Dinosaurs began life out of the nest as small, vulnerable hatchlings. Within 20 years, most reached their adult size, whether that was as small as a chicken or as big as a bus. They had growth spurts, much like human teenagers do, and they got big quickly (compare the sizes of adult and juvenile specimens of the ornithopod dinosaur Camptosaurus dispar). Counting the annual growth rings in fossil bone reveals just how rapidly dinosaurs grew up. Getting big fast gave dinosaurs a chance against predators, and maybe it also gave baby dinosaurs an edge in “outgrowing” competition for food with other animals, even mammals. Many Jurassic ecosystems contained more species of mammals than dinosaurs.

Bony armor helped plant eaters avoid predation. This specimen of Stegosaurus stenops displays distinctive pointed plates that protected the spine and bony nodules around the throat. For decades, paleontologists puzzled over how the plates and spikes had been arranged on the living animal.

Stegosaurus wielded several defensive spikes on its tail.

Both these Camptosaurus dispar skeletons are from the Morrison Formation in Wyoming. The smaller specimen was first thought to be from another species, but now it’s recognized as a juvenile Camptosaurus.