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

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.


Introduction
The Stegosauria of Marsh (1877) is a clade of quadrupedal graviportal herbivorous ornithischian dinosaurs with a bizarre array of dermal plates and spines arranged as two rows along the top of the body from the neck to the end of the tail. The group is represented by bones from the Middle Jurassic to the Late Cretaceous and from all continents except Antarctica and Australia (Galton & UpchUrch 2004;Maidment et al. 2008;Maidment 2010;Galton 2012). Benton & spencer (1995: 125) listed "Stegosaurus" spines from a shallow marine deposit in the Inferior Oolite Group (Aalenian-Bajocian, early Middle Jurassic) of Bradford Abbas (National Grid Reference SY 5915) near Yeovil in Dorset, southern England.
These bones were cited as stegosaurian by naish & martill (2008: 616), so this would be the earliest record for stegosaur bones, but maidment et al. (2008) re-identified the spine as Thyreophora indet.; they also mentioned a fragmentary vertebral centrum and two indeterminate fragments.
These bones were collected and subsequently donated in 1951 to the NHMUK by david meredith seares Watson (1886Watson ( -1973, a distinguished vertebrate paleontologist and professor at University College of Lon-don University from 1921to 1951(see parrinGton & Westoll 1974. There is no information on the position of these bones as found and they are the only ones in the Watson Collection from this locality (S.D. Chapman, pers. comm. 2016). The results of an examination of these potentially important but previously unillustrated bones is given here.

Description and comparisons
The bone fragments of Benton & spencer (1995) were identified as NHMUK PV R.6333-36 by maidment et al. (2008: 386, but incorrectly cited as from Lower Jurassic) as follows: "R.6334 is a small fragment of a dermal spine. It is expanded slightly at the base and appears to be rounded in cross-section. R.6336 may be a partial vertebral centrum, although (it) is extremely fragmentary. R.6333 and R.6335 are indeterminate bone fragments. Just a small part of the dermal spine is present, and morphologically it could be referable to either Ankylosauria or Stegosauria. These elements are therefore regarded as Thyreophora indet."  Galton 1985Galton , 2016 and Stegosaurus (Upper Jurassic, USA; Galton 2016). The proximal surface is smooth (Fig. 1B), rather than being very irregular with foramina ( Fig. 1L), and the rim is smooth rather than being indented by numerous grooves. The distal broken cross-section (Fig. 1D, E) lacks a thick outer layer of compact bone bordering a well-defined central cavity as in Dacentrurus (Fig. 1N) and old adult individuals of Stegosaurus (Upper Jurassic, USA; Main et al. 2005;hayashi et al. 2008, 2012. Instead it resembles the dermal spines of juvenile to young adult individuals of Stegosaurus and those of ankylosaurs in having a thin outer layer of compact bone bordering cancellous bone, with no well-defined central cavity (hayashi et al. 2010, 2012). Proximally this spine differs from those of nodosaurid ankylosaurs from the Upper Jurassic of the USA in which this surface is markedly concave ("hollow") with smoothly rounded edges (KirKland & carpenter 1994;KirKland et al. 1998;Galton 2016).
The cross-section of the broken end is roughly Ushaped (Fig. 1D), rather than sub-circular to oval (Fig.  1N), so the presumed posterior outline is indeterminate. Because the cited histology (Fig. 1D, E) matches that of a vertebral centrum, as does the morphology, R.6334 is re-identified as the half centrum of a caudal vertebra ( Fig. 1B-F). The form of the articular end surface corresponds to that of a centrum and the oblique ventral surface (Fig. 1C) has facets for a chevron (Fig. 1B). In ventral view (Fig. 1F) the sides initially converge but then diverge, as do the sides of a centrum, rather than continuing to converge distally as on a stegosaurian dermal spine (Fig. 1L, M; Galton 2016). The original length of this centrum was at least twice its height (Fig.  1C), quite unlike the proportionally short caudal centra of stegosaurs in which the height exceeds the length except in the most distal part of the tail (Gilmore 1914;ZhoU 1984). More elongate caudal centra occur in many groups of dinosaurs ) and in marine crocodylomorphs (andreWs 1913), so this half centrum is tentatively identified as Archosauria indet.
NHMUK PV R.6336 ( Fig. 1G-K): this is a ventrally incomplete vertebral centrum and, from the presence of a parapophysis anterodorsally ( Fig. 1G-J), it is from a cervical vertebra. The centrum is slightly asymmetrical in ventral view and amphicoelous (gently concave at both ends), with the maximum anterior width greater than the length (Fig. 1G, H). Originally the centrum was low, being wider than high (Fig. 1G, J), and it probably came from the posterior part of the neck.
The Inferior Oolite Group (Middle Jurassic) of England is a marine deposit but the remains of marine reptiles from it are rather limited, consisting mostly of isolated bones and teeth of crocodylomorphs, plus fragments of ichthyosaurs, plesiosaurs and pliosaurs (Benton & spencer 1985). These groups are very well represented in the more recent marine Peterborough Member of the Oxford Clay Formation (basal middle Callovian, Middle Jurassic) of Fletton near Peterborough, England (andreWs 1910(andreWs , 1913. The crocodylomorphs do not have short cervicals. The ichthyosaurs have extremely short centra that are thick plate-like discs. Short wide cervical centra occur in some sauropterygians (plesiosaurs and pliosaurs) but the parapophysis is situated ventrally at mid-length, with the diapophysis sometimes fused to it dorsally (andreWs 1910, figs. 50, 51, pl. 3, fig. 4;andreWs 1913, figs. 17-19). The proportions of the caudal centra of some plesiosaurs, such as in some elasmosaurids (andreWs 1910, figs. 58, 85), resemble R.6336 in being wider transversely than long. u n c o r r e c t e d p r o o f s However, the base of the transverse process originates from the middle of the centrum that is longitudinally symmetrical in ventral view with paired chevron facets at both ends.
The most common terrestrial tetrapod remains from the English Inferior Oolite are isolated teeth and jaws of the tetanuran theropod dinosaur Megalosaurus (Benton & spencer 1985). Only an incomplete cervical vertebra is described for  Britt 1987). In these taxa the cervical centra are very different from R.6336 (Fig. 1G-K), being very strongly opisthocoelous (articular surfaces strongly convex anteriorly and concave posteriorly) with well-developed lamellae bordering prominent lateral fossae or pleurocoels (side cavities) for pneumatic diverticular for extensions of the air sacs (Britt 1997). Cervical vertebrae are not known for the ceratosaurian theropod Sarcosaurus (Lower Jurassic, England; AndreWs 1921) but in the ceratosaurian Dilophosaurus (Lower Jurassic, USA; Welles 1984) the centra are similar to those of megalosaurids but only weakly opisthocoelous. The same is true for the proportionally more elongate centra of the sauropod dinosaur Cetiosaurus (Middle Jurassic, Bajocian, England) in which the parapophysis is on the anteroventral corner and the fossae and lamellae are very prominent (UpchUrch & martin 2002).
As Britt (1997) notes, the interior of the centra of most saurischian dinosaurs, i.e., theropods and sauropod sauropodomorphs, contain sizable cavities that were filled with marrow and fatty tissues or, if connected to the outside by a large pneumatic foramen, then with air as part of a pneumatic diverticulum. In underived or camarate vertebrae, as occur in sauropods and non-tetanuran theropods (includes Ceratosauria, e.g., Ceratosaurus Upper Jurassic, USA; Madsen 1976, fig.  26B), the walls are relatively thick with several large internal chambers. In derrived or camellate vertebrae, as occur in tetanuran theropods (most theropods, e.g., Allosaurus, Upper Jurassic, USA; Madsen 1976, fig.  26A; includes Megalosauridae), the walls are thin with supernumerary, small internal chambers. However, the Dorset centrum shows no evidence of any internal cavities or lateral foramina (Fig. 1G, J), as is also true for all ornithischian dinosaurs including ornithopods (e.g., NHMUK PV R.2477, Hypsilophodon, Lower Cretaceous, England; hUlKe 1882, pl. 76, fig. 2 fig. 12).

