The adaptive significance of endothermy and salt excretion amongst the earliest archosaurs

Abstract

Evidence, amongst Triassic thecodonts, of energetic upright postures and advanced bipedalism (or the vestiges of such characteristics) reveals that both endothermic and ectothermic thecodonts were descended from warm-blooded archosaurs with rapid metabolisms. Arboreal and aquatic adaptations in bipedal iguanians, and in extinct thecodonts, suggest that the emergence of reptilian bipedal locomotion is related to an arboreal-aquatic existence. But endothermy-required obligatory bipedalism could not have arisen in fauna-poor freshwater environments. The possible existence, in faunivorous thecodonts, of functional (or vestigial) cranial salt glands may suggest that the earliest semiaquatic archosaurs originated in fauna-prolific marine environments. If archosaurs were originally semiaquatic (marine) predators, then the existence of archosaurian endothermy could have initially emerged as an adaptation to increase metabolic heat production under the enhanced thermal cold stresses of the marine hydrosphere, while concurrently augmenting the rate of metabolic water production within saltwater environments inherently lacking in palatable freshwater resources.