Lycorhinus consors (Thulborn, 1974)

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 : 236-238

= LYCORHINUS CONSORS THULBORN, 1974

Revised diagnosis: Heterodontosaurid that lacks enlarged caniniform premaxillary and dentary teeth; anterior end of dentary ramus dorsoventrally deeper than mid-section; 12 maxillary and 14 dentary crowns; crowns are generally leaf-shaped in profile, but show little development of a median (primary) ridge and show only weak development of anterior and posterior ridges; prominent laterally projecting maxillary shelf is absent, and there is, consequently, no well-marked cheek recess;

Holotype: NHMUK RU B54 (formerly UCL B54), partial skull and incomplete postcranial skeleton ( Fig. 39A, B ; Thulborn, 1974: figs 2–4, Hopson, 1975: fig. 3c, d, Galton, 1986: fig. 16.6m, Smith, 1997: fig. 3c, d).

Holotype horizon and locality: Upper Elliot Formation , from a streamside exposure at the village of Noosi (30°03′S, 28°32′E; Kitching & Raath, 1984: table 1), 5.1 miles east of Whitehill, southern Lesotho.

Discussion: Thulborn (1974) described a new species of heterodontosaurid based upon a single specimen (NHMUK RU B54 – Fig. 39A, B ) and referred this new species to the genus Lycorhinus as Lycorhinus consors . He distinguished it from previously named heterodontosaurids on the basis of several craniodental characters, notably the absence of caniniform teeth. The postcranial material of NHMUK RU B54 was not described. There are a number of errors in Thulborn’s description, some of which are highlighted below, and a full redescription is required, although this requires further preparation of the specimen. Thulborn used the new species name as a means of highlighting his suspicion that the lack of caniniform teeth indicated that NHMUK RU B54 represented a female individual.

Hopson (1975) provided a detailed account of the anatomical differences between the holotypes of Ly. angustidens and Ly. consors , and erected the new genus Abrictosaurus for the latter species. Hopson also provisionally referred NHMUK RU A100, described by Thulborn (1970b), to A. consors . With the exception of Thulborn (1978), later authors have accepted the generic distinctiveness of A. consors . Most subsequent authors, with the exceptions of Thulborn (1978), Gow (1990), and Butler et al. (2008a), have uncritically accepted the referral of NHMUK RU A100 to Abrictosaurus ( Galton, 1986; Weishampel & Witmer, 1990; Smith, 1997; Norman et al., 2004c). Weishampel & Witmer (1990) and Norman et al. (2004c) regarded the type specimen (NHMUK RU B54) of Abrictosaurus as juvenile and/or female and NHMUK RU A100 as adult and/or male, and thus reaffirmed assumptions about the probability of sexual dimorphism as a factor to be considered in the taxonomy of some heterodontosaurs. Norman (1985) suggested that the holotype specimen of Abrictosaurus might represent a female individual of Heterodontosaurus .

The ontogenetic stage of NHMUK RU B54 is undetermined: the neurocentral sutures of the vertebrae are not sufficiently well exposed to assess the presence/absence of fusion and histological analysis has not yet been carried out. The estimated anteroposterior length of the orbit in NHMUK RU B54 is estimated at 24 mm, 73% of the estimated preorbital length (33 mm) and 34% of the estimated total skull length (70 mm). In SAM-PK-K1332 (the referred skull of H. tucki ) the corresponding ratios are 73 and 32%, whereas in SAM-PK-K337 (the holotype of H. tucki ) the corresponding ratios are 69 and 31%. In contrast, the immature skull of H. tucki (SAM- PK-K10487, Fig. 28A ) the orbit is approximately 90% of preorbital length. The proportions of the skull of NHMUK RU B54 are therefore more similar to those of presumed adult individuals of H. tucki than juveniles.

Although there have been several unpublished suggestions that NHMUK RU B54 contains elements from more than a single individual (M. Evans, A. Yates, unpublished notes associated with NHMUK RU B54) this observation cannot be confirmed. This suggestion results from a block containing two scapulae from the syntype specimen ( NHMUK RU B17 – Thulborn, 1972: fig. 2) of Lesothosaurus diagnosticus having been mistakenly catalogued under the specimen number NHMUK RU B54 (A. Yates, pers. comm. 2007, R. J. B., pers. observ.). However there is no clear evidence for duplication of elements in the remaining material and appears to represent a single individual, pending preparation and redescription .

