Lycorhinus angustidens Haughton, 1924

Sereno, Paul C., 2012, Taxonomy, morphology, masticatory function and phylogeny of heterodontosaurid dinosaurs, ZooKeys 226, pp. 1-225 : 96-108

Lycorhinus angustidens Haughton, 1924 Figs 37380Tables 13

Lycorhinus angustidens Haughton, 1924 - Thulborn (1970, Figs 1-5); Gow (1975, 336, Figs 1, 2, pl. 1); Hopson (1975, Figs 1, 2, 3a, b, e); Hopson (1980, Figs 2, 3); Weishampel (1984, fig. 12a, b); Gow (1990, Figs 1-7); Porro et al. (2011, Figs 4-6); Norman et al. (2011, Figs 37B, 38, 39C)

= Lanasaurus scalpridens Gow 1975

Holotype.

SAM-PK-K3606, left dentary with 11 teeth (preserved mostly as a natural mold; UCRC PVC10 was cast from the natural mold; Fig. 3D)

Referred material.

NHMUK RU A100 (formerly BMNH A100), partial skull preserving the right premaxilla, right nasal, right maxilla, the posterior two-thirds of the right lower jaw, and the anterior third of the left lower jaw; BP/1/4244, left maxilla with 12 teeth (holotype Lanasaurus scalpridens ); BP/1/5253, left maxilla with 15 teeth.

Type locality.

Paballong, near Mount Fletcher, Transkei (Herschel) District, Cape Province, South Africa; S30°26', E28°31' ( Kitching and Raath 1984) (Fig. 1B).

Horizon.

Upper Elliot Formation; Lower Jurassic, Hettangian, ca. 200-196 Ma ( Thulborn 1970; Gow 1990; Knoll 2005; Gradstein and Ogg 2009).

Revised diagnosis.

Heterodontosaurid ornithischian characterized by the following two autapomorphies: (1) prominent ridge on the distal crown margin running from the first denticle to the cingulum (labial aspect of maxillary crowns, lingual aspect of dentary crowns); (2) lingually curved maxillary and dentary tooth rows.

Description.

The validity of Lycorhinus angustidens and referral of an important partial skull (NHMUK RU A100; Thulborn 1970) have been at the center of debate between several authors ( Thulborn 1974, Charig and Crompton 1974; Hopson 1975, Gow 1975, 1990). In the course of this argument, the holotype of Lycorhinus angustidens has been described in detail ( Hopson 1975; Gow 1990). The same cannot be said for the partially disarticulated skull of NHMUK RU A100. Several misidentifications in Thulborn’s original description have gone unnoticed. Proper referral of this specimen is critical, as the holotype for Lycorhinus angustidens is limited to a single dentary that now is preserved largely as an impression ( Hopson 1975; Gow 1990; Fig 3). Here I describe the partial skull NHMUK RU A100 and later discuss the history and resolution of the taxonomy of Lycorhinus (see Discussion, Taxonomy of Lycorhinus ).

Specimen map.

The NHMUK RU A100 is preserved on a single block of matrix with bone exposed on both sides. In its current state of preparation, the block preserves the right premaxilla (medial view), right maxilla (lateral view), anterior end of the left lower jaw (medial view), and mid and posterior parts of the right lower jaw (partial medial and lateral views) (Figs 73, 74). Thulborn (1970) misidentified a portion of the right maxilla as part of a nasal, the right lower jaw as a partial cranium, and the anterior end of the left lower jaw (medial view) as the end of the right lower jaw (lateral view) (Fig. 4). First I provide evidence for identification of these bones as shown here (Figs 73, 74).

A flat broad bone next the right maxilla is here tentatively identified as the posterodorsal ramus of the maxilla, preserved near its natural position relative to the remainder of the bone (Figs 75, 77B). It is difficult to envision this platelike bone as the nasal as identified by Thulborn (Fig. 4), as it is very broad (especially for a heterodontosaurid), lacks a median suture, and shows a fluted surface posteriorly that is difficult to understand as an articular surface for the frontal. The posterodorsal ramus of the maxilla is very broad as preserved in NHMUK RU A100 and BP/1/4244 ( Gow 1975). Most of the surface is devoted to the antorbital fossa, which in heterodontosaurids approaches the nasal suture. The relatively flat bone piece in question, however, does not have any external rim between the surface of the fossa and the edge in contact with the nasal. As a result I tentatively identify the bone as a continuation of the maxilla, although it was not included in the skull reconstruction (Fig. 80). The identity of the bone could be determined with more confidence after further preparation of the specimen.

