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Re: Biplane birds of the Mesozoic

> Xiaoting Zheng, Zhonghe Zhou, Xiaoli Wang, Fucheng Zhang, Xiaomei
> Zhang, Yan Wang, Guangjin Wei, Shuo Wang & Xing Xu (2013)
> Hind Wings in Basal Birds and the Evolution of Leg Feathers.
> Science 339 (6125): 1309-1312
> DOI: 10.1126/science.1228753
> http://www.sciencemag.org/content/339/6125/1309

One hypothesis is that these "hind wings" were used to provide extra
lift (and/or drag) and improve maneuverability.  If so, the hind wing
might have been superseded by the aerodynamic tail-fan (rectricial
fan) that evolved in ornithuromorph birds.

So I'm not convinced that the loss of the hind wings in modern birds
(and other ornithuromorphs) has anything to do with the role of the
hindlimbs in terrestrial locomotion, as proposed by the authors.  In
the concluding paragraph the authors propose that...

     "The reduction and loss of distal
      feathers on the legs reflect decoupling of the
      forelimbs from the hindlimbs in the locomotor
      system of ornithuromorph birds, in which the
      arms became specialized for flight and the legs
      for terrestrial locomotion (39)."

I don't entirely agree.  Sure, in ornithuromorphs the hindlimbs were
decoupled from the flight apparatus, with the disappearance of the
hind wings.  But... there is no evidence that the hindlimbs of basal
birds ever stopped being used for terrestrial locomotion.  In fact,
ref. 39 (Gatesy and Dial, 1996) actually says the hindlimb retained
its ancestral role in terrestrial locomotion as flight evolved in
basal birds.  So it is odd that Gatesy and Dial (1996) is cited here
by Zheng &c.

A better explanation for the reduction and eventual disappearance of
the hind wings (shown in Fig. 4) is that firstly the forelimbs became
more specialized for aerial locomotion, and then the tail became
integrated into the flight apparatus (i.e., with the development of an
aerodynamic tail-fan).  These steps reduced the hindlimb's role in
aerial locomotion: the hind wings diminished in size, and then
disappeared altogether.  But along the way, the hindlimb was used for
BOTH terrestrial and aerial locomotion IMHO.  (This likely explains
the increased range of femoral abduction in paravians.)

I suspect the disappearance of the hind wing has more to do with the
appearance of an aerodynamic tail-fan than any alleged change in role
of the hindlimb in terrestrial locomotion.  Although basal birds like
confuciusornithids and sapeornithids show improved perching abilities,
that doesn't mean they avoided the ground altogether.  (We know
confucusiornithids probably spent time on the ground - because one
ended up inside the belly of a fully terrestrial hunter
_Sinocalliopteryx_.)  There is no reason to think that basal birds
used their hindlimbs any less than modern birds in terrestrial
locomotion.  Basal birds such as _Archaeopteryx_, _Jeholornis_ and
probably even _Confucisuornis_ probably spent most of their time on
the ground, based on the proportions of the legs and feet.