purported earliest bones of a plated dinosaur (ornithischia: stegosauria): a "dermal tail spine" and a centrum from the aalenian-bajocian (middle jurassic) of england, with comments on other early thyreophorans

Abstract: The supposed base of a slender dermal tail spine from the Inferior Oolite Group (shallow marine deposit, early Middle Jurassic, Aalenian-Bajocian) of Dorset, England, previously reported as “Stegosaurus” and Thyreophora indet., is a half centrum of a caudal vertebra, Archosauria indet. A dorsally and ventrally incomplete vertebra from the same locality has a low centrum that is slightly wider transversely than it is long and, as there is a parapophysis anteriorly, it is part of a cervical vertebra. However, it does not match those of marine reptiles from the Middle Jurassic (Callovian) of England in which the cervical centra are elongate in crocodylomorphs, plate-like in ichthyosaurs, and short and wide in some sauropterygians (plesiosaurs and pliosaurs) but the parapophysis is mid-ventrally situated. The Dorset centrum does not correspond to those of most contemporaneous dinosaurs, viz. theropods, basal sauropodomorphs, basal sauropods and ornithopods. However, it proportions correspond to posterior neck vertebrae of the basal thyreophoran Scelidosaurus (Lower Jurassic, England) and those of eurypod thyreophorans, the dacentrurine stegosaur Dacentrurus (described as Miragaia, Upper Jurassic, Portugal) and the nodosaurid ankylosaur Mymoorapelta (Upper Jurassic, USA), so the Dorset centrum is tentatively identified as Thyreophora indet. The earliest skeletal records for armored dinosaurs are from the Middle Jurassic: for Eurypoda a proximal ulna from the early Bajocian of Scotland, for Stegosauria two large sub-vertically plates from the earliest Bathonian of England, and for Ankylosauria the ?Bathonian-Callovian of China or the early middle Callovian of England. However, the stegosaurian footprint taxon Deltapodus brodricki (Aalenian) of England pushes the origin of Stegosauria (and sister group Ankylosauria) down into the Early Jurassic.