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

[Jaime Headden <jaimeheadden@gmail.com>]

Oh hey, look what's behind paywalls. Even Nature and Science make
their correspondence on articles openly accessible.

On Tue, Dec 16, 2014 at 12:34 PM, Ben Creisler <bcreisler@gmail.com> wrote:
> 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.



-- 
Jaime A. Headden
The Bite Stuff: http://qilong.wordpress.com/


"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)