Lycorhinus angustidens, HAUGHTON, 1924

Norman, David B., Crompton, Alfred W., Butler, Richard J., Porro, Laura B. & Charig, Alan J., 2011, The Lower Jurassic ornithischian dinosaur Heterodontosaurus tucki Crompton & Charig, 1962: cranial anatomy, functional morphology, taxonomy, and relationships, Zoological Journal of the Linnean Society 163, pp. 182-276 : 233-236

LYCORHINUS ANGUSTIDENS HAUGHTON, 1924

Revised diagnosis: Lycorhinus angustidens is retained provisionally as a valid taxon on the basis of the following character combination: dentary caniniform present; diastema between caniniform and first postcaniniform tooth is short, equal in length to the anteroposterior length of the first postcaniniform crown; postcaniniform crowns are closely packed and adjacent crowns contact one another (with some overlap); lateral and medial surfaces of the postcaniniform crowns possess a distinct basal swelling (‘cingulum’) and are therefore expanded mediolaterally above the root; postcaniniform crowns are expanded anteroposteriorly above the root and therefore possess a ‘neck’.

Holotype: SAM-PK-3606, an incomplete left dentary with caniniform and 11 postcaniniform crowns. ( Fig. 37B, C ; Haughton, 1924: fig. 8; Broom, 1932: fig. 104i; Charig & Crompton, 1974: figs 8, 9; Hopson, 1975: figs 1, 2, 3e; Hopson, 1980: fig. 3; Galton, 1986: fig. 16.6q; Gow, 1990: figs 2, 3, 7; Weishampel & Witmer, 1990: fig. 23.2c; Smith, 1997: fig. 3e; Norman et al., 2004c: fig. 18.2c).

Holotype horizon and locality: Upper Elliot Formation at Paballong (30°26′S, 28°31′E; Kitching & Raath, 1984: table 1), near Mount Fletcher, Eastern Cape Province, South Africa. Broom (1932) mistakenly suggested that the holotype specimen was collected from Witskop near Burgersdorp.

Discussion: Haughton (1924) named Ly. angustidens on the basis of a partial left dentary exposed in lateral view (SAM-PK-3606), identifying it as a cynodont. Broom (1932) considered Lycorhinus a possible ictidosaur (Trithelodontidae), but also noted similarities between the postcaniniform dentition and that of dinosaurs. By the time of Broom’s publication the specimen had already been damaged, and most of the specimen (except for part of the crown of the caniniform tooth) was subsequently lost. Crompton & Charig (1962) first noted the heterodontosaurid affinities of SAM-PK-3606 on the basis of similarities to the dentition of H. tucki .

The incomplete preservation of SAM-PK-3606 ( Fig. 37B, C ) makes assessing the validity of Lycorhinus difficult, especially given the brief description provided by Haughton (1924). More recent descriptions of the dentary and dentition have been based upon silicone moulds of the original specimen and casts of the remaining impressions ( Hopson, 1975, 1980; Gow, 1990). Although recognizing autapomorphies is difficult given the nature of the holotype material, Lycorhinus should be retained provisionally as a distinct taxonomic entity on the basis of a unique character combination, absent in other heterodontosaurids ( Hopson, 1975).

Comparison with A. consors ( Fig. 39A, B – discussed below) is difficult, because in Lycorhinus the dentary teeth have been best described in medial view, whereas the holotype of Abrictosaurus (NHMUK RU B54) only exposes the dentition laterally. One striking difference is the absence of a dentary caniniform in Abrictosaurus (cf. Thulborn, 1974 – as Lycorhinus consors ), which is likely to be a feature of genuine taxonomic significance (as opposed to ontogenetic or sexually dimorphic significance, as originally suggested, see below).

Heterodontosaurus tucki differs from Lycorhinus in that the dentary cheek teeth do not substantially overlap one another (although minor imbrication is present – see Figs 25 , 27 ); have crowns that are not expanded labiolingually or mesiodistally above the root (i.e. a distinct ‘cingulum’ and a distinct ‘neck’ are both absent); have a nearly continuous wear surface developed across all teeth; and have a primary ridge on the medial surface that is offset anteriorly ( Hopson, 1975) with secondary ridges commonly developed on the medial surface posterior to the primary ridge. Comparisons of Lycorhinus with La. scalpridens ( Fig. 39C ) are not possible because the holotypes are lower and upper jaws, respectively.

NHMUK RU A100

This specimen is represented by an incomplete and disarticulated skull ( Fig. 38 ), and was collected from the same locality as SAM-PK-3606 (the holotype of Ly. angustidens ). It was referred to the genus Lycorhinus by Thulborn (1970b) who suggested that NHMUK RU A100 and SAM-PK-3606 might represent different parts of a single individual; this interpretation is not credible: Thulborn (1970b: 236) acknowledged the substantial hiatus (~40 years) between the collection of SAM-PK-3606 and the collection of NHMUK RU A 100 in 1960–1961 and this was further reinforced by the observations of Charig & Crompton (1974: 176) on the nature of the Red Bed exposure in this area and the annual rates of erosion. Furthermore, Thulborn (1970b: fig. 5) misidentified the dentary of NHMUK RU A100 as the right dentary; it in fact appears to be the left dentary exposed in medial view (R. J. B., pers. observ.), the same element as SAM-PK-3606. Charig & Crompton (1974) discussed the possibility that NHMUK RU A100 might represent more than a single individual; however, we agree with their conclusion that this is unlikely given that the specimen contains no duplicate elements.

