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Re: Ah ha! That's where therizinosaurs came from
David Marjanovic <firstname.lastname@example.org> wrote:
> Oh no. They retain a _cranially_ directed hallux. A medially directed one is
> found in *Confuciusornis* (and for instance the kiwis).
Sorry, I worded this poorly. In _Archaeopteryx_, the first metatarsal
attaches to the medial surface of the second metatarsal (based on Mayr
et al., 2005, 2007).
But I had thought _Confuciusornis_ had a hallux that was more
posteromedially oriented. Metatarsal I was slightly twisted and
distally deflected. Still a long way from a fully reversed hallux -
but on the right path.
> Somebody should have a look at *Changchengornis* and the scansoriopterygids.
> The latter have a pretty long and fully descended hallux...
Yes, the articulation of metatarsal I of _Epidendrosaurus_ with
metatarsal II was so low that all four toes inserted on the metatarsus
at around the same level. So technically, _Epidendrosaurus_ had an
incumbent hallux, as in perching birds. The first pedal digit was
also quite long, and the penultimate phalanges of the other toes were
elongated. However, the hallux of _Epidendrosaurus_ was not reversed.
So although _Epidendrosaurus_ had a functionally tetradactyl pes that
was adapted for clinging, it was not adapted for perching.
Derived therizinosaurs also had a tetradactyl pes, but the hallux was
not incumbent. Here, metatarsals I-IV were short but massive, and
metatarsal I was enlarged. But metatarsal I articulated quite high on
metatarsal II, and was much shorter than the other three metatarsals.