Before asking what a frill was for, it helps to look at an early horned dinosaur. Early horned dinosaurs such as Protoceratops had a thin frill and no long brow or nose horns, so the frill's first job was probably display rather than neck protection. Triceratops had two long horns above its eyes, and the 2009 study of museum skulls found far more lesions on the frill's squamosal bone in Triceratops than in Centrosaurus. The authors did not claim a precise cause for any one injury. They said only that the overall pattern fits fighting among Triceratops. They also left out juvenile and subadult skulls, to control for possible changes in behavior as the animals grew up.

Longrich and Field concluded that Torosaurus is a distinct genus of horned dinosaur, not the adult of Triceratops, and the argument depends on small differences in bone. The underside depressions of the Triceratops frill straddle the parietal and squamosal bones, while the openings in the Torosaurus frill are located on the parietal. No specimen has ever been described with an intermediate number of epiparietals between Triceratops and Torosaurus. Longrich and Field argued that the Nedoceratops skull, cataloged as USNM 2412, is clearly pathological, with holes through both squamosals as well as the parietal. Scannella and Horner argued from the other side that Triceratops skulls changed radically as they matured: horn cores changed direction, the frill-edge bones changed shape, and the frill expanded and became fenestrated. The two teams were reading the same kind of bones and drawing different lessons from them.

Rock layers add a second line of evidence about the two forms of Triceratops. In the 2014 study, the T. horridus form was found low in the Hell Creek Formation and the T. prorsus form in the upper third. The authors said that the stratigraphic separation of T. horridus and T. prorsus makes the idea that they are sexes or growth stages of one species untenable. Skulls from the middle of the formation show mixed traits, consistent with anagenesis, the transformation of one lineage over time. One idea was that one Triceratops species changed into the other over time, with the earlier one dying out before the later appeared. A 1992 study had suggested this but used too few samples.

Teeth reveal another surprise. Dinosaurs evolved precise tooth-to-tooth chewing four times, in heterodontosaurid ornithopods, hadrosaurids, giant sauropods and ceratopsians. That matters because most reptile teeth are used for grabbing and crushing and do not meet like mammal teeth. In Triceratops, the vasodentine in the teeth was so porous that it wore unusually fast, which formed fullers. Triceratops teeth wore down while it fed, making a groove in each tooth's chewing surface that cut friction. The teeth also had five layers of tissue, more than the four in the teeth of horses and other hoofed mammals.

The newest puzzle is inside the nose. As of November 2026, University of Tokyo researchers have used CT scans of fossil Triceratops skulls, compared with the snouts of birds and crocodiles, to work out what filled the huge nasal space. As of November 2026, in Triceratops the skull blocks the jaw route that nerves and blood vessels use in most reptiles, so they reach the nostrils by the nasal branch. Respiratory turbinates are thin, scroll-shaped bones in the nasal cavity that let blood and air exchange heat more efficiently, and as of November 2026 researchers think Triceratops had them. As of November 2026, Triceratops was likely not fully warm-blooded, but nasal turbinates may still have helped it control temperature and keep moisture.

Words to know

• lineage: line of descent
• anagenesis: one lineage changing
• turbinates: scroll-shaped nose bones
• fenestrated: having window-like holes
• pathological: injured or diseased