Scelidosaurus cranial anatomy

Scelidosaurus cranial anatomy

Premaxilla ( Figs 8–13 View Figure 8 View Figure 9 View Figure 10 View Figure 11 View Figure 12 View Figure 13 ): In the lectotype ( NHMUK R1111 : Fig. 12A View Figure 12 , Pm) only the distal tip of the posterolateral process of the premaxilla is preserved. This is a tapering sliver of bone lying along the dorsal edge of the maxilla and evidently pinching out between the nasal and maxilla, before it can form an external contact with the lacrimal. The external surface of the posterior tip of the premaxilla is rugose (p.os). This exostosis appears to indicate part of the attachment area for collagenous webbing that anchored a superficial epidermal keratinous scale .

Even tone: broken/eroded surfaces.

In the slightly smaller skull (BRSMG LEGL 0004 – Fig. 14 View Figure 14 ), the anterior portion of the snout is a separate block of bone that has been eroded by water-rolling prior to its discovery. The anterior tip of the premaxilla is eroded, but on its right side ( Fig. 24 View Figure 24 ), the premaxilla can be seen to extend posterodorsally as a tapering rod along the anterodorsal margin of the maxilla (from which it is slightly separated).

In the yet smaller partial skull (BRSMG Ce12785 – Fig. 13 View Figure 13 ), both premaxillae are nearly complete, well-preserved and partly separated from the adjacent skull bones. In articulation with the maxilla, the ventral edge of the premaxilla is inclined slightly anteroventrally, continuing the gentle downward curvature seen toward the anterior end of the maxilla ( Figs 12 View Figure 12 , 13A, B View Figure 13 ). There is no obvious transverse expansion (flaring) of the ventral margin of the external naris. The anterior surface of the premaxilla forms a small rostrum that is rugose, grooved ( Fig. 13D View Figure 13 , gr) and pitted with tiny vascular openings. Viewed in profile, this rostrum forms a discrete, edentulous projection that is set off anteriorly relative to the base of the median dorsal process by a transverse crease; the rostrum has a slightly downturned tip ( Figs 8 View Figure 8 , 13 View Figure 13 ). It seems certain that the rostrum was ensheathed in a keratinous rhamphotheca (beak). Posterior to the small rostrum, the ventral edge of the external surface is roughened ( Fig. 13C View Figure 13 , rug) and forms a curtain-like structure lateral to the dentition. The surface morphology of this bone suggests that the premaxillary dentition may have been at least partially sheathed (and perhaps mechanically supported) by a posterior extension of the rhamphotheca.

The most substantial portion of each premaxilla is a low, roughly rectangular block, the lateral sides of which converge anteromedially toward the rostrum ( Figs 9 View Figure 9 , 13C View Figure 13 ). The dorsolateral surface of this block is smooth and shallowly concave, forming part of the ventromedial wall to the external naris ( Fig. 13D View Figure 13 , nf). The ventrolateral surface of the premaxilla forms a longitudinal alveolar channel that houses five premaxillary teeth ( Fig. 13C View Figure 13 ). This channel is bounded laterally by a curtain-like edge and, medially, by a shallower alveolar parapet that is scalloped. Each tooth is associated with a ‘special’ foramen [ Fig. 13C View Figure 13 , sf – equivalent to the ‘alveolar foramina’ of Horner et al. (2004)] and these foramina are linked by a shallow interconnecting groove that defines the dorsal margin of the parapet ( Fig. 13C View Figure 13 ; see also: Fig. 41A View Figure 41 ). Thin, vertical partitions separate individual alveoli, but overall the alveoli are not symmetrical, so each tooth root is not as well supported lingually as it is buccally. This arrangement gives some notional support to the idea that the rhamphotheca may have extended posteriorly to partially envelope (and support) the premaxillary teeth. There is no clear gap (‘diastema’) or offset between the premaxillary and maxillary dentitions, as seen in some early Jurassic taxa such as Heterodontosaurus ( Norman et al., 2011) , and as has been hinted at in Lesothosaurus ( Porro et al., 2015) . It is also worth noting in passing that, in general, the premaxillary teeth are a little taller and more conical, pointed and recurved compared to those on the maxilla ( Fig. 13B View Figure 13 ). However, the two posterior premaxillary teeth, though a little larger, exhibit a transitional morphology and more closely resemble, in shape and proportions, those of the maxillary series.

Mediodorsal to the alveolar parapet, the margins to the ventral roof of the premaxilla display a row of foramina (cf – Edmund, 1960; Horner et al., 2004) connected by a shallow groove. Between the rows of special foramina, the premaxillae form a shallowly vaulted, triangular anterior palatal roof, subdivided by an irregular median sutural line ( Fig. 13B View Figure 13 , pms). Anterior to the tooth row, and on the ventromedial edge of the premaxillary rostrum, there is a pit ( Fig. 13C View Figure 13 , apo) marking the opening for a canal that runs posterodorsally through the premaxilla and connects to an anterior premaxillary foramen located in the anterior corner of the external naris ( Fig. 8 View Figure 8 , apf – Norman & Porro, in preparation). There is no indication of an inter-premaxillary pocket or fossa in the palatal roof, as described in Heterodontosaurus ( Norman et al., 2011: fig. 13) and Hypsilophodon ( Galton, 1974: fig. 4B), and some other ornithischians. In this latter respect, the premaxillary palate of Scelidosaurus more closely resembles that described in Lesothosaurus ( Porro et al., 2015) . The premaxillae are butt-sutured along the midline and this suture was evidently unfused in immature specimens (e.g. BRSMG Ce12785) because the left and right premaxillae have separated and are offset from one another ( Fig. 13C View Figure 13 , pms). Dorsal and posterior to the rostrum, each premaxilla preserves the base of a short, curved, tapering dorsal process (dp) that, when complete, evidently made sutural contact against a facet preserved on the anterolateral margin of the anterior tip of the nasal in BRSMG Ce12785 ( Fig. 15 View Figure 15 , pms). This sutural arrangement is unusual because in ornithischians the nasals usually diverge anteriorly and clamp against either side of a tapering, median dorsal process formed by the conjoined premaxilla. Clearly there was a complete bony rim to the external naris. There is no obvious external evidence for an anterior premaxillary foramen in the anterior corner of the narial fossa, as reported in Huayangosaurus ( Sereno & Dong, 1992) , Lesothosaurus ( Porro et al., 2015) and Heterodontosaurus ( Norman et al., 2011) . However, MicroCT scan evidence confirms its presence (Norman & Porro, in preparation).

Posterolaterally, each premaxilla produces an oblique, slightly sinuous, tapering posterolateral process ( Fig. 13D View Figure 13 , pp) that wedges itself between the maxilla and nasal, and extends to a point just beyond the medial anterior lamina of the lacrimal (although these bones do not make sutural contact): premaxilla and nasal appear to be separated by the dorsal lamina of the maxilla. The dorsomedial edge of the maxillary lamina bears a narrow groove ( Fig. 13B View Figure 13 , gr) that represents a narrow sutural contact area for the posterior premaxillary process. The nasal also bears a step on its lower external surface and below this step is a medially offset vertical ‘flange’ or, more accurately, a thin curtain wall ( Fig. 15 View Figure 15 , mf) that offers a more substantial contact surface for the medial side of the posterolateral process of the premaxilla.

The anterior sutural contact between the maxilla and premaxilla is not clear. It is probable that this was strong and firm, and required an anteromedially directed robust process of each maxilla to meet in the midline, creating a wedge-like structure that slotted into a recess in the posterior margin of the body of the premaxilla dorsal to the premaxillary palatal roof ( Fig. 10 View Figure 10 ). The disarticulated maxilla of Emausaurus exhibits just such a structure ( Haubold, 1990: taf. III).

Apart from the extreme rostral tip and adjacent surfaces, which are all somewhat rugose and pitted (reflecting the active growth and mechanical support for a keratinous rhamphotheca), there is little indication of a superficial coating of periosteally derived bone, prevalent on the exterior surfaces of many of the other skull bones. However, this may well be a juvenile feature in the instance of BRSMG Ce12785 because the distal tip of the posterolateral premaxillary process of the lectotype ( Fig. 12A View Figure 12 ) has a rugose surface texture .

Comparisons: In the only other reasonably well preserved and morphologically comparable, stratigraphically early (Lower Toarcian) and systematically basal thyreophoran Emausaurus ernsti ( Haubold, 1990: figs 2, 4), the left premaxilla is well preserved. Five premaxillary tooth positions have been reported, as described in Scelidosaurus . The premaxilla is not laterally flared and tapers anteriorly. Unfortunately, the anterior tip and much of the dorsal portion of this bone is missing. However, the posterolateral process is preserved. It is transversely flattened and becomes broad and spatulate posteriorly, unlike the morphology seen in Scelidosaurus . Among eurypodans, the basal stegosaur Huayangosaurus is reported to possess seven premaxillary teeth ( Sereno & Dong, 1992: fig. 6A). However, in the more derived Stegosaurus , the premaxilla is edentulous. Ankylosaurid premaxillae are edentulous and some have large subsidiary foramina positioned dorsally and laterally ( Maryańska, 1977: fig. 3). In contrast, several nodosaurids – Pawpawsaurus ( Lee, 1996) , Silvisaurus Eaton, 1960 ), Gargoyleosaurus (Carpenter et al., 2000) and Hungarosaurus ( Ösi, 2005) – are reported to possess premaxillary teeth (as many as eight in the case of Silvisaurus ).

Maxilla ( Figs 8 View Figure 8 , 12 – 14 View Figure 12 View Figure 13 View Figure 14 ): In the lectotype, NHMUK R1111 ( Fig. 12 View Figure 12 ), the left maxilla (Mx) is disarticulated, but truncated by an oblique abrasion surface anteriorly; this latter surface reveals cortical and trabecular bone exposed by erosion. The partially prepared but articulated skull BRSMG LEGL 0004 ( Fig. 14 View Figure 14 ), reveals most of the superficial features of the maxilla, and these are echoed in the smaller, partly disarticulated specimen ( BRSMG Ce12785 – Fig. 13A View Figure 13 ) .

