Filling in Gaps in the Ceratopsid Histologic Database: Histology of Two Basal Centrosaurines and an Assessment of the Utility of Rib Histology in the Ceratopsidae.

Bone histology grants substantial insight into the growth and biology of fossil vertebrates. Many of the major non-avian dinosaurian clades have been extensively sampled for bone histologic data allowing reconstruction of their growth as well as the assessment of the evolution of growth changes along phylogenies. However, horned ceratopsians are poorly represented in paleohistologic studies. Further, the ceratopsian taxa that have been examined are unevenly sampled phylogenetically with very basal forms and highly derived forms making up the majority of studied taxa. In order to rectify this, we have histologically sampled Avaceratops from Montana and Yehuecauhceratops from northern Mexico to assess how mid-sized basal centrosaurines grew relative to more basal and derived forms. Based on results from these taxa, basal centrosaurines present a mosaic of growth characters intermediate between those seen in basal ceratopsians and more derived centrosaurines. Further, Yehuecauhceratops has many lines of arrested growth preserved, suggesting that the large number of lines of arrested growth found in a high-latitude Pachyrhinosaurus specimen may be a result of phylogeny rather than geography. Since lines of arrested growth are not preserved in long bones of many ceratopsians, especially chasmosaurines, we also histologically sampled ribs of Avaceratops and Pachyrhinosaurus. However, the largest ribs were highly remodeled obscuring lines of arrested growth, making it unlikely that rib histology will clarify growth trends in ceratopsians. These centrosaurines add to the growing ceratopsian histological database and demonstrate that basal centrosaurines grew in a manner intermediate between non-ceratopsid taxa and derived centrosaurines. Anat Rec, 303:935-948, 2020. © 2019 Wiley Periodicals, Inc.