Aerodynamics and thermoregulatory function of the dorsal sail of Edaphosaurus

Abstract

Dorsal sails supported by hyperelongate neural spines of dorsal vertebrae were evolved by various tetrapods, but most work on their function has centered on the pelycosaur Dimetrodon, in which the sail has generally been interpreted as a thermoregulatory structure that would permit rapid warming in the morning and cooling during the hot midday. The pelycosaur Edaphosaurus differed from other sailed tetrapods in that the neural spines supporting the sail had laterally directed tubercles or cross-bars. Past interpretations of Edaphosaurus suggested that the cross-bars were embedded in a thick fat-storage structure or extended from a thin sail to enhance its utility for intraspecific display. However, wind tunnel modeling of air flow over a thin sail with laterally projecting cross-bars supports a thermoregulatory interpretation of the sail of Edaphosaurus. The cross-bars would produce a turbulent flow, which would increase the effectiveness of convective cooling. Measurements of heat flow in an instrumented model show that cross-bars increase heat loss from the sail. The cross-bars may have enabled Edaphosaurus to thermoregulate effectively with a smaller and lower dorsal sail than would have been required without them.