Although seemingly entrenched in the literature, the referral of NHMUK RU A100 to Abrictosaurus cannot be supported. An enlarged and serrated premaxillary caniniform tooth is present in NHMUK RU A100, the apicobasal height of which significantly exceeds the height of both the maxillary dentition and the preceding premaxillary dentition. By contrast, the two preserved premaxillary incisiform teeth of NHMUK RU B54 [note that, contra Thulborn (1974), a possible cross-section of the root of a third premaxillary crown appears to be present just anterior to those described and illustrated in NHMUK RU B54 – see Fig. 39A ] are subequal in size and do not exceed the apicobasal height of the maxillary dentition. NHMUK RU B54 lacks the prominent maxillary ridge that, in NHMUK RU A100, forms the ventral margin of the external antorbital fenestra and the dorsal margin of a well-developed cheek recess. In contrast, the tooth row of NHMUK RU B54 is positioned marginally so there is no wellmarked cheek recess. The maxilla is proportionally deeper below the antorbital fossa in NHMUK RU B54 and numerous foramina are present on its lateral surface; such foramina have not been recognized in an examination of NHMUK RU A100 (R. J. B., pers. observ.). The maxillary teeth of NHMUK RU A100 possess a well-developed swelling (‘cingulum’) on their lateral surfaces, an inflated, apicobasally extending, median swelling (in an equivalent position to, but considerably less well developed than, the primary ridge of Heterodontosaurus ); and a well-developed ridge along the posterior margin of the crown that is substantially better developed than the equivalent ridge on the anterior margin. By contrast, the maxillary teeth of Abrictosaurus (NHMUK RU B54) lack both a basal swelling (cingulum) and a median swelling or ridge; they are also less strongly expanded anteroposteriorly above the root and more closely packed than in NHMUK RU A100. Although anterior and posterior ridges are evident on at least some of the crowns of Abrictosaurus , these ridges are very subtle and weak, and the posterior ridge is not better developed than the anterior ridge. The differences between the maxillary dentitions of NHMUK RU A100 and NHMUK RU B54 are striking. This observation contrasts with that of Hopson (1975:304) who stated (based largely on comparisons of the dentary teeth: see his fig. 3) that the dentition of NHMUK RU A100 was ‘not distinct enough from that of the type of ‘ L.’ consors [NHMUK RU B54] to merit its specific separation’.

Differences between NHMUK RU A100 and NHMUK RU B54 are also evident in the mandible. The anterior end of the dentary of NHMUK RU B54 is dorsoventrally expanded relative to more caudal parts of the element (see Fig. 39A ); in contrast, the anterior end of the dentary tapers in dorsoventral height in NHMUK RU A100. Perhaps more significantly, there is an enlarged and serrated caniniform tooth at the anterior end of the tooth row in NHMUK RU A100 which is, of course, absent in NHMUK RU B54. Further comparison between the dentary teeth of NHMUK RU A100 and NHMUK RU B54 is limited because the teeth of NHMUK RU B54 are exposed in lateral aspect only, whereas those of NHMUK RU A100 are exposed in medial view (contra Thulborn, 1970b).

Characters that distinguish Abrictosaurus from Lycorhinus and Heterodontosaurus are discussed above. Abrictosaurus can further be distinguished from the holotype specimen of Lanasaurus (BP/1/4244 – Fig. 39C , discussion follows) by the same characters of the maxillary dentition that distinguish Abrictosaurus from NHMUK RU A100.

Suggestions that the absence of caniniform teeth might indicate that Abrictosaurus represents an early ontogenetic stage or gender differences within another Stormberg taxon are unlikely for several reasons:

1. Differences between Abrictosaurus and other Stormberg taxa are numerous and substantial and are not simply limited to the presence/absence of caniniforms;

2. It has not been demonstrated that NHMUK RU B54 is a juvenile individual;

3. Evidence from a juvenile specimen of H. tucki (SAM-PK-K10487, Figs 28 , 29 ) indicates that the dentary and premaxillary caniniforms were present at a comparatively early developmental stage in this taxon and are unlikely to have been late ontogenetic or secondary sexual characters;

4. Eleven Stormberg heterodontosaurid specimens of which we are aware have the anterior end of the dentary preserved. Ten of these (SAM-PK- 1871, 3606, K337, K1332, K10487 , K10488 ; NHMUK RU A100, RU C68, RU C69; NMQR 1788) have evidence of an enlarged caniniform; only in NHMUK RU B54 is the caniniform missing. Although larger sample sizes would be informative, a Chi-squared test of goodness-of-fit indicates that the observed ratio differs significantly from the predicted 50: 50 ratio expected if caniniforms are sexually dimorphic (c 2 = 5.82; d.f. = 1; P = 0.016).

On this (admittedly limited) evidence it is considered that A. consors represents a valid species and we reject the possibility that NHMUK RU B54 is either a juvenile or female individual of one of the currently recognized species. No additional specimens of Abrictosaurus have yet been recognized.

Figures

Figure 28. Heterodontosaurus tucki. SAM-PK-K10487 (referred specimen). Partial skull of a juvenile heterodontosaur as preserved in: A, left lateral aspect; B, right lateral aspect. For full list of abbreviations see end of paper. (See also Butler et al., 2008b.)

Figure 29. Heterodontosaurus tucki. SAM-PK-K10487 (referred specimen). Partial skull of a juvenile heterodontosaur as preserved in: A, oblique left lateral aspect to show details of the maxillary dentition; B, ventral view. For full list of abbreviations see end of paper.

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).