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RE: Gansus Bird-Dino Connection - Penn Press Release
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.