The maxilla has a low, triangular outline. The extreme anterior tip of the maxilla is not directly visible in any specimen, but most probably formed a medially stepped, peg-like extension that was sutured against its neighbour and, together, were securely locked into a complementary recess in the posterodorsal midline between the premaxillae (cf. Haubold, 1990). Viewed externally, the anterior end of the maxilla has a smoothly rounded edge that contacts the premaxilla ( Fig. 8 View Figure 8 ). There is no obvious offset between the alveolar margins of these two bones as is also the case in Emausaurus ( Haubold, 1990) and Huayangosaurus ( Sereno & Dong, 1992) . On the lateral surface of the maxilla, close to the anteroventral tip of the maxilla, there is a prominent, slit-like foramen ( Figs 8 View Figure 8 , 14 View Figure 14 , amf). The dorsolateral surface of the maxilla is laterally compressed, forming a thin lamina (lacrimal lamina) that forms the lateral wall of the nasal cavity. It displays exostotic bone on its external surface that indicates the probable existence of overlying keratinous scales. The dorsal edge of the maxilla is obliquely inclined and has a slightly sinuous contact with the premaxilla. In BRSMG Ce 12785 ( Fig. 13B View Figure 13 , gr), the edge of the lamina is grooved to provide a very narrow, trough-like suture with the posterolateral premaxillary process. Beyond its premaxillary contact, the dorsal edge of the maxilla continues posterodorsally along a suture with the nasal and then meets the lacrimal at its posterior edge. Beneath and anterior to the suture line with the lacrimal there is an extensive scarf suture with the lateral surface of lacrimal ( Fig. 12 View Figure 12 , ls). The apex of the maxilla is rounded and the suture line between maxilla and lacrimal descends in a slightly sinuous fashion ventrally. This suture line terminates at a notch that forms the smooth anterior margin of the antorbital fenestra ( Fig. 12A View Figure 12 , af) located at the rear of the antorbital fossa (aof). At this point, the antorbital fenestra forms a smooth-surfaced channel that curves anteromedially, beneath the base of the lacrimal lamina of the maxilla and opens out on to the lateral floor of the nasal cavity ( Fig. 12B View Figure 12 , af). In lateral view, this foramen widens on to the external surface and merges with the smooth medial wall of the shallow antorbital fossa.

The antorbital fossa ( Figs 8 View Figure 8 , 12–14 View Figure 12 View Figure 13 View Figure 14 , aof) has an ovoid outline in lateral view; it is marked off from the surrounding area of the maxilla because it has a smooth, concave medial wall that is delineated ventrally by an everted and rugose ledge. The latter also forms the dorsal margin of the buccal emargination (be) and, farther posteriorly, a laterally everted shelf that forms the jugal suture. Anteriorly, along the ventral margin of the antorbital fossa, this rough edge develops into a smooth, sharp-edged rim that curves tightly, but smoothly, posterodorsally in order to define the upper margin of the antorbital fossa. The anteromedial corner of the antorbital fossa forms a pocket (sediment-filled) that indicates the position of the anterior antorbital foramen (aaf). A clear, narrow ridge marks the dorsal margin of the fossa anteriorly but this gradually fades away as the ridge subsides and merges with the base of the dorsal (lacrimal) lamina of the maxilla. The posterior border of the antorbital fossa is closed by a bridge formed by the jugal process of the lacrimal; the latter forms an oblique overlapping suture against the anterior tip of the jugal ( Fig. 14 View Figure 14 ). The entire lacrimal (suborbital) process of the jugal is sutured (ventromedially) against a laterally flaring ledge formed along the entire posterodorsal surface of the maxilla ( Fig. 12A View Figure 12 , js).

Immediately beneath the area occupied by the antorbital fossa, the body of the maxilla reaches its maximum transverse thickness. Beneath the rugose ledge that forms the upper boundary of the buccal emargination, the external surface of the maxilla is punctured by a number of larger foramina that lead into a large sinus within the body of the maxilla.

The portion of the maxilla immediately posterior to the antorbital fossa forms an oblique, shelf-like structure that underlies the anterior process of the jugal. The dorsal surface of this shelf is rugose and transversely concave, forming an elongate trough into which the ventral surface of the maxillary (suborbital) process of the jugal appears to fit snugly ( NHMUK R1111 ). The posterior extremity of the maxilla is acutely pointed, lies beneath the main body of the jugal and is wedged between the jugal dorsally and the ectopterygoid medially ( Fig. 12B View Figure 12 , ecs). The posterior ventral margin of the maxilla is sharp-edged beneath its contact with the jugal and is angled anteroventrally in a slightly concave curve. This edge is interrupted by a small pit, which appears to be an incipient alveolus (plugged with calcite in the lectotype). This curved edge is finally interrupted by the concave scalloped edge and alveolus representing the last (preserved) maxillary tooth position (18 th) .

In lateral view, the entire alveolar border of the maxilla is scalloped, where the bone of each alveolus is molded to form a collar around the root of each tooth. The alveolar margin is mildly sinuous along its length – slightly arched anteriorly and bowed ventrally posteriorly – mirroring the profile of the mandibular dentition; this configuration is seen in all three examples ( Figs 12–14 View Figure 12 View Figure 13 View Figure 14 ) and is not an artefact of preservation. Medially, the alveolar wall of the maxilla shows evidence of tooth replacement along the entire length of the tooth row. A row of ‘special’ foramina ( Edmund, 1960; Horner et al., 2004) is located parallel to, and just a few millimetres above, the alveolar margin of each functional tooth ( Fig. 12B View Figure 12 , sf – see Fig. 44 View Figure 44 ). Similar foramina were reported in a variety of ankylosaurs and stegosaurs ( Edmund, 1960: fig. 48). These foramina do not form an arched array, as seen in more derived forms such as ornithopods and ceratopians. In almost all instances, the foramina show the presence of a replacement crown (see Fig. 44 View Figure 44 for more detail). These range from small emergent crown tips through various stages to emerged replacements. Unusually, among ornithischians, the widest part of the emergent crowns is adjacent to the regions of destruction of the medial wall of the alveolus. This creates a visual equivalent of incised interdental plates at intervals along the dentition – see the Dentition section later in this article. Interdental plates are more commonly found on the alveolar walls in theropod dentitions (Holtz et al., 2004).

The inner surface of the main body of the maxilla is smooth and bulges slightly medially, above the dental margin, before curving laterally to form a rounded shelf ( Fig. 12B View Figure 12 , mxs) that partially floors the nasal passage before contacting the lacrimal, nasal and premaxilla. In combination, all these bones form the floor, lateral and dorsal walls of the snout/ nasal passages. The lacrimal lamina is triangular and marked medially by a concave area ( Fig. 12 View Figure 12 , ls – shaped like a thumbprint) that is covered with low ridges indicating that connective tissue secured this surface against the lacrimal in an extensive scarf suture. This area is marked off from the main body of the maxilla by the smooth notch ( Fig. 12B View Figure 12 , af) that forms the anterior edge of the antorbital fenestra. Posterior to the lacrimal lamina, the maxilla attains its maximum width. Its medial and dorsal surfaces bear closely packed ridges running subparallel to one another. This large area represents the sutural attachment for the palatine. The palatine has a dorsolateral process that caps the maxilla and contacts the jugal, as well as an extensive ventral lamina ( Fig. 12B View Figure 12 , mwp) that covers the medial wall of the maxilla. Posteriorly, the border of the maxilla is reflected sharply laterally and displays more sutural scarring, this time for the attachment of the anterolateral edge of the ectopterygoid (ecs); the latter bone connects the body of the pterygoid to the maxilla and jugal.

The dorsal surface to the maxilla, immediately posterior to the lacrimal lamina, is marked by a large elliptical trough/fossa ( Fig. 27F View Figure 27 , mxf) that communicates with a row of large foramina that emerge on the lateral surface of the maxilla. This fossa is sandwiched between the elongate sutural surface for attachment of the jugal and the broad mound that forms the sutural surface for the palatine. The lateral edge of the base of the palatine forms a narrow suture against the medial edge of the jugal and jugal process of the lacrimal, forming a bridge across the central part of the maxillary fossa.

Three areas on the external surface of the maxilla are uncoated with either granular or fibrous exostoses: a posterior portion of the lacrimal lamina dorsal to the antorbital fossa ( Figs 8 View Figure 8 , 14 View Figure 14 ), the medial wall of the antorbital fossa itself and the buccal emargination; these areas would have been lined with soft epidermal tissue. The small unadorned patch of bone on the lacrimal lamina is difficult to explain, but may have some relationship with the antorbital fossa that is positioned directly beneath.

The dorsal surface of the maxilla, between the boundary to the buccal emargination and the premaxilla, is coated with low-relief bony strands; these are mostly arranged longitudinally, but farther posteriorly, near the lacrimal, a pattern of curved ridges develops. A similar pattern of ‘swirly’ ridges is also seen just dorsal of this area on the nasal (particularly well displayed on the nasals of BRSMG Ce12785 – Fig. 15 View Figure 15 ). The prominent ledge beneath the antorbital fossa is rugose, which may indicate the anchorage of the ornithischian equivalent of a mammalian buccinator muscle. The rugosities extend posteriorly as irregular strands of exostotic bone on the lateral surface of the lacrimal process of the jugal beneath the orbit ( Fig. 14 View Figure 14 ). Comparisons: In Emausaurus ernsti ( Haubold, 1990: fig. 5), the maxilla has, overall, a similar morphology to that seen in Scelidosaurus . A buccal emargination is well developed, the sinuous dental profile is the same and the dentition appears to be bowed medially (better seen in the dentary than the maxilla). An oblique, flared sutural shelf, for attachment of the jugal, is similar to that described in Scelidosaurus . An extensive rugose area on the posteromedial wall of the maxilla ( Haubold, 1990: taf. III: 4) indicates the attachment of a medial lamina of the palatine, as seen clearly in Scelidosaurus . Additionally, the anteromedial (premaxillary) process is well preserved. This region is not well exposed in all known specimens of Scelidosaurus . However, in striking contrast, the preserved margin of antorbital fossa ( Haubold, 1990: taf. III, 2) suggests that this is considerably larger than that seen in Scelidosaurus and more reminiscent of that seen in basal ornithischians such as Lesothosaurus and heterodontosaurids ( Norman et al., 2011; Porro et al., 2015). Among eurypodans, the maxilla is excavated laterally and backs a deep buccal emargination (covered by prominent cheek osteoderms in ankylosaurs), but no antorbital fossa has been recorded. In the basal stegosaur Huayangosaurus ( Sereno & Dong, 1992: fig. 6), the maxilla resembles, in its general proportions, those of Scelidosaurus and may well have a large elliptical foramen close to the premaxilla–maxilla suture. It also displays a triangular antorbital fossa. In the more derived Stegosaurus the maxilla is lower and more elongate compared to that of Scelidosaurus ( Gilmore, 1914: pl. 5). It also has a long anterior process that underlies the premaxilla. A small antorbital fossa is present and resembles that seen in Scelidosaurus .

Nasal ( Figs 8 View Figure 8 , 9 View Figure 9 , 14–16 View Figure 14 View Figure 15 View Figure 16 ): The nasals are visible in ventral view in the lectotype skull and are smooth and shallowly arched. Their dorsal surfaces are roughly textured but the skull as preserved is far too fragile to examine closely (see Fig. 4D View Figure 4 ). Fortunately, the nasals are visible externally on the referred skull ( BRSMG LEGL 0004 : Figs 14 View Figure 14 , 16 View Figure 16 ), whereas in the intermediate-sized specimen ( BRSMG Ce12785) both nasals are preserved separated from the remainder of the skull and folded together about the dorsal midline and are nearly complete ( Fig. 15 View Figure 15 ) .