Discussion
The large thyreophoran centrum NHMUK PV R.6336 ( Fig. 1G-K)  Earliest bones of Stegosauria: Very large (estimated original length ~1000 mm), columnar partial femoral shafts from the Upper Triassic (Rhaetian) of Avon, England were tentatively referred to the Stegosauria by Galton (2005). However, redelstorff et al. (2014) concluded that they are probably dinosaurian, as indicated by the presence of fibrolamellar bone, and represent an unknown lineage of the Sauropodomorpha or, alternatively, they represent an unknown pseudosuchian lineage that independently evolved fibrolamellar bone as an adaptation for reaching giant size.
Earliest bones of Ankylosauria. Nath et al. (2002) described jaw bones, a few vertebral centra, fragments of scapula and ilium, and dermal armor from the Lower Jurassic of southern India and referred this material to the Ankylosauria. However, the jaws are crocodylomorph and the rest of the bones represent a basal thyreophoran (see Galton & carpenter 2016: 208). The mandible of Sarcolestes leedsi from the basal middle Callovian of Fletton near Peterborough, England (Galton 1983) is recognized as the oldest valid genus but it is Ankylosauria incertae sedis (vicKaryoUs et al. 2004). Earliest footprint record of Stegosauria. Deltapodus brodricki Whyte & romano, 1995, which was based on a supposed sauropod dinosaur trackway (Fig. 2N) from the basal Middle Jurassic (Aalenian, 174-170 Ma) of Yorkshire, England, was re-identified as representing the prints of a three toed stegosaur like Stegosaurus by Whyte & romano (2001, 130A, B, 131A, B). The quadrupedal trackway shows several distinctive characters, viz., for manus prints (m, Fig. 2N): arcuate (transversely wide so arc shaped), entaxonic (medial digits more strongly developed), pollex (digit I) print evident, and digit prints indistinct; for pes prints (pes. Fig. 2N): triangular with elongate heel, digitigrade, three very wide blunt digits not separated by well-developed hypicies (angles), weakly mesaxonic (digit III only slightly longer than II and IV), and toes radiating (Whyte & romano 2001;Li et al. 2012). These characters agree with the foot skeleton and anticipated footprints of a quadrupedal stegosaur such as Stegosaurus (see Gilmore 1914) and not with those of other potentially contemporaneous dinosaurs (see Whyte & romano 2001, table 1). This referral is supported by subsequent studies (milàn & chiappe 2009;CoBos et al. 2010;MateUs et al. 2011;PascUal et al. 2012;li et al. 2012;xinG et al. 2013;romano & Whyte 2015;LocKley et al. 2017). As mateUs et al. (2011: 656) noted, the "occurrence of Deltapodus in the Aalenian of England and its confirmation as a stegosaur implies an earlier cladogenesis for the Stegosauria, than the Bajocian or Bathonian age implied by the skeletal record." This earlier origin is also true for the sister group, the Ankylosauria, with both groups probably originating sometime in the Early Jurassic at the latest.