Megalosaurus hesperis, Benson, Roger B. J., 2008

Benson, Roger B. J., 2008, A redescription of ' Megalosaurus ' hesperis (Dinosauria, Theropoda) from the Inferior Oolite (Bajocian, Middle Jurassic) of Dorset, United Kingdom, Zootaxa 1931, pp. 57-67 : 59-64

Duriavenator hesperis ( Waldman, 1974)

Figs 1–2

Owen (1883:pl. 11); Waldman (1974:pl. 42–43); Molnar et al. (1990:fig. 6.29A)

1883 Megalosaurus bucklandi von Meyer; Owen, p. 334. 1964 Megalosaurus sp.; Walker, p. 115.

1974 Megalosaurus hesperis sp. nov.; Waldman, p. 326. 1988 Megalosaurus ? hesperis Waldman ; Paul, p. 294.

2004 Unnamed taxon (=' Megalosaurus' hesperis ); Holtz et al., p. 99.

Holotype. NHM R332. Premaxillae, right maxilla, vomer, paired dentaries, right surangular, fragmentary unidentified elements, and associated teeth.

Hypodigm. The holotype and casts of the holotype: NHM R333, cast of the maxilla/premaxilla block as figured by Owen (1883:pl. 39) prior to preparation; NHM R334–335, casts of the right and left dentaries.

Locality and horizon. Greenhill, Sherborne, Dorset. The material was collected from the Upper Inferior Oolite, Parkinsonia parkinsoni ammonite Zone, Garantiana garantiana Subzone ( Waldman 1974), late Bajocian ( Callomon 2003; Gradstein et al. 2004).

Previous diagnosis. "A large megalosaurid, fifteen to eighteen maxillary teeth, seventeen or eighteen dentary teeth; only apical part of dentary teeth recurved" ( Waldman 1974, p. 326).

Revised diagnosis. Megalosaurid theropod with the following autapomorphies of the maxilla: deep groove on dorsal surface of jugal process containing numerous pneumatic foramina; array of small foramina in ventral part of articular surface for premaxilla.

Description. The anterior part of the left premaxilla ( Fig. 1 A–D) and the posterior part of the right premaxilla ( Fig. 1 E–I) are preserved. The left premaxilla preserves only the first two alveoli and the base of the nasal process and the right element preserves only the posterodorsal part of the bone. Walker (in Waldman 1974) commented that five alveoli may have been present in the premaxilla and Waldman (1974) noted the presence of four premaxillary teeth, two preserved as crowns, one preserved as a cross-section of its original position, and one as an imprint of a juvenile tooth. This observation is confirmed by Owen's (1883:pl. 11, fig. 1) figure. Unfortunately, only the two adult teeth are evident in the casts of the specimen prior to Waldman's preparation (NHM R333) and photographs taken during preparation ( Fig. 2 A), so the specimen as it existed when these comments on premaxillary tooth count were made cannot be examined directly. If Duriavenator possessed five premaxillary teeth then it was unique among spinosauroids: Dubreuillosaurus (Allain 2002) and Eustreptospondylus ( Sadleir et al. 2008) possess four; spinosaurids such as Baryonyx ( Charig & Milner 1997) and Suchomimus ( Sereno et al. 1998) possess six or seven; and Torvosaurus possesses three, although a small fourth alveolus is present posteriorly and filled with rugose bone that may reflect pathological loss ( Britt 1991; BYU 4882).

The anterior surface of the premaxilla is inclined posterodorsally at an angle of around 50 degrees relative to the ventral margin. This angle is comparable to that in Torvosaurus ( Britt 1991) , but is higher than the angle in Dubreuillosaurus (around 45 degrees; Allain 2002) and Eustreptospondylus (around 40 degrees; OUMNH J.13558; contra Sadleir et al. 2008). Although the lateral surface of the bone is poorly preserved, a number of randomly distributed nutrient foramina are visible anteriorly. The medial bone surface is flat anteriorly and scored by posterodorsally oriented striations where it articulated with the opposing premaxilla. A foramen is present on the medial surface immediately ventral to the anterior end of the naris.

