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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.

Mickey Mortimer