Thursday, July 2, 2015

Fingers - Quick Summary
There is a debate between embryologists and paleontologists whether the hands of theropod dinosaurs and birds are essentially different, based on phalangeal counts, a count of the number of phalanges (fingers) in the hand. This is an important and fiercely debated area of research because its results may challenge the consensus that birds are descendants of dinosaurs.
Dinosaurs have hands with digits 2-3-4-x-x. (Roman numerals represent fingers, numbers represent phalanges).
But birds have hands with digits x-2-3-4-x.
This is a problem for the dino to bird theory.
To overcome this problem, the dino to bird folk propose changes that include the following:
  • the loss of digit I 
  • the reappearance of the lost digit IV
  • digits II-III-IV adopting the phalangeal count and characteristics of the earlier digits I-II-III (via a frame shift) resulting in x-2-3-4-x with digit III the longest.  

The pterosaur transition (from 2-3-4-5-x) would be:
  • the loss of digit I
  • digits II-III-IV lose one phalange each, resulting in x-2-3-4-x with digit IV the longest (as in scansoriopterids). 
Note: The dino to bird folk do not consider scansoriopterids to be on the line leading to birds, but to have branched from that line.

Frame shift
Thus the change of the phalangeal formula (as in the PRH) 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).
Dinosaur to bird (notice the original loss of digit IV and its re-appearance)
In the diagram, Neotheropoda ( 1 ), basal tetanurae ( 2 ), a coelurosaurian ( 3 ), the bird (?)Archaeopteryx ( 4 ) and modern bird ( 5 ).

If scansoriopterygids are the basalmost members of paraves, then the very first paraves were (or were very similar to) the scansoriopterygids, which have x-2-3-4-x with digit IV being the longest.
Which is consistent with a pterosaur ancestry and contrary to a dino ancestry.

See here for more details:

1 comment:


    The theory of descent of birds from theropod dinosaurs demands that their fore-limb digit identities (1,2,3) are the same as those of birds and thus the conventional embryological identification of these as 2,3,4 remains a major problem for acceptance of this theory. Are the 2,3,4 identities of bird wing digits correct? The paper analyses the developing bird wing as a specialisation of the general developmental ‘bauplan’ for the pentadactyl skeleton. Evidence from the chick embryonic skeletogenic pattern supports interpretation of the main digits as 2,3,4 on the basis of timing, position & connections, using comparative methods eg comparison with other amniote patterns of limb skeletogenesis. Fresh support for 2,3,4 identity comes from evidence of i) a temporary embryonic digit 1 in the ostrich and of ii) a condensation-specific Sox9 molecular domain in a digit 1 position in the chick wingbud. In contrast, the recent ‘frame shift’ hypothesis of Wagner proposes molecular identity transformation by which theropod identities for 1,2,3 have become shifted to avian digit condensations 2,3,4. Support for ‘frame shift’ is claimed from evidence (Vargas & Fallon 2004) that the expression domain for Hox d 13 alone characterises digit 1, but a domain for Hox d 12 and 13 characterises digits 2-5. Here it is argued that this evidence for ‘frame shift’ is speculative and insufficiently convincing to support reinterpretation of the wing digits as 1,2,3. Evidence that wing digits have been correctly identified as 2,3,4 continues to provoke doubts about the ‘dinosaur-bird’ theory. The presence of well-defined feathers in the bird-like hands of certain dromaeosaurs (from Liaoning, China) may be due to their possibly being secondarily flightless birds, thus suggesting that birds may derive from a lineage separate from that of theropods.