Scelidosaurus harrisonii, OWEN, 1861

Norman, David B, 2020, REVIEW Scelidosaurus harrisonii from the Early Jurassic of Dorset, England: the dermal skeleton, Zoological Journal of the Linnean Society 190 (1), pp. 1-53 : 8-13

SCELIDOSAURUS HARRISONII OWEN, 1861

Lectotype: NHMUK R1111 ( Charig & Newman, 1992; BZN, 1994). The remains of a medium-sized (5–6-m long) armoured ornithischian dinosaur. Comprising the skull, lower jaws (missing the rostral portion of the snout parts of the skull roof and occiput) and the majority of the articulated postcranial skeleton (missing a major part of the cervical region, some caudal vertebrae and the distal halves of both forelimbs).

Etymology: From the Greek skelos (σκέλος) = limb or leg and sauros (σαυρος) = lizard/reptile. The epithet harrisonii refers to belonging to James Harrison, the collector. The generic name was probably chosen to distinguish this taxon from the paddle-limbed reptiles that predominate in the Liassic beds of the Charmouth/ Lyme Regis area.

Stratigraphic range: Charmouth Mudstone Formation (Upper Sinemurian), Black Ven Marl Member, Asteroceras obtusum Zone, Obtusum Subzone ( Hesselbo & Jenkyns, 1995) . Nodule-bearing beds that occasionally reveal these remains are locally referred to as the Topstones Bed (David Sole pers. comm., 2018) and (perhaps) the Stonebarrow Flatstones Bed ( Fig. 2).

There is an unverified report ( Ensom, 1987) of scelidosaur remains having been recovered from the base of the Pliensbachian (Belemnite Marl Member, Uptonia jamesoni Zone ). Precise details concerning the provenance of scelidosaur specimens has always been particularly problematic in this area, because fossils are often recovered from the shoreline below the cliffs or on the scree slopes. The degradation of the cliffs at Charmouth [landslides (=cliff collapses) is a consequence of the poorly consolidated Liassic sediments and is enhanced by spring-line induced rotational slips] and the inherent cyclicity of the ‘Lias’ shale and marly limestone bands, are confounding factors. Longshore drift may account for specimens washing up at Seatown where the Pliensbachian is exposed.

Locality: Cliff exposures on The Spittles–Black Ven between Charmouth and Lyme Regis, Dorset. Weathered nodules containing bones or eroded individual bones are occasionally found along the foreshore beneath these cliffs ( Fig. 1) .

Diagnosis: Synthesis based on diagnoses in Norman (2020a,b) with the addition of features associated with the dermal skeleton. Premaxilla has a mound-shaped rugose exostosis on its tip; facet for the posteromedial process of the premaxilla positioned on the laterodorsal surface of the nasal; the sagittal crest on the parietals comprises two parallel crests separated by a smooth, narrow midline trough; the angular covered by an exostosis; epivomer bones contribute to the roof of the nasal chambers; epipterygoid bone forms a vertical, conical structure sutured to the dorsolateral surface of the pterygoid; large, oblique (epistyloid), articular facets on the ventrolateral wall of the basioccipital; facetted pedicle on the opisthotic; elongate, distally bladed, epistyloid bones project obliquely from the posterior of the skull; spur-shaped structure on the dorsal edge of the paroccipital partly encloses the posttemporal fenestra; a pair of subconical osteoderms project from the posterodorsal surface of the occiput; a pair of tricorn cervical osteoderm arrays lie immediately behind the occipital region; four partial, collar-like arrays of osteoderms in the cervical region. Caudal 1 has a convex posterior articular surface on its centrum and caudal 2 is correspondingly procoelous (in one skeleton only – this may prove to be either a sexual dimorphism or a character that defines a new taxon). Small, fusiform clavicles present. A convex, ovoid ‘antitrochanteric’ pad is present on the lateral surface of the ischiadic peduncle of the ilium. The prepubic process twists laterally. The proximal end of metatarsal 5 appears to have been sutured to the lateral margin of distal tarsal 4. Pedal digits 2–4 curve medially along their lengths. Pedal unguals of digits 2–4 are curved medially. Pedal ungual of digit 1 is short, more nearly symmetrical, mildly arched and bluntly pointed.