The left lower jaw (Figs 73, 74) preserves the anterior portion of the splenial broadly overlapping the medial aspect of the dentary, a raised dentary symphysis, and anteromedially canted crowns that are exposed in medial view. Despite the assertion that no crowns are worn ( Thulborn 1970: 240), almost all of the preserved left dentary crowns are truncated by prolonged tooth-to-tooth abrasion. Only the truncated apical edge on some of the anterior dentary crowns is visible, however, because the left dentary teeth are exposed in medial view (Fig. 74). The axis of each exposed left dentary tooth is medially convex with thicker, orange-tinted enamel as is typical of the medial side of dentary crowns. The last preserved left dentary tooth, which is exposed on the other side of the block, preserves a broad, low-angle, wear facet on the lateral side of the crown (Fig. 73).

The more complete right lower jaw is also exposed on both sides of the block (Figs 73, 74). On one side of the block, the right lower jaw is broadly exposed in lateral view (Fig. 74). Many observations support the identification of these bones as the right lower jaw rather than a partial cranium in dorsal view as suggested by Thulborn (Fig. 4). The buccal emargination is on the lateral aspect of the dentary (previously identified as a left frontal), there are sutures typical of that between the surangular, angular and dentary, a strong surangular ridge is present (previously considered part of the left jugal), and an external mandibular fossa is present as in Heterodontosaurus (pr eviously identified as the laterotemporal fenestra). Finally, although the anterior end of the right lower jaw is embedded in matrix, the enlarged dentary caniniform tooth passes through to the opposite side of the block, its tip emerging near the right maxilla (Figs 73, 74). The description of the frontal, postorbital and jugal by Thulborn (1970) and remarks concerning the postorbital-jugal suture by Weishampel (1984: 44), thus, were based on the right lower jaw of NHMUK RU A100.

With the identifications proposed here, there is no reason to doubt that NHMUK RU A100 represents a single, probably adult individual with worn upper and lower cheek of similar form, worn premaxillary teeth, and large upper and lower caniniform teeth. The specimen preserves the right side of the snout and lower jaw and a matching anterior portion of the left lower jaw. This important specimen, the salient features of which are outlined below, should be fully prepared and described in more detail.

Premaxilla.

The right premaxilla is preserved in ventromedial view, exposing the median articular surface for the left premaxilla and the palatal shelf medial to the premaxillary teeth (Figs 73, 75, 76). The internarial and posterolateral processes are broken, the former very narrow and the latter quite broad as in Abrictosaurus and Heterodontosaurus . The only lateral exposure of the premaxilla occurs along its posterior margin, where the anterior portion of an arched, inset upper diastema is preserved (Fig. 76). The arched diastema for the dentary caniniform tooth is broadly open laterally. A secondary lateral wall enclosing a space within the diastema is initiated in the upper half of the premaxillary margin of the diastema and continues onto the maxilla as in Heterodontosaurus (Fig. 75).

The palatal surface is flat with a near horizontal orientation as in Heterodontosaurus and most ornithischians. Judging from the location of the anterior portion of the arched diastema, the palate and premaxillary tooth row would have been set below the maxillary tooth row. Just above the posterior end of the palate, an anteriorly tapering slot accommodates the anteromedial process of the maxilla (Fig. 76). This maxillary process, therefore, inserts above the palate as in Heterodontosaurus ( Norman et al. 2011), Hypsilophodon ( Galton 1974a), and many other ornithischians, rather than against the ventral surface of the premaxillary palate as previously suggested (contra Weishampel 1984: 34). The posterior end of the palatal surface turns sharply upward to join the anterior portion of the arched diastema. Two small foramina are present near the bases of the first and second premaxillary teeth. Although it is possible that they represent replacement foramina, only two are present (rather than three). Unlike replacement foramina, in addition, they are not positioned immediately adjacent to premaxillary alveoli and lack a connecting groove for the dental lamina. It is probable, therefore, that they represent neurovascular foramina associated with the palate, an interpretation that could be verified with computed-tomographic imaging of the bone.

Maxilla.

