Aside from its massive size--at up to 10 tons, it was the largest carnivorous dinosaur ever to walk the earth, outweighing even the fearsomely gigantic Giganotosaurus and Tyrannosaurus Rex--the most notable feature of Spinosaurus was the long, roughly semicircular, sail-like structure along its back. This adaptation had not been seen in such prominence in the reptile kingdom since the heyday of Dimetrodon, which lived over 150 million years earlier, during the Permian period (and which wasn't even technically a dinosaur, but a type of reptile known as a pelycosaur).
The function of Spinosaurus' sail is a continuing mystery, but paleontologists have narrowed the field down to four plausible explanations:
Theory Number One: The Sail Was About Sex
The sail of Spinosaurus may have been a sexually selected characteristic--that is, males of the genus with bigger, more prominent sails would have been favored by females during mating season. Big-sailed Spinosaurus males would thus have transmitted this genetic trait to their offspring, perpetuating the cycle. Simply put, the sail of Spinosaurus was the dinosaur equivalent of a peacock's tail--and as we all know, male peacocks with bigger, flashier tales are more attractive to females of the species.
But wait, you may ask: if the sail of Spinosaurus was such an effective sexual display, why weren't the other meat-eating dinosaurs of the Cretaceous period equipped with sails as well? The fact is that evolution can be a surprisingly capricious process; all it takes is a random Spinosaurus ancestor with a rudimentary sail to get the ball rolling. If that same forebear had been equipped with an odd bump on its snout, its descendants millions of years down the line would have sported horns rather than sails!
Theory Number Two: The Sail Was About Body Temperature
Might Spinosaurus have used its sail to help regulate its internal body temperature? During the day, the sail would have absorbed sunlight and helped perk up this dinosaur's metabolism, and at night, it would have radiated away excess heat. One piece of evidence in favor of this hypothesis is that the much earlier Dimetrodon seems to have used its sail in exactly this way (and probably even more dependent on temperature regulation, since its sail was so much bigger relative to its total body size).
The main problem with this explanation is that all the evidence we have points to theropod dinosaurs being warm-blooded--and since Spinosaurus was a theropod par excellence, it was almost certainly endothermic as well. The more primitive Dimetrodon, by contrast, was almost certainly ectothermic (i.e., cold-blooded), and needed a sail to regulate its metabolism. But if that was the case, then why didn't all cold-blooded pelycosaurs of the Permian period have sails? No one can say for sure.
Theory Number Three: The Sail Was For Survival
Might the “sail” of Spinosaurus actually have been a hump? Since we don't know how the neural spines of this dinosaur were covered by its skin, it's possible that Spinosaurus was equipped with a thick, camel-like hump containing deposits of fat that could be drawn down in times of scarcity, rather than a thin sail. This would necessitate a major overhaul in how Spinosaurus is depicted in books and on TV shows, but it's not outside the realm of possibility.
The trouble here is that Spinosaurus lived in the wet, humid forests and wetlands of middle Cretaceous Africa, not the water-parched deserts inhabited by modern camels. (Ironically, thanks to climate change, the jungle-like region of northern Africa inhabited by Spinosaurus 100 million years ago is today mostly covered by the Sahara Desert, one of the driest places on earth.) It's hard to imagine that a hump would have been a favored evolutionary adaptation in a place where food (and water) was relatively plentiful.
Theory Number Four: The Sail Was For Navigation
Recently, a team of paleontologists came to the surprising conclusion that Spinosaurus was an accomplished swimmer--and may, in fact, have pursued a semi- or almost fully marine lifestyle, lurking in the rivers of northern Africa like a giant crocodile. If this is the case, then we have to accept the possibility that the sail of Spinosaurus was some kind of marine adaptation--like the fins of a shark or the webbed hands of a seal. On the other hand, if Spinosaurus was able to swim, then other dinosaurs must have possessed this ability, as well--some of which didn't possess sails!
And the Most Likely Answer Is…
Which of these explanations is the most plausible? Well, as any biologist will tell you, a given anatomical structure can possess more than one function--witness the variety of metabolic tasks performed by the human liver. The odds are that Spinosaurus' sail served primarily as a sexual display, but it may secondarily have functioned as a cooling mechanism, a storage place for fat deposits, or a rudder. Until more fossil specimens are discovered (and Spinosaurus remains are rarer than mythical hens' teeth), we may never know the answer for sure.