Three prominent processes extend posteriorly from the dorsal part of the premaxillary body. The most ventrally positioned is the palatal process. It originates on the medial bone surface and is triangular ( Fig. 1 F– H). This process is prominent in Duriavenator relative to other basal tetanurans ( Madsen 1976; Currie & Zhao 1993) and neoceratosaurs ( Madsen & Welles 2000; Sampson & Witmer 2007). Some other theropods such as Compsognathus ( Peyer 2006) and Coelophysis rhodesiensis ('ventral process' of Rauhut 2003: p. 45) are known to possess large palatal processes. However, the corresponding region of the premaxilla is not preserved in any spinosauroid other than the spinosaurids Baryonyx ( Charig & Milner 1997) and Suchomimus ( Sereno et al. 1998) , in which this region is heavily modified. The two more dorsal posterior processes of the Duriavenator premaxilla form a biramous subnarial process ( Fig. 1 H–I). The medial ramus is shorter and thicker than the lateral ramus. A foramen, visible in lateral view, opens posteriorly between the palatal and subnarial processes, and a second foramen opens posterolaterally from the lateral surface of the bone immediately adjacent to this. It is probable that one of these foramina opened into a canal for the same nerve or blood vessel as the dorsally located foramen on the maxillary articular surface for the premaxilla ( Fig. 1 K). This foramen on the maxilla is also present in Megalosaurus (RBJB, unpublished data) and other theropods including Carcharodontosaurus (SGM-Din 1), but is absent in Baryonyx (NHM R9951).

The maxillary articular surface for the premaxilla is slightly concave transversely. Eleven foramina approximately two millimetres in diameter, and a number of additional smaller foramina, are present ventrally on this surface ( Fig. 1 K). These foramina are absent in most basal tetanurans, including the spinosauroids Baryonyx (NHM R9951) and Megalosaurus (RBJB, unpublished data), and the carcharodontosaurid Mapusaurus (MCF-PVPH-108.169), but are present in Carcharodontosaurus (SGM-Din 1), in which the region covered by the foramina extends further dorsally than in Duriavenator . The condition in Duriavenator is considered here as an autapomorphy.

The anterior process of the maxilla is approximately as long anteroposteriorly as it is high dorsoventrally. This is long compared to some theropods such as non-tetanurans, carcharodontosaurines and sinraptorids which either lack an anterior process or have an anteroposteriorly short process (e.g. Currie & Zhao 1993; Madsen & Welles 2000; Coria & Currie 2006). An elongate anterior process was recovered as a spinosauroid synapomorphy by Sereno et al. (1996). However, it is also seen in Allosaurus ( Madsen 1976) , Guanlong ( Xu et al. 2006) , Monolophosaurus (Zhao & Currie 1993) and Neovenator (MIWG 6348; Brusatte et al. in press). A tab-like flange of bone projects anteriorly from the lateral margin of the anterior process. This tab must have overlapped the lateral surface of the premaxilla, and it is possibly homologous with the 'anterior peg' described by Charig & Milner (1997) in Baryonyx . A similar structure is present in Torvosaurus (BYU 9122; Britt 1991) and absent in Dubreuillosaurus (MNHN 1993-18), but preservation of the maxillae of other megalosaurids, including Afrovenator ( UC OBA 1), Eustreptospondylus (OUMNH J.13558; Sadleir et al. 2008) and Megalosaurus (RBJB unpublished data), is not sufficient to confirm its presence or absence.

The lateral surface of the maxilla adheres to the counterpart slab. Consequently, it is not possible to observe texture or foramina on this surface. Similarly, the exact position of the ventral margin of the antorbital fossa cannot be determined, but it seems to have been in the ventral half of the lateral surface of the jugal process. The lateral bone surface in the anteroventral corner of the antorbital fossa is broken, exposing two pneumatic chambers ( Fig. 1 J). The anterior of these is the maxillary 'fenestra'. Its lateral opening has been exposed in the bony fragment from this region that remains in the counterslab and it is closed medially to form a fossa rather than a true fenestra. The posterior chamber is the anterior extension of a pneumatic excavation on the dorsal surface of the jugal process that is only visible in dorsal and medial views ( Fig. 1 L). This excavation takes the form of a longitudinal groove containing numerous foramina. Although a pneumatic excavation is present in this position in other basal tetanurans such as Piatnitzkysaurus ( Bonaparte 1986) and spinosauroids such as Eustreptospondylus (OUMNH J.13558) and Megalosaurus (RBJB unpublished data), these take the form of two semioval excavations that do not extend far posteriorly. The morphology in Duriavenator is therefore autapomorphic.