The right maxilla, which is broadly exposed in lateral view (Figs 73, 75, 77A), has sustained some fracturing and loss during the time of burial as well as some breakage after exposure of the fossil. Fracturing and loss during burial appears to have isolated the base of the posterodorsal ramus of the maxilla, leaving it near its natural position relative to the main body of the maxilla (Fig. 75). The distal portion of this ramus may also be preserved near its natural position, although the identification of this plate-shaped bone remains tentative (Figs 75, 77B).

The body of the maxilla thickens transversely from the alveolar margin to the ventral rim of the antorbital fenestra, which protrudes laterally as in other heterodontosaurids (Figs 75, 77A). Several neurovascular foramina lie just under this rim, which forms the upper margin of a deeply inset buccal emargination (Fig. 80). Dorsal to the rim, the smooth medial wall of the antorbital fossa is exposed, which increases in depth anteriorly. Near the anteroventral corner of the antorbital fossa are located two oval depressions (Figs 77A, 80). The broadly arched anteroventral corner of the antorbital fossa is not invaginated as occurs in Abrictosaurus and Heterodontosaurus .

The edges of the arched diastema and the anterior end of the anteromedial process of the maxilla are broken away (Figs 75, 77A). This portion of the maxilla is best preserved in an isolated referred left maxilla ( Gow 1975, 1990). In NHMUK RU A100, some of the inset medial wall of an arched diastema is preserved. The posterodorsal ramus is divided between a raised external margin and the recessed medial wall of the antorbital fossa (Figs 75, 77A). Unlike Abrictosaurus and Heterodontosaurus , the fossa is not deeply invaginated, and the raised external margin separates the antorbital fossa from the premaxilla by a greater distance. A low prominence is present about halfway along the anterior margin of the antorbital fossa (Fig. 80).

Lower jaw.

The right lower jaw is exposed mainly in lateral view and is complete except for the retroarticular region and small areas of breakage (Figs 74, 79). The anterior end, however, remains embedded in the matrix, and information on this end of the lower jaw comes from the left side, which is exposed in medial view (Figs 74, 78). There is some chance that the predentary is hidden within the matrix of the block, given that both dentary rami are present.

The dentary is a robust bone approximately 17 mm in depth at mid-length and some 70 mm in length. The ventral margin of the ramus is thickened, and the tooth row is inset by a ventrally arched, deep buccal emargination (Fig. 80). Although partially visible in NHMUK RU A100, the lateral aspect of the dentary is best exposed in the holotype (Fig. 3D). Anteriorly, the dentary curves toward an oval symphysis (Fig. 78) that is deeper than that in Heterodontosaurus (Fig. 61A). In medial view the end of the dentary is beveled anteroventrally (Fig. 78), unlike the rounded and expanded end of the dentary in Abrictosaurus and Heterodontosaurus (Figs 35, 59).

Medially the dentary is overlapped by a broad, tongue-shaped splenial, which appears to have been displaced anteriorly toward the symphysis on the right side (Fig. 78), and a long strap-shaped coronoid (Fig. 73) similar to that in Heterodontosaurus (Fig. 56). The coronoid curves posterodorsally to overlap the junction between the dentary and surangular on the coronoid process. An external mandibular fossa occupies the central portion of the surangular and angular below the coronoid process and is filled with matrix (Fig. 79). Additional preparation is needed to determine if a mandibular fenestra is present at the anterior end of this fossa, as occurs in Heterodontosaurus .

The retroarticular region is broken away, so the exact location of the jaw articulation is unknown. Judging from the preserved margin of the surangular and the location of the dentary tooth row, the jaw articulation was probably offset ventrally at least as much as preserved in Abrictosaurus (Fig. 35).

Premaxillary teeth.

The three premaxillary teeth (pm1-3) are preceded by an edentulous margin approximately one-third of the length of the ventral margin of the premaxilla (Figs 76, 80). The first two premaxillary crowns may have slid partially out of their sockets. A gentle constriction is present on these teeth where the crown and root meet, and the second more complete crown is clearly recurved. A broad wear facet on pm2 may have obliterated the midline of the crown. No ornamentation is visible as preserved. The basal swelling of the crown of pm2 and posterior deflection of its tip resembles the form of pm2 in Heterodontosaurus (Fig. 91) and pm1 in Lesothosaurus ( Sereno 1991: fig. 6C). Thulborn (1970: 239) described the crowns of pm1 and 2 as "conical pegs".

