That means the majority of living birds and mammals are smaller than the tiniest dinosaurs ever discovered.” They compiled body-mass data for thousands of species across six groups: mammals, birds, lizards, snakes, turtles, and crocodylians, plus 515 non-avian dinosaur species. “Everyone agrees that dinosaurs restricted mammals to small sizes, before the end-Cretaceous mass extinction,” Dr. Benson said. “Here we suggest that mammals in turn restricted dinosaurs to large sizes, preventing the evolution of tiny dinosaur species.” But dinosaurs themselves seemed to be forbidden from being tiny,” Dr. Benson said.