Glossary

**Ectothermic** describes an animal whose regulation of body temperature depends primarily on external heat sources—such as solar radiation, heated substrate, or ambient water temperature—rather than on internally generated metabolic heat. Ectotherms encompass the vast majority of animal species, including all fishes, amphibians, non-avian reptiles, and invertebrates. The resting metabolic rate of an ectotherm is roughly one-tenth to one-half that of an endotherm of equivalent body mass, even at identical body temperatures. This low-energy physiological strategy means ectotherms require far less food—endothermic mammals and birds consume approximately eight to eleven times more food per unit body mass than comparably sized active reptiles—and can survive extended periods of fasting. However, their dependence on ambient temperature constrains activity levels, geographic range, and the capacity for sustained aerobic exertion. To maintain preferred body temperatures, ectotherms rely heavily on behavioral thermoregulation: basking in sunlight (heliothermy), absorbing conductive heat from warm substrates (thigmothermy), and shuttling between thermally distinct microhabitats. In paleontology, the question of whether dinosaurs were ectothermic, endothermic, or metabolically intermediate has been one of the most enduring debates, with evidence now suggesting that thermoregulatory strategies varied substantially among different dinosaurian lineages.

**Endothermy** is the physiological capacity of an organism to generate and regulate internal body heat through metabolic processes, maintaining a relatively stable core temperature independent of external environmental conditions. Endothermic animals (endotherms) sustain basal metabolic rates approximately 5 to 10 times higher than those of similarly sized ectotherms, using this metabolic heat to keep body temperature within a narrow homeostatic range. The primary endothermic groups are mammals and birds, though regional endothermy has evolved independently in several fish lineages, including tunas, lamnid sharks, and billfishes. The fundamental functional advantage of endothermy lies in its support for sustained aerobic activity. High resting metabolic rates are coupled with a cardiovascular system capable of delivering oxygen at rates sufficient to power prolonged muscular exertion, freeing endotherms from the reliance on anaerobic metabolism that limits the stamina of ectotherms. Stable body temperature also optimizes enzyme kinetics and neural conduction velocity, enabling rapid, precise responses across a wide range of ambient conditions, including cold temperatures and darkness. These capabilities allowed endotherms to colonize virtually every terrestrial climate zone, from polar regions to deserts, and to evolve energy-intensive life strategies such as powered flight, long-distance migration, and sustained pursuit predation. However, endothermy carries substantial energetic costs: endotherms require far more food than ectotherms of comparable size, and the high rates of oxidative metabolism generate reactive oxygen species (ROS) that can damage cellular components.

A **gastrolith** is a hard, non-caloric object—typically a stone—voluntarily ingested and retained within the gastrointestinal tract of an animal. Gastroliths are best documented in living birds, where the muscular gizzard (ventriculus) contracts rhythmically around ingested grit to mechanically triturate and mix food, effectively compensating for the absence of teeth. They are also reported in crocodilians, pinnipeds (seals and sea lions), cetaceans, and numerous extinct taxa including non-avian dinosaurs and marine reptiles such as plesiosaurs. The most widely accepted function of gastroliths is the **mechanical breakdown of plant material** in herbivorous animals. In birds, gastrolith mass consistently approaches approximately 1% of body mass, a ratio that is maintained across species spanning four orders of magnitude in body size. Alternative functional hypotheses include hydrostatic ballast for buoyancy control in aquatic animals, mineral supplementation (particularly calcium from limestone), stomach cleaning (especially in raptors), and stimulation of digestive secretions. However, the degree of empirical support varies widely among these proposals. Gastroliths are significant in paleontology as indirect evidence of diet, digestive physiology, and even migratory behavior in extinct animals. The discovery of bird-like gastrolith clusters in derived theropods such as *Caudipteryx* and ornithomimosaurs indicates that the avian gastric mill evolved deep within the theropod stem lineage, well before the origin of crown-group birds. Provenance analysis of gastrolith lithologies has also been used to infer long-distance dinosaur migrations spanning hundreds of kilometers.

**Mesothermy** is a thermoregulatory strategy intermediate between cold-blooded ectothermy and warm-blooded endothermy, in which an organism produces metabolic heat to elevate its body temperature above ambient levels but does not maintain a constant, precisely regulated internal temperature as birds and mammals do. This strategy confers greater activity levels and performance advantages over ectotherms while incurring lower energetic costs than full endothermy, since mesotherms require less food than a comparably sized endotherm. The concept was formally applied to dinosaurs in a 2014 study by John M. Grady and colleagues published in *Science*, in which the authors analyzed growth rate and metabolic rate data across 381 vertebrate species—including representatives of all major dinosaur clades—and found that dinosaur metabolic rates fell intermediate to those of modern ectotherms and endotherms. Among extant animals, mesothermic physiology is observed in tunas, lamnid sharks (including the great white shark), leatherback sea turtles, and monotremes such as the echidna. The concept challenges the traditional endotherm–ectotherm dichotomy and supports a view of thermoregulation as a continuous spectrum, suggesting that the modern binary classification is overly simplistic.