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Re: [dinosaur] Fossil feathers from Santana Formation, Brazil + flapping wings before birds (free pdfs)



> Also, an older paper now in final form:
>
> T. Alexander Dececchi, Hans C.E. Larsson & Michael B. Habib (2016)
> The wings before the bird: an evaluation of flapping-based locomotory
> hypotheses in bird antecedents.
> PeerJ 4:e2159

When it comes to the origin of flight in birds, I think this is one of
the most pivotal papers in recent years.  Using biomechanical models
based on aerodynamic principles this study finds that, for small
pennaraptorans, the size of the wings was the most important factor in
determining flight ability.  Thus, _Microraptor_, _Archaeopteryx_ and
other basal birds could potentially take off from the ground.  All had
enough lift potential to achieve powered take-off ("ground up"),
removing the need for gravity-assisted launches ("trees down").  This
fits with the terrestrial adaptations (and lack of any clear arboreal
adaptations) in these pennaraptorans.

Taking this further, I'd say that this means that the evolution of
perching abilities (such as seen in confuciusornithids and
sapeornithids) could have come about *because* birds could fly up into
trees.  So flight led to arboreality in birds, not the other way
around (which is the traditional "trees-down" view).  This bypasses
the need for climbing up or within trees by avian ancestors.  (I've
never been comfortable with the idea that _Microraptor_ or basal birds
were adapted for quadrupedal climbing.)

Also, under this 'wings-first' model, the expansion/hypertrophication
of the pectoral muscles and the associated refinement of the pectoral
girdle came later (enlarged & ossified sternum; sternal keel;
strut-like coracoid; more dorsal glenoid orientation; etc).  These
features evolved in order to improve flight abilities (especially
level flight). In fact, _Microraptor_ (which had an alula) was more
'advanced' in some respects than certain basal birds.  _Microraptor_
and birds (= _Archaeopteryx_ + more crownward taxa) may therefore have
evolved powered powered flight independently.  Otherwise, this ability
was a basal paravian trait, and lost in multiple taxa but retained in
_Microraptor_ and the bird line (beginning with _Archaeopteryx_); but
this scenario is highly non-parsimonious.  (If _Rahonavis_ was also
capable of a ground-level take-off, as mentioned in the CSVP 2016
abstract, then things become even more interesting if _Rahonavis_ is a
unenlagiine rather than a basal bird.)

Dececchi &c also found that other non-avian maniraptorans showed no
flight ability - although the wings of some small pennaraptorans might
have conferred some benefit in terms of vertical and/or horizontal
leaping (_Anchiornis_, _Eosinopteryx_, _Changyuraptor_, _Aurornis_).
However, there is no evidence that WAIR played any role in the
development of the avian wing and/or flight ability.  (WAIR, or
wing-assisted incline running, is when juveniles of some bird species
flap their under-developed wings to help them scale steep surfaces
with the hindlimbs.)  Dececchi &c propose that the advent of
pennaceous feathers and enlargement of the wing was driven by
selective factors entirely unrelated to flap-based aerodynamic
locomotion, such as WAIR.  In fact, the study includes a thorough and
detailed refutation of the notion that WAIR led to the evolution of
powered flight.  Instead, Dececchi &c propose that the reverse is more
likely to be true: that WAIR evolved in extant birds only because of
their ancestral highly refined flight abilities.