Exploring the ceratopsid growth record: A comprehensive osteohistological analysis of Triceratops (Ornithischia: Ceratopsidae) and its implications for growth and ontogeny

Abstract

Ceratopsids represent one of the most iconic groups of non-avian dinosaurs. These large quadrupedal ornithischians are well-known for their bizarre cranial ornamentations, which are distinctive among different ceratopsids. However, only very little data exist on ceratopsid osteohistology and growth rates. Here, we present a detailed osteohistological analysis on Triceratops horridus preserved in a relatively large bonebed from the Lance Formation (eastern Wyoming, USA) as well as additional Triceratops cf. prorsus specimens from Canada. Deciphering the bone microstructure of this iconic dinosaur allows to better understand the growth and development of ceratopsids. The Triceratops limb elements show a distinct pattern of slower growing parallel-fibred and faster growing woven-parallel bone tissue that serves as basis for the definition of histologic ontogenetic stages (HOS). Lower (i.e., younger) HOS correspond to woven-parallel tissue while higher (i.e., older) HOS correspond to parallel-fibred tissue. The intraskeletal variation in histology is best explained through the Three-Front Model, indicating significant differences in cortical thickness between different limb bones. The Triceratops primary growth record is poorly expressed, and the few growth marks preserved show irregular spacing inconsistent with expected growth patterns. The HOS scheme provides seven stages that correspond to biological age classes and that show a correlation with body size. Our analysis suggests that the taxonomic ambiguity between Torosaurus and Triceratops cannot be solved based purely on histological data, but requires additional taphonomic, taxonomic and histological analyses. This study expands the current ceratopsian histological database and helps to better understand ceratopsid growth patterns.