Re: Archie a bird not a dino

[Tim Williams <tijawi@gmail.com>]

>From Lee & Worthy (2011):

"Archaeopteryx is restored as a basal bird with bootstrap frequency of
73 per cent and posterior
probability of 1. These results are consistent with a single origin of
typical (forelimb-powered) bird flight."


This is assuming that _Archaeopteryx_ was actually capable of powered
flight.  Several recent studies have called this into question.


If these studies are correct, and forelimb-powered flight emerged
later in evolution (i.e., after _Archaeopteryx_), then the
phylogenetic position of _Archaeopteryx_ has little bearing on the
origin of forelimb-powered bird flight.


Also from the paper:


"The flight capabilities of some deinonychosaurs remain contentious:
the most proficient, _Microraptor gui_, used both
the forelimbs and hindlimbs, and has been argued to be a glider".


Two points. (1) It has been argued that _Archaeopteryx_ might have
been a glider (see above), although possibly capable of some flapping
ability.  (2) As in _Microraptor_, _Archaeopteryx_ had wings developed
from both the forelimbs and hindlimbs, giving it separate sets of
airfoils; so it too might have used both the forelimbs and hindlimbs
in aerial locomotion.  _Pedopenna_ also had hindwings.


Thus, in terms of aerodynamic capabilities, the distinction between
_Archaeopteryx_ and _Microraptor_ is not clear-cut.  The "four-winged"
condition might have preceded the "two-winged" condition typical of
modern birds.  Or the four-winged condition might diagnose the
Deinonychosauria, if _Archaeopteryx_ and _Pedopenna_ belong in this
clade.  (Although at least one Early Cretaceous enantiornithean also
has long hindlimb feathers.)


I only mention all this because Lee & Worthy (2011) focus a lot of
attention on the origin of forelimb-powered flight, and argue that
removing _Archaeopteryx_ from the bird clade implies two separate
origins of forelimb-powered flight (= powered by the front limbs
only), or was lost or modified in deinonychosaurs.  In reality, the
aerodynamic capabilities of deinonychosaurs and primitive birds appear
to be far more murkier - and the phylogenetic position of
_Archaeopteryx_ may have little impact on how and when
forelimb-powered flight arose in birds.






Cheers

Tim