Figure TEXT-FIG. 1.. Interpretive drawing of lectotype right dentary (OUMNH J.13505) of Megalosaurus bucklandii from the Taynton Limestone Formation of Stonesfield, Oxfordshire, UK. A, medial and B, lateral views; idp, interdental plates; mg, Meckelian groove; mf, Meckelian foramen; mfo, Mecklian fossa; nvf, neurovascular foramina. Light grey represents tooth; medium grey, areas restored in plaster; dark grey, broken surface. Scale bar represents 100 mm.
Download figure to PowerPoint
1827 Megalosaurus bucklandii Mantell, p. 67, pl. 18, fig. 2; pl. 19, figs 1, 8, 12, 14–16.
Remarks. The binomen Megalosaurus conybearei was used by Ritgen (1826) but not accompanied by any description, diagnosis or indication as required under Article 12 of the International Code of Zoological Nomenclature (International Commission on Zoological Nomenclature 1999) and is thus a nomen nudum. Megalosaurus bucklandii was used by Mantell (1827) in a publication including descriptions and figures of the syntype material. This spelling of the species epithet was used subsequently by only a few early authors (e.g. Mantell 1833; Eudes-Deslongchamps 1838) prior to Molnar et al. (1990) who incorrectly attributed the name to Ritgen (1826). Since that time it has been used as the presumed valid name in a handful of publications. Megalosaurus bucklandi was proposed by von Meyer (1832) and this spelling has been used by the vast majority of subsequent authors. Nevertheless it is an objective junior synonym of M. bucklandiiMantell, 1827.
The lectotype appeared in the collections of the Oxford Anatomy School at Christchurch College in 1797. The School’s minute book for 24 October 1797 records this entry: ‘Large jaw bone with two serrated teeth, in calc schistus from Stonesfield, purchased for 10s 6d’ (Günther 1925, p. 191). At this time the collection was being built by Dr (later Sir) Christopher Pegge, the Reader in Anatomy. Buckland was friendly with Dr Pegge but even if he had not been he would surely have known the collection as he was also a fellow of Christchurch.
The original syntype series of Megalosaurus bucklandii includes the various isolated, unassociated elements mentioned and figured by Mantell (1827) following the material originally attributed to this taxon by Buckland (1824), including: dorsal (OUMNH J. 13577) and caudal (OUMNH J. 13579) vertebrae, a sacrum (OUMNH J.13576), a right ilium (OUMNH J.29881: misidentified as a coracoid), a pubis (OUMNH J. 13563: misidentified as a fibula), an ischium (OUMNH J.13565: misidentified as a clavicle), a right femur (OUMNH J.13561), a metatarsal (OUMNH J. 13572), and two ribs (OUMNH J.13580, J.29792). Unfortunately, other material included in the original description (including a tooth and some fragments of bone) cannot be reliably identified among the OUMNH collections. Molnar et al. (1990) formally proposed designation of the dentary (OUMNH J.13505) as the lectotype, rendering the rest of the syntype series paralectotypes. However, as none of the paralectotypes can be convincingly referred to the same taxon as the lectotype dentary we follow Allain and Chure (2002) in restricting the hypodigm of Megalosaurus bucklandii to the lectotype pending the discovery of additional comparable material.
Description. The type right dentary, OUMNH J.13505 is almost complete though missing much of the delicate portion posterior to the opening of the Meckelian fossa. The lateral and medial surfaces have been restored in part as indicated in Text-figure 1: however, there is no indication of damage in the figures of Buckland (1824, pl. 40, fig. 2) or Owen (1857, pl. 11, fig. 2). It is straight in dorsal view, unlike the medially curving dentary of Allosaurus (UMNH VP 9366). In lateral view it has a slightly sinuous ventral margin, being weakly convex anteriorly and weakly concave posteriorly, and a weakly convex dorsal margin, both of which are usual for carnivorous theropods.
Eleven alveoli are preserved, although more were certainly present. Waldman (1974) estimated that 12–13 teeth would have been present in the complete dentary based on comparison with Allosaurus. However, 13 teeth are present in the type of Dubreuillosaurus valesdunensis (MNHN 1998–13) with only ten positioned anterior to the position of the break in OUMNH J.13505. As such it is likely that more teeth, perhaps 14, were originally present in the latter specimen. The alveoli have transversely compressed oval outlines in dorsal view except for the anteriormost alveolus, which is small and subcircular. The fourth alveolus is the largest, having the greatest anteroposterior length. The third and fifth alveoli appear to have greater transverse widths (Text-fig. 2B): however, this is the result of resorption of the medial walls of the alveoli to accommodate replacement teeth and does not reflect their original dimensions. Although it is largest, the fourth alveolus is not transversely expanded. In coelophysoids (Dilophosaurus, UCMP 37303) and spinosauroids (Baryonyx, BMNH R9951; Dubreuillosaurus, MNHN 1998–13; Eustreptospondylus, OUMNH J.13558; Magnosaurus, OUMNH J.12143/1a–b; and Torvosaurus, BYU 2003) the third alveolus is the largest, and in these taxa the lateral wall of the dentary swells laterally to accommodate the enlarged alveolus. In OUMNH J.13505 the dentary is transversely expanded between the third and sixth alveoli but this is a much more gentle expansion and is subequal on both sides of the tooth row.
