Material referred to Megalosaurus (Dinosauria: Theropoda) from the Middle Jurassic of Stonesfield, Oxfordshire, England: one taxon or two?

Material referred to Megalosaurus (Dinosauria: Theropoda) from the Middle Jurassic of Stonesfield, Oxfordshire, England: one taxon or two? Julia J. Day' & Paul M. Barretr' DAY, J. J. & BARR ETT, P. M. 2004. Materi al referred to Meg alosaurus (Dinosauria: Theropoda) from the Middle Ju rassic of Stonesfield, Oxford shire, England: one taxon or two? Proceedings of the Geologists' Association, 115, 359- 366. Megalosaurus buck landii von Meyer, 1832 is one of the most widely known , but poorly under stood , of all theropod dinosaurs. It was recently claimed that the large theropod material from the Stone sfield Slate of Oxfordshire, which is usually attributed solely to M . bucklandii, represent s not one, but two taxa on the basis of differences in the scapul ocor acoids , ilia and femora. Here we evaluate this claim follow ing examination of almost all relevant examples of these elements from the UK. A consistent suite of four differing femor al char acter states cannot be accounted for by sexual, ontogenetic or individual variation, and supports the presence of two morphotypes that probably represent distinct taxa. However , other purported differences between these two morphotypes are shown to be either ambiguous or erroneous. One taxon ('mo rphotype B', corresponding to that represented by one of the paralectotypes of M. bucklandih appears to be a basal tetanuran, whereas the other ('morphotype A') cannot be confidently assigned to any major theropod clade. Phylogenetic analyses of theropod interrelationships should not include any nom inal species of M egalosaurus until a full taxonomic appraisal of this material has been completed. Key words: Theropoda, Megalosaurus, Middle Jurassic, taxonom y I Department of Biology . University of Konstanz , 78457 Konstanz, Germany 2Department of Palaeontology. The Natural History Museum. Crom well Road. London SW7 5BD. UK (e-mail: [email protected] )

1. INTRODUCTION M egalosaurus bucklandii von Meyer, 1832 is a large theropod dinosaur from the Middle Jurassic of England. This historically important taxon has occupied a central position in dino saur studies, as a result of its status as the first dino saur to be described scientifically (Buckland, 1824). Moreover, it is also significant because of the rarity of dinosaur remains, especially those of theropods, from this period in Earth history . In spite of its potential importance in studies of theropod phylogeny and evolutionary history, the taxonomy of the genus Megalosaurus has been severely neglected and is in urgent need of revision. M egalosaurus was named on the basis of a collection of theropod material from the Stonesfield Slate of Stone sfield, Oxfordshire, England [National Grid Ref. SP 387171]. Recent litho- and biostratigraphical work has demonstrated that the Stonesfield Slate is not a formal lithostratigraphical unit, but should be regarded as a recurrent, sporadic lithofacies of the Taynton Limestone Formation (Boneham & Wyatt, 1993). Ammonites from the Taynton Limestone Formation indicate referral of this unit (including three separate Stonesfield Slate horizons) to the Procerites pro gracilis Zone of the lower part of the middle Bathonian stage (Middle Jurassic) (Torrens, 1980; Boneham & Wyatt, 1993). Proceedings of the Geologists ' Association, 115, 359- 366.

The original syntype series (see Appendix for definition of taxon omic terms) of Megalosaurus bucklandii includes the anterior portion of a right dentary, partial ribs, a sacrum, several caudal vertebrae, pelvic element s, a right femur and a metatarsal (Buckland, 1824). Man y additional theropod specimens from both the UK and elsewhere have been referred to Megalosaurus and several species were proposed for the reception of this material (e.g. Lydekker, 1888; Woodward, 1910; von Huene , 1923, 1932; Waldman, 1974), some of which were later transferred to other genera (see reviews by Steel, 1970; Padian, 1997). This practice continued unchecked throughout the 19th and 20th centuries ; as a result , Megalosaurus became a 'wastebasket' taxon that was used extensively as a convenient receptacle for any large, generalized theropod material, particularly that from the Middle Jurassic of Europe (Steel, 1970; Padian, 1997). This problem has been compounded by the taxonomic instability of the type species, M. bucklandii, and the validity of the genus has been questioned as a result (e.g. Allain & Chure, 2002). This instability stems from the fact that neither Buckland (1824) nor von Meyer (1832) proposed a type specimen for M. bucklandii. Subsequently, several workers have suggested that the syntype dentary (OUM J. (3505) should be regarded as either the type or lectotype of this taxon 00 16-7878/04 SI5.00

