Evolution of Bone Histological Characters in Amniotes, and the Implications for the Evolution of Growth and Metabolism

Histological studies have established relationships between the microstructural features of bone, the growth rates of primary cortical bone, and whole-body growth rates of the animal. For animals of a given body size, the density and connectivity of vascular canals and the disorganization of collagen fibers increase with the rate of bone deposition, and osteocyte density is positively correlated with metabolic rate.I first review and refine several methods to improve the quantification of growth-related patterns in fossil bone tissue, focusing on specific microstructural characters known to correlate with growth and metabolic rates in living tetrapods. The most critical histological indicator of growth, the rate of bone deposition, is rarely reported in fossil studies. However, zonal area and average zonal width directly measure annual deposition, and can be used to bracket daily deposition rates. Estimating bone tissue growth based on vascularization pattern ("Amprino's Rule") likely confounds three separate vascular signals: density, connectivity, and orientation/patterning. I discuss ways to measure these separately, using qualitative and quantitative means. Collagen fiber orientation, a sensitive indicator of bone deposition rate that may resolve seasonal shifts, is sometimes obscured in fossils by diagenetic alteration. Patterns of osteocyte organization and orientation, more than cell shape, are highly associated with fiber orientation and may be more appropriate proxies. Osteocyte and canal density, not typically reported in paleohistological studies, are easily measured using digital boxplots along radial transects through the cortex. These measures suggest the possibility of more useful quantification of osteohistological indicators as proxies for growth and metabolic rates in extinct and extant vertebrates.I next investigate the origins of avian growth rates. Birds grow much faster than other extant reptiles, a trait that is reflected in the appearance of their bone microstructure. However, some of these traits are shared by their dinosaurian ancestors, and it is not known when this condition first evolved. I expand the histological database of archosaurs and their ancestors to include early archosauromorphs, pseudosuchians, and dinosauromorphs. By sampling through deep time and in taxa whose character states are not represented among living animals, I show that the avian histological features associated with faster growth and higher metabolic rates evolved not among birds or dinosaurs, but earlier than the common ancestor of birds and crocodylians. Most of these character changes accumulated in a short segment of the archosauriform tree before the end of the Triassic.Finally, I examine histological patterns of growth in marsupial mammals. Among extant mammals, the bone tissue of placentals has been fairly well characterized, and is known to vary with size and ecology. Comparative data on marsupials, however, are lacking. I sampled the mid-diaphyseal femora of more than 50 extant and extinct marsupial species, as well as some afrotherian, xenarthran, and laurasiatherian placentals. My marsupial sample encompasses all extant orders, spans a 10 g-2500 kg size range, and comprises mainly wild-caught animals. The main factors influencing marsupial bone microstructure are life history and body size. The histological differences resulting from body size are subtle, occur gradually, and hold across six of the seven extant orders. The uniformity of marsupial bone histology reflects uniformity of their life history, especially related to the ontogeny of growth rates. Across all body sizes, marsupials share a common ontogeny: they are extremely altricial, experience their time of fastest growth at or just prior to weaning, and then continue to grow at lower rates for an extended period relative to their lifespan. Among placental mammals, histological variability likely reflects greater diversity in the ontogeny of growth rates. It is likely that sampling biases have obscured both size and phylogenetic signals in the distribution of mammalian bone growth patterns.By incorporating natural history and life history, the fossil record and the modern record, the study of bone microstructure can facilitate a much richer understanding of growth at the organismal level, and the evolution of growth strategies at higher levels