Inventory of the key osteoderm-bearing material referred to this in monograph

A full listing of presently known scelidosaur material can be found in Norman.

1. NHMUK R1111 (lectotype). The articulated skeleton of a large scelidosaur (4.4 m long). The skull is largely complete and preserves exostoses and osteoderms. Missing parts of the postcranial endoskeleton include some cervical vertebrae and their ribs, portions of the dorsal ribcage, a few caudal vertebrae, parts of the pectoral girdle, the right forelimb, the left forearm and manus. Most of the craniocervical osteoderms are missing and only isolated elements from the osteoderm rows that covered the dorsal and lateral surfaces of the torso are preserved. Some of the caudal osteoderms are still associated with caudal vertebrae, but the majority were removed during preparation and are now stored separately in the collections .

2. NHMUK R12019. A broken marlstone block containing a fragment of the occiput linked to an articulated series of six (visible) cervical vertebrae of a large-sized scelidosaur (~ 4 m long). The vertebrae are flanked by several eroded subconical osteoderms and their attached base-plate.

3. CAMSM X39256 . Associated skeleton of a small scelidosaur (2.1 m long). This specimen includes a collection of fully prepared, but disassociated, cervical osteoderms. These include isolated base-plates (a few of which have adhering osteoderms) and a variety of isolated ridged osteoderms.

4. BRSMG LEGL 0004. Articulated skeleton of a scelidosaur of large size (3.9 m long) comprises a skull, the presacral vertebral column, the shoulder girdle and proximal portions of the left forelimb, most of the pelvic girdle and most of both hind limbs (except for the distal halves of the metatarsals and phalanges of both feet) and the proximal part of the tail (the first 13 caudals). A substantial portion of the dermal skeleton is preserved close to its natural layout. This specimen was partly recovered and later assembled by David Sole; it is now on permanent display (and available for research) at Bristol City Museum. A good-quality cast of this specimen is also held in the collections of the St. George Discovery Site (Utah) and is accessioned as SGDS 1311. A similar cast is also on display at the Charmouth Heritage Coast Centre, Dorset UK.

5. BRSMG LEGL 0005. Partly articulated specimen of an intermediate-sized scelidosaur (3.1 m long). Major parts of the appendicular skeleton have been fully prepared, while the axial skeleton remains partly embedded in marlstone. The specimen includes a small portion of the braincase and a fully articulated cervical series that has been partprepared, but remains embedded in an ellipsoid bed of marlstone. Associated with this cervical series is a well-preserved and little disturbed group of cervical osteoderms. This specimen, discovered by David Sole, is also on permanent display (and available for research) at Bristol City Museum.

ANATOMICAL INTRODUCTION

The skull: The structure of the skull and mandible ( Norman (2020a)) reveals that the external surface of many of the endochondral bones of the skull exhibit a range of textures and surface features that indicate the existence of both exostoses and genuine dermal bones.

The neck: In 1986, an interesting (but as yet unprepared) marlstone slab was recovered from Charmouth (NHMUK R12019; Fig. 7). The block is broken into two pieces along the midline and includes a small portion of the occiput ( Fig. 7, pro, paro) and, slightly disconnected from the latter, an articulated series of longitudinally sectioned cervical vertebrae. On either side of the latter lie the remnants of a bilateral arrangement of eroded osteoderms. Unlike the osteoderms of the tail that Owen had described (1863), these neck osteoderms exhibit a different morphology. The osteoderms per se can be differentiated into two distinct regions. The most superficial part of each osteoderm comprises an elongate, conical and slightly curved structure – termed herein the osteoderm. The uneroded surfaces of these roughly cone-shaped osteoderms are moderately roughened, being covered by a network of fine grooves and small, vascular foramina. The base of each osteoderm is girdled by a rounded rim, beneath which the surface is slightly constricted, creating a waist-like zone (w). Below this waist, the bone surface re-expands and its texture is noticeably coarser and spongier in appearance ( Fig. 7, ba). This portion is termed the base-plate. In some examples, the base-plate has an irregular ‘torn’ edge ( Fig. 7, re), whereas other examples are tilted to reveal an innermost surface to the base-plate that is smoother and shallowly arched ( Fig. 7, arc).