The medial surface of the maxilla is smooth. An anteroventrally curving anteromedial process extends from this surface. As in most theropods it bears two distinct longitudinal grooves on its medial surface. Posteriorly it merges with the medial surface of the maxilla and does not extend further as a lingual bar.

Emerging or fully-erupted teeth are present in all eleven alveoli of the maxilla. Walker (1964) estimated that 15–18 alveoli were present when the element was complete. Among spinosauroids the maxilla of Dubreuillosaurus is most similar to Duriavenator in overall shape (Allain 2002). Based on comparison with Dubreuillosaurus , 14–16 teeth may have been present. Fewer than 14 alveoli are present in an estimate based on the relative proportions of Megalosaurus . Replacement teeth at varying stages of development are visible between the interdental plates medial to these teeth. The teeth are recurved and serrated, as are the majority of theropod teeth. Weak enamel wrinkles are present and have a band-like morphology, extending between the mesial and distal carinae across the labial and lingual tooth surfaces. Short interdenticular sulci ( Smith 2007) are also present. The posterior carina continues to the base of the crown and the anterior carina extends approximately half way to the base of the crown. The interdental plates are sub-pentagonal and unfused. They extend to the ventral level of the lateral wall of the maxilla. The tallest plate is that between the second and third alveoli, and the plates diminish in height anterior and posterior to this. The medial surfaces of the plates are lightly pitted and weakly striated.

The anterior portion of the vomer is preserved. It is similar in morphology to that of Allosaurus ( Fig. 2 F– H).

Parts of both dentaries are preserved. The right dentary is missing most of the delicate portion posterior to the opening of the Meckelian fossa and retains parts of the first thirteen alveoli only. Although Waldman (1974) estimated a total of seventeen or eighteen alveoli in the dentary, comparison with the dentary of Allosaurus suggests that only one or two further alveoli were present, thereby totalling fourteen or fifteen. This lower estimate is comparable to the new lower estimate for the number of maxillary alveoli. In the right dentary, emerging teeth are present in the second, fourth, sixth, eighth and tenth alveoli. Replacement teeth are visible between the interdental plates medial to the fourth, fifth, sixth and eighth alveoli. The left element is less complete and preserves parts of the second to the ninth alveoli. The bases of emerging teeth are present in the third, fourth, fifth and ninth alveoli and replacement teeth are visible adjacent to the third, fifth, seventh, and ninth alveoli. The first three alveoli are subcircular and the third is the largest of all the alveoli. Alveoli posterior to the third are subrectangular and decrease in size posteriorly. The dentary expands laterally at the level of the third alveolus to accommodate the enlarged alveolus as in Magnosaurus ( Rauhut 2003) and megalosaurids other than Megalosaurus (Benson et al. 2008) . The tooth in the second alveolus of the right dentary is the only well-preserved adult mesial dentary tooth preserved in any non-spinosaurid spinosauroid. It is longer than more distal teeth and has a circular cross-section, consistent with the circular outline of the alveolus. It is also inclined anterodorsally. The anterior carina only extends a short distance basally from the tip, whereas in more distal crowns it extends just over a third of the crown height basally. The most mesial dentary teeth have a similar morphology in coelophysoids ( Tykoski & Rowe 2004) and may also be cylindrical and slightly anterodorsally inclined in other theropods with enlarged, subcircular anterior alveoli such as other spinosauroids, Dilophosaurus (UCMP 37303), and the allosauroids Acrocanthosaurus (NCSM 14345) and Neovenator (Brusatte et al., in press); however, none of the specimens representing these taxa preserve mesial dentary teeth. An extreme development of this condition is seen in Masiakasaurus , in which a larger number of mesial dentary teeth are longer and more strongly inclined in such a way that they can be described as procumbent ( Carrano et al. 2002).