The interdental plates are poorly preserved but were probably subpentagonal in outline, with vertical, subparallel sides basally, but becoming pointed towards the apex. This shape is common among theropods, although some of the interdental plates of abelisaurids (Majungatholus, FMNH PR 2100) have a rectangular outline. In OUMNH J.13505 the interdental plates posterior to the fifth and sixth tooth positions respectively have the appearance of having rectangular outlines. However, at this level both the lateral wall of the dentary and the interdental plates are broken dorsally and the shape of the interdental plates cannot be assessed with certainty. On the basis of the shapes of more anteriorly and posteriorly positioned interdental plates these plates were probably subpentagonal and not square in outline. The interdental plates of Megalosaurus are unfused and do not have the rugose medial surfaces that are present in abelisaurids (Carrano et al. 2002).
The paradental groove for the dental artery separates the medial faces of the interdental plates from the medial wall of the dentary. It is open dorsally, defining a distinct gap between those two structures anteriorly until the fifth interdental plate where it becomes closed, allowing the two structures to contact one another. In most other Jurassic theropod taxa the groove is either open for the entire length (Allosaurus, UMNH VP 9366; Dilophosaurus, UCMP 37303; Magnosaurus, OUMNH J.12143/1a–b) or closed for the entire length (Torvosaurus, BYU 2003) of the dentary. However, in ‘Megalosaurus’hesperis (BMNH R332) and Dubreuillosaurus valesdunensis (MNHN 1998–13) the situation is similar to that in OUMNH J.13505. This character should be treated cautiously as it may be the result of post-burial plastic deformation of the element: however, it appears to be present in both left and right elements of ‘Megalosaurus’hesperis (BMNH R332).
The medial wall of the dentary beneath the interdental places is relatively flat. Anteriorly, there is no evidence of a definite symphysial area, though this region has been slightly abraded. Only the anterior part of the Meckelian fossa is preserved. The Meckelian groove continues along the ventral part of the medial surface from the anterior and anterodorsal bounds of the fossa, paralleling the ventral margin of the element. Posteriorly it is broad and indistinct, housing two finer, more distinct and slightly sinuous grooves. Anterior to the level of the sixth alveolus it becomes narrow and more distinct, terminating at the level of the posterior margin of the third alveolus. Ventral to this termination an oval foramen is present. Anterior to the termination a distinct, slit-like, opening is present that may be a second foramen (Text-fig. 2A). Two such foramina are present in most Jurassic theropods in which this region is preserved [Allosaurus, UMNH VP 9366; Ceratosaurus, UMNH VP 5278; Dubreuillosaurus, MNHN 1998–13; Eustreptospondylus, OUMNH J.13558; Magnosaurus, OUMNH J.12143/1a–b); Monolophosaurus (Zhao and Currie 1993); and Sinraptor (Currie and Zhao 1993)], although it is possible that only one is present in Dilophosaurus (UCMP 37303). The presence of only a single foramen is a feature shared with some abelisaurids (e.g. Majungatholus, FMNH PR 2100) and the Rhaetian taxon ‘Zanclodon’cambrensis (BMNH R2912). None of the listed taxa possesses an anterior foramen that is dorsoventrally compressed.
A number of subcircular foramina are present with a random distribution in the anterior parts of the lateral and ventral surfaces of the dentary. A further four are widely spaced in a longitudinal row posterior to these and just dorsal to mid-height on the lateral surface (Text-fig. 2). They open dorsolaterally and are anteroposteriorly long ovals in shape. In common with Allosaurus (UMNH VP 9366) these are not set in a lateral groove, as are at least some in Ceratosaurus (UMNH VP 5278), Dilophosaurus (UCMP 37303), Dubreuillosaurus (MNHN 1998–13), Eustreptospondylus (OUMNH J13558), Magnosaurus (OUMNH J.12143/1a–b), Majungatholus (FMNH PR 2100), Monolophosaurus (N. Smith, pers. comm. 2006), Sinraptor (Currie and Zhao 1993; Gao 1999), Torvosaurus (BYU 2003). The dorsal part of the lateral wall is damaged and restored but only in places, giving it an unnatural jagged appearance. Parts have also been restored centrally and in the ventral part posteriorly. A longitudinal groove is present near the ventral margin and parallel to it (Text-fig. 1A). This has not been observed in any other non-avian theropod dinosaur.
Only one fully-erupted tooth is present, in the sixth alveolus. Partially erupted teeth are present in alveoli 3, 5 and 8, and replacement teeth are visible between the interdental plates adjacent to the fourth, fifth, sixth, eighth and ninth alveoli. The teeth are recurved, transversely compressed and serrated as is generally typical of theropods. They are almost symmetrical: the posterior carina, which continues to the base of the tooth, is positioned on the midline and the anterior carina, continuing about one-third of the way down the tooth, is only slightly laterally deflected basally. ‘Blood grooves’, fine grooves continuing onto the crown from between the serrations, are not present. Gentle transverse wrinkles continuing around the side of the crown and slightly upturned towards the carinae anteriorly and posteriorly are distinct laterally but only weak medially.