The neck, torso and tail: The discovery and collection, during the winter/spring of 1999/2000, of substantial portions of another large but subadult specimen (3.9 m long), led to the assembly of a scelidosaur skeleton by a consortium of private collectors led by David Sole (Lyme Regis). This specimen supplements our understanding of the variation in structure and distribution of osteoderms across the body of a scelidosaur. The presence of portions of the dermal skeleton was recognized early on, during the removal of the marlstone matrix in which the skeleton was embedded, and considerable effort was expended to ensure that this component of the skeleton was recovered in its post-mortem arrangement. As can be appreciated from the shape of the reassembled skeleton ( Figs 8, 9), the body of this animal was subjected to oblique, lateral compression and displacement during the time that elapsed between the settling of its carcass on the sea-floor and its subsequent decay, burial, compaction and the cementation (lithification) of the surrounding sediment. As a result of these taphonomic processes, the left side of the body of the scelidosaur ( Fig. 8), upon which the carcass appears to have lain, has not been greatly disturbed.

The right side of the same skeleton ( Fig. 9) has suffered more disruption owing to a combination of scavenging, decay and compaction.

NB: Because this referred specimen remains in private ownership, to describe it would appear to some to violate the ethical guidelines adopted by the Society of Vertebrate Palaeontology. To clarify matters as they stand, the original specimen is on permanent loan to the Bristol City Museum. It is provided with the reference number BRSMG LEGL 0004 and is freely available to researchers (and has been so for the past decade). In addition, good-quality casts of this specimen are exhibited at Charmouth Heritage Coast Centre , Dorset, England and at the St. George Discovery Site , Utah, USA ( SGDS 1311 ). The osteoderm arrangement in the cast specimens ( Figs 8, 9) faithfully represents that seen in the original specimen. All observations provided in this description are the same for the original specimen and its replicates .

Intermediate-sized neck osteoderms: A second, partly articulated, incomplete and intermediate-sized skeleton (BRSMG LEGL 0005: 3.1 m long) includes an articulated cervical series embedded in a marlstone slab. Preparation has exposed the dorsal halves of the cervical vertebrae and their associated osteoderms ( Fig. 10).

Juvenile neck osteoderms: Another fully prepared but disassociated partial skeleton of a small individual (CAMSM X39256 : 2.1 m long) includes an articulated cervical and dorsal series of vertebrae and ribs. Associated with this vertebral column is a collection of osteoderms. The osteoderms comprise a series of base-plates and osteoderms from the cervical region and these represent an early stage in the growth of the dermal skeleton ( Fig. 11)

CRANIAL ORNAMENT AND OSTEODERMS: INTRODUCTION

Vickaryous et al. (2001) reviewed the anatomical evidence concerning the origin of cephalic ornament in ankylosaurs using a combination of evidence from fossil specimens and those of recent lizards that exhibit bony cranial ornament and osteoderms. Their study reviewed the understanding that there are alternative developmental pathways that may account for the bony ornamentation seen in ankylosaurs. These pathways were considered to involve either exostotic elaboration through surface deposition of bone minerals on endochondrally-derived cranial bones (as advocated by Coombs, 1971), or the attachment of overlying, dermally derived osteoderms that are at first anchored to underlying cranial bones by connective tissue before becoming fused in position ( Brown, 1908; Sternberg, 1928; Romer, 1956). Coombs & Maryańska (1990) later moderated the views of Coombs (1971) by allowing that cranial ornamentation might be the result of either elaboration of the chondrocranium (exostoses) or the co-ossification of dermally derived bones. Both phenomena are present on the skull of Scelidosaurus , although the former pattern is more widespread.

CRANIAL ORNAMENTATION