Stygimoloch ("river devil") was a pachycephalosaurid that lived during the Cretaceous Period between 68-65 million years ago in what is now North America. This dinosaur measured about 10 feet long and bore a thick skull that was decorated with six large spikes that covered the top rim of its head. This may have served the dinosaur as a display of recognition, helping the dinosaur to identify its companions. This dinosaur was a plant-eater and it lived in herds for protection from predators such as Tyrannosaurus, Dromaeosaurus, and Gorgosaurus. Males would often challenge one another to battle, either for females with which to mate, or for supremacy in the herd. [citation needed] It was previously thought that, like goats, they would butt heads with each other until one weakened and backed off, leaving the victor to choose a female or to lead a herd. However, the lack of apparent cranial damage exhibited by recent fossil discoveries has given rise to the hypothesis that they did not actually butt heads with other members of the herd. Some conjecture that stygimoloch headbutted other dinosaurs in their much softer underbelly regions.