological) and compositional (phenotypic) homology criteria, we argue that the identity of the avian digits is II, III,IV, despite a partially altered phenotype. Finally, we introduce an alternative digit reduction scheme that reconciles the current fossil evidence with the presented molecular‐morphogenetic model. Ourapproach identifies specific experiments that allow to test whether gene expression can be shifted and digit phenotypes can be altered by induced digit loss or digit gain.J. Exp. Zool. (Mol. Dev. Evol.)322B:1–12, 2014.©2013 Wiley Periodicals, Inc.

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  5. Sapeornis:
    http://img04.deviantart.net/5d8a/i/2015/113/2/0/sapeornis_skeleton_by_franz_josef73-d5q5aq2.jpg

    ReplyDelete