The idea that Triceratops fought its own kind rests on a careful reading of damaged skulls. In a 2009 study of museum skulls, Triceratops had far more lesions, or damaged spots, on the frill than Centrosaurus did. The most common type of lesion was periosteal reactive bone, which made up 22 of the 26 lesions the study counted. The authors ruled out predator attacks as the main cause of the frill lesions. Tyrannosaurs lived in the habitats of both Triceratops and Centrosaurus, so both would show similar damage. The study found the evidence most consistent with most Triceratops skull injuries being made by the horns of other Triceratops. The researchers speculate that many injuries may have come from misplaced horn thrusts by rival animals, as seen in modern horned animals. They also warn that some supposed injuries on horned dinosaur skulls were later found to be random bone resorption that was not traumatic, not wounds.

Skull injuries are not the only puzzle. The frill has attracted several explanations. By National Geographic's account, as of November 2026, early ideas that the frill was armor or a radiator have been discredited, and the leading idea is that it helped Triceratops choose mates. Scientists think the frill may have helped it attract mates, and the frill grew bigger compared to the skull as the animal grew up. The mate idea is the leading one, but it is still an idea about the past, not something anyone has watched happen.

Teeth and noses offer a different kind of evidence. Tests on 66-million-year-old Triceratops teeth showed that their wear properties were still preserved, which let researchers build a 3D wear model. Triceratops teeth wore down while it fed, making a groove called a fuller in each tooth's chewing surface, which cut friction. As of November 2026, researchers also note something about the head's plumbing. 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.

Meanwhile, a dispute about growth has run for years. In 2009, Scannella and Horner reported that fossils called Torosaurus may be Triceratops that reached full adulthood before dying. They argued that every feature once used to name Nedoceratops can be found in Triceratops specimens. In their view, allowing for variation within species, including change with growth, cut 18 latest Cretaceous horned dinosaur kinds down to two. Longrich and Field disagreed. In 2012 they concluded that Torosaurus is a distinct genus of horned dinosaur, not the adult of Triceratops. They found a sequence of changes in horned dinosaurs: ornament develops in juveniles, the skull roof fuses in subadults, and the epoccipitals, epijugals and rostral fuse to the skull in adults. Fewer than a dozen adult Torosaurus specimens have been found, compared with many dozens of younger Triceratops.

Rock layers add a second kind of question. By 1949, as many as 16 species of Triceratops had been named, because nearly every difference between skulls was treated as grounds for a new species. In the 2014 study of Hell Creek, the T. horridus form was found low in the formation and the T. prorsus form in the upper third. One idea was that one Triceratops species changed into the other over time, called anagenesis, with the earlier one dying out before the later appeared. A 1992 study had suggested this but used too few samples. That leaves open how the two forms are related.

Words to know

• lesions: damaged spots
• anagenesis: one lineage changing
• genus: group of species
• Torosaurus: a horned dinosaur
• fuller: tooth groove