The medium-sized chasmosaurine ceratopsid grew up to 4.9m (16ft) long and weighed about 2.2 tonnes. Skulls of many individuals have been discovered in western North America, where Chasmosaurus pre- dated its close relative Triceratops by several million years. 

Chasmosaurus skull reveals that the ceratopsid had a small nose horn and two long brow horns rather like those of a cow. It had narrow, bony struts: scaffolding to support the frilland reduce its weight. Name has reference to the openings in the skull, more particularly to the great size of the intraparietal fontanelles" in the frill; for a form supposedly ancestral to Torosaurus. (To replace preoccupied Protorosaurus Lambe.) 

In 1997 new scrappy material was described of juvenile chasmosaurines that question whether any real difference exists with the centrosaurines relative to the attachment of the frontal on the medial side of the postorbital. The juvenile chasmosaurines  look like centrosaurines.