A number of characters of NHMUK RU A100 has been reported to differ from SAM-PK-3606 ( Charig & Crompton, 1974: 179; Hopson, 1975), and on this basis it has been suggested that NHMUK RU A100 may represent either a distinct but unnamed genus ( Charig & Crompton, 1974) or a second individual of A. consors ( Hopson, 1975, 1980; Weishampel & Witmer, 1990; Norman et al., 2004c). Butler et al. (2008b) treated NHMUK RU A100 as a distinct taxon for phylogenetic analysis because of this uncertainty. The characters by which NHMUK RU A100 is suggested to differ from SAM-PK-3606 are:

1. The dentary caniniform of NHMUK RU A100 serrated on its anterior margin only ( Thulborn, 1970b), whereas both mesial and distal margins are serrated in SAM-PK-3606 ( Hopson, 1975).

2. The base of the dentary caniniform is more slender in SAM-PK-3606 than in NHMUK RU A100 ( Charig & Crompton, 1974).

3. The dentary is deeper beneath the caniniform in SAM-PK-3606 when compared to NHMUK RU A100 ( Charig & Crompton, 1974).

4. The length of the diastema between the caniniform and the first postcaniniform dentary tooth is greater in NHMUK RU A100 than in SAM- PK-3606 ( Charig & Crompton, 1974).

5. The tip of the caniniform is bevelled anteromedially in SAM-PK-3606 but not in NHMUK RU A100 ( Charig & Crompton, 1974).

6. Postcaniniform dentary crowns are inclined slightly mesially [anteriorly] in SAM-PK-3606 but not in NHMUK RU A100 ( Charig & Crompton, 1974).

7. Dentary crowns are closely packed with the cusps of adjacent crowns contacting one another in SAM-PK-3606 but not in NHMUK RU A100 ( Charig & Crompton, 1974).

8. Dentary crowns are heavily worn in SAM-PK- 3606 but not in NHMUK RU A100 ( Charig & Crompton, 1974).

9. Dentary crowns asymmetrical in labial view in SAM-PK-3606 but symmetrical in NHMUK RU A100 ( Charig & Crompton, 1974).

10. Bases of the dentary crowns are notably more strongly swollen in SAM-PK-3606 than in NHMUK RU A100 ( Hopson, 1975).

Character 1 cannot be assessed for NHMUK RU A100 (contra Thulborn, 1970b) because the posterior margin of the caniniform is largely covered by sediment. The exposed apicothecal height of the caniniform of NHMUK RU A100 is approximately 3.1 times the mesiodistal length of its base ( Thulborn, 1970b); the corresponding ratio as preserved is ~ 2.8 in SAM-PK-3606 ( Thulborn, 1970b) but the tip of the caniniform is missing. There is therefore little difference in Character 2 between NHMUK RU A100 and SAM-PK-3606. Adequate assessment of Character 3 is precluded by the incomplete exposure of the dentary of NHMUK RU A100 and the incomplete ventral margin of the dentary of SAM-PK-3606 ( Hopson, 1975: fig. 1a). Character 4 does represent a valid difference: in NHMUK RU A100 the postcaniniform diastema is equivalent in mesiodistal length to the mesiodistal length of the first two postcaniniform crowns, and is ~70% of the length of the base of the caniniform. By contrast, the diastema is significantly shorter in SAM-PK-3606, being equivalent only to the mesiodistal length of the first postcaniniform crown, and ~25% of the length of the base of the caniniform. Character 5 cannot be adequately assessed for NHMUK RU A100 because the dentary is only visible in medial view (contra Thulborn, 1970b) and the tip of the caniniform is, in any case, inadequately exposed. Character 6 does not appear to be valid: a slight anterior inclination of several of the postcaniniform crowns (notably crowns 4 and 5) is evident in NHMUK RU A100, although not in the drawings of NHMUK RU A100 provided by Thulborn (1970b: fig. 5). Character 7 is a valid distinction: the exposed postcaniniform crowns of NHMUK RU A100 are comparatively broadly spaced and the margins of adjacent crowns do not contact or overlap one another, unlike the tightly packed dentition of SAM-PK-3606. Character 8 is difficult to assess adequately for NHMUK RU A100 because so few of the dentary crowns are adequately exposed, and those that are exposed are exposed in medial view (wear facets on dentary crowns occur on the lateral surfaces). However, the preservation of denticles along the anterior and posterior margins of the anterior dentary crowns of NHMUK RU A100 does suggest that the crowns have received little wear – this may therefore represent a valid difference from SAM-PK- 3606 in which the crowns are heavily worn and cusps are rarely preserved (see Fig. 37C ). Characters 9 and 10 cannot be assessed adequately because the dentary teeth of NHMUK RU A100 are exposed only in medial view, whereas those of SAM- PK-3606 are best documented in lateral view ( Hopson, 1975; Gow, 1990). However, it is clear that in medial view, the dentary teeth of NHMUK RU A100 are not symmetrical: the primary ridge is offset towards the anterior margin, so Character 9 is invalid. Although the anterior dentary crowns of NHMUK RU A100 are well differentiated from the root and expanded anteroposteriorly above, this is also true of the anterior dentary crowns of Lycorhinus ( Hopson, 1975: fig. 1B), so Character 10 seems to be invalid.

