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Cerebavis (Aves) from Cretaceous of Russia redescribed + dinosaur-bird ankle evolution



Ben Creisler
bcreisler@gmail.com


Two recent papers:



Stig A. Walsh, Angela C. Milner and Estelle Bourdon (2015)
A reappraisal of Cerebavis cenomanica (Aves, Ornithurae), from
Melovatka, Russia.
Journal of Anatomy (advance online publication)
DOI: 10.1111/joa.12406
http://onlinelibrary.wiley.com/doi/10.1111/joa.12406/abstract


The evolution of the avian brain is of crucial importance to studies
of the transition from non-avian dinosaurs to modern birds, but very
few avian fossils provide information on brain morphological
development during the Mesozoic. An isolated specimen from the
Cenomanian of Melovatka in Russia was described by Kurochkin and
others as a fossilized brain, designated the holotype of Cerebavis
cenomanica Kurochkin and Saveliev and tentatively referred to
Enantiornithes. We have previously highlighted that this specimen is
an incomplete skull, rendering the diagnostic characters invalid and
Cerebavis cenomanica a nomen dubium. We provide here a revised
diagnosis of Cerebavis cenomanica based on osteological characters,
and a reconstruction of the endocranial morphology (= brain shape)
based on μCT investigation of the braincase. Absence of temporal
fenestrae indicates an ornithurine affinity for Cerebavis. The brain
of this taxon was clearly closer to that of modern birds than to
Archaeopteryx and does not represent a divergent evolutionary pathway
as originally concluded by Kurochkin and others. No telencephalic
wulst is present, suggesting that this advanced avian neurological
feature was not recognizably developed 93 million years ago.


===


Luis Ossa-Fuentes, Jorge Mpodozis & Alexander O Vargas (2015)
Bird embryos uncover homology and evolution of the dinosaur ankle.
Nature Communications 6, Article number: 8902
doi:10.1038/ncomms9902
http://www.nature.com/ncomms/2015/151113/ncomms9902/full/ncomms9902.html
Free pdf:
http://www.nature.com/ncomms/2015/151113/ncomms9902/pdf/ncomms9902.pdf




The anklebone (astragalus) of dinosaurs presents a characteristic
upward projection, the ‘ascending process’ (ASC). The ASC is present
in modern birds, but develops a separate ossification centre, and
projects from the calcaneum in most species. These differences have
been argued to make it non-comparable to dinosaurs. We studied ASC
development in six different orders of birds using traditional
techniques and spin–disc microscopy for whole-mount
immunofluorescence. Unexpectedly, we found the ASC derives from the
embryonic intermedium, an ancient element of the tetrapod ankle. In
some birds it comes in contact with the astragalus, and, in others,
with the calcaneum. The fact that the intermedium fails to fuse early
with the tibiale and develops an ossification centre is unlike any
other amniotes, yet resembles basal, amphibian-grade tetrapods. The
ASC originated in early dinosaurs along changes to upright posture and
locomotion, revealing an intriguing combination of functional
innovation and reversion in its evolution.