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Birds exhibit an enormous diversity in adult skull shape (disparity), while their embryonic chondrocrania are considered to be conserved across species. However, there may be chondrocranial features that are diagnostic for bird clades or for Aves as a whole. We synthesized and analyzed information on the sequence of chondrification of 23 elements in ten bird species and five outgroups. Moreover, we critically considered the developmental morphology of the chondrocrania of 21 bird species and examined whether the diversity in adult skull shape is reflected in the development of the embryonic skull, and whether there are group-specific developmental patterns.
Results
We found that chondrocranial morphology is largely uniform in its major features, with some variation in the presence or absence of fenestrae and other parts. In kiwis (Apteryx), the unique morphology of the bony skull in the orbito-nasal region is reflected in its chondrocranial anatomy. Finally, differences in morphology and chondrification sequence may distinguish between different Palaeognathae and Neognathae and between the Galloanserae and Neoaves. The sequence of chondrification is largely conserved in birds, but with some variation in most regions. The peri- and prechordal areas in the base of the chondrocranium are largely conserved. In contrast to the outgroups, chondrification in birds starts in the acrochordal cartilage and the basicranial fenestra is formed secondarily. Further differences concern the orbital region, including early chondrification of the pila antotica and the late formation of the planum supraseptale.
Conclusion
Synthesizing information on chondrocranial development confronts terminological issues and a lack of comparable methods used (e.g., different staining; whole-mounts versus histology). These issues were taken into consideration when assessing differences across species. The summary of works on avian chondrocranial development, covered more than a century, and a comparison of the chondrification sequence among birds could be conducted. Future studies could test the hypothesis that chondrocranial disparity in Aves, in terms of the shape and proportion of individual elements, could be as large as adult skull disparity, despite conserved developmental patterns and the richness of forms in other (dermal) portions of the skull.
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Birds share an array of unique characteristics among extant land vertebrates. Among these, external and microstructural characteristics of extant bird eggs have been linked to changes in reproductive strategy that arose among nonâavian theropod dinosaurs. More recently, differences in egg proportions recovered in crown birds relative to other dinosaurs were suggested as possibly linked to avian flight, but dense sampling close to its proposed origin was lacking. Here we assess the evolution of eggshell thickness in a targeted sample of 114 dinosaurs including birds, and test the relationship of eggshell thickness with potential life history correlates and locomotor mode using phylogenetic comparative methods. Only egg mass and flight are identified as significant predictors of eggshell thickness. While a high correlation between egg mass and eggshell thickness is expected, that relationship is much stronger in flying taxa, which show a significantly higher slope and lower residual variance than flightless species. This suggests stabilizing selection of eggshell thickness among theropods, as recovered for other traits in extant birds (e.g. genome size, metabolic rate). Within living birds, Eufalconimorphae present an apomorphic increase in relative eggshell thickness which remains unexplained, as few morphological synapomorphies of this clade have been identified.
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