If it weren't for one tell-all feature, Spinosaurus might have been indistinguishable from any other large carnivore stalking the swamps of the Cretaceous period. That feature, of course, was the fin-shaped sail on its back, a thin flap of skin supported by sharp needles of bone that protruded from Spinosaurus' vertebrae. (See more facts, figures and news about Spinosaurus and 10 Facts About Spinosaurus.)
Why did Spinosaurus have this strange-looking sail? The most likely explanation is that this structure evolved for cooling purposes in the hot Cretaceous climate (much like the big, floppy ears of African elephants). It may also, as a byproduct, have been a sexually selected characteristic--perhaps male Spinosauruses with bigger sails were more successful at mating with females of the species.
By the way, paleontologists now believe that Spinosaurus was the biggest carnivore that ever lived--outclassing even T. Rex by one or two tons.