The nasals are elongate, transversely arched plates, broad posteriorly and tapering gradually to a bluntly rounded anterior tip. Just posterior to the anterior tip, each nasal bears a small facet on its lateral surface that must represent the sutural contact for the median dorsal processes of the premaxilla ( Fig. 15A, B View Figure 15 , pms). Unusually among ornithischians, the conjoined tips of the nasals interpose themselves between the posterior ends of the premaxillary processes. The nasals meet in the midline along an elongate, irregular, tongue-in-groove suture ( Fig. 15B View Figure 15 , ns). The ventral edge of the anterior tip of the nasal is transversely rounded and gently arched, and meets the posterolateral process of the premaxilla, pinching off the border of the external naris. The suture with the premaxilla bows ventrally along its length following the dorsal profile of the premaxilla and then meets the lacrimal lamina of the maxilla. In addition, this suture is stepped medially to produce a curtain-like flange ( Fig. 15 View Figure 15 , mf) that lies against the medial surface of the posterolateral process of the premaxilla and probably backs the lacrimal. Posteriorly, the external sutural margin of the nasal curves dorsomedially forming an embayment where it contacts the prefrontal ( Figs 15A View Figure 15 , pfs, 16). However, the ventral aspect of the nasals of the lectotype reveals that this latter suture was scarf-like, forming a continuation of the medial flange that underplates the prefrontal. The well-preserved isolated nasals ( Fig. 15 View Figure 15 ), as well as those of the lectotype, are smooth internally, which contrasts with the external surface that is coated with an array of linear, irregular and curved ridges and some strand-like exostoses ( Figs 8 View Figure 8 , 9 View Figure 9 , 15 View Figure 15 , 16 View Figure 16 ). The posterior suture with the frontals is scarf-like, but here the nasals overlap the frontals, reversing the pattern seen on the lateral side of the snout.

Comparisons: In Emausaurus the nasals are not preserved. Among eurypodans more generally, ankylosaurs have nasals that are obscured by large caputegulae (with the exception of very juvenile examples, e.g. Burns et al., 2011), whereas in the basal stegosaur Huayangosaurus and the more derived Stegosaurus nasals resemble more closely in general shape those seen in Scelidosaurus . However, they are not so obviously encrusted with exostoses and their anterior tips are illustrated lapping laterally against the sides of a common median dorsal premaxillary process (as seen more typically in ornithischians).

Frontals ( Figs 9 View Figure 9 , 16 View Figure 16 ): In the lectotype, the frontals (F) are visible in ventral view. The dorsal surface was evidently rough-textured originally ( Fig. 4B View Figure 4 ), but seems to have been chemically eroded and is coated in consolidant. The dorsal surface cannot be examined without considerable risk to the remainder, because what remains of the articulated skull is exceedingly fragile. In contrast, the articulated skull of the referred specimen ( BRSMG LEGL 0004 ; Fig. 16 View Figure 16 ) is well preserved, but visible only in dorsal view .

The frontals are fused together along the midline (the straight, but irregular butt-jointed suture line is visible in ventral view of the lectotype). In dorsal view ( Fig. 16 View Figure 16 ), the frontals form a roughly trapezoidal (kiteshaped) plate at the centre of the skull roof. There is a shallow but distinct groove along the midline of its posterior half. This groove is undoubtedly exaggerated by the substantial layer of exostotic tissue that coats the dorsal surface on either side of the midline. Posteriorly, the central portion of the frontal plate contacts and overlaps the parietals along a broad, slightly coarsely interdigitated, convex and open scarf suture. Laterally, the frontoparietal suture line continues in a slightly concave irregular curve contacting the dorsal edges of the lateral wings of the parietal and skirting the anterior margin of the supratemporal fenestrae, before butting up against the blunt medial processes of the postorbitals. Anterior to the postorbital suture, each frontal contacts the middle supraorbital (mso) forming a curved and crenulate scarf suture that cuts back anteromedially across the roof of the skull before extending anteriorly to meet the edge of the prefrontal. The margin then turns medially to contact the posterior edge of the nasals on either side of the midline. The nasals and frontals meet along an irregular scarf suture. The nasals overlap the dorsal surface of the frontals. Ventrally, the outline of the frontals and relationship to most of the surrounding skull bones can be discerned. The most noteworthy features are the smoothness of the bone surface (in comparison with the dorsal surface texture) and the ‘hourglass’ ridges that mark the upper internal edges of the ethmoid cartilages that walled the orbital cavities, leaving a median channel that represents the roof of the passage for the olfactory lobes.

The bony textures preserved on the dorsal surface of the frontals (see Figs 8 View Figure 8 , 16 View Figure 16 ) are striking. Adjacent to the parietal suture, the posterior portion of the frontal plate bears a patch of longitudinal strands of bony tissue. Anterior to this patch, a median trough subdivides the frontals and the surrounding area is covered by densely granular-textured bone. The trough flattens out anteriorly and the granular texture becomes one of strands of radiating bony fibres that extend beyond the nasal suture. These bony fibres extend laterally as a sort of ‘zone’ surrounding the central (granular) area of the frontal plate. These radiating fibres extend toward all the peripheral skull roofing bones. Fibrouslooking strands, as well as irregular arrays of bony fibres or grains, and some bony pustules, coat the external surfaces of all adjacent skull-roofing bones, except in the areas of bone in which muscles were either attached directly or where bone forms the smooth inner walls of the adductor chambers. The texture on the external surface of the skull bones of Scelidosaurus most nearly resembles that seen on the skulls of chelonians that are covered in life by a mosaic of large, keratinous scales (pers. coll. Chelonia mydas Linnaeus, 1758 ).

Comparisons: In Emausaurus the structure of the frontals is not well preserved. In outline, its proportions resemble those of Scelidosaurus , but the same is true of many ornithischians. There is no description of surface textures equivalent to those described in Scelidosaurus , but this may also be an artefact of its preservational condition (e.g. the palpebral is one of the few skull bones to exhibit surface texturing equivalent to that seen more extensively on the skull of Scelidosaurus ). It appears, based upon the reconstruction ( Haubold, 1990: fig. 2), that the frontal forms the dorsal orbital margin, even though it is flanked laterally by the palpebral. The presence of a middle supraorbital (or its cartilaginous equivalent), between the frontal margin and the palpebral, is suspected, and this suspicion is reinforced by the structure of the dorsal orbital margin of the postorbital. The latter displays an oblique rugose facet for attachment of a supraorbital. The frontals of the eurypodan Stegosaurus retain a midline suture but otherwise have similar relationships to the surrounding skull bones ( Gilmore, 1914: pl. 6), despite the skull being narrower and much more anteroposteriorly stretched. In contrast, the basal stegosaur Huayangosaurus has been illustrated with a frontal that is excluded from contact with the middle supraorbital by the prefrontal and postorbital ( Sereno & Dong, 1992: fig. 6B).

Parietals ( Figs 9 View Figure 9 , 16 View Figure 16 ): The parietals (P) are completely fused together to create a saddle-shaped structure ( Figs 4D View Figure 4 , 9 View Figure 9 , 16 View Figure 16 ). The lectotype has the parietal plate exposed dorsally ( Fig. 4D View Figure 4 ), but it was evidently substantially eroded prior to collection and has also been somewhat degraded after long periods of acid immersion. The referred skeleton ( BRSMG LEGL 0004 : Fig. 16 View Figure 16 ) has a well-exposed parietal plate, although it is visible only in dorsal aspect .

On either side of the midline, the sides of the parietal plate are concave and form the dorsal portion of the sloping internal walls of the adductor chamber. These surfaces are smooth to allow movement of the adductor musculature that was attached along the margins of the supratemporal fenestra. The ventral edges of the parietal plate contact the proötic and opisthotic along a horizontal suture line. The internal structure of these sutural surfaces is visible as counterparts on the corresponding surfaces of the laterosphenoid, opisthotic and supaoccipital in the lectotype ( Figs 31A View Figure 31 , 35B). These include mortice-and-tenon-like structures that would have reinforced the suture with the skull roof.

The sagittal crest(s) is (are) rugose and prominent but, unusually, comprise a pair of crests separated by a narrow median gully ( Figs 9 View Figure 9 , 16 View Figure 16 ). The midline gully has a smooth surface, whereas the raised parasagittal crests are rough-textured; no doubt reflecting the ligamentous attachment of medial portions of the adductor musculature. Anteriorly, these raised crests diverge to form the anteromedial margins of the supratemporal fenestrae. The dorsal area between these divergent ridges flattens out (becoming mildly concave transversely) and terminates anteriorly along an irregular (coarsely crenulate and interdigitate), open suture line that is concave anteriorly where it meets the central portion of the frontal plate ( Fig. 16 View Figure 16 ). Anteriorly, the dorsal surface between the divergent parasagittal crests is roughened by the presence of anteroposteriorly oriented superficial bony strands. Posteriorly, the sagittal crests diverge above the occiput and produce a pair of diverging gullies that cap the mediodorsal edge of the occiput and overlap the squamosals. Immediately anteroventral to these crested edges (and lining the posterior face of the adductor cavity) lie thin, curved laminae of the squamosals. The lateral extent of the divergent parietal wings is uncertain, but they clearly capped the supraoccipital medially, the paroccipital farther laterally and backed the medial part of the ‘shoulder region’ of each squamosal. The robustness of these occipital parietal wings makes it seem probable that one of their roles was to offer support and/or anchorage for the occipital osteoderm horns ( Fig. 16 View Figure 16 , oc.ost).

Comparisons: Parietals are not preserved in Emausaurus . Among eurypodans, the parietal is poorly known but forms a broad, saddle-shaped cap to the braincase in stegosaurs and nothing is known of the form of the sagittal crest (or if it even existed). The parietal plate of mature ankylosaurs is covered by dermal ossifications (although parietals are unfused in a juvenile Pinacosaurus ; Maryańska, 1977) and cannot be usefully compared to Scelidosaurus .

Lacrimal ( Figs 8 View Figure 8 , 12–14 View Figure 12 View Figure 13 View Figure 14 ): The external surfaces of both left and right lacrimals (La) of the lectotype ( NHMUK R1111 : Fig. 12A View Figure 12 ) appear coated by rough-textured exostotic bone that partly obscures sutural contacts with adjacent bones. The internal surface of the disarticulated left lacrimal ( Fig. 12C View Figure 12 ) helps to clarify some of its internal structure. A left lacrimal is visible in external view on the articulated skull ( BRSMG LEGL 0004 : Fig. 14 View Figure 14 ) and its sutural relationships are a little clearer. The left and right lacrimals of the intermediate-sized specimen ( BRSMG Ce12785; Fig. 13A View Figure 13 ) are also preserved in partial articulation with their respective maxillae .

