Nature Communications: Evolution of the avian sternum

[Tim Williams <tijawi@gmail.com>]

This new study proposes that the ossified sterna of ornithuromorphs
and enantiornitheans evolved independently:

Zheng X, Wang X, O'Connor J, Zhou Z. (2012). Insight into the early
evolution of the avian sternum from juvenile enantiornithines.  Nat.
Commun. 2012 Oct 9;3:1116. doi: 10.1038/ncomms2104


Abstract

"The sternum is one of the most important and characteristic skeletal
elements in living birds, highly adapted for flight and showing a
diverse range of morphologies. New exceptional material of young
juvenile specimens from the Early Cretaceous Jehol Group in
northeastern China reveals the unique sequence of development in the
sternum of Enantiornithes, the dominant clade of Cretaceous birds. We
recognize six ossifications that together form the sternum, three of
which were previously unknown. Here we show that although basal living
birds apparently have retained the dinosaurian condition in which the
sternum develops from a bilateral pair of ossifications (present in
paravian dinosaurs and basal birds), the enantiornithine sternal body
primarily develops from two unilateral proximo-distally arranged
ossifications. This indicates that although superficially similar, the
sternum formed very differently in enantiornithines and
ornithuromorphs, suggesting that several ornithothoracine sternal
features may represent parallelism. This highlights the importance of
ontogenetic studies for understanding homology and the evolution of
skeletal features in palaeontology."


This is potentially very important in the context of recent studies.
If - and this is still an *if* - non-ornithothoracean avialans
(archaeopterygids, jeholornithids, sapeornithids, confuciusornithids)
were incapable of powered flight, then there is a real possibility
that true powered flight might have arisen independently in
Ornithuromorpha and Enantiornithes.

The ossified sternum was previously held to be a flight-related
feature that was plesiomorphic for Ornithothoraces.  But Zheng &c
dispute this, and raise the possibility that the ornithothoracean
common ancestor did not have an ossified sternum.

By contrast, the pygostyle, another flight-related feature in modern
birds, is likely plesiomorphic for Ornithothoraces.  However, there is
no evidence that the pygostyle was used for flight in the first
ornithothoraceans: rather, it was originally aerodynamically
non-functional.  Derived ornithuromorph birds used (and still do) the
pygostyle to support the aerodynamic tail-fan, which is important in
flight; but it seems that this tail-fan was not present in the
ornithothoracean common ancestor, and was a later innovation (possibly
independently acquired in some derived enantiornitheans).

Finally there's the articulation between the scapula and the coracoid,
with Enantiornithes having an opposite convex-concave articulation
compared to Ornithuromorpha.  Presumably, in the most recent common
ancestor of Enantiornithes and Ornithuromorpha this articulation was
flat.

Thus, it is possible that the ornithothoracean common ancestor had
flight abilities closer to gliding than to true flight. Enantiornithes
and Ornithuromorpha then each evolved flapping flight in their own
way.  The alternative (and IMHO more likely) hypothesis is that the
ornithothoracean common ancestor had rather weak powered flapping
flight abilities, and the flight apparatus was refined independently
by Enantiornithes and Ornithuromorpha.  Still very interesting though.





Cheers

Tim