Glossary

**Growth rate** is a quantitative measure of how rapidly an organism increases in body mass or size per unit of time. In paleontology, the growth rates of extinct animals are inferred primarily through bone histology (osteohistology), in which thin-sections of fossilized long bones are examined microscopically to reveal annually deposited lines of arrested growth (LAGs) and the type of bone tissue present. Fibrolamellar bone, characterized by disorganized collagen fibers with abundant vascular canals, is widely accepted as an indicator of rapid growth, whereas lamellar bone reflects slower deposition. Research on dinosaur growth rates has demonstrated that non-avian dinosaurs did not grow slowly throughout life in the manner typical of extant ectothermic reptiles; instead, they exhibited accelerated, sigmoidal growth patterns more comparable to those of endothermic mammals and birds. These findings have been pivotal to the debate on dinosaur thermophysiology, providing some of the strongest evidence that dinosaurs possessed metabolic rates elevated well above those of modern cold-blooded reptiles. The study of growth rates in fossil vertebrates continues to refine our understanding of life-history strategies, ontogeny, and the evolution of endothermy across the archosaur lineage.

**Ontogeny** refers to the entire sequence of developmental events that occur during the life history of an individual organism, from fertilization through embryonic development, hatching or birth, postnatal growth, the attainment of sexual maturity, and eventual senescence. In paleontology, ontogenetic research is critical because many extinct organisms — dinosaurs in particular — underwent dramatic morphological transformations during growth. Cranial ornamentation such as horns, domes, and frills, as well as body proportions, tooth counts, and skeletal architecture, could change so profoundly between juvenile and adult stages that specimens of the same species were frequently classified as separate taxa. Modern ontogenetic analyses using bone histology, computed tomography, and morphometrics have revolutionized dinosaur taxonomy by revealing these growth-dependent changes, thereby reducing inflated species counts and clarifying the true diversity of Mesozoic ecosystems.

**Sexual dimorphism** refers to systematic differences in morphology and appearance between males and females of the same species, encompassing variations in body size, skeletal structure, coloration, and ornamentation. These differences arise primarily through sexual selection—a process operating via intrasexual competition (e.g., males competing for access to mates) and intersexual choice (e.g., females preferring males with elaborate display structures). In extant animals, sexual dimorphism manifests in diverse ways, from the manes of male lions and the tail plumage of male peacocks to pronounced body size differences in baboons and sea lions. In paleontology, identifying sexual dimorphism in extinct organisms, particularly non-avian dinosaurs, remains one of the discipline's most challenging problems. The fragmentary nature of the fossil record, small sample sizes, difficulty distinguishing sex-based variation from ontogenetic, individual, or interspecific variation, and the loss of soft-tissue features during fossilization collectively hinder statistically robust identification. Nevertheless, sexual dimorphism in fossils provides critical insights into reproductive strategies, social behavior, and evolutionary pressures in deep time.