The lacrimal has comparatively little external exposure on the skull. It is overlapped anteriorly by the lacrimal lamina of the maxilla and is overlapped by the prefrontal posterodorsally. It is also partly overlain by the base of the large palpebral ( Fig. 14 View Figure 14 , Pp). The laterally exposed surface of the disarticulated lacrimal of the lectotype ( Fig. 12 View Figure 12 ) and referred skull ( Fig. 14 View Figure 14 ) is partly coated with bony strands. In the lectotype ( Fig. 12A View Figure 12 ) the osseous coating that covers the posterior portion of the lacrimal ends abruptly at an oblique line and step. Anterior to the step, the external surface of the lacrimal is smooth and seems to represent an exposed part of the scarf suture with the lacrimal lamina of the maxilla; the lamina of the maxilla appears to have partly broken away. The equivalent area in the articulated referred skull ( Fig. 14 View Figure 14 ) also shows the posterior half of the lateral wall of the lacrimal covered by strand-like superficial bony fibres. The strands terminate abruptly (in the area of the suture between maxilla and lacrimal) and the external surface of the lamina of the maxilla becomes perfectly smooth and unadorned for a short distance before becoming once again coated in rough-textured curved ridges.

Posteroventrally, the lacrimal produces a tapering, curved jugal process ( Fig. 12C View Figure 12 , jp) that forms the anteroventral margin of the orbit. The posterior tip of the jugal process is arched ventrally and twisted axially, and wraps around the mediodorsal surface of the finger-like anterior end of the jugal ( Figs 12A View Figure 12 , 21 View Figure 21 ). The medial edge of the jugal process also has a thin contact with the lateral edge of the base-plate of the palatine.

The foramen for the nasolacrimal duct ( Fig. 12 View Figure 12 , ld) is transversely compressed and positioned low down on the posterior (orbital) surface of the lacrimal. The nasolacrimal duct is enclosed in a narrow, sinus-like chamber in the body of the lacrimal. Medial to the nasolacrimal foramen there is a thin, transverse orbital wall. Farther dorsally, this partition wall becomes thicker as it approaches the area of the suture with the prefrontal, but this area is badly broken in the lectotype lacrimal. Just anterior to the orbital partition there is a recess floored by a short, blunt, anteromedial extension of the lacrimal. This would have been roofed by the prefrontal/ nasal in the articulated skull. Anterior to the orbital partition wall, the lacrimal comprises two laminae: the lateral and more substantial one fits snugly against the medial surface (ls) of the dorsal lamina of the maxilla; the medial lamina ( Fig. 12C View Figure 12 , mw) of the lacrimal is a thin partition wall that lined the nasal passage. The central area of this medial lamina appears to be pierced by a fenestra, but this is, in fact, a fractured area in the thin bony wall (through which can be seen the cavity for the nasolacrimal duct). Farther anteriorly, the nasolacrimal duct opens into the nasal cavity via a slot-like anteriorly directed opening ( Fig. 13C View Figure 13 , ld).

Externally, the lacrimal seems not to have contacted the nasal, but the nasal has a long, recessed, posteroventral curtain-like sutural surface that supported (backed on to) the premaxilla–maxilla contact, and this may well have overlapped the anterodorsal portion of the lacrimal. Posterodorsally, the lacrimal is overlapped/capped by the ventral ‘foot’ of the prefrontal. Posteriorly, the dorsolateral portion of the lacrimal bears a small shoulder ( Fig. 13 View Figure 13 ) for attachment of the prefrontal. The latter bone wraps around on to the medial edge of the lacrimal so that the prefrontal is locked in place on the anterior orbital margin.

Comparisons: The lacrimal of Emausaurus is incomplete, but includes a long, curved jugal process that evidently wrapped itself around the anterior tip of the jugal in a similar fashion to that seen in Scelidosaurus . The lacrimal also formed the posterodorsal margin of the antorbital fossa and, unlike Scelidosaurus , backed the dorsal portion of the antorbital fossa and bordered the dorsal half of the antorbital fenestra, and its sutural relationship with the maxilla and prefrontal are not clear. Among more derived eurypodans, such as Huayangosaurus and Stegosaurus , the lacrimal resembles that of Scelidosaurus in its superficial characters, although it does have a short, pillar-like jugal process that forms the posterior boundary to the antorbital fossa. No particularly fruitful comparisons can be drawn with currently known ankylosaurs. The juvenile Pinacosaurus skull ( Maryańska, 1977: fig. 3) reveals a superficially oblong bone with a long jugal process, but there is no antorbital fossa for it to bridge.

Prefrontal ( Figs 8 View Figure 8 , 9 View Figure 9 , 14 View Figure 14 , 16 View Figure 16 , 17): The left prefrontal (Pf) of NHMUK R1111 is crushed and broken, and was mostly destroyed by mishandling of the acid-prepared skull. Its external features are obscured by a superficial layer of bony tissue that may also be a portion of the fused ‘footplate’ (basal body) of the overlying palpebral bone ( Fig. 12 View Figure 12 , pp?). The right prefrontal of the lectotype is visible internally, so its general shape and sutural relationships can be determined. The external (dorsal) surface of the prefrontal is well displayed in the referred skull ( BRSMG LEGL 0004 ; Fig. 16 View Figure 16 ), where it is seen to be covered by an irregular array of bony strands that extend to the lateral surface, but its lateral surface is mostly obscured by the overlying palpebral (Pp). An isolated, partial, right prefrontal is preserved in the intermediate-sized specimen ( BRSMG Ce12785; Fig. 17A, E) .

The medial edge of the prefrontal curves mediodorsally and forms a slight crenulate suture against the nasal ( Fig. 16 View Figure 16 ). The latter turns abruptly anteromedially to form an extensive scarf suture with the frontal. The posteromedial sutural edge of the prefrontal is horizontally recessed, creating upper and lower flanges of bone that clamp the lateral edges of the frontal (Fig. 17E, fs). Posteriorly, the prefrontal lodges against a recess on the frontal, swinging abruptly laterally and suturing to the middle supraorbital (mso). The latter would have formed the dorsal portion (and margin) of the orbital cavity were it not for the presence of the posterior process of the palpebral. The prefrontal forms the anterodorsal margin of the orbit and has a thickened edge that is distinctly rugose (Fig. 17A). Following this margin as it curves ventrally (toward the lacrimal), the orbital margin becomes more smoothly rounded and then thickens to form a mound that culminates in a relatively prominent Figure 17. Scelidosaurus cf. harrisonii . Orbital View Figure 1 View Figure 2 View Figure 3 View Figure 4 View Figure 5 View Figure 6 View Figure 7 elements assembled in the referred specimen (BRSMG Ce12785). A, lateral view of the right postorbital, the posterior supraorbital, middle supraorbital and prefrontal. B, postorbital, posterior supraorbital and middle supraorbital in articulation – note the pitting/foramina on the dorsal surface of the middle supraorbital, the entire bone has a spongy texture. C, postorbital and posterior supraorbital in anterior view. D, middle supraorbital in ventral view. E, prefrontal in medial view. Abbreviations: do – postorbital osteoderm, fs – frontal suture, ls – lacrimal suture, mso – middle supraorbital, ns – nasal suture, os – orbital roof, Pf – Prefrontal, Po – Postorbital, pos – postorbital suture, ppf – palpebral facet, Pr – Prefrontal, prs – prefrontal suture, pso – posterior supraorbital, sq.s – squamosal suture, vas – vascular openings.

lateral facing, roughly triangular facet against which the footplate of the palpebral was anchored (Fig. 17A, ppf). Medially (Fig. 17E), the prefrontal develops a transverse partition wall that extends from the frontal suture on the mediodorsal surface, above the orbital cavity and descends across the front of the orbital cavity as a sharp edge that widens as it approaches the equivalent partition wall on the medial side of the lacrimal.

Comparisons: In Emausaurus , the prefrontal has a broadly similar shape to that seen in Scelidosaurus ( Haubold, 1990: fig.2). Its morphology is partly obscured by a large shield-like palpebral that is clearly sutured to the lateral edge of the prefrontal alone ( Haubold, 1990: figs 2, 6). Eurypodans (and basal ornithischians more generally) exhibit rather similarly proportioned prefrontals.

Palpebral (anterior supraorbital) ( Figs 8 View Figure 8 , 16 View Figure 16 , 18 View Figure 18 ): The nearly complete articulated and partly prepared skull (BRSMG LEGL 0004) reveals, for the first time, that Scelidosaurus possessed a well-developed palpebral bone (Pp). The bone is securely sutured to the anterodorsal portion of the orbital margin (across the prefrontal–lacrimal suture). There is an enlarged base-plate that envelopes, and was sutured to, an elongate boss-like facet (Fig. 17A, ppf) on the lacrimal. From the base-plate, the palpebral extends posterodorsally in an arc parallel to the orbital margin before terminating bluntly at, or close to, the suture between the middle supraorbital and the posterior supraorbital (pso, see also Fig. 9 View Figure 9 ). The palpebral, in dorsal view, is tightly bound to the margin of the prefrontal (in all probability by a combination of bony exostotic strands and connective tissue), but there is a narrow unossified gap ( Figs 9 View Figure 9 , 16 View Figure 16 , gap) between the edge of the middle supraorbital and palpebral. The palpebral has a generally rugose, irregular bony texture.

Comparisons: In Emausaurus ( Haubold, 1990: figs 2, 6), the palpebral is well preserved. It exhibits an extensive sutural surface medially, showing that it was firmly attached to the lateral margin of the prefrontal and that its base did not encroach upon the lacrimal. This bone is roughly triangular in outline, with a slightly concave orbital margin. Its external surface is markedly sculptured and, when articulated with the prefrontal, there is a tapering gap between the orbital margin of the frontal and the inner margin of the palpebral. The orbital margin of the frontal is rugose and bevelled, perhaps indicating an area for the attachment of the middle supraorbital (whether ossified or unossified) that spanned the gap between the frontal and palpebral. Among more derived eurypodans, the stegosaurs Huayangosaurus and Stegosaurus possess similarly robust palpebrals attached to the prefrontal and curving backwards parallel to the orbital margin. Gilmore (1914: pl. 6) also illustrated an unossified gap between the palpebral and adjacent middle supraorbital. The juvenile ankylosaur Pinacosaurus ( Maryańska, 1977: figs 2, 3) displays a broadly similar pattern.

Middle supraorbital: In NHMUK R1111, an angular, slightly domed bone is visible on both sides of the dorsal aspect of the skull roof ( Fig. 4D View Figure 4 ). The bone is positioned on the dorsal orbital margin, lateral to the frontal and forms coarse interdigitate sutures with the prefrontal, frontal and postorbital. This bone is seen clearly in the dorsal view of the slightly smaller, referred specimen (BRSMG LEGL 0004; Fig. 16 View Figure 16 ). Its dorsal surface is roughened by a coating of bony strands, whereas its shallowly domed ventral (orbit) wall is smooth.