Three valid distinctions between NHMUK RU A100 and SAM-PK-3606 remain: the difference in the relative length of the diastema (Character 4); the difference in the packing of the postcaniniform teeth (Character 7); and differences in the degree of dental wear (Character 8). The last character is inadmissible because it is a developmental and functional variable between individuals. NHMUK RU A100 and SAM- PK-3606 are commensurate so it seems unlikely that the first two of these distinctions can be ascribed to ontogenetic stage differences. The degree of intraspecific variation in the length of the diastema in heterodontosaurids is unknown: in Heterodontosaurus the diastema is only adequately preserved in the referred specimen SAM-PK-K1332. Heterodontosaurus shows little ontogenetic or individual variation in the packing of cheek teeth.

It is impossible to evaluate the significance of the anatomical differences between NHMUK RU A100 and SAM-PK-3606 because the material is too imperfect to establish whether they represent individual variations, species-specific features or components of higher-level taxonomic separation. In view of these differences, and in view of the inadequate nature of the holotype specimen of Ly. angustidens (SAM-PK- 3606), we feel that referral of NHMUK RU A100 to Ly. angustidens is not justified at this stage.

Note. NHMUK RU A100 will be referred tentatively to the taxon La. scalpridens Gow, 1975 (see below).

Figures

Figure 25. Heterodontosaurus tucki Crompton & Charig, 1962. SAM-PK-K1332 (referred specimen). Dentary dentition. A, right dentition in lateral view (top) and medial view (bottom). B, left dentition in lateral view (top) and medial view (bottom).

Figure 27. Heterodontosaurus tucki Crompton & Charig, 1962. SAM-PK-K1332 (referred specimen). Line drawings based on scanning electron micrograph images of right dentary teeth: De.7–De.9. A, worn surfaces of the teeth in oblique dorsolateral aspect showing the lipped lower edges of the wear facets and the minor imbrication between crowns (the overlap pattern is the reverse of that seen in the maxillary dentition – see Fig. 21B). B, close-up of De.9 to show details of the wear facet and the lower ‘lip’ at the base of the wear facet. For full list of abbreviations see end of paper.

Figure 37. A, Geranosaurus atavus Broom, 1911. SAM- PK-1871 (holotype). Upper image: dorsal view of the fragmentary (and subsequently damaged) remains of the anterior part of the dentaries and predentary; lower image, the extremely fragmentary right maxilla (with traces of the premaxilla), showing the broken crowns and roots of cheek teeth of a heterodontosaurid from Eastern Cape Province. Matrix on dentary block signified using irregular tone. B, Lycorhinus angustidens Haughton, 1924. SAM-PK-3606 (holotype). Partial left dentary, based on the original illustration by Haughton (re-drawn by Hopson, 1975: fig. 1a). The specimen can no longer be found (apart from the dentary crown) and is now represented only by a moulded impression of the original. C, L. angustidens Haughton, 1924. Lateral view of the left dentary dentition (from Hopson, 1975: fig. 1B) retrieved from an impression of the dentition left on the sandstone matrix preserved with the original specimen.

Figure 38. NHMUK RU A100 (BMNH A100). Unnamed heterodontosaur remains based on a partial skull that was originally referred to the genus Lycorhinus by Thulborn (1970b) – composite image manipulated to create the effect of a partial anterior skull viewed from left side. A, left premaxilla in medial view (reversed). B, left maxilla in lateral view. C, maxillary dentition in medial view (reversed). D, left dentary in medial view (reversed). Illustrations derived from Charig & Crompton (1974: figs 4–7).

Figure 39. A, Abrictosaurus consors (Thulborn, 1974). NHMUK RU B54 (holotype). A partial skull (illustrated in abstract form – and based on the reconstruction in Thulborn, 1978). Shading represents known elements of the skull and dentition. The specimen includes postcranial elements that (along with the skull) are in need of further preparation and re-description. B, Abrictosaurus consors (Thulborn, 1974). NHMUK RU B54 (holotype). The maxillary and dentary dentitions, as drawn by Thulborn (1974: fig. 3). C, Lanasaurus scalpridens Gow, 1975. BP/1/4244 (holotype). The maxillary dentition in medial aspect [reversed for comparison with (B)] showing the almost complete dentition (crown 12 obliquely truncated) and with the individually angled wear facets accentuated by cross-hatching (redrawn from Hopson, 1980: fig. 2).