| Summary: | Bone histology of crown-group birds is a research topic of great interest, permitting insight into the evolution of remarkably high growth rates in this clade and variation across the altricial-precocial spectrum. In this study, we describe microanatomical characteristics of the humerus and femur in partial growth series from 14 crown group birds representing ten major clades (Struthioniformes, Galliformes, Apodiformes, Columbiformes, Charadriiformes, Accipitriformes, Strigiformes, Psittaciformes, Falconiformes, and Passeriformes). Our goals were to: (1) describe the microanatomy of each individual; (2) make inter-and intra-taxonomic comparisons; (3) assess patterns that correspond with developmental mode; and (4) to further parse out phylogenetic, developmental, and functional constraints on avian osteological development. Across taxa, the femoral and humeral tissue of neonates can be broadly characterized as highly-vascularized, disorganized woven bone with great variation in cortical thickness (inter-and intrataxonomically, within an individual specimen, and within a single section). The tissue of precocial chicks is relatively more mature at hatching than in altricial, but other categories along the developmental spectrum were less easy to distinguish, thus we were unable to identify a definitive histological proxy for developmental mode. We did not find evidence to support hypotheses that precocial chicks exclusively have thicker cortices and more mature bone in the femur than the humerus at time of hatching; instead, this is a characteristic of nearly all taxa (regardless of developmental mode), suggesting deep evolutionary origins and the effects of developmental channeling. Bone tissue in adults exhibited unexpected variation, corresponding to differences in body size. Large-bodied birds have cortices of fibrolamellar bone, but organization of tissue increases and vascularity decreases with diminishing body size. The outer circumferential layer (OCL) also appears at earlier growth stages in small-bodied taxa. Thus, while the OCL is indicative of a cessation of appositional growth it is not always indicative of cortical maturity (that is, maximum organization of bony tissue for a given taxon). Small size is achieved by truncating the period of fast growth; manipulation of the timing of offset of bone growth is therefore an important factor in changing growth trajectories to alter adult body size.
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