The intermediate-sized specimen (BRSMG Ce12785; Fig. 17) preserves an isolated right middle supraorbital that articulates loosely against the postorbital and lies anteromedial to the tent-shaped posterior supraorbital. This small, inverted-dished bone has an approximately pentagonal outline. Its sutural edges are rough and slightly thickened and the entire bone is vaulted because it roofs a portion of the orbital cavity. The dorsal surface of this bone does not bear any irregular, superficial, bony fibres and this is probably a reflection of the comparative ontogenetic immaturity of the skeleton. Nevertheless, its dorsal surface is pock-marked by a number of foramina – perhaps hinting at elevated levels of biological activity associated with the growth of overlying tissue. In contrast, the ventral (orbital) surface appears comparatively smooth and unmarked. The orbital margin, which lies adjacent to the medial edge of the distal process of the palpebral, is round-edged, but slightly rugose.

Comparisons: In Emausaurus , a middle supraorbital is not reported as being present. Eurypodans generally exhibit a similar pattern of circumorbital bones with both stegosaurs and ankylosaurs possessing middle supraorbitals that widen the roof of the orbital cavity.

Postorbital ( Figs 8 View Figure 8 , 9 View Figure 9 , 16–18 View Figure 16 View Figure 18 ): In the lectotype ( NHMUK R1111 ), the postorbitals (Po) are fragmentary because they appear to have been damaged and almost completely removed by erosion. The now disarticulated portion of the skull, comprising the complete jugal, quadratojugal and quadrate, displays just the jugal process of the postorbital and its extensive oblique suture to the jugal ( Fig. 21A View Figure 21 ). The dorsal part of the jugal-postorbital suture is open. Nevertheless, the distal tip of the jugal process of the postorbital appears to be fused to the jugal (apparently bound by strands of exostotic tissue). The orbital rim of this process is concave and, viewed anteriorly, there is a minor degree of interdigitation between the jugal and postorbital ( Fig. 21C View Figure 21 ). There is also a line of foramina (vas) lying medial and parallel to the lateral edge of the orbital margin of the postorbital. These foramina seemingly mark the existence of an interface between the lateral wall of the postorbital and a crater-shaped area of osteodermal tissue plastered against the lateral surface of the postorbital (see also Figs 14 View Figure 14 , 17A, C) .

The orbital surface of the postorbital is smooth and slightly concave. The medial margin of the internal orbital wall of the postorbital forms a raised ridge that marks the incomplete bony partition (probably completed by connective tissue) between the orbital and adductor chambers. Posteromedially, the jugal process is smooth and flat, and its ventral edge forms a slightly curved diagonal line that represents the upper extent of the overlap by the jugal internally. Similar anatomical features can be seen on the slightly smaller, better-preserved referred specimen ( Figs 14 View Figure 14 , 18 View Figure 18 , 19 View Figure 19 ). In the latter, the postorbital is triradiate and displays a diffuse, but large, depressed patch of rugose bony tissue on its dorsolateral surface. The postorbital of Emausaurus has a similarly depressed area on the lateral surface of the postorbital ( Haubold, 1990: taf. I, 2), but the surface does not appear to be so strongly textured.

Above the laterally positioned osteodermal crater on the postorbital is a discrete pup-tent-shaped posterior supraorbital that was bound in position to the distal end of the palpebral by connective tissue ( Fig. 18 View Figure 18 , pso). This suture becomes ossified later in development and may be overgrown by strands of periosteal bone.

The dorsal part of the postorbital–jugal suture is open, as it is in the lectotype. Another curious feature, relating to the superficial layering of bone on the skull surface, is visible in the dorsal aspect of the skull ( Fig. 16 View Figure 16 ). It seems that this surface of the postorbital is encrusted with strands of exostotic tissue and an area of little ‘islets’ of superficial bone in the area of the skull roof medial to the cap-shaped posterior supraorbital. However, in the area surrounding the supratemporal fenestra, where the adductor mandibulae musculature is anchored, the squamosal and parietal processes are smooth-surfaced and free of exostotic bone. The small islets of superficial bone encroach on the periphery of these areas as part of what has the appearance of an on-going process of competitive spreading.

The intermediate-sized specimen (BRSMG Ce12785) has a complete, disarticulated right postorbital ( Fig. 19 View Figure 19 ). The entire bone is roughly triangular in lateral view. There is a posteriorly tapering process (sqp) that ends in a sharp point, The dorsal surface of this process bears an elongate, flat facet for the sutural attachment of the anterior (postorbital) process of the squamosal ( Fig. 19C, D View Figure 19 , sqs). The lateral surface of this process is slightly convex, dorsoventrally, while its medial surface is shallowly concave and forms the lateral rim of the supratemporal fenestra. The ventral process (jp) curves anteriorly and forms a spatulate distal tip; the whole process twists about its long axis and displays a large, posteromedially facing suture ( Fig. 19C, j.s View Figure 19 ). The dorsal edge of this sutural surface is marked by a curved contour line (see Fig. 21B View Figure 21 ). In anterior view, the orbital margin is smooth and shows a line of vascular foramina positioned laterally, beyond which there is an area of ornamented bone: an osteoderm. The dorsal portion of this osteoderm forms a hook-shaped projection (hk) that protrudes slightly into the orbital margin. Immediately above this bony projection, the surface of the postorbital is excavated into a crude, bowl-shaped structure ( Fig.19A View Figure 19 , bwl) that merges into the base of the anterior process of the postorbital above the orbital margin. The latter process projects anteriorly in lateral view ( Fig. 19A View Figure 19 ), but when viewed in dorsal aspect ( Fig. 19D View Figure 19 ) this process can be seen to bend medially, before ending in a truncated, rough-textured sutural surface against the frontal (fs). In isolation, this curious superficial structure on the postorbital would be puzzling. Fortunately, other material preserved with BRSMG Ce12785 provides an explanation. The recess above the postorbital osteoderm proves to be a surface for attachment of a pup-tent-shaped posterior supraorbital.

Comparisons: Among eurypodans, the fine details of the structure of the postorbital are not well preserved. The basal stegosaur Huayangosaurus has been described as possessing a distinctive horn-like mound positioned immediately anterior to the supratemporal fenestra ( Sereno & Dong, 1992: fig. 6A–C, poh). Its external surface was covered by a large posterior supraorbital, which looks similar to that seen in Stegosaurus ( Fig. 20 View Figure 20 ). The structure of the suture between the postorbital and jugal of Stegosaurus ( Gilmore, 1914: fig.5) is similar to that of Scelidosaurus . This may also be true for Huayangosaurus , but cannot be established with certainty on the basis of the published illustrations. Among ankylosaurs, the postorbital cannot be described because it is obscured by osteoderms (caputegulae).

Posterior supraorbital ( Figs 8 View Figure 8 , 9 View Figure 9 , 16 View Figure 16 , 17): This small osteoderm (Fig. 17A, B, pso) fits snugly into a bowl-shaped recess above and, medial to, the hook-like projection formed by the osteoderm on the lateral surface of the postorbital. It projects laterally as a horizontal mound (Fig. 17A, B) that is a continuation of the brow ridge formed by the palpebral bone. The anteromedial edge of this supraorbital is truncated and rugose and forms a sutural attachment site for the middle supraorbital (Fig. 17B, mso). It also bears a short facet anteriorly that is a contact point for the distal end of the palpebral process. The combination of palpebral and posterior supraorbital, create a brow ridge (see Figs 8 View Figure 8 , 18 View Figure 18 ) that would have been sheathed by keratin and doubtless served as a shield for the eye, but may also have served as a signalling device.

Jugal ( Figs 8 View Figure 8 , 13 View Figure 13 , 14 View Figure 14 , 21 View Figure 21 ): The left jugal (Ju) is well preserved in the lectotype ( Fig. 21 View Figure 21 ), as part of a broken/ disarticulated portion of the suspensorium. Its form is partially obscured by the scarf suture with the postorbital externally, but its internal aspect is also visible ( Fig. 21B View Figure 21 ). The jugal of the intermediate-sized specimen ( BRSMG Ce12785: Fig. 13 View Figure 13 ) is well preserved, but firmly attached to the maxilla anteriorly, whereas its posterior (quadratojugal) and dorsal (postorbital) sutural attachments are cleanly separated. The larger referred specimen ( BRSMG LEGL 0004 ; Fig. 14 View Figure 14 ) displays well-preserved, partly disarticulated jugals, but these are only visible externally .

In the lectotype ( Fig. 21 View Figure 21 ), the anterior (lacrimal) process of the jugal has been disarticulated from the maxilla and lacrimal. The lacrimal process is slender and rests in a trough that runs along the shelf-like dorsolateral edge of the maxilla ( Fig. 12A View Figure 12 ), and its external surface is roughened by exostotic bony tissue. The anterior tip of the lacrimal process is expanded dorsoventrally and compressed laterally so that it slots, wedge-like, into the maxilla. Its ventral surface is smooth (rather than suturally rugose) in this area. Medially, there is a longitudinal groove adjacent to the lateral edge of the palatine and it is clear that the base-plate of the palatine was sutured to the jugal along this edge, which presumably helped to secure the jugal. The anteromedial edge of the anterior tip also bears a curved, saddle-shaped sutural surface that fits snugly against the ventral surface of the posterior (jugal) process of the lacrimal; the latter wraps itself (medially and ventrally) around the lacrimal process of the jugal. The orbital margin of the lacrimal process widens posteriorly (flooring the orbital cavity) as it curves into the base of a stout postorbital process. The medial side of jugal, beneath the orbit rim, bears by a triangular sutural facet for the ectopterygoid ( Fig. 21B View Figure 21 , ecs). Farther posteriorly, the body of the jugal begins to narrow transversely and deepen dorsoventrally. Its dorsal edge forms the posteroventral quadrant of the orbital margin. The anterolateral surface of the postorbital process bears a broad, angled scarf suture for the postorbital and this process is obliquely truncated dorsally ( Fig. 21B, C View Figure 21 ). The posterior margin of the dorsal process descends almost vertically before sweeping posteriorly to form the anteroventral rim of the infratemporal fenestra. The posterior edge contacts the quadratojugal in an overlapping (scarf) suture. Medially, this suture surface bears an oblique sinuous ridge running diagonally from the anteroventral margin of the jugal to the posterodorsal edge ( Fig. 21B View Figure 21 ). The external manifestation of the suture is different because the jugal is notched along its posterior edge. This notch separates two flattened laminae that overlie the anterolateral face of the quadratojugal, a feature not visible on the lectotype because of damage in this area ( Fig. 21 View Figure 21 , br), but well preserved in the intermediate-sized specimen ( Fig. 13A View Figure 13 ). The notch on the posterior edge of the jugal is reflected in a complementary wedge-shaped ridge on the lateral surface of the quadratojugal. The ventral edge of the jugal is comparatively thick and describes a shallow arch anteroposteriorly.