The large, caniniform third premaxillary tooth lacks any constriction between the crown and root (Fig. 76). The crown base is cylindrical but flattens distally with development of anterior and posterior carinae. The smooth anterior carina is convex in lateral view, whereas the posterior carina is nearly straight. The crown apex thus is only slightly recurved (Fig. 80). The straight posterior carina has about five serrations per millimeter as noted by Thulborn (1970).

The wear facets on pm2 and 3 are planar but are not “flat” as described by Thulborn (1970: 239). The facets are mesiodistally flat but concave apicobasally (Fig. 76). These facets are the result of shearing wear against the straight keratinous edge of the predentary bill. The tip of the pm3, in addition, is broken along a horizontal fracture as preserved in matrix. The crown tip thus either was broken in life or during transport after death. Wear alone cannot account for the straight edge that truncates the tip of this caniniform tooth. Breakage and subsequent apical abrasion characterizes the teeth of adult Heterodontosaurus , as described in more detail below.

Maxillary teeth.

The right maxilla of NHMUK RU A100 has alveoli for 14 fully developed teeth (Fig. 75) rather than 13 as described by Thulborn (1970: 239). A count of 14 fully developed maxillary teeth matches that in another complete maxillary tooth row, which has one peglike distalmost crown ( Gow 1990). Judging from the empty first alveolus in NHMUK RU A100, m1 is smaller than more posterior maxillary teeth as preserved in a referred maxilla ( Gow 1975). An edentulous margin about the width of one tooth is present between the arched diastema and first maxillary tooth in NHMUK RU A100 (Fig. 75). The edentulous margin is much shorter in referred maxillae ( Gow 1975, 1990). Maxillary crowns are set at an angle to the root and are subject to considerable wear against the dentary crowns (Fig. 77C). Wear facets truncate the distal portion of all of the preserved maxillary crowns. Unlike Abrictosaurus and Heterodontosaurus , all but the most mesial and distal maxillary crowns are fairly similar in size and shape (Fig. 80).

The distinguishing feature of the lateral aspect of the maxillary crowns is the accentuated distal edge, which is raised as a distal marginal ridge (Figs 10, 77C). The dentary crowns, in contrast, are more symmetrical as seen in lingual view (Fig. 78). A rounded median eminence is present but not developed into a distinct primary ridge, and no secondary ridges are present on either maxillary or dentary crowns. The medial aspect of the maxillary crowns is not exposed; the description of the medial side of the maxillary crowns by Thulborn (1970: 240) is based on misidentification of the dentary as a maxilla (Fig. 4B). Besides the distal marginal ridge, the only other distinguishing feature in the maxillary dentition is the medial curvature of the tooth row ( Gow 1990). In Echinodon , Abrictosaurus and Heterodontosaurus , in contrast, the maxillary tooth row is nearly straight.

Dentary teeth.

The posterior portion of a small dentary tooth (d1) is preserved anterior to the caniniform tooth (Fig. 78). Its crown appears to have been subconical or peglike with a rounded distal margin. Although figured by Thulborn (1970: fig. 5), this tooth was not described. The second dentary tooth (d2) is caniniform, with anterior and posterior carinae. The crown is more than twice the height of dentary crowns in the center of the tooth row (Fig. 80). The premaxillary caniniform tooth (pm 3) is approximately 70% the height and basal width of d2. Unlike pm3, d2 is noticeably recurved with convex and concave carinae anteriorly and posteriorly, respectively (Fig. 78). The mesial carina of d2 is serrate, whereas the distal carina is not fully exposed. The supposed absence of serrations on the distal carina ( Thulborn 1970) thus cannot be used to differentiate NHMUK RU A100 from the caniniform in the holotype, which has a serrate distal carina ( Hopson 1975; Gow 1990). An edentulous margin the width of one tooth is situated between the dentary caniniform tooth and d3 in NHMUK RU A100 (Fig. 78), which closely matches the condition in the holotypic dentary of Lycorhinus angustidens (Fig. 3).

Eleven postcaniniform teeth (d3-13) are present in the holotypic dentary, which lacks the distalmost teeth. In NHMUK RU A100, the first and second postcaniniform crowns (d3, 4) are smaller and proportionately taller than more distal dentary crowns (Fig. 78). The tip of the crown of d5 is truncated by shearing wear on the unexposed labial side of the crown. More substantial wear is present on d6 and 7 that truncates more of the crown tip. Thinking they were maxillary teeth, Thulborn (1970: 240) reported that all of the exposed dentary crowns are unworn. His lower count of only two or three denticles on mesial and distal crown margins, rather than four or five, is based on dentary crowns truncated by wear facets.

