Some of the most exciting fossil finds over the last fifty years have been those that have something to do with young dinosaurs – their nests, eggs, or skeletons. When skeletons are found as part of a herd or as a nesting group, it is much easier to tell the young from the adults. The size of a dinosaur’s head, eyes, and feet can give clues. The bone itself, if well-enough preserved, sometimes has textures and structures that show different growth rates.
If a tree is cut down, an examination of the trunk will show rings in the wood from the core of the tree to the bark. Each new ring represents a year of growth of the living tree. The rings are known as growth rings. Sometimes this effect can be seen in dinosaur bones as well. However, it is not possible to simply count the rings to tell what age the dinosaur was when it died. Often the structure of the older bone will have changed during the lifetime of the animal, and the growth rings that were formed earlier will have disappeared
For a time, the prosauropod Mussaurus (“mouse lizard”) was thought to be the smallest dinosaur known. Then scientists realized that the skeletons that had been found were all of young hatchlings. The skeletons found were only 18 cm (9 ins) long and could have fitted in a person’s hand. The adults would in fact have been about 3 m (10 ft). Some of the bones of the hatchlings had not developed fully, and like most young animals, the skull, eye sockets, and feet were big for the size of the animal.
Herds of the horned dinosaur Protoceratops roamed the Cretaceous plains of Asia like flocks of sheep. Hundreds of fossils of these dinosaurs have been found buried in desert sandstones. They were probably overwhelmed and suffocated in the sandstorms of the period. The remains are at all stages of growth, showing that they moved about in herds or in family groups. The fossils provide very good evidence of how this species developed as they grew older. For example, the skulls and jaws developed at different rates between the time that the Protoceratops hatched and the time it reached full adulthood.