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Evolution of sternal ossification patterns in ornithothoracine birds

Ben Creisler

A new online paper:

Jingmai K. O'Connor, Xiaoting Zheng, Corwin Sullivan, Cheng-ming
Chuong, Xiaoli Wang, Ang Li, Yan Wang, Xiaomei Zhang and Zhonghe Zhou
Evolution and functional significance of derived sternal ossification
patterns in ornithothoracine birds.
Journal of Evolutionary Biology (advance online publication)
DOI: 10.1111/jeb.12675

The midline pattern of sternal ossification characteristic of the
Cretaceous enantiornithine birds is unique among the Ornithodira, the
group containing birds, non-avian dinosaurs and pterosaurs. This has
been suggested to indicate that Enantiornithes is not the sister group
of Ornithuromorpha, the clade that includes living birds and their
close relatives, which would imply rampant convergence in many
non-sternal features between enantiornithines and ornithuromorphs.
However, detailed comparisons reveal greater similarity between
neornithine (i.e. crown group bird) and enantiornithine modes of
sternal ossification than previously recognized. Furthermore, a new
subadult enantiornithine specimen demonstrates that sternal
ossification followed a more typically ornithodiran pattern in basal
members of the clade. This new specimen, referable to the
Pengornithidae, indicates that the unique ossification pattern
observed in other juvenile enantiornithines is derived within
Enantiornithes. A similar but clearly distinct pattern appears to have
evolved in parallel in the ornithuromorph lineage. The atypical mode
of sternal ossification in some derived enantiornithines should be
regarded as an autapomorphic condition rather than an indication that
enantiornithines are not close relatives of ornithuromorphs. Based on
what is known about molecular mechanisms for morphogenesis and the
possible selective advantages, the parallel shifts to midline
ossification that took place in derived enantiornithines and living
neognathous birds appear to have been related to the development of a
large ventral keel, which is only present in ornithuromorphs and
enantiornithines. Midline ossification can serve to medially reinforce
the sternum at a relatively early ontogenetic stage, which would have
been especially beneficial during the protracted development of the
super-precocial Cretaceous enantiornithines.