Evidence of combat in triceratops

Abstract

Background: The horns and frill of Triceratops and other ceratopsids (horned dinosaurs) are interpreted variously as display structures or as weapons against conspecifics and predators. Lesions (in the form of periosteal reactive bone, healing fractures, and alleged punctures) on Triceratops skulls have been used as anecdotal support of intraspecific combat similar to that in modern horned and antlered animals. If ceratopsids with different cranial morphologies used their horns in such combat, this should be reflected in the rates of lesion occurrence across the skull.

Methodology/principal findings: We used a G-test of independence to compare incidence rates of lesions in Triceratops (which possesses two large brow horns and a smaller nasal horn) and the related ceratopsid Centrosaurus (with a large nasal horn and small brow horns), for the nasal, jugal, squamosal, and parietal bones of the skull. The two taxa differ significantly in the occurrence of lesions on the squamosal bone of the frill (P = 0.002), but not in other cranial bones (P > 0.20).

Conclusions/significance: This pattern is consistent with Triceratops using its horns in combat and the frill being adapted as a protective structure for this taxon. Lower pathology rates in Centrosaurus may indicate visual rather than physical use of cranial ornamentation in this genus, or a form of combat focused on the body rather than the head.

Figure 1. Cranial lesions in horned dinosaurs.

Schematics of the skulls of (A) Triceratops and (B) Centrosaurus, showing incidence rates of lesions (periosteal reactive bone and fracture calluses) on each cranial element (number of abnormal elements / total number of elements). Not to scale.

Figure 2. Examples of periosteal reactive bone in selected specimens of Triceratops.

Arrows indicate lesions on (A) the left jugal of YPM 1822 (Yale Peabody Museum, New Haven, Connecticut, USA) and (B) the right squamosal of YPM 1828. The inset skull graphics indicate the approximate area of each photograph with a gray box.

Figure 3. Example of a fracture callus in Centrosaurus.

Arrows indicate a lesion on the ventral surface of the isolated right squamosal (an incomplete element) of TMP 82.18.108 (Royal Tyrrell Museum of Paleontology, Drumheller, Alberta, Canada), in ventral view. The inset skull graphic indicates the approximate area of the photograph with a gray box.