Scelidosaurus, Owen, 1859

Scelidosaurus : general skull form

The skull of Scelidosaurus harrisonii comprises, in essence, a rectangular box that, amongst other roles, surrounds and provides anchorage for the braincase. On the front of this ‘box’ is a gently tapering snout whose rostrum bends slightly ventrally and ends in a smoothly rounded, slightly pendent, toothless tip ( Fig. 8 View Figure 8 ). The dorsal profile of the premaxilla is notched transversely, marking off a discrete ‘rostrum’ supporting the short, edentulous upper beak. The external nares (nf) are small and the premaxillary margin beneath them is sloped, but not flared laterally. There is a shallow, subtriangular (roughly beanshaped) antorbital fossa (aof). In the anterior corner of this fossa is a discrete foramen (aaf) that extends anteriorly within the maxilla to emerge as a slit-like anterior maxillary foramen (amf). In the posterior corner of the antorbital fossa is a larger fenestra (af) that dips backward into the floor of the orbit, and also opens medially on to the internal maxillary shelf. The posterior margin of the antorbital fossa is formed by a sloping bar of the lacrimal that separates the fossa from the much larger, laterally facing, circular orbit. A few sclerotic ossicles have been found attached to the medial surface of the left jugal ( Fig. 21B View Figure 21 ) and on the isolated cultriform process, as well as lying loose in the matrix, so there is no doubt that there was a sclerotic ring in the orbit, but the remains are insufficient to allow the sclerotic ring to be reconstructed with any confidence. The posterodorsal margin of the orbit is slightly encroached upon by the overgrowth of a craterlike osteoderm on the lateral face of the postorbital.

Beneath the rugose horizontal ledge that borders the antorbital fossa and orbit, the sidewall of the skull is smooth but deeply inset, creating a pronounced horizontal buccal emargination (be) or cheek recess, adjacent to the upper dentition and co-extensive with a similar feature on the mandible. The infratemporal fenestra is large and ovoid with its long axis is somewhat inclined posterodorsally. This fenestra is not occluded by the overgrowth of any of the surrounding bones, or overlying osteoderms. The quadrate is completely excluded from the margin of the infratemporal fenestra by contact between finger-like processes of the squamosal and quadratojugal. The jugal is elongate and deep posteriorly. Its external surface is plastered by an extensive covering of periosteally derived, accreted bony tissue (exostoses). This latter tissue comprises irregular bony strands in the area beneath the orbit; on the quadratojugal wing of the jugal there are distinct patches of accreted tissue: these have a granular texture in places and strand-like in others. These discrete patches are separated by smooth-surfaced grooves. The quadratojugal bridges and seals off the posteriorly facing quadrate (paraquadrate) foramen. The main body of the quadrate is pillar-like but has a shallowly concave posterior margin, a short laterally flared jugal wing and a considerably deeper, more expanded, pterygoid wing.

The dorsal surface ( Fig. 9 View Figure 9 ) and sides of the skull are extensively ornamented by exostoses comprising fine bony fibres, as well as areas that have a coarser, more granular texture. The equivalent periosteal surface texture on the nasals is patterned by a set of curved, low-relief ridges. The central (frontal) portion of the skull roof has a shallow median groove and the periosteal accretions on either side have a granular texture (as if there were a large, median, keratinous scale attached to this area). Radiating away from this central (granular) region is an array of elongate bony strands that extend across the surface of adjacent skull roofing bones. In the area adjacent to the supratemporal fenestrae the postorbitals show small islets of periosteal bone that give the impression that there is a process of encroachment around the adjacent margin of the supratemporal fenestra. The internal surfaces of the supratemporal fenestrae are smooth, providing areas that allowed the unhindered movement of the adductor musculature that was attached to the margins of these fenestrae. The margins of the supratemporal fenestra are extremely rugose (notably along the pair of ridges that form the sagittal crest) because they represent the anchoring sites for the jaw adductor musculature.

The snout tapers gently anteriorly and the premaxillae are firmly butt-sutured. There is a distinct transverse crease on its dorsal surface (better seen in lateral view) that marks the base of the projecting portion that supported a toothless ‘beak’ (rhamphotheca). The dorsolateral edge of the skull, above the orbit and anterior part of the intertemporal arch is dominated by a prominent, rugose, overhanging brow-ridge ( Fig. 9 View Figure 9 ) formed by the arched and conjoined palpebral (anterior supraorbital) and posterior supraorbital. The latter is sutured to a stubby tabshaped projection (and adjacent bowl-shaped slot) along the top edge of a laterally positioned postorbital osteoderm. A narrow unossified space (see Fig. 16 View Figure 16 , gap) remains between the posterior process of the palpebral and the lateral margins of the prefrontal and middle supraorbital. This gap (probably plugged with connective tissue and visible in the subadult specimen; Fig. 16 View Figure 16 ) might well become occluded by the continued spread of exostotic tissue in fully adult individuals.

Ventrally ( Fig.10 View Figure 10 ) thereisashortpremaxillarypalate, behind which the deep nasal cavities are subdivided by the keel-like vomers and seem to be at least partly roofed by a pair of thin, curved bones referred to (herein) as epivomers (epv). Farther posteriorly the pterygoids form broadening, arched plates that separate the nasal cavity from the ethmoid region/orbital cavities and shield the braincase; they also link laterally to the upper jaw via the palatines and ectopterygoids. There is a narrow interpterygoid vacuity, above and between which can be seen a narrow parasphenoid rostrum (cultriform process). Farther posteriorly, the barrellike floor of the braincase is visible and flanked by two unusually large, oblique, rugose tuberosities (bot), as well as prominent triangular pedicles (op.ped) formed by ventral extensions of the opisthotics.

Perhaps most striking of all (in view of our previous dependence upon the accuracy of Owen’s original description) is that the posterodorsal margin of the occiput is ornamented by a pair of prominent, horn-shaped osteoderms ( Figs 16 View Figure 16 , oc.ost, 47). The reconstruction of the dorsal ( Fig. 9 View Figure 9 ) and occipital aspects of the skull ( Fig. 11 View Figure 11 ) indicate the likely position of the attachment sites (oc.f) for the pad-like base-plates that supported the osteodermal horns. The occiput is reconstructed only tentatively because no specimens show this surface particularly well. The lectotype provides information about the general disposition of the occiput, but none of the cranial sutures are clearly shown and seem to be obscured by a patina of periosteally derived (exostotic) bone. The supraoccipital contributed to the dorsal margin of the foramen magnum, and the pedicles of the exoccipitals formed dorsolateral mounds on the occipital condyle. There is an unusual spur-shaped process ( Figs 11 View Figure 11 , 33 View Figure 33 , psp) on the dorsal margin of the paroccipital process that surrounds most of the remnant of the post-temporal fenestra ( Fig. 11 View Figure 11 , ptf). A more medial position for any other opening seems improbable in the skull of more mature individuals, given the need for a sutural surface for anchorage of the occipital horns and the extensive exostotic overlay. However, one juvenile specimen ( CAMSM X39256 View Materials – Fig. 32B, C View Figure 32 , can) reveals the presence of a smooth, oblique groove representing what appears to be the lateral wall of a more medially positioned oblique canal running from the occiput toward the cranial cavity along the suture between the opisthotic and supraoccipital. This canal presumably became plugged by accretionary bone growth during ontogeny. The precise layout of the squamosals and parietals on the occiput is not known .