Dorsal sails supported by hyperelongate neural spines of dorsal verteb
rae were evolved by various tetrapods, but most work on their function
has centered on the pelycosaur Dimetrodon, in which the sail has gene
rally been interpreted as a thermoregulatory structure that would perm
it 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. Fast interpretations of Edaphosaurus suggested that th
e cross-bars were embedded in a thick fat-storage structure or extende
d from a thin sail to enhance its utility for intraspecific display. H
owever, wind tunnel modeling of airflow over-a thin sail with laterall
y projecting cross-bars supports a thermoregulatory interpretation of
the sail of Edaphosaurus. The cross-bars would produce a turbulent flo
w, which would increase the effectiveness of convective cooling. Measu
rements of heat flow in an instrumented model show that cross-bars inc
rease heat loss from the sail. The cross-bars may have enabled Edaphos
aurus to thermoregulate effectively with a smaller and lower dorsal sa
il than would have been required without them.