The external surface of the jugal has superficial patches of granular textured and more strand-like arrays of exostotic bone ( Figs 13 View Figure 13 , 14 View Figure 14 , 21A, p View Figure 21 .os).

In the intermediate-sized specimen (BRSMG Ce12785; Fig. 13 View Figure 13 ) the external surface of the lacrimal process is smooth near its tip, but beneath the orbit, longitudinal strands of exostotic tissue lead posteriorly into a mound-shaped area with a more granular texture on the central body of the jugal (directly beneath the postorbital process). There is then an oblique gap before another area of dense bony strands is found on the posteroventral portion of the jugal (adjacent to the quadratojugal suture). The larger specimen (BRSMG LEGL 0004; Fig. 14 View Figure 14 ) displays external textures on the left jugal that have a similar pattern to those seen on the intermediate-sized individual. However, the larger more granular area is found on the deeper posteroventral blade-like part of the jugal. This latter area is raised and displays an array of foramina and what appear to be associated vascular grooves (or natural channels between bony strands) that all share a generally posteroventral orientation. In contrast, other areas, such as the postorbital process and the margin of the lateral temporal fenestra, have a smooth, cortical bone surface.

Comparisons: In Emausaurus , the left jugal is well preserved ( Haubold, 1990: fig. 7) and has a similar morphology to that seen in Scelidosaurus . The lacrimal process is finger-like and wedges between the lacrimal and maxilla, and there is an ectopterygoid facet on its medial surface ventral to the orbital margin. The postorbital suture is large, pocket-like and faces the orbit, and the quadratojugal process is blade-like and notched along its posterior edge to accommodate a ridge on the external surface of the quadratojugal. Little is known of the detailed structure of the jugal in eurypodans. Superficially, the jugal of Huayangosaurus and Stegosaurus has an elongate, narrow lacrimal Figure 22. Scelidosaurus harrisonii . The right View Figure 2 quadratojugal of the lectotype (NHMUK R1111): A, medial view. B, lateral view. The right quadratojugal of the referred specimen (BRSMG Ce12785): C, medial view. D, lateral view. Abbreviations: for – vascular foramina, j.s – jugal suture, qs – quadrate suture, sqs – squamosal suture.

process and the body of the jugal is undercut and appears shelf-like. The relationship between jugal and quadratojugal is vague and somewhat stylized ( Sereno & Dong, 1992: fig. 6), but has been given a structure that resembles that of Scelidosaurus , whereas in Stegosaurus the quadratojugal process of the jugal is somewhat abbreviated. The jugal of ankylosaurs also displays an elongate lacrimal process and the body of the jugal forms a distinct undercut shelf beneath the orbit ( Maryańska, 1977: fig. A 2), but the remainder of its relationships are obscured by dermal bone.

Quadratojugal( Figs8 View Figure 8 , 14 View Figure 14 , 21 View Figure 21 ,22): Theleftquadratojugal (Qj) of the lectotype is partially obscured because it is articulated between the jugal and quadrate, and this area is also fractured ( Fig. 21 View Figure 21 ). This is also the case with the large referred specimen ( BRSMG LEGL 0004 ; Fig. 14 View Figure 14 ). However, the right quadratojugal of the lectotype is disarticulated and complete (Fig. 22A, B), as also is that of the intermediate-sized specimen ( BRSMG Ce12785; Fig. 22C, D) .

Roughly triangular in outline, the anterior part of its external surface is extensively overlapped by the jugal. This sutural surface is smooth and bifacetted (in the sense that a tapering, oblique ridge on the external surface of the quadratojugal bisects the jugal suture – j.s). This portion of the quadratojugal is relatively thin, although the ventral edge is thicker because it represents a continuation of the thickened ventral margin of the jugal. Posteriorly, the ventral surface of the quadratojugal forms a spatulate process that contacts and overlaps the ventral, external margin of the jugal wing of the quadrate immediately above the quadrate-articular condyle (see Fig. 30C View Figure 30 ). This overlapping sutural surface narrows as it extends dorsally as a kind of spire, which curves slightly anteriorly, up along the leading edge of the jugal wing of the quadrate and, in so doing, spans (and closes off) a nearly circular notch in the quadrate that forms a quadrate (paraquadratic) foramen (qf). Immediately above the quadrate foramen, the posterior margin of the quadratojugal is notched (more obviously so in the lectotype) and twists itself around the edge of the jugal wing of the quadrate to help secure its position.

The quadratojugal thus becomes reduced to a tapering splint of bone that is bound to the leading edge of the jugal wing of the quadrate (Fig. 22A, qs) until it contacts an equivalent descending process of the squamosal (sqc). The apical portion of the left and right quadratojugals exhibit a short, concave sutural surface medially. In the articulated portion of the left jugal arch in the lectotype ( Fig. 21 View Figure 21 ), the distal tip of the squamosal can be seen contacting the quadratojugal. Vascular foramina (Fig. 22C, for) are present on the medial side of the quadratojugal of the smaller referred specimen. These are not apparent in the larger specimen and may well represent a juvenile feature linked to active bone growth.

Comparisons: The quadratojugal of Emausaurus is similar in morphology to that described above and, although not completely preserved, probably had a similar dorsal contact with the squamosal (as suggested by Haubold, 1990: fig. 2). Eurypodans, particularly stegosaurs, display part of the structure of this bone. In Huayangosaurus , this bone is overlain by the jugal anteriorly and has been reconstructed with a tapering dorsal (squamosal) process that fails to meet the squamosal. It also appears that the quadratojugal extends ventrally to lie adjacent to the condylar surface of the quadrate, as in Scelidosaurus . In Stegosaurus , the quadratojugal is more restricted, having a narrow jugal process and posteriorly ascends the quadrate for a short distance; it may not extend ventrally to reach the condylar region, although this apparent abbreviation may also be a quirk of preservation (see Gilmore 1914: fig. 6). In ankylosaurs, the quadratojugal is completely obscured because of fusion between adjacent bones and its being is overlain by large osteoderms.

Squamosal ( Figs 8 View Figure 8 , 9 View Figure 9 , 16 View Figure 16 , 18 View Figure 18 ): The squamosals (Sq) are broken and eroded on both sides of the skull of the lectotype ( NHMUK R1111 ; Fig. 21A View Figure 21 ) but are well exposed on the surface of the articulated referred skull ( BRSMG LEGL 0004 ; Fig. 18 View Figure 18 ). None are preserved in the intermediate-sized individual ( BRSMG Ce12785) .

On the left side of the lectotype ( Fig. 21A View Figure 21 ), the only part remaining is the anterior descending process that forms a tapering structure attached along the anterior edge of the jugal wing of the quadrate. Its upper end is broad and clearly expanded as it approaches the central body of the squamosal. The anterior surface is smoothly rounded transversely, while the posterior surface is grooved along its length so that it can wrap itself tightly around the edge of the jugal wing. Distally, the squamosal process tapers to a point that overlaps the dorsal tip of the quadratojugal (excluding the quadrate from the margin of the infratemporal fenestra). The right squamosal is poorly preserved.

As can be seen in the articulated skull of the referred specimen (BRSMG LEGL 0004; Figs 16 View Figure 16 , 18 View Figure 18 ), the squamosal has a typically dinosaurian morphology. The bone has a robust central portion that houses the vaulted recess that is the cotylus for the quadrate head. From this central region radiate a number of processes: a long, slightly arched, anterior (postorbital) process, a shorter anteromedially curved (parietal) process and a posteroventral wing-like (paroccipital) process. The postorbital process tapers anteriorly and contacts a complementary process of the postorbital (creating the intertemporal bar). The suture between these bones appears to be a diagonal overlap in dorsal ( Fig. 16 View Figure 16 ) and lateral ( Fig. 18 View Figure 18 ) aspects. However, this was evidently more complex: the process of the squamosal appearing to wrap itself around the dorsal and medial sides of the postorbital as it develops (ontogenetically). The lateral surface of the postorbital process of the squamosal has an arched ledge that curves ventrally beyond the quadrate cotylus ( Fig. 18 View Figure 18 ). This ledge, and the area beneath, would have anchored a superficial portion (MAMES) of the adductor musculature. The lower edge of the postorbital process is transversely rounded and, along with the postorbital, forms the dorsal margin of the infratemporal fenestra. Its medial surface is smooth and slightly concave (facing medioventrally), and rises to an acute internal edge. This latter edge is sculpted with ropeytextured bony fibres indicating the attachment site for deeper portions (MAMEP) of the adductor musculature ( Fig. 16 View Figure 16 ). This dorsal margin traces the lateral rim of the supratemporal fenestra. Posteromedially the dorsal edge of squamosal becomes noticeably thinner and sharper, curving medially to create a deep lamina that overlaps the posterolateral process of the parietal within the adductor chamber. The ventral edge of this squamosal lamina overlies the opisthotic.

Posterolaterally, the squamosal forms a thick shoulder that curves ventrally toward the paroccipital process and forms a wing-like structure that overlies its anterior surface. The occipital portion of the squamosal is mostly hidden from view in the referred specimen (BRSMG LEGL 0004; Fig. 16 View Figure 16 ) by the presence of two horn-shaped occipital osteoderms ( Fig. 16 View Figure 16 ). The two occipital osteoderms are attached to the sloping dorsal portion of the occiput, judged by the position that they now occupy – as suggested in Fig. 11 View Figure 11 (see also Fig. 47 View Figure 47 ). The left occipital horn comprises a superficially positioned subconical osteoderm that is attached to the left occiput via a wedge-like base-plate ( Fig. 16 View Figure 16 , ba). The latter is sutured to the posterodorsal occipital wall and would have been anchored, in part at least, to the exposed occipital surfaces of the adjacent squamosal, parietal and supraoccipital. The right horn is a little displaced, as preserved ( Fig. 16 View Figure 16 ).

Comparisons: The right squamosal of Emausaurus appears to be damaged ( Haubold, 1990: fig. 8) but exhibits a long, tapering quadratojugal process, a well-developed cotylus and a lateral ledge on the postorbital process, all of these features resemble those seen in Scelidosaurus . Among eurypodan ankylosaurs, the squamosal tends to be obscured by dermal bone. The basal stegosaur Huayangosaurus appears to show a squamosal with a curved, external ledge and a tapering quadratojugal process. However, as reconstructed by Sereno & Dong (1992: fig. 6), the squamosal and quadratojugal did not make contact on the margin of the infratemporal fenestra. The squamosal of Stegosaurus is highly modified to accommodate the oblique, laterally compressed head of the quadrate. There is no quadratojugal process, but the dorsolateral margin of the postorbital process is ledge-like.

