Why Did Some Dinosaurs Have a Sail on Their Back? Several Answers, None Settled
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Elongated spines rising from the backbone supported a structure in several unrelated animals, which suggests it was useful more than once. What it was for has been argued about since the first specimens were described.
What the structure was
The evidence is bone, specifically neural spines projecting upward from the vertebrae and extended to many times their usual length, in some animals reaching over a metre and a half. What connected them is the question, since soft tissue is rarely preserved. Two reconstructions are possible from the bone alone. The spines may have supported a thin membrane of skin stretched between them, producing a sail, which is the traditional interpretation and which the spacing and shape support in several cases. Or they may have been buried within a thick ridge of muscle and fat, producing a hump rather than a sail, which has been argued for particular species on the basis of spine cross-section and of comparison with living animals carrying humps.
The animals that had them
The feature appears repeatedly in unrelated lineages, which is informative:
- •Spinosaurus and its relatives, among the largest predatory dinosaurs known
- •Ouranosaurus, a plant-eating dinosaur from the same region and period
- •Dimetrodon, which is not a dinosaur at all and predates them by tens of millions of years
- •Several other synapsids contemporary with that animal
- •Amargasaurus, a long-necked dinosaur with paired spines along the neck
- •Independent appearance in separate lineages suggests a repeated advantage rather than inheritance
The explanations offered
Several functions have been proposed and the evidence for each is uneven. Temperature regulation is the oldest proposal, with the sail acting as a radiator to shed or absorb heat, and it was modelled in detail for the earliest examples and found plausible for animals of that size, while scaling arguments make it less convincing for the largest dinosaurs. Display for species recognition and for mate choice is the leading current explanation, supported by the same reasoning that applies to crests, since the structures differ between related species while the rest of the skeleton does not. Storage of fat for lean periods is proposed for the hump interpretation. A hydrodynamic function has been suggested for the aquatic predator. Stability, intimidation and shading have all been proposed and are weakly supported.
Testing the temperature idea
The radiator proposal is the one that has been examined quantitatively, and the methods are worth describing because they apply to any such question. Modelling treats the structure as a surface exchanging heat with the air, calculates how much heat it would move given its area, the blood flow assumed and the conditions, and compares that with the animal's estimated heat production and with the mass it has to warm or cool. The results depend heavily on the assumptions, particularly on blood supply, which bone cannot establish. Scaling works against the idea for large animals, since heat production rises with volume and surface area rises more slowly, so a structure sufficient for a smaller animal is inadequate for a larger one. The honest conclusion is plausible for some and unlikely for others.
The animal that keeps changing
The best-known sailed dinosaur has been reconstructed several different ways within two decades, which illustrates how provisional this material is. The original specimens were destroyed in a bombing raid in 1944, leaving only drawings and descriptions, so everything since has depended on new and frequently incomplete material. Reconstructions have shifted from a conventional bipedal predator to a short-legged quadruped, then to a substantially aquatic animal with a paddle-like tail, with each revision resting on newly described specimens and each contested by other researchers. The sail has been reinterpreted alongside each change. The honest summary is that the animal is known from fragmentary and disputed material and that confident depictions of it should be read as one interpretation among several.
The takeaway
Elongated neural spines are the evidence, and whether they supported a thin membrane or were buried in a fleshy hump is genuinely disputed for some species. The feature appears independently in unrelated lineages including animals that are not dinosaurs. Display is the leading explanation, and the best-known example has been reconstructed several different ways within two decades.