Yinlong

A “Mosaic” Connecting Ceratopsians and Pachycephalosaurs

Yinlong is the most primitive ceratopsian, appearing shortly after the split between pachycephalosaurs and ceratopsians.
Its skull had a remarkable structure that could be called an evolutionary “missing link,” as it combined features from both groups.

Ceratopsian Features

At the tip of its upper jaw, it had a rostral bone, a characteristic ceratopsian feature that connects it to later species like Triceratops.

Pachycephalosaur Features

On the other hand, the squamosal bone at the back of its head had a knob-like ornamentation, a feature seen in pachycephalosaurs like Pachycephalosaurus.

It also had long, sharp, fang-like teeth, a primitive feature of ornithischians.
These characteristics are conclusive evidence that ceratopsians and pachycephalosaurs are very closely related and evolved from a common ancestor.

The Possibility of an Omnivorous Diet and the Discovery of Gastroliths

Because Yinlong had some sharp teeth, it is believed not to have been a purely herbivorous dinosaur.
Gastroliths, or stomach stones, were found in its abdominal area, which it likely used to efficiently grind and digest tough plants in its stomach.
Based on this evidence, it is highly likely that Yinlong was an omnivore that also ate insects and small animals, in addition to its plant-based diet.

The discovery of the Yinlong fossil in 2006 was a very important event for unraveling the evolution of ceratopsians.