RE: Bird-like tracks from Late Triassic (new Nature issue)

["Williams, Tim" <TiJaWi@agron.iastate.edu>]

Ben Creisler wrote:

> The new issue of Nature has the following article of interest:
> MELCHOR, RICARDO N., SILVINA DE VALAIS & JORGE F. GENISE, 
> 2002. Bird-like fossil footprints from the Late Triassic. 
> Nature 417, 936 - 938 (2002).
>
> "Whatever the ichnotaxonomic affinities of these 
> footprints, their producers are unknown from Late Triassic 
> skeletal remains. In particular, the Late Triassic 
> theropodan record is sparse and no theropod shows evidence 
> of an avian-like reversed hallux. Consequently, these bird-
> like footprints can only be attributed to an unknown group 
> of theropods showing some avian characters."


The paper also states, "The shallow hallucal impression, commonly
disconnected with the rest of the foot, suggests that the hallux contacted
the ground, but that it was slightly raised and probably not adapted for
perching as in some birds..."

Thus, the hallux was reversed, but evidently not descended or elongated such
that the hallux is at the same level as the other three digits, as in
specialized anisodactyl perchers.  (Note: This does not preclude arboreal
habits; many wading or ground-dwelling birds with short or raised halluces
are capable of perching in trees.)  Also, footprints that are tetradactyl
and tridactyl (without hallucal impression) are found at the same site (even
in the same track) and were probably made by the same biped, according to
the authors.  The footprints also show no evidence of webbing.

I'd very much like to hear Greg Paul's interpretation of this discovery -
especially after his words on theropod hallucal orientation in _DA_.
(Wonderful book, BTW.)



Tim


-------------------------------------------------------

Timothy J. Williams, Ph.D. 

USDA-ARS Researcher 
Agronomy Hall 
Iowa State University 
Ames IA 50014 

Phone: 515 294 9233 
Fax:   515 294 9359