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Re: [dinosaur] Hoatzin nestling quadrupedal limb coordination + flightless ducks + extant crocodilian phylogeny



> Anick Abourachid, Anthony Herrel, Thierry Decamps, Fanny Pages, Anne-Claire 
> Fabre, Luc Van Hoorebeke, Dominique Adriaens and Maria Alexandra Garcia Amado 
> (2019)
> Hoatzin nestling locomotion: Acquisition of quadrupedal limb coordination in 
> birds.
> Science Advances  5(5): eaat0787
> DOI: 10.1126/sciadv.aat0787
> https://urldefense.proofpoint.com/v2/url?u=https-3A__advances.sciencemag.org_content_5_5_eaat0787&d=DwIBaQ&c=clK7kQUTWtAVEOVIgvi0NU5BOUHhpN0H8p7CSfnc_gI&r=Ry_mO4IFaUmGof_Yl9MyZgecRCKHn5g4z1CYJgFW9SI&m=69tkkZ7YQuH2tzPneOTK_2JnfaG-n1WnZ43GfqnZqJc&s=nz8gCt2kMBVjf-s1XDxQTKqT1gyfSqOqlWIY6RgpnG8&e=
>
> The evolution of flight in birds involves (i) decoupling of the primitive 
> mode of quadrupedal locomotor coordination, with a new synchronized flapping 
> motion of the wings while conserving alternating leg movements, and (ii)
> reduction of wing digits and loss of functional claws. Our observations show 
> that hoatzin nestlings move with alternated walking coordination of the four 
> limbs using the mobile claws on their wings to anchor themselves to
> the substrate. When swimming, hoatzin nestlings use a coordinated motion of 
> the four limbs involving synchronous or alternated movements of the wings, 
> indicating a versatile motor pattern. Last, the proportions of claws
> and phalanges in juvenile hoatzin are radically divergent from those in 
> adults, yet strikingly similar to those of Archaeopteryx. The locomotor 
> plasticity observed in the hoatzin suggests that transitional forms that 
> retained
> claws on the wings could have also used them for locomotion.


This is great work on hoatzin nestlings (note that juveniles of other
extant bird species also use wing-claws for quadrupedal locomotion
e.g. turacos; so hoatzin chicks are not unique in this regard).

It's unfortunate that _Archaeopteryx_ was dragged into this study.
Apart from a superficial resemblance between the phalangeal and claw
proportions of the manus, there is nothing similar between
_Archaeopteryx_ and juvenile hoatzins.  _Archaeopteryx_ has no
adaptations for arboreality, and its overall limb proportions are
nothing like those of a hoatzin chick (which have proportionally huge
feet, and a long and opposable hallux).  Hoatzin chicks are completely
irrelevant to the ecology of _Archaeopteryx_ (or _Microraptor_,
_Anchiornis_, etc) and/or the origin of avian flight.

The quadrupedal locomotion used by hoatzin chicks is likely a derived
avian behavior, like WAIR, not basal or ancestral for birds.