First evidence for the frameshift hypothesis?

[David Marjanovic <david.marjanovic@gmx.at>]

Xu Xing & James Clark: Homologies in the hand of theropods, SVP meeting 
abstracts 2008, 163A

"Homologies are not self-evident. Topology and special quality of structures 
are the principal criteria used in the conjecture of primary homology, and 
intermediate forms can provide the basis for alternative primary homology 
conjectures. In case of inferring the homologies of [the] theropod hand, 
most features such as manual phalangeal formula and innermost metacarpal 
morphologies of tetanurans (a theropod subgroup including birds) suggest 
that their three manual digits correspond to the digit I-III in basal 
theropods (the second criterion). However, often ignored is that some 
features such as the asymmetrical distal end of metacarpal II in some 
*Syntarsus* [oops!] specimens and digit II being the most robust one in 
non-tetanuran theropods (resembling in this respect the innermost digit in 
most tetanurans) support the unconventional hypothesis that the manual 
digits of Tetanurae are II-III-IV, which is also supported by embryological 
studies on extant tetanurans [but only if you presuppose the "primary axis" 
hypothesis, see my post from 2 days ago]. Given the contradictory 
information generated between the different criteria and even within the 
same criterion, new information from intermediate taxa can be critical. 
Ceratosaurs, an intermediate group positioned between coelophysoids and 
tetanurans, differ from other theropods in many manual features including a 
very slender digit I. Here we report a new basal ceratosaur with a strongly 
reduced digit I and a distally asymmetrical metacarpal II, which supports 
the intermediate status of the ceratosaurian hand and provides key 
information concerning the homology of the tridactyl tetanuran hand. A 
scenario proposed here to explain the evolution of the theropod hand 
suggests the reduction of digit I and enlargement of digit II in the 
ceratosaur-tetanuran common ancestor, and complete loss of digit I and an 
enlargement of digit IV early in tetanuran evolution. Although it requires 
an evolutionary increase in phalangeal counts and possibly the re-emergence 
of a small metacarpal V in basal tetanurans,  it otherwise fits well with 
the distribution of manual morphological characters on the currently 
accepted theropod phylogeny, and is explicable fromt he perspectives of 
adaptive advantage and developmental plausibility."

Discuss.

Incidentally, the new ceratosaur is _yet_ _another_ animal that's convergent 
on [possibly embargoed]. That ecomorphotype is slowly getting annoying. 
Anyway, the talk had impressive pictures.