Neural spine elongation in dinosaurs: Sailbacks or buffalo-backs?

Abstract

Several dinosaurs, notably Ouranosaurus and Spinosaurus, have vertebral columns marked by prominent arrays of elongated neural spines. Using pelycosaurian sailbacks like Dimetrodon as analogies, popular orthodoxy holds that the tall spines served as supporting struts for dorsal sails of purported thermoregulatory function, especially heat dissipation in tropical climates. It is argued here that the neural spines of Ouranosaurus, Spinosaurus, and several other long-spined dinosaurs favor bison-like humps rather than sails: 1) in functional morphology and relative elongation they are dissimilar to pelycosaur spines but homoplastically converge on the spines of high-withered ungulates; 2) the usefulness of a sail in thermoregulation has been exaggerated - in large tetrapods it would have been fairly efficient as a thermal amplifier but ineffective as a radiator; hence large sail-hearing dinosaurs in open tropical climates are improbable; 3) the insulation properties of humps favor gigantothermy, the most likely thermobiological model for large dinosaurs. Dinosaur humps are probable adaptations for: 1) energy storage, maintenance of gigantothermy, and heat-shielding in unshaded habitats; 2) long-distance migration from feeding to nesting grounds across terrains of variable productivity; and 3) lipid conservation for production of large clutches of eggs at the nesting site. Because sacral, caudal, or dorsal humps were relatively common traits among certain groups, the fashionably anorexic image of many large dinosaurs must he emended.