Elaborate CNS cooling structures in large dinosaurs

Abstract

DURING sustained activity, or when exposed to high ambient temperatures, terrestrial animals often experience periods of core hyperthermia. The central nervous system (CNS) is very sensitive to elevated temperatures^1–3, and consequently, both bradymetabolic and tachymetabolic⁴ terrestrial vertebrates have evolved physiological mechanisms which effect localised cooling of the brain, and thereby reduce any thermal impairment of CNS functioning. In modern reptiles this temperature gradient is produced by evaporative cooling from the buccal cavity and upper respiratory tract^5,6, conducting heat from the brain through the floor of the cranium⁷. Mammals dissipate heat, by evaporation, from the nasal mucosa to the air flowing through the nasal passages. The cooled venous blood draining from this highly vascularised mucosa flows into the cavernous sinus, where counter-current heat exchange with the carotid arteries, elaborated into a rete in many forms, results in brain cooling^8,9 (Fig. 1a). Dinosaurs would have experienced similar thermal problems to those of modern vertebrates, and these would have been particularly acute in the larger forms whose low surface area to volume ratio would have restricted dissipation of the enormous amounts of heat generated by the skeletal muscles during activity. It is therefore proposed that they required and possessed comparable physiological mechanisms to protect the brain during core hyperthermia.