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Re: Early evolution of avian sternum based on absence in Anchiornis and Sapeornis



The study makes the entirely logical and well-supported claim that
_Archaeopteryx_ lacked a sternum - ossified or otherwise.  This goes
against the grain of 150 years of prevailing wisdom/assumption that
_Archaeopteryx_ had a sternum; if it was chondrified/cartilagionous,
it would not be preserved.  I agree with the interpretation that its
absence from  _Archaeopteryx_ was most likely real.

The authors make the statement that "It is commonly accepted that
_Archaeopteryx_ and _Sapeornis_ were volant" with two references cited
in support (Burgers and Chiappe, 1999; Zhou et al., 2002).  Even if
"commonly accepted", the evidence is hardling compelling - especially
for _Archaeopteryx_.  It is not even settled if the shoulder joint of
either bird was capable of executing an upstroke (recovery stroke),
necessary for powered flight.

Given their claim that both _Archaeopteryx_ and _Sapeornis_ were
volant, the study then discusses how these birds might have 'made do'
without one in terms of their ability to fly.  To me, the absence of a
sternum increases the likelihood that _Archaeopteryx_ did not fly at
all - it was either primitively or secondarily flightless.  Ditto for
_Sapeornis_; there is not much daylight between _Archaeopteryx_ and
_Sapeornis_ in terms of shoulder anatomy.  But at least _Sapeornis_
has clear perching adaptations, suggesting it was arboreal (= spending
most or all of its time in trees).

Thus, _Archaeopteryx_'s flight abilities may have been fairly similar
to those of _Anchiornis_ and _Xiaotingia_ (all three were
morphologically very similar, as the study concedes).  This agrees
with the authors' later statement that "Most researchers tend to agree
that _Archaeopteryx_ had "extremely limited flight capabilities".  In
the case of _Archaeopteryx_, "extremely limited" could mean "none at
all" IMHO.  The wings could have been used for maneuverability on the
ground, for example.  As for _Sapeornis_, it might have been an
arboreal glider, although this implies it was also a good climber
(which seems plausible, using all four limbs to climb trunks).

Slipping into nit-picking mode, there's a few oddities lurking in the
text.  Firstly, "Chiropterygidae" is obviously a typo for "Chiroptera"
(bats). Secondly, I'm not sure what is meant by "centripetal role" (of
the sternum).  Thirdly, there is a possible nomen nudum (a new
enantiornithean bird).





Cheers

Tim

On Wed, Sep 10, 2014 at 12:25 AM, Ben Creisler <bcreisler@gmail.com> wrote:
> Ben Creisler
> bcreisler@gmail.com
>
> A new online paper:
>
>
> Xiaoting Zheng, Jingmai O’Connor, Xiaoli Wang, Min Wang, Xiaomei
> Zhang, and Zhonghe Zhou (2014)
> On the absence of sternal elements in Anchiornis (Paraves) and
> Sapeornis (Aves) and the complex early evolution of the avian sternum.
> Proceedings of the National Academy of Sciences (advance online publication)
> doi:10.1073/pnas.1411070111
> http://www.pnas.org/content/early/2014/09/03/1411070111.abstract?sid=e8e6d5a9-4fe5-4ffa-a6d2-6063160e0f8e
>
> Significance
>
> We have observed more than 200 specimens of Anchiornis, the earliest
> known feathered dinosaur, and nearly 100 specimens of Sapeornis, one
> of the basalmost birds, and recognize no sternal ossifications. We
> propose that the sternum may have been completely lost in these two
> taxa (and Archaeopteryx as well) based on histological analysis and
> the excellent preservation of soft-tissue structures, thus suggesting
> the absence of a sternum could represent the plesiomorphic avian
> condition. Our discovery reveals an unexpected level of complexity and
> high degree of inherent developmental plasticity in the early
> evolution of the avian sternum.
>
> Abstract
> Anchiornis (Deinonychosauria: Troodontidae), the earliest known
> feathered dinosaur, and Sapeornis (Aves: Pygostylia), one of the
> basalmost Cretaceous birds, are both known from hundreds of specimens,
> although remarkably not one specimen preserves any sternal
> ossifications. We use histological analysis to confirm the absence of
> this element in adult specimens. Furthermore, the excellent
> preservation of soft-tissue structures in some specimens suggests that
> no chondrified sternum was present. Archaeopteryx, the oldest and most
> basal known bird, is known from only 10 specimens and the presence of
> a sternum is controversial; a chondrified sternum is widely considered
> to have been present. However, data from Anchiornis and Sapeornis
> suggest that a sternum may also have been completely absent in this
> important taxon, suggesting that the absence of a sternum could
> represent the plesiomorphic avian condition. Our discovery reveals an
> unexpected level of complexity in the early evolution of the avian
> sternum; the large amount of observable homoplasy is probably a direct
> result of the high degree of inherent developmental plasticity of the
> sternum compared with observations in other skeletal elements.