Tall, subpentagonal interdental plates are present medial to the tooth row. The medial surfaces of these plates have a weakly pitted texture. The plates are separated from the medial wall of the dentary by the paradental groove. The groove is wide anteriorly, defining a space between the interdental plates and the medial wall of the dentary, but narrow posterior to the fourth alveolus, where the medial wall of the dentary contacts the interdental plates. The symphyseal region at the anterior end of the dentary is poorly preserved, but appears to have been much less distinct than that of Allosaurus ( Madsen 1976) . A series of three anterodorsally curving ridges oriented parallel to the anteroventral surface of the dentary is present. This is similar to the arrangement in Magnosaurus (OUMNH J.12143) and other spinosauroids. The Meckelian groove is inclined anteroventrally and is wide and shallow. The ventral margin of the groove is better defined than the dorsal margin, especially in the region anterior to the Meckelian fossa where it forms a sharp, dorsally-convex lip ( Fig. 1 O, T–U). Two Meckelian foramina are present close to the anterior termination of the groove. Both have anteroposteriorly oriented, suboval outlines. They are arranged so that one is anterior and slightly dorsal to the other.

The lateral surface of the dentary is smooth. Approximately fourteen randomly-arranged neurovascular foramina are located anteriorly in the right dentary. More posteriorly, a longitudinal row of anteroposteriorlyoriented, slit-like foramina are situated in a groove just dorsal to mid-height. They are positioned ventrally in the groove and open dorsolaterally such that they are not visible in lateral view. The groove is deep, but not so sharply-incised as that of Dubreuillosaurus (Allain 2002) , Eustreptospondylus or Magnosaurus ( Rauhut 2003).

The anterior portion of the right surangular is a plate-like bone fragment with a convex anteroventral margin. It bears a slot-like longitudinal groove dorsally on the medial surface for the coronoid ( Fig. 2 I–J). A small bone fragment was recovered by Waldman (1974) from matrix within the antorbital fossa ( Fig. 2 A–E, I), but this has not been identified.

Figures

FIGURE 1. Tooth-bearing bones of Duriavenator hesperis (NHM R 332) in lateral (A, J, F – G, M, P – Q), anterior (B, K), medial (C, H, L, O, T – U), posterior (D – E), and dorsal (I, N, R – S) views. A – D, left premaxilla; E – I, right premaxilla; J – L, right maxilla with magnified (x 2) sections showing the anteroventral region of the antorbital fossa (J) and the pneumatic region on the dorsal surface of the jugal process (L); M – O, left dentary; P – U, right dentary. In line drawings (G, Q, S, U) light grey tone indicates tooth, mid grey tone indicates restored surface, and dark grey tone indicates broken bone surface. Abbreviations: 3 rd, third dentary alveolus; amp, anteromedial process; idp, interdental plate; for, foramen; mef, Meckelian fossa; meg, Meckelian groove; mfr, Meckelian foramen; mxf, maxillary fenestra (imperforate); pal, palatal process; pdg, paradental groove; pnu, pneumatic fossa; snp, subnarial process. Scale bar equals 100 mm.

FIGURE 2. Cranial bones of Duriavenator hesperis (NHM R 332) showing parts of NHM R 332 during (A) and prior to (O) preparation reported by Waldman (1976), and after preparation by S. Moore-Fay in 2008 (I – J), and fragmentary elements in unknown (B – E), left lateral (F), ventral (G), dorsal (H), lateral (K – L), medial (M – N) views. A, I – J, block originally containing right maxilla, posterior portion of right premaxilla and unidentified bone fragment (B – E) during preparation by Waldman (1974); F – H, posterior portion of vomer; K – N, right surangular; O, NHM R 334, cast of block containing right dentary, fragmentary right surangular, and anterior portion of right premaxilla prior to preparation. In line drawings (D – E, L, N) dark grey shading indicates broken bone. Abbreviations: cor, articular facet for the coronoid; frag, unidentified bone fragment; imp, tooth impression. Scale bars equal 50 mm (B – H, K – N) and 100 mm (A, I – J, O).