Spinosaurus is called 'spiny lizard' because it had a series of spines up to 6 feet (1.8 m) long coming out if its back, perhaps forming a sail-like fin that may have helped in thermo-regulation. Spinosaurus dated from the late Cretaceous period and was a bipedal (walked on two legs) predator about 40-50 feet long (12-15 m), weighing 4 tons or more.