DESCRIPTIVE OSTEOLOGY: THE PALATAL COMPLEX

Little is known of the palate of eurypodans, so few comparisons are possible.

Vomer ( Figs 23–25 View Figure 23 View Figure 24 View Figure 25 , 28 View Figure 28 ): The vomers ( V) are preserved almost in their entirety in the lectotype ( NHMUK R1111 ). Figure 23 View Figure 23 presents sketches that indicate the deterioration of these bones that has occurred in a decade. Figure 23A View Figure 23 represents an accurate sketch that I made in 1999. It can be compared directly with a similar sketch made in 2009 ( Fig. 23B, C View Figure 23 ). The posterodorsal edges of both vomers have been broken and lost. This is unfortunate because the loss of these parts of the vomers means that the evidence regarding the positioning of these bones with respect to the pterygoid has also been lost. A small square fragment of vomer preserved attached to the pterygoid ( Fig. 23E View Figure 23 , vfr) fitted perfectly in the ‘notch’ ( Fig. 23A View Figure 23 ) present on the posterodorsal corner of the intact vomers and permitted the sketched reconstruction seen in Figure 23E View Figure 23 . The anterior portion of the vomers is visible in articulation in the partially prepared detached snout nodule of the smaller referred specimen ( BRSMG LEGL 0004 ; Fig. 24 View Figure 24 ) .

The paired vomers are fused along the ventral midline ( Fig. 28 View Figure 28 , vs). They form a deep, triangular plate that bisects the nasal cavity. In cross-section the dorsal portion of each vomer separate above the midline suture, creating a narrow V-shape. The anterior tip of the conjoined vomers is sheared off.What remains forms a cylindrical structure that would have been wedged between medial processes of the premaxillae. This region of the skull is rarely preserved in much detail, but what is known of the anatomy in Scelidosaurus does not contradict the description of this region in the remarkably well-preserved, disarticulated skull of Hypsilophodon ( Galton, 1974: fig. 4B). The vomers are evidently clamped in position by flattened, tapering splints that project from the posterior margin of the premaxilla ( Fig. 23 View Figure 23 , Pm). The lateroventral margin of the sutural surface is notched and has a small ventral projection (lip) that secures this suture.

The ventral edge of the conjoined vomers forms a narrow keel running horizontally between the maxillae at about the level of the maxillary shelf. Just before the keel terminates posteriorly, the two vomers separate slightly and form slight rounded projections (bif). Above this projection each lamina of the vomer rises steeply anterodorsally and the laminae diverge a little: the line of fusion between these two bones follows almost the entire ventral edge and expands dorsally across the central portion of the two laminae ( Figs 23A View Figure 23 dashed line, 28). Near the dorsal tip of the posterior border, the edge hooks posteriorly and then extends anteriorly as a convex edge that is dorsolaterally bevelled. This edge then descends anteroventrally so that the dorsal and ventral edges converge to form the cylindrical anterior (premaxillary) process.

The bevelling continues along the dorsolateral edge of each vomer and fades just before the constriction for the premaxillary process. These bevelled surfaces undulate slightly along their length and are pock-marked by small, irregularly placed foramina. These edges have the general appearance of suture surfaces, and may be associated with equivalent bevelled facets found on two isolated epivomers (see below) that are among other loose cranial fragments associated with the lectotype.

Epivomers ( Figs 25 View Figure 25 , 26 View Figure 26 ): Two small, roughly triangular bones were found among the residue of palatal fragments that had been assembled after the skull was severely damaged by a palaeoichthyologist. Photographic images of the skull made at intervals during the phases of the acid preparation of the skull do not (unfortunately) reveal the original position of these two bones. I suspect that they were positioned deep within the inverted skull: that is to say, close to the roof of the snout (and effectively ‘invisible’ to the lighting and photographic techniques used at the time), otherwise there would have been some indication of their whereabouts on these photographs. Checking back through my notes, I found that in the late 1970s (prior to the event that so badly damaged the skull) I had made crude pencil sketches ( Fig. 25 View Figure 25 ) of the articulated, incompletely acid-prepared, skull of the lectotype (NHMUK R1111). I was, at this time, attempting to decipher the layout of the palate of Iguanodon , in preparation for two monographic studies ( Norman, 1980, 1986). The sketches indicate the presence of two thin sheets of bone lateral to the vomers. Judged from my annotations at the time, I clearly assumed that these were probably anterior extensions of the palatine bones. However, they are more likely to be the two bones in question ( Fig. 25 View Figure 25 ,?epv), because the palatines of Scelidosaurus do not extend either anteriorly or, more importantly, dorsally toward the roof of the snout.

These bones are thin and mildly dished. Their edges are damaged in places, but the two bones (allowing for damage) appear to be mirrored structures ( Fig. 26C, D View Figure 26 ) that can be positioned on either side of the sagittal plane. As preserved ( Figs 26A, B View Figure 26 ), these bones are roughly triangular and each has a somewhat thickened and bevelled edge (bev) that has been found, by cautious manipulation, to be complementary in shape to the bevelled dorsolateral edges on each vomer. One of these bones ( Fig. 26B View Figure 26 ) has a few irregular perforations that may have been created by excessive acid leaching of these rather thin bony plates. The other bone is imperforate. When an attempt is made to articulate these bones with the vomers they form vaulted ‘wings’ ( Fig. 26E View Figure 26 ) that project laterally, perpendicular to the principal planes of the vomers. These bones, if they are articulated correctly, formed a substantial part of the roof to the nasal cavity (nc). The epivomers extend laterally toward the internal surface of the nasals. Whether they contacted the deep curtain-like premaxillary flange on each nasal cannot be ascertained. The incomplete free anterior and posterior edges of these bones may indicate that these structures were ossified portions of more extensive sheets of tissue that roofed the nasal cavity.

These bones appear to be unique to Scelidosaurus .

Palatine ( Figs 10 View Figure 10 , 12B View Figure 12 , 27A, B, F, G View Figure 27 ): As preserved, the medial portion of both palatines (Pal) seem roughly oblong, and they are shallowly vaulted bones that roof the space between the maxilla and pterygoid at the back of the oral cavity. They simultaneously form much of the floor of the orbital cavity. When complete, the medial wing of the palatines was probably slightly fan-shaped, with a more anteroposteriorly elongate pterygoid suture. An isolated dorsomedial portion of the right palatine is illustrated ( Fig. 27A, B View Figure 27 ). The medial palatal wing is gently concave dorsally, forming the floor of the orbit. Its anterior margin is concave and somewhat thicker than the remainder of the bone, and has a smoothly rounded edge. Laterally, the dorsal surface of its anterolateral corner bears a roughened facet representing the sutural contact with the jugal process of the lacrimal and, ventral to this facet, there is a rugose strip representing a thin contact with the medial edge of the lacrimal process of the jugal ( Fig. 27A, B View Figure 27 , js). Farther posteriorly, and medially, the palatine contacts the ectopterygoid along an irregular, butt-style suture, which abruptly changes into a ventrally facing, scarf-style suture with the pterygoid ( Fig. 27G View Figure 27 ), which extends along its entire medial edge. Ventrally, there is a well-developed sutural surface (laterally) for the maxilla and an arched gap (the palate roof), and then medially the scarf suture for the pterygoid. The right palatine has what appears to be an almost circular, but blind, fenestra (fen) near the anterior border of the palatal roof, but no equivalent fenestra is preserved on the left palatine.

The left palatine is more complete and preserved in articulation with the maxilla ( Fig. 12B View Figure 12 , 27F, G View Figure 27 ). As shown in medial view ( Fig. 12B View Figure 12 ), the maxillary suture is extensive: as previously described, it caps the dorsal surface of the maxilla and, furthermore, develops a thin, deep, ventral flange forming a sheet that is sutured to the medial maxillary wall (mwp). This palatine sheet extends ventrally to just above the row of special foramina that lie parallel to the alveolar margin. In ventral view ( Fig. 27G View Figure 27 ) where it is articulated with the ectopterygoid and maxilla, the posterolateral corner of the left palatine spans a small gap in the anteromedial corner of the palatine (suborbital) fenestra ( Fig. 27G View Figure 27 , pf).

Ectopterygoid ( Fig. 27 View Figure 27 ): The left ectopterygoid (Ec) is disarticulated in the lectotype ( NHMUK R1111 ; Fig. 27C, D, E, G View Figure 27 ). It is a robust, U-shaped bone forming a smooth, concave, anteroventral border to the adductor chamber/subtemporal fenestra. The ectopterygoid forms an important structural link, binding together the jugal, maxilla, palatine and pterygoid, as well as bordering the palatine/suborbital fenestra and reinforcing the lateral edge of the pterygoid flange. Laterally, the ectopterygoid produces a posteriorly pointed, transversely flattened process that bears a facet on its lateral surface (js) that contacts an equivalent facet on the medial wall of the jugal ( Fig. 21B View Figure 21 , ecs). Anterior to this facet its anterior edge curves medially and becomes immediately suturally bound to the medial surface of the maxilla ( Fig. 27F, G View Figure 27 ). This edge then becomes smooth and concave anterolaterally, because it forms the posterior margin of the palatine/ suborbital fenestra (pf). Farther medially, the edge of the ectopterygoid becomes rugose again, where it forms a sutural surface for the palatine (pal.s) before the edge turns posteriorly as a scarf-style sutural flange (on its ventral surface; Fig. 27G View Figure 27 , pt.s) for contact with the pterygoid and backing the pterygoid flange. The ventral surface ( Fig. 27C, D, G View Figure 27 ) shows the pterygoid suture clearly, as well as the oblique channel that is directed medioventrally from the palatine/suborbital fenestra (pf) .

Pterygoid ( Figs 10 View Figure 10 , 23 View Figure 23 , 28 View Figure 28 ): The pterygoid (Pt) is an elongate and delicate, but complex, bone that spans (and links) the nasal cavity and adductor chamber via the palate and basal articulation. Anteriorly, each pterygoid produces an elongate sheet-like anterodorsally directed vomerine process (vp) that has an overlapping contact with the medial surface of the posterior portion of the vomer. A small, rectangular fragment of the left vomer ( Fig. 28A View Figure 28 , vfr) is attached to the lateral wall of this vomerine process and this, fortunately, confirmed the relative positioning of these two bones precisely (before the vomers were damaged). The dorsal edge of the vomerine process is notched where it would have interlocked with the (now missing) posterodorsal process of the vomer. The anterior end of the vomerine process is laterally flattened and vertically oriented, so that it can slot into the narrow gap between the vomers. Posterior to the vomer contact, the dorsal edge of the vomerine process arches laterally forming an internal wall and partial roof of the nasal passage. Closer to the central portion of the pterygoid the arched roof extends laterally toward the maxilla and underlaps the medially directed wing of the palatine ( Figs 10 View Figure 10 , 27G View Figure 27 , 28 View Figure 28 , pal.s). The ventral edge of the vomerine process is narrow and straight, running parallel to the midline, but never makes contact with the adjacent pterygoid, leaving a narrow interpterygoid slot (vacuity) through which the parasphenoid rostrum (cultriform process) can be seen ( Fig. 10 View Figure 10 ). The base of the anterior (vomerine) process becomes twisted and expands laterally, forming a broad shield at the back of the palate, and supporting (dorsally) a platform for an anteroposteriorly orientated concave trough that forms the basal articulation ( Fig. 28B, b View Figure 28 .ar).

