Protoceratopsidae

Protoceratopsidae were a family of early ceratopsians that lived during the Late Cretaceous. While other ceratopsians roamed across the globe, these dinosaurs are only known from Asian rock layers, especially in China and Mongolia. Like all ceratopsians, they ate plants, using constantly replaced tooth batteries to slice through vegetation and a hooked beak to grasp it. They were small, measuring about 1 to 2.5 meters long, and their bony frills and horns were much smaller than those of later, more advanced ceratopsians.

Their skulls had the frill and rostral bone typical of ceratopsians, but the snout was wedge-shaped with tall, narrow nostrils set high up. The antorbital fenestra was unusually tiny, and the antorbital fossa sat high on the skull, connected by a slit to a sinus in the maxilla—a feature also seen in other ceratopsians like Leptoceratopsidae and some basal neoceratopsians. They may have had cheeks to hold food, suggested by well-defined ridges on the maxilla and dentary where cheek muscles would have attached, along with small holes (foramina) on the maxilla for nerve branches. The upper jaw tip was likely covered in horn-like material, and the upper and lower jaws curved toward each other. Compared to later ceratopsians, their oral cavity was deep and wide, though narrower than in earlier forms like *Psittacosaurus*, possibly aiding breathing or temperature control. The nasal cavity, originally a single large space, was split into two by the hard palate, likely to make room for the deeper mouth.

The vertebral column was S-shaped, with unusually long neural spines. On the tail vertebrae, these spines were five times taller than the centrum, and longer in the middle of the tail than at the base, making the tail taller and flatter. The middle of the tail was rigid and straight, and while the whole tail could bend sideways easily, up-and-down movement was limited. The neck had little side-to-side mobility but could bend up and down, allowing the animals to reach food.

The family Protoceratopsidae was first named in May 1923 by Walter W. Granger and William King Gregory for the single species *Protoceratops andrewsi*. They saw it as primitive enough to warrant its own family. Later, the group was expanded to include ceratopsians more advanced than psittacosaurids but less so than ceratopsids.