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

[Ben Creisler <bcreisler@gmail.com>]

Ben Creisler
bcreisler@gmail.com

A comment and reply online in PNAS:

Christian Foth (2014)
Comment on the absence of ossified sternal elements in basal paravian dinosaurs.
Proceedings of the National Academy of Sciences 111 (50) E5334;
published ahead of print December 5, 2014
doi:10.1073/pnas.1419023111
http://www.pnas.org/content/111/50/E5334.extract?sid=ee1560e3-b8fe-487c-acc0-9ea231cb2d43


In a recent paper, Zheng et al. (1) studied the presence and absence
of sternal elements within the pectoral girdle of three paravian
theropods (Anchiornis, Jeholornis, and Sapeornis) from the Jehol Biota
of China, which are each known from a number of specimens. Based on
the consistent absence of any traces of sternal elements in Anchiornis
and Sapeornis, but the frequent perseveration of soft tissues (e.g.,
feathers) in the sediments of the Jehol Biota, the authors conclude
that sternal elements were completely absent in these taxa, even as
chondral elements. Because of the lack of ossified sternal elements,
Zheng et al. conclude the …

REPLY:
Jingmai K. O’Connor, Min Wang, Xiaoting Zheng, and Zhonghe Zhou (2014)
Reply to Foth: Preserved cartilage is rare but not absent: Troodontid
sternal plates are absent, not rare.
Proceedings of the National Academy of Sciences 111 (50) E5335;
published ahead of print December 5, 2014
doi:10.1073/pnas.1419403111
http://www.pnas.org/content/111/50/E5335.extract?sid=ee1560e3-b8fe-487c-acc0-9ea231cb2d43


Foth (1) critiques the conclusions of a previous work by Zheng et al.
(2), which describes the complete absence of sternal elements in
Anchiornis and Sapeornis. Although initially we were equally
skeptical, we feel that the evidence continues to be strongly
compelling, and we thank Foth (1) for giving us the opportunity to
further address potential skepticism. First, it is argued that most
theropod dinosaurs lack ossified sternal elements; this may be true of
basal members of the clade and indeed these elements are rare.
However, recent work on derived maniraptoran theropods strongly
suggests that these elements are present in most taxa but ossify late
in skeletal …

On Tue, Sep 9, 2014 at 7:25 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.