Histological and chemical diagnosis of a combat lesion in Triceratops

In the collective imagination derived from scientific and popular literature, Triceratops often faced each other in combat. Thus, from the second half of the twentieth century, these ceratopsids were described as pugnacious animals. This arises primarily from the interpretation of extracranial fenestrae in ceratopsids being the result of combat trauma. However, the diagnosis of the traumatic nature of these anatomical variants of their neck frill requires evidence of bone healing and remodelling by microscopy analysis. Here, we present the case of the Triceratops horridus known as Big John, which is one of the largest specimens discovered in the Hell Creek Formation (Upper Cretaceous; South Dakota, USA). Its right squamosal bone shows an extrafenestra with irregular margins and signs of inflammation. Microscopy analysis revealed newly formed and healing bone, with histological signs typical of the bone remodelling phase. Chemical analysis revealed sulphur that was derived from glycosaminoglycan’s and sulphated glycoproteins of the preosseous osteoid substance present in the healing phases of a bone trauma. Histological and microanalytical analyses confirm that the squamosal fenestra of Big John is the result of a traumatic event, which might indeed have occurred during combat with another Triceratops.