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Re: Yet more on pterosaur quad arm posture



Jason Brougham <jaseb@amnh.org> wrote:


> As for the 40 million year interval, that is surely an artifact. By the
> time we see anything with a fully reversed hallux it is the Barremian, and
> birds have already radiated and diversified. Basal avialans,
> enantiornithines and ornithurines all had memebres with reversed halluces
> already. I would bet most workers imagine the radiation beginning in the
> Lat Jurassic, closer to these basal paravians.


Ornithothoraceans can (and do) have fully reversed halluces - though
interestingly, hallucal reversion was achieved in different ways in
enantiornitheans (especially avisaurids) and ornithuromorphs
(especially ornithurans).  In basal avialans, the situation is more
complicated.  The hallux seems to be more posteromedially oriented in
some forms (like sapeornithids and confuciusornithids), compared to
the ancestral condition.  In archaeopterygids and jeholornithids, the
hallux is medially oriented, as in their close (non-avialan)
relatives.  So translocation of the hallux was a gradual process, and
the most parsimionious hypothesis is that it was linked to increased
arboreality.


A "perching foot" involves more than just a more posteriorly
(caudally) directed hallux (for the anisodactyl condition).  It is
also entails a longer hallux that is located lower on the foot, plus
larger unguals.  Having the toes articulating at the same level is
also helpful.


> You make  a good point about parallel halluces. They are very close to
> parallel in Epidendrosaurus, right? And in Turner et al. 2012 they find
> Epidexipteryx may be very close to the last common ancestor of Paraves
> (figure 57).


In _Epidendrosaurus_ the hallux is descended to a point well down on
the foot (there is no sign of reversion).  We don't yet know if this
is a juvenile trait.  _Epidexipteryx_ has a more elevated hallux.





Cheers

Tim