Glossary

**Bone histology** is the study of the microstructure of bone tissue at the microscopic level. In paleontology, it is more specifically known as **paleohistology** or **osteohistology** and refers to the analysis of fossilized skeletal tissue microstructure to reconstruct the biology of extinct organisms. The method involves cutting thin sections from bones and examining them under polarized light microscopy to observe features such as vascular canal density and orientation, osteocyte lacunae, collagen fiber organization, and lines of arrested growth (LAGs). These microstructural features are interpreted on the basis of 'Amprino's rule'—the principle first proposed by Rodolfo Amprino in 1947 that local bone tissue type reflects its rate of deposition. By applying this principle to fossils, researchers can estimate individual growth rates, age at death, skeletal maturity, and metabolic status. Highly vascularized fibrolamellar bone indicates rapid growth typical of endotherms, while poorly vascularized lamellar-zonal bone suggests slower growth more characteristic of ectotherms. Bone histology has been transformative in vertebrate paleontology. It fundamentally changed scientific perceptions of non-avian dinosaurs from sluggish, cold-blooded reptiles to fast-growing animals with relatively high metabolic rates. It has also been instrumental in resolving taxonomic debates by demonstrating that morphologically distinct specimens can represent different growth stages of the same species, and in elucidating major evolutionary transitions such as the shift from dinosaurian to avian growth strategies.

Lines of arrested growth (LAGs) are thin, hyper-mineralized lines approximately 10 μm thick that form within the cortical bone of vertebrates when periosteal appositional growth temporarily ceases or markedly decelerates. Visible in transverse thin sections under a petrographic or polarizing microscope, LAGs appear as concentric rings analogous to tree rings. Their formation is driven primarily by seasonal environmental stressors such as low temperatures, drought, or reduced food availability, though physiological factors including reproductive energy expenditure, hormonal cycling, and disease can also trigger growth arrest. Because LAGs are generally deposited once per year, counting them provides a minimum estimate of an individual's age at death—a technique known as skeletochronology. This method has become central to paleobiology, enabling reconstruction of age, growth rate, maturation timing, and population structure in both extant and extinct vertebrates, from amphibians and sea turtles to non-avian dinosaurs.