The posterior portion of the ventral edge of the pterygoid (below the platform for the basal articulation) forms a flattened and slightly medially expanded plate (ms) with its medial edge curving dorsolaterally into the base of a vertical wall with a convex dorsal margin that is the medial wall of the basal articulation ( Fig. 28B View Figure 28 ). This medial wall curves laterally and then swings dorsally as it creates the dorsal edge of the quadrate wing (qw) of the pterygoid. The lateral surface of the basal articulation is at the base of the quadrate wing of the pterygoid. Near the origin of the dorsal edge of the quadrate wing sits a discrete epipterygoid (Ep). The dorsal edge of the quadrate wing is thin and curves dorsally, following the margin of the pterygoid wing of the quadrate. In contrast the ventral edge of the wing runs horizontally posteriorly, is considerably thicker and it supports on its medial side what appears to be an unusual, narrow, pocket-like slot ( Fig. 28B View Figure 28 , sl). The posterior margin of the quadrate wing is broadly emarginate at mid-height so that the wing appears to be broadly forked.

Lateral to the base of the quadrate wing of the pterygoid there is a posteroventrally oriented shallow trough. The lateral border of this trough is the deflected dorsal edge of the vomerine process. This angled trough provides room for a scarf-style suture with the overlying medial wing of the palatine. The trough continues posteroventrally and forms a rather thin and insubstantial pterygoid flange (fl); the latter is evidently reinforced (structurally) by the attachment of the robust medial arm of the ectopterygoid, to which it is sutured (ecs).

Epipterygoid ( Fig. 28 View Figure 28 ): Epipterygoids (Ep) are rarely recorded as being present in archosaurs generally ( Romer, 1956; see also: Holliday & Witmer, 2009 for its elimination in crocodile evolution) and are rarely reported among dinosaurs (e.g. some theropods – see: Weishampel et al., 2004). Among ornithischians, Maryańska& Osmólska (1974) reported an epipterygoid in the well-preserved skull of the pachycephalosaurid Prenocephale . Maryańska (1977: 116) was also able to report the presence of epipterygoids in the ankylosaurids Euoplocephalus , Pinacosaurus and Saichania . Epipterygoids have so far not been reported in stegosaurs or in other ornithischian subclades.

A left epipterygoid is attached to the quadrate wing of the pterygoid of the lectotype ( Fig. 28 View Figure 28 , Ep). It has an anteroposteriorly expanded and laterally flattened base that is firmly bound/sutured against the lateral surface of the pterygoid.The dorsal edge of the pterygoid is sculpted adjacent to the area of attachment of the epipterygoid: producing a flattened, pillar-shaped structure (pil) that backs the medial side of the base of the epipterygoid. Equally, the anteroventral edge of the epipterygoid is warped (w) so that it wraps around the edge of the pterygoid creating an evidently snug fit. Dorsally, the epipterygoid is recurved and tapers to a narrow point that would have been positioned close to the proötic, although there is no clear evidence of sutural or articular contact between the epipterygoid and any of the adjacent braincase bones.

Comparative comments: The palatal structure of the basal thyreophoran Emausaurus is unknown and the palatal elements of eurypodans, such as ankylosaurs, are highly derived. The palate of Huayangosaurus is largely unknown, but the palate of Stegosaurus was illustrated by Gilmore (1914: pl. 7) and bears some general similarity to the features seen in Scelidosaurus , although there is not enough information to allow for detailed comparisons to be made.

Quadrate ( Figs 29 View Figure 29 , 30 View Figure 30 ): The quadrate (Q) of the lectotype, is well-preserved ( Fig. 29 View Figure 29 ). Some of the referred specimens include undistorted examples that complement the description of this bone ( Figs 29C View Figure 29 , 30 View Figure 30 ). The general morphology of the quadrate gives the slightly misleading impression of its having been twisted outward along its length, away from the cranial midline, because of the asymmetry of its two anterior wings: pterygoid (ptw) and jugal (jw) and the obliquity of the mandibular condyle.

In lateral profile, the posterior edge of the shaft of the quadrate is concave and comprises a central, thickened shaft, which is convexoconcave in cross-section, transversely rounded posteriorly and broadly embayed anteriorly. Each end of the shaft is capped by articular condyles of different morphologies. The head or dorsal condyle ( Fig. 30 View Figure 30 , qh) is smooth, convex and comparatively small relative to the overall size of the quadrate and its ventral (mandibular) condyles. Although smooth and articular in appearance, the quadrate head is located in a deep socket in the body of the squamosal. The presence of anterior and posterior processes that clamp the proximal portion of the quadrate shaft would have limited palinal rotation (fore–aft) between the head of the quadrate and the squamosal cotylus. In contrast, mediolateral rotation and consequent flexure of the quadrate shaft cannot completely be ruled out. The posterior edge of the shaft, directly beneath the quadrate condyle, descends vertically as a transversely compressed buttress (cb). This condylar buttress extends for about 20% of the length of the shaft, beyond which the shaft is bowed evenly anteriorly, creating an overhang. The anterior edge of the quadrate head descends obliquely before dividing into widely divergent laminae, the jugal and pterygoid wings.

The jugal wing (jw) is less expanded than the pterygoid wing, but is more robust. Its anterior edge describes a convex curve and flares laterally. The leading edge of this wing is etched by low-relief ridges ( Fig. 30A View Figure 30 ), which indicate attachment scars for the ligaments securing the anteroventral process of the squamosal, as well as the ascending process of the quadratojugal. Just below its mid-length, the jugal wing is notched by a comparatively small, but incomplete, quadrate (paraquadratic) foramen ( Fig. 30 View Figure 30 , qf). This foramen is subcircular in outline and runs obliquely through the jugal wing. The notch for this foramen is almost entirely enclosed in the jugal wing of the quadrate, but a small residual gap remains that is spanned by the quadratojugal.

It is interesting to note that in the partial, but well preserved, left quadrate of the referred specimen (CAMSM X35296 View Materials : Fig. 29C View Figure 29 , pn) there is a distinct pit positioned just medial to the quadrate foramen. This structurally and positionally resembles a remnant of the posterior pneumatic foramen ( Hendrickx et al., 2015), a feature that was also noted in the quadrate of Heterodontosaurus ( Norman et al., 2011: fig. 14,?pn). The ventral portion of the jugal wing of the quadrate is overlain by the posterior edge of the quadratojugal ( Fig. 29 View Figure 29 , 30C View Figure 30 ) and there is a well-marked elongate sutural surface ( Fig. 30 View Figure 30 , qjs). Beneath this scarred surface, the jugal wing merges with the main body of the quadrate above the lateral expansion that supports the oblique surangular portion (sac) of the mandibular condyle.

The pterygoid wing originates as a ridge on the anteromedial corner of the quadrate condyle. This ridge becomes progressively narrower, and the wing deep, as it expands medioventrally in a nearly straight line to overlap the quadrate wing of the pterygoid ( Figs 28B View Figure 28 , 29 View Figure 29 , 30 View Figure 30 ). The pterygoid wing reaches its greatest expansion at about mid-shaft, where it forms a convex bulge before contracting back sharply, to merge with the lower part of the shaft well above the expansion for the mandibular condyle. The medial surface of the pterygoid wing bears an unusual deep, sutural area for the thin, bifurcated quadrate wing of the pterygoid. Just posterior to this sutural surface, the pterygoid wing bears a broad depression ( Fig. 30 View Figure 30 , dep), although the presence of this feature varies between individual specimens: well developed in CAMSM X39256 View Materials and BRSMG Ce12785, yet poorly developed in the lectotype. The quadrate wing of the pterygoid comprises a thin, tapering process that is pressed against the mediodorsal portion of the pterygoid wing, and a slightly more robust process that is pressed against the medioventral surface of the pterygoid wing of the quadrate ( Fig. 28 View Figure 28 ). The degree of development of the sutural surfaces for the pterygoid varies between the left and right quadrates of NHMUK R1111 , but this may well be an artefact of preparation .

The mandibular condyle of the quadrate is expanded transversely and has the rough outline of an elongate, rounded triangle in distal end view ( Fig. 10 View Figure 10 ), the apex curving laterally and the base directed medially. The posterior edge of the articular surface lies roughly perpendicular to the long axis of the skull. The lateral corner is relatively narrow anteroposteriorly, but roller-like anteroposteriorly with an oblique axis (sac). More medially the condylar surface expands steadily to form an enlarged and more anteroposteriorly expanded, near-spherical, medial condylar surface (ac). A shallow, oblique groove seems to separate what are distinguished as two condylar areas. The articular surfaces are comparatively smooth and would have been coated with articular cartilage. A framework of thin, curved ridges that surround the margins of the condylar area mark the limits of the articular cartilage. The asymmetry evident in the structure of the lateral and medial portions of this quadrate articular surface reflects the structure of the mandibular cotylus and this may well have constrained the motion of the lower jaw when in articulation.

Comparisons: The quadrate of Emausaurus is not sufficiently well-preserved to allow comparison to be made. In eurypodans, the quadrate is inconsistently described. Among stegosaurs, the basal taxon Huayangosaurus exhibits a quadrate that is similar in shape to that of Scelidosaurus ( Sereno & Dong, 1992: fig. 6C): it is illustrated with a larger medial condyle and an oblique lateral condyle, as seen in Scelidosaurus , and there is even an indication of a pit, notch or recess dorsomedial to the quadrate foramen. The quadrate of Stegosaurus is highly specialized in having an obliquely compressed proximal head ( Gilmore, 1914: fig.5), but it does have the asymmetrical distal articular condyles, as seen in Scelidosaurus and as reconstructed in Huayangosaurus . Among ankylosaurs, the quadrate tends to be cryptically positioned beneath a shield of dermal osteoderms (caputegulae) in ankylosaurids, although it is visible in posteroventral view. In nodosaurids the quadrate is a little better known and illustrated. The ventral portion of the quadrate, where visible, conforms in its sutural relationships with those seen in Scelidosaurus and the mandibular condyle displays a similar asymmetry.

DESCRIPTIVE OSTEOLOGY: THE BRAINCASE