Scelidosaurus harrisoniifrom the Early Jurassic of Dorset, England: the dermal skeleton
Authors: Norman David B 
Published: August 2019

Cranial exostoses (areas of periosteal ornamentation) are present on the external surfaces of the skull and mandible of Scelidosaurus harrisonii. True osteoderms have also been identified on the skull, forming a ‘brow-ridge’ of three supraorbital bones, dished plates that are attached to the lateral surface of the postorbitals and a pair of larger, horn-shaped structures that project from the posterodorsal surface of the occiput. Postcranial osteoderms form an extensive series of oval-based, ridged osteoderms that extend backward across the dorsal and lateral surfaces of the neck and torso. Smaller, narrow-based ridged osteoderms are also found on the lateral surfaces of the limbs. The tail is surrounded by four longitudinal rows of large, narrow-based, ridged or keeled osteoderms.

The neck, unlike the rest of the body, is encased dorsolaterally by a variety of osteoderms. These can be differentiated into two fundamental types: base-plate osteoderms that develop deep within the compact layers of the dermis and, superficial to each base-plate, tall, ridged or cap-like osteoderms. These latter, project outward from the skin surface and were covered by an epidermal scale or a rigid keratinous sheath. The base-plates are true osteodermal components, but to differentiate them from the more familiar superficial osteoderms, they will be called here simply ‘base-plates’. Lying on the dorsal midline between and beneath the occipital horns is a single, ridged, nuchal osteoderm comprising a base-plate and osteoderm cap. The nuchal plate is flanked by a pair of prominent ‘tricorn’ osteoderm arrays mounted on shallowly arched blocks of fused base-plate osteoderms. Behind the tricorn arrays is a succession of four partial collar-like arrays of osteoderms formed (at least in ontogenetically mature specimens) by coalesced base-plates that anchor tall and either carinate or more plate-like osteoderms. The largest of these are always positioned on the ventrolateral margin of each collar. The osteoderms become progressively smaller toward the midline. It is at present unclear whether the base-plate supported collar arrays on either side fuse together along the midline to form cervical half-rings, as is often reported in more derived ankylosaurian thyreophorans. Individual collar arrays do not imbricate with each other, but are likely to have been interconnected by sheets of tough connective tissue.

On the ventrolateral flanks of the pectoral region are found the largest, bladed osteoderms. In two partly articulated skeletons an osteoderm is preserved on the posterodistal surface of the scapular blade. Although this position is reminiscent of the parascapular spines found in some stegosaurs, these bones are not regarded as homologues; their placement is a coincidence of positioning an osteoderm row adjacent to the scapular blade.

The torso preserves three principal rows of large, ridged osteoderms that show no evidence of accompanying base-plates. The ventrolateral row has the largest osteoderms and these are succeeded in size by the lateral row and dorsolateral row, respectively. There is no evidence to support the existence of a midline dorsal row of osteoderms. The principal rows extend backward across the dorsal and lateral flanks of the body as far as the pelvic area. Smaller cap-shaped osteoderms are scattered between the principal rows, but whether they were organized into subsidiary rows or were more randomly distributed cannot be ascertained. Smaller, narrow-based, ridged osteoderms are found in oblique rows across the anterior chest; they also flank the proximal half of the forelimb (as far as the elbow) and extend to the ankle region in the hind limb.

The tail is surrounded by large, narrow-based, high-ridged osteoderms. Unlike the neck and torso, there is a row of dorsal midline osteoderms that are flanked by large, lateral osteoderms, and beneath these there is a midline ventral row. The latter are close-set and particularly deeply keeled in the area nearest to the pelvis.

Osteoderms vary considerably both in structure and texture. Base-plates have a rough, porous external texture as a consequence of the abundant vascular canals that penetrate these bones. Internally, their surface is arched and has a woven-textured fabric comprising bundles of mineralized fibres interspersed with large vascular foramina. Accompanying osteoderms are generally a little denser than their base-plates and have a smoother cortex, although abundant small foramina and shallow vascular channels pit and groove this external surface. The pair of occipital osteoderms closely resemble bovid (ungulate mammal) horn-cores and are likely to have been sheathed by keratin (as preserved exceptionally in the ankylosaurians Zuul and Borealopelta). Farther posteriorly, the principal osteoderms in the major rows along the torso and tail are generally thin-walled, cap-shaped and ridged. They have a rough and porous external surface, which suggests that the bone surface was covered by keratinous scales. The generally porous fabric of these osteoderms has been remarked upon and it is probable that these were flushed with blood. Interspersed between the visually dominant parasagittal rows of osteoderms is a scattering of smaller cap-shaped osteoderms and polygonal or rounded, flat ossicles. Scattered populations of these ossicles were probably lost because they were, in effect, ‘invisible’ during excavation and skeletal preparation, being of millimetric dimensions. These smaller osteodermal ossicles formed a mosaic-like pattern on the skin surface and toughened the flexible portions of the skin of the animal. Skin impressions and epidermal peels, probably deriving from the ventral surface of the body, reveal a closely packed mosaic of smaller flat osteoderms that underlie similarly shaped keratinous scales.

The discovery of smaller, partly articulated skeletons has revealed aspects of the growth and development of the cervical osteoderm arrays. Individual base-plates begin to form deep in the dermis through mineralization of the woven connective tissue fibres in the stratum compactum and, as these thicken, they also involve the looser and more irregular fibres of the stratum superficiale. Individual base-plates expand peripherally, deepen and form shallowly convex pads externally upon which primordial osteoderms developed. The latter form initially as narrow, elongate, pup-tent-shaped structures with a posteriorly off-set apex and arched, slightly hollow bases. Differential patterns of mineral deposition progressively modify these ‘templates’ into the range of osteoderm morphologies seen in ontogenetically mature skeletons: from subconical curved horns, through tall, carinate blades, to extremely tall, plate-shaped structures, as well as to the simpler oval-based, ridged, pup-tent-shaped osteoderms. As the skeleton approaches full size, in the neck region the base-plates and their osteodermal caps fuse together, and adjacent base-plates interlock before finally fusing together to form partial collars that anchor and support transverse arrays of prominent osteoderms. Osteoderms had the potential to contribute to a number of biological roles in the life of these animals, including protection (defence-retaliation), thermoregulation and more subtle aspects of their behaviour.