The lingual side of the dentary crowns has a prominent, slightly mesially offset, vertical eminence that is stronger than that on the labial aspect of the maxillary crowns (Figs 77C, 78). Unlike the labial surface of the maxillary crowns, neither margin is raised as a ridge nor are the crowns tilted relative to the axis of the root. The labial side of the dentary crowns is exposed in the holotype but all are heavily worn (Fig. 3). The fourth postcaniniform tooth in the holotype dentary (Fig. 3C; labeled “4”)-the only one preserving some of the crown edge-has a prominent distal marginal ridge comparable to that on the labial aspect of the maxillary crowns in NHMUK RU A100 (Fig. 77C). A rounded median eminence is present which joins a labial prominent, swollen cingulum, which is well-demarcated form the root. Thus the labial aspect of the dentary crowns in the holotype of Lycorhinus angustidens bears a striking resemblance to the labial aspect of the maxillary crowns in NHMUK RU A100. In both the cingulum is swollen and there is a prominent distal marginal ridge.

The roots of the dentary teeth in NHMUK RU A100 appear to be swollen. The left dentary and one of its posterior teeth are exposed in cross-section (Fig. 73). Although there is a gentle constriction below the crown, the hollow root expands to a width equal to that of the crown, resembling the condition in Abrictosaurus and Heterodontosaurus . Hopson (1975: 304) also noted swelling of the roots of the dentary teeth in the holotype of Lycorhinus angustidens .

Maxillary and dentary crowns have an imbricate arrangement with the mesial edges of the crowns angled mesiolingually. Although some adjacent crowns are in contact, there is little overlap of crown edges in either the holotypic dentition, the tooth rows of NHMUK RU A100, or the two referred maxillae. As described by Gow (1990) in the worn dentition of the holotype, wear facets in Lycorhinus do not join to form a single surface nor are they oriented in a single occlusal plane (contra Weishampel 1984: 54). The enamel on the dentary crowns is distributed asymmetrically with a thicker layer on the lingual side in NHMUK RU A100. A similar asymmetrical distribution of enamel (reversed) is probably present in maxillary crowns but cannot be unequivocally demonstrated.

Skull reconstruction.

The partial skull reconstruction (Fig. 80) is the first for Lycorhinus angustidens . Previously Gow (1990: fig. 3) had juxtaposed the maxillary and dentary tooth rows of BP/1/5253 and SAM-PK-K3606. The reconstruction presented here is based on the holotypic dentary (UCRC PVC10; silicone cast from the natural mold of (SAM-PK-K3606; Hopson 1975), two referred maxillae (BP/1/4244, BP/1/5253; Gow 1975, 1990), and specimen NHMUK RU A100. The premaxilla and maxilla of the new reconstruction are based primarily on NHMUK RU A100. The anterior end of the dentary in NHMUK RU A100 is exposed only in medial view, and so the lateral aspect of the dentary is based on the holotypic natural mold ( Hopson 1975; Gow 1990). The dentary of NHMUK RU A100, however, shows that its anterior end is less expanded and lacks the rounded profile of that in Heterodontosaurus and Abrictosaurus (Figs 35, 59). The predentary is not preserved in available material of Lycorhinus .

The premaxillary teeth, which are based on NHMUK RU A100, are inset from the anterior margin of the premaxilla with the first two premaxillary teeth set back into their sockets. The premaxillary tooth row is positioned below the level of the maxillary tooth row (Fig. 80), judging from the low position of the maxillary anteromedial process (BP/1/4244; Gow 1975: fig. 1) and the slot on the premaxilla for that process (Fig. 76). Maxillary and dentary crown shape is best preserved in BP/1/4244 and NHMUK RU A100, respectively (Figs 77C, 78). The maxillary tooth count of 15 (the last a rudimentary crown) is based the nearly complete tooth rows of NHMUK RU A100 and BP/1/1523 ( Gow 1990). The dentary tooth count of 15 is an estimate. The holotype preserves 11 dentary teeth posterior to the caniniform tooth for a total of 13 teeth (d3-13), but the posterior end of the row is probably lacking one or two teeth. The right dentary in NHMUK RU A100 may well preserve a complete tooth row but is not fully exposed. Dentary crown shape is based on the exposed and unworn anterior dentary crowns in NHMUK RU A100 (Fig. 78).