An osteohistological analysis of <i>Triceratops</i> (Ornithischia: Ceratopsidae) cranial ornamentation

Ceratopsids are among the most distinctive and well known extinct Cretaceous vertebrates, yet many details regarding the growth and composition of their cranial features are still not fully anatomically described or understood. In particular, striking cranial adornments such as the postorbital horns and parietal-squamosal frill of Triceratops experience a series of major morphological and histological shifts throughout ontogeny. While previous osteohistological studies have largely focused on the parietal of Triceratops, there are relatively few studies of postorbital horn and squamosal histology. Here we present an in-depth osteohistological description of the postorbital horns and squamosal from a single subadult Triceratops horridus specimen and compare similar histological features among ceratopsian cranial ornaments. Analysis revealed the postorbital horns display considerable histological variation based on sampling location, with our proximal section describing an extensive range of vascularity across three notable horizons previously undocumented, suggesting that histological ontogenetic assessments could differ across ornamental structures. Although the Triceratops squamosal maintains similarities with remodeled bone among other ceratopsid parietals, distinct growth marks are preserved near the periosteal surface containing at least three lines of arrested growth, citing a novel developmental shift from rapid azonal to slowed zonal growth late in ontogeny. Additionally, although some metrics of cranial size remain relatively unreliable characteristics for assessing ontogeny, occipital condyle dimensions may offer an avenue of delineating ontogenetically older Triceratops individuals. Alongside morphological measurements, extensive histological sampling may provide a means to clarify currently perceived anatomical inconsistencies and heterochronic variation between putative adults and subadults of Triceratops and other ceratopsids.