RE: Gansus Bird-Dino Connection - Penn Press Release

[Tim Williams <twilliams_alpha@hotmail.com>]

Mickey Mortimer wrote:

> Well, the situation is more complicated.  Only a few Mesozoic 
> ornithuromorphs have had their metatarsal torsion described.
> - Patagopteryx lacks it, but is terrestrial, so that's no surprise.
> - Yixianornis lacks it.
> - Apsaravis lacks a hallux entirely.
> - Gansus has it.
>
> So based on You et al.'s cladogram, the fully reversed hallux evolved 
> sometime after Yixianornis, but before Gansus.

Although _Gansus_ shows a reversed hallux, it does not have a perching pes.  
The hallux is too short to be used for perching.  Torsion of the first 
metatarsal is essential for hallucal reversion (in ornithurans, anyway), but 
the hallux has to be long enough to oppose the other three toes to give an 
anisodactyl perching grip.  It is possible that the ancestors of _Gansus_ 
had a perching pes, and the shortness of its hallux is secondary (associated 
with its webbed feet and amphibious lifestyle).  But AFAIK no Mesozoic 
ornithuromorph has all the characters necessary for perching.  When the 
hallux is present in these taxa, it is very short.

> And thus definitely before neognaths.

Yes, metatatarsal torsion appeared well before neognaths, but there is no 
direct evidence of perching in any Mesozoic ornithurmorph.

Cheers

Tim