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RE: Gansus Bird-Dino Connection - Penn Press Release
Tim Williams wrote-
I don't think this point gets emphasized enough. There is precious little
evidence for perching in the vast majority of Mesozoic birds - the perching
enantiornitheans are the exception. The feet of certain enantiornitheans
(avisaurids) show perching specializations, though the retroverted hallux
was achieved in a very different manner to that of modern perching birds.
Confuciusornithids (maybe) show some adaptations for perching. But the
rest (including _Archaeopteryx_, other enantiornitheans, basal
ornithuromorphs, basal ornithurans) show no perching adaptations at all.
For ornithuromorphs, a perching pes does not appear until quite late in
their evolution, well into the crown-group Neornithes. The most basal
neornithean group to exhibit perching might be certain galloanserine birds
that roost in trees - like modern tree-ducks, or guans. Either perching
ornithuromorphs were around in the Mesozoic, and the fossil record is doing
a really good job keeping them secret; or this is a relatively recent
innovation for this group, and ornithurans (as neognaths) did not venture
into the trees until the Cenozoic.
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. And thus definitely before
neognaths. There is a first metatarsal of Hesperornis known, but whether it
will be useful for resolving the precise time of origin for the reversed
hallux is uncertain, because hesperornithines were so modified for diving
they may have lost it.