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



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.