This remarkable sequence of Protoceratops skulls is a growth series. Starting with the smallest one, examine the changes in each skull as they become larger. In particular, notice the frills and the eye sockets. The frills are quite reduced on the smallest skulls, but they grow more rapidly than the rest of the skull as the animals mature. By the time the animal is grown, the frill has become quite large in proportion to the rest of the skull. The eye sockets, on the other hand, start out large and seem to grow smaller. Actually, they stay about the same size, but the bones of the skull grow larger around them.

Protoceratops' eyes didn't grow much during aging—and yours don't either. That's what gives babies that cute, big-eyed look compared with their elders. This growth sequence is an approximation, based solely on the size of the skulls. The skulls were found on several expeditions to Mongolia conducted by the Museum over a ten-year period, beginning in 1922. We can tell generally how old a fossil is by the age of the surrounding strata - the rocks and sediments in which the fossil is embedded. These skulls are about 72 million years old. But we can't tell how long a particular dinosaur fossil lived. It's quite possible that this growth sequence is out of chronological order, if one of these animals grew faster than another, or if some are males and others females.