The “mammal theory” of dinosaur extinction (which falls under the theory of gradualism) states that dinosaurs were slowly wiped out by mammals, animals that only appeared around the end of the Mesozoic era (the end of the Cretaceous period). The mammals could have eaten many of the dinosaurs eggs (thus, the dinosaurs had difficulty reproducing); or the mammals could have taken over territory from the dinosaurs. On a smaller scale, such events happen today, especially when an introduced species takes over another species by eating the native organism’s young, or by taking over the territory and eating the available food supply.