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(e.g. Swinton , 1934; Moln ar et al., 1990; Benton & Spencer , 1995; Rauhut, 2000; Allain, 2002; Allain & Chure, 2002). The latter suggestion has never been propo sed as part of a formal taxonomic review but , for publication s dated prior to 2000, merely stating that a particular specimen is the lectotyp e of a specific taxon constitute s a valid lectot ype designation under Article 74 of the International Code on Zoological Nomenclatur e (Intern ation al Commission on Zoological No menclature, 1999); the other syntypes should therefore be regarded as paralectotypes. The lectotype dentary apparently lacks apomorphic character states, making diagnosis of the taxon problematic (Molnar et al., 1990; Rauhut, 2000; Allain & Chure , 2002). Moreover, the specimens that form the syntype series were not associated when collected (Buckland , 1824), probably pert aining instead to several different mines in the Stone sfield area, each of which may have been working one of several different Stonesfield Slate horizons (see Boneham & Wyatt, 1993). Despite recent claims to the con trary, size discrepancies between the various syntype elements clearly demonstrate that more than one individual is represented by the type series (Buckland, 1824; H. P. Powell, pers . comm. 2000; contra Delair & Sarjeant , 2002), although most workers have suggested that only a single taxon of large theropod was present (e.g. Owen, 1842; Molnar et al., 1990; Benton & Spencer, 1995). Furthermore, previous diagnose s of Megalosaurus have not relied on explicit autapomorphic (uniquely diagnostic) character sta tes or character state combinatio ns and th is problem has been exacerbated by the referral of additional specimens to the genus in the absence of rigorous compara tive observation s (e.g. Buckland , 1824; von Huen e, 1926; Steel, 1970). Several diagnoses have depended , at least in part, on referred specimens and species, adding fur ther to the confusion that surrounds this taxon (e.g. von Huene, 1926; Steel, 1970). Nevertheless, despite these problem s, various species of Megalosaurus (excluding M. bucklandiii have been incorporated in cladistic ana lyses of the Theropoda, which have all suggested that at least some of the material referred to this genus occupied a basal position within Tetanurae (Holtz, 1994, 2000; Rauhut, 2003). Allain & Chure (2002) recently suggested that two large ther opod taxa were present in the Stonesfield faun a, on the basis of apparent differences in the morphology of the scapul ocoracoid, ilium and femur. As part of a continuing project to reassess the taxon omy and systematics of Middle Ju rassic British theropod s, we examined exampl es of these element s from the Stonesfield Slate that have been assigned to Mega losaurus bucklandii. Instituti onal abbreviations: BMNH , The Natural History Museum, London , UK; O UM NH , Oxford University Museum of N atural History, Oxford , UK; MNHN, Museum National d'Histoire Naturelle, Pari s, France; SDM, Stroud District Museum, Stroud, UK.

