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The origin of novelties and the evolution of biodiversity during the radiation of birds

Final Report Summary - ERB (The origin of novelties and the evolution of biodiversity during the radiation of birds)

Final Summary Report of the Research Project (ERB, 624732); covering all two years:

Introduction: Birds represent one of the most remarkable radiations in the history of life. The origin of flight and the diversification of birds have become classic ‘textbook’ examples for discussions about evolutionary models, biomechanics, the history of life, and phylogenetics. Much attention has focused on the discovery and description of new fossils, both derived theropods and basal birds: in particular, sediments of the Jehol Group in Liaoning and other provinces in China have doubled the number of Mesozoic bird taxa and have considerably expanded our knowledge of derived theropod dinosaurs. All this wealth of fossil data and new phylogenetic studies have established the relationships of basal bird groups, as well as their phylogenetic nesting among theropod dinosaurs, with dromaeosaurid and troodontid theropods as their closest sister groups. The mode and tempo of avian evolution have received much attention in recent years, thanks to the assembly of more complete datasets, the ongoing flow of new specimens and taxa, and the development of new techniques of macroevolutionary analysis. Recent studies revealed an avian radiation characterized by the gradual acquisition of apomorphies followed by accelerated (yet clade heterogeneous) rates of evolution. However, in the context of Coelurosauria (a diverse clade including T. rex and its kin, the ostrich-mimic ornithomimosaurs, and the sickle-clawed dromaeosaurids, among others), previous analyses have considered the skeleton as a whole, apart from studies focusing on single continuous traits. Here, I document the evolutionary dynamics of each anatomical region of the coelurosaurian skeleton. I aim at disentangling the relative role of each body part in the evolutionary dynamics of Coelurosauria. In doing so, I have the goal of resolving the timing of the origin and acquisition of key evolutionary novelties during the evolution of Avialae (birds) and closely related theropod dinosaurs and identifying the crucial characters in driving the initial (and subsequent) radiation(s) of birds

1. When considered as a whole, the cranium of coelurosaurian dinosaurs displayed accelerated evolutionary rates at the base of tyrannosauroids (also among derived members of the clade), alvarezsaurids, oviraptorosaurs, and paravians.
2. In the neurocranium (the skull area that protects the brain and associated neuronal structures;), only derived tyrannosauroids displayed higher rates of evolution.
3. The facial skeleton showed accelerated rates in derived tyrannosauroids and in the earliest stages of the evolution of paravian theropods. In contrast, slower evolutionary rates appeared among derived dromaeosaurid theropods.
4. The coelurosaurian mandible displayed accelerated evolutionary rates at the base of alvarezsaurids and oviraptorosaurs, and in a few bird species and the most derived tyrannosaurids.
5. Significantly higher rates of dental evolution (were found in early ornithomimosaurs; no significantly different (whether higher or lower) rates were present in other clades.
6. Considering the postcranium as a whole (, accelerated rates of evolution took place at the base of Maniraptora and at the base of maniraptoran clades like therizinosauroids, alvarezsaurids, and paravians (notably also in birds). Such high rates are widespread among numerous bird lineages. Significantly low rates are present among maniraptoran outgroups such as ornithomimids and also within some dromaeosaurid paravians.
7. The coelurosaurian axial skeleton showed accelerated evolutionary rates at the base of alvarezsaurids and lower rates in some early dromaeosaurid paravians.
8. In the pectoral girdle , higher evolutionary rates are present at the base of Paraves and several bird lineages.
9. The forelimb experienced accelerated rates of evolution ancestrally in alvarezsaurids and among derived bird lineages.
10. The pelvic girdle shows accelerated evolutionary rates ancestrally in Maniraptora, Oviraptorosauria+Paraves, and Paraves.
11. In the hindlimb , paravians and this clade plus oviratorosaurs showed accelerated evolutionary rates ancestrally. Notably, various bird lineages also display significantly higher rates.
12. These results highlight the important role in the postcranium in the evolution of birds, with a particular focus on limb module evolution in the transition to flight. All postcranial regions in Avialae display greater disparity than in most other clades. Overall, these postcranial regions increase in disparity during the Late Jurassic and most of the Cretaceous, all of them showing a post-Santonian disparity decrease. In the avian cranium, significantly higher rates of evolution are found in the facial skeleton.

Impact: These results represent the most comprehensive and detailed study of morphological evolutionary dynamics in coelurosaurian dinosaurs, including birds and their closest relatives. This study allows us to understand the relative influence of each anatomical region of the coelurosaurian skeleton in the macroevolution of the clade, particularly the relative role of each body part in the accelerated rates of evolution previously reported in early avian theropods. The results of the project will form the foundation for future analyses seeking to disentangle the role of particular traits in the radiation of birds from coelurosaurian theropods, from a functional, morphological and phylogenetic perspective.