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A neural spine sail is a large, flattish protrusion from the back of an animal colinear with the spine. Many extinct species of amphibians and reptiles have very extended neural spines growing from their back vertebrae. These are thought to have supported a sail. Paleontologists have proposed many ways in which the sail could have functioned in life.
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Function
Many suggestions have been made for the function of the sail. The consensus amongst modern scholars is that, at least for the pelycosaurs, the sail was used for thermoregulation.
Thermoregulation
The structure may have been used for thermoregulation. The base of the spines have a channel which it is proposed contained a blood vessel supplying abundant blood to the sail. The animal could have used the sail's large surface area to absorb heat from the sun in the morning. As ectotherms they required heat from an external source before their muscles would start to function properly. A predator would thus have an advantage over its slower moving prey. The sail could be used in reverse if the animal was overheating. By standing in the shade, the sail would radiate heat outwards.
However, recent studies have put in doubt the efficiency of this purported means of thermoregulation, and indeed no extinct sailed animal is currently assumed to have used its sails for thermoregulation.
Sexual selection
Elaborate body structures of many modern-day animals usually serve to attract members of the opposite sex during mating. This has been proposed in the past as a function of the sail but is now largely discredited.
Food storage
The structure may also have been more hump-like than sail-like, as noted by Stromer in 1915 ("one might rather think of the existence of a large hump of fat [German: Fettbuckel], to which the [neural spines] gave internal support") and by Jack Bowman Bailey in 1997. In support of his "buffalo-back" hypothesis, Bailey argued that in Spinosaurus, Ouranosaurus, and other dinosaurs with long neural spines, the spines were relatively shorter and thicker than the spines of pelycosaurs (which were known to have sails); instead, the dinosaurs' neural spines were similar to the neural spines of extinct hump-backed mammals such as Megacerops and Bison latifrons.
Camouflage
It has been suggested that the Dimetrodon used the sail on its back to help camouflage itself when hiding among reeds and waiting to ambushing its prey.
Sound display
Gregory Paul argued that parallel neck sails of Amargasaurus would have reduced neck flexion. Instead, he proposed that, with their circular rather than flat cross-sections, these spines were more likely covered with a horny sheath. He even suggests that they could have been clattered together for a sound display.