2. MATERIALS The material used in this study comes from the Stonesfield Slat e (Tay nton Limestone Fo rmation) in the area around Stonesfield, Oxfordshire, UK. 'R' and 'L' denot e right and left elements, respectively. Mea surements for the femora and scapulocoracoids are given in Table I. Scapulocoracoids . OUMNH J.13574 [R]. Largely compl ete, with a minor amount of restoration. OUMNH J.29879 [L]. Scapula largely complete, but broken proximally and coracoid missing. OUMNH J.29887a [R]. Proximal part of scapula onl y. OUMNH J.29888 [R]. Scapula broken dor sally and distally; proximal part of the coracoid missing. OUMNH J.29889 [L]. Badly damaged, missing most of the scapula blade and proximal coracoid. BMNH 31810 [R]. Isolated, but complete, coracoid. BMNH R1099 [L]. Scapula broken distally, with some restoration of proxim al plate and coracoid. Femora. OUMNH J.13561 [R]. Partially restored , but oth erwise complete . Forms a part of the syntype series of Megalosaurus from Stone sfield (Buckland, 1824). OUMNH J.29753a [R]. Th e proxim al third only. OUMNH J.29802 [R]. Mostly compl ete. Th e femor al head and greater trochanter are broken proximally and posteriorl y and the lateral margin of the anterior trochanter has been lost. The distal condyles are broken and the epicond yles have been sheared off. Von Huene (1926) sta ted that this specimen was from Endslow Bridge, but the lithology of the adhered matrix strongly suggests that it pertains to the Stone sfield Slate (H. P. Powell, pers. comm. 2000). OUMNH J.29803 [R]. Damaged pro ximal end . Th e greater trochant er, anterior trochant er and ant erior surface of the proximal end have been broken . The distal end is slightly crushed dor soventrally, which has caused slight skewing of the condyles. BMNH 31804 [L]. Mostly complete, anterior surface of pro ximal and distal ends is incomplete. BMNH 31806 [R]. Mostly complete. BMNH 31808 [L]. Mostly comple te, some damage to the pr oximal and distal ends. MNHN 9630 [L]. Mostly complete, some damage to the proximal end . Ilia. OUMNH J.13560 [R]. Part of the syntype series (Buckland , 1824). OUMNH J.29881 [R], BMNH RI100 [L (not right: cont ra Lydekk er, 1888, p. 162)] and BMNH RIlOI [R]. Three complete ilia. BMNH 318/ 1 [R (not left: contra Lydekker, 1888, p. 162)]. Partial ilium dam aged anteriorly and dorsally. Figured (contra Allain & Chure, 2002) as a coracoid byOwen (1857, plate VI), but noted as an ilium by Lydekker (1888, p. 162). BMNH R283 [R]. Parti al ilium lackin g part s of the anterior and posteri or processes. 3. ANATOMICAL DATA A full redescriptio n of Megalosaurus buck landii lies beyond the scope of this paper and will be addressed

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Table 1. Comparative measurements of the femora and scapulocoracoids attributed to Megalosaurus bucklandii.

Element Femur [R] Morph B Femur [R] Morph A Femur [R] Morph B Femur [L] Morph A Femur [R] Morph A Femur [1] Morph B Femur [R] Morph B Femur [R] Morph A Femur [1] Morph A Scapulocoracoid [R] Scapulocoracoid [R] Scapulocoracoid [L] Scapulocoracoid [L] Scapula [L] Scapula [R] Coracoid [R]

Specimen number

Maximum length (mm)

Maximum dorsoventral height of proximal plate of scapula (mm)

740 685 735 795 800 815 820 800 710 845 795 NA a 830 765 NA a 150

240 210 185 210 220 320 240b

OUMNH J.13561 OUMNH J.29802 OUMNH J.29803 BMNH 31804 BMNH 31806 BMNH 31808 SDM 44.23 SDM 44.24 MNHN 9630 OUMNH J.13574 OUMNH J.29888 OUMNH J.29889 BMNH RI099 OUMNH J.29879 OUMNH J.29887a BMNH 31810

The table demonstrates that most of the elements fall within a narrow size. This observation excludes the possibility of ontogenetic variation accounting for the observed morphological differences that distinguish morphotypes A and B. Measurements are given as preserved: details of the preservation of each element are given in the text. Maximum lengths are either proximodistal (femora) or anteroposterior (scapulocoracoids). "Not available becauseof breakage. "Maximum dorsoventral height of coracoid.

elsewhere; here we emphasize character states of potential taxonomic significance and have attempted to summarize the range of morphological disparity within the collection of theropod material from Stonesfield. Scapulocoracoids

Phillips (1871, p. 208) mentioned two distinct types of Megalosaurus pectoral girdle from Stonesfield. In one form (exemplified by a single individual) the scapula and coracoid were separate from each other, whereas in the other morphotype (represented by several individuals) these two elements had fused. This observation was cited in support of the hypothesis that more than one large theropod was present in the Stonesfield fauna (Allain & Chure, 2002). Unfortunately, the unfused pectoral elements mentioned by Phillips (1871) could not be located during this study, as these specimens were not adequately figured, or identified by catalogue numbers. The majority of other pectoral girdle elements attributed to Megalosaurus show evidence of extensive fusion between the scapula and coracoid (BMNH RI099, OUMNH 1.13574, OUMNH 1.29888 and OUMNH 1.29889). An isolated coracoid (BMNH 31810) and the proximal part of a scapula (OUMNH 1.29887a) indicate that fusion may not have occurred in some individuals, which might be taxonomically significant. However, the articular surfaces of these isolated elements are irregular and unfinished, representing either poor preservation or over-preparation. Consequently, it is not possible to deduce whether they articulated with, or were fused to,

the other part of the scapulocoracoid in life, rendering this potential difference ambiguous at present. Moreover, OUMNH 1.29887a is significantly larger and more robust than any of the other theropod scapulae from the Stonesfield Slate (see Table I): indeed, the very large size suggests this specimen might pertain to a sauropod, although more work is needed to confirm this. Femora

Allain & Chure (2002) recognized two femoral morphotypes in the Megalosaurus sample: one with a sigmoidal shaft and an anteromedially inclined femoral head (based on OUMNH 1.13561; also noted by Owen, 1842, p. 108), the other with a straight shaft and medially oriented caput (based on BMNH 31806). Our observations indicate that OUMNH 1.29802, OUMNH 1.29753a, BMNH 31804, BMNH 31806 and MNHN 9630 are straight in both anterior and posterior views and are only very gently bowed anteriorly along their length in lateral view (Fig. I). This contrasts with femora OUMNH 1.29803, OUMNH 1.13561 and BMNH 31808, which in anterior view are almost sigmoidal in outline, bowing first laterally and then medially. In lateral view, these femora are also slightly sigmoidal (Fig. 1). Unfortunately, most of the femora have damaged proximal and distal ends, so it is not always possible to assess the distribution of some potentially interesting character states in each of the two morphotypes. Herein, we refer to the 'straight' femora as morphotype A, and the 'sigmoidal' femora as morphotype B.

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ar

eg Fig. l. Representative right femora attributed to Megalosaurus bucklandii, in anterolateral view: (A) morphotype B (OUMNH J.29803: 'sigmoidal'); (B) morphotype A (OUMNH J.29802: 'straight'). Scale bar represents 100 mm. Arrows indicate some of the features that can be used to differentiate the two morphs. In the morphotype B, the extensor groove (eg) is present and the anterior ridge (ar) is absent: the converse is true in morphotype A. (See text for further details.)

In both morphs, in proximal view, the femoral head projects anteromedially from the proximal end of the femur (contra Allain & Chure, 2002). The ventral margin of the femoral head forms a distinct, acute angle with the proximal part of the shaft where preserved. In proximal view, the femoral head of both morphotypes has an elliptical outline and a deep groove extends medial to the 'ball' of the caput. The remaining portion of the greater trochanter is narrower than the head and decreases in anteroposterior width laterally, so that it is sub-triangular in proximal outline, tapering laterally. The anterior trochanter

forms a prominent alariform process (e.g. BMNH 31806) that is separated from the greater trochanter by a distinct cleft. The proximal extremity of the anterior trochanter terminates at a point approximately level with the ventral extremity of the femoral head. In morphotype A the anterior trochanter is supported ventrally by a robust ridge that extends along the anterolateral margin of the femur to a point approximately one-third of the way down the shaft, migrating medially as it does so (Fig. I). Although this region has experienced varying degrees of crushing in some of the femora leading to breakage along the ridge (e.g. OUMNH J. 29802), in all specimens assigned to this morphotype it remains a prominent structure. The area between this ridge, the base of anterior trochanter and the proximal end of the femur is strongly concave transversely. This region is partially bounded dorsally by an additional low ridge that extends ventrolaterally from the underside of the midpoint of the ventral margin of the femoral head. Both of these ridges are absent in morphotype B. In morphotype A, the ridge ventral to the anterior trochanter gives the shaft a sub-triangular cross-section at femoral mid-length; in morphotype B, the shaft mid-length cross-section is sub-circular, as a result of the absence of this ridge. Both morphs have distal shafts that are sub-circular in cross-section. The fourth trochanter (which is damaged distally in many specimens) is a prominent ridge-like structure that is situated on the posteromedial margin of the femur in both morphotypes. The ventral end of the trochanter is situated above femoral mid-length. Proximally, the fourth trochanter continues as a low ridge that merges with the posterior surface of the shaft just distal to the proximal end of the femur. In some specimens, this ridge is associated with two proximodistally elongate muscle scars, which are situated one each side of the ridge, but poor preservation obscures the distribution of this character state in morphotypes A and B. In both morphotypes, the distal end of the femur expands markedly transversely and slightly anteroposteriorly relative to the distal shaft. A shallowly concave area is situated on the anterior surface of the distal end of the bone. In specimens that are well preserved distally, the posterior intercondylar groove is a prominent structure. Femora of morphotype B possess a distinct anterodistal intercondylar (extensor) groove, whereas those of morphotype A lack this structure (Fig. 1): this groove is particularly deep in OUMNH J.29803, but has been accentuated by crushing in this specimen. In distal view, morphotype B has a medial condyle that is almost twice the size of the lateral one, whereas in morphotype A the condyles are more symmetrical in size (although the medial condyle is very slightly larger than the lateral condyle: Fig. IB). One specimen (morphotype A: BMNH 31806) possesses a shallow, but discernible, groove that traverses the crista tibiofibularis (Fig. 2: see also Owen, 1857,

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1.29 803, but this is probably d ue to post mortem crushing.

Ilia

Fig. 2. Morphotype A femur (BMN H 31806) in distal end

view, showing the position of the groove traversing the crista tibiofibularis (get). Scale bar represents 50 mm.

plate VIII). In severa l specimens from both morphotyp es, short ridges extend proximally from the epicondyles alo ng the posterio r surfac e of the sha ft for a short di stance. These rid ges a re absent from O UM NH

Two iliac m orphotyp es were recognized by Allain & Chure (2002 ) on the basis of co mp a risons between OUMNH 1.13560 (o ne o f th e original Mega losaurus syntypes: Buckland , 1824) and BMNH 3 1811. Those workers stat ed that ' they [the ilia] a re clearl y different in sha pe a nd proport ion s' (All ain & Chure, 2002 , p. 118), but did not provide any descriptive data or figure s to suppo rt thi s co nclus ion . On first ins pection, O UMNH ] . 13560 and BMNH 31811 do appear to be rather diffe rent in two major respect s (Fig . 3). Firstly, th e a nte rio r and poster ior iliac lobes o f BMNH 31811 arc proportionally much sho rter a ntero pos teriorly (relative to total ilium length o r height) than in OUMNH ] .13560. Secondly, in lateral view, the outline of th e anterio r lobe of BMNH 3 1811 is subrectangular, with a n a lmos t straig ht anter ior margin th at meets the dorsal margin of the ilium a t a n ab ru pt a ngle of approxima tely 80 0 • In co nt rast, OUM N H ] . 13560 ha s a mu ch more rounded anter ior lobe, in wh ich the an te rior and dorsal margin s merge into each o ther along a smoothly convex arc (Fig. 3). However, these differences in sha pe are mi sleading a nd spurious. Most of th e a nter ior lobe a nd a nte ro do rsa l portion of BMN H 3 1811 is recon struc ted from plaster, th ou gh the recon struct ed a reas a re pa inted to

11-

c

1)

-

Fig. 3. Ilia attributed to M egalosaurus bucklandii in lateral view: (A) right ilium BMNH R3122 (cast of OUMNH J.13560): (B) left ilium BMNH RI IOO (reversed to allow easier comparison); (C) right ilium BMNH 31811 ; (D) outline drawing of BMNH 31811 with reconstructed areas shaded in grey. Dotted lines indicate the possible continuation of the broken posterior lobe. It should be noted that although BMNH 31811 does appear to differ in outline and proportions from the other ilia referred to Megalosaurus, the former is broken anteriorly and posteriorly and is extensively reconstructed anteriorly and anterodorsally, obscuring the tru e shape of the ilium, which is more similar to that of BMNH RIIO O and OUMNH 1.13560. Scale bars represent 100mm.

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look like the original bone (Fig. 3D). Moreover, the posterior lobe of the ilium is broken and it is likely that a substantial portion is missing. Thus, any proportional or shape differences related to these areas of the ilium cannot be reliably determined in this specimen. When the actual preserved part of BMNH 31811 (rather than the partially reconstructed 'whole') is compared with OUMNH J.13560 (or indeed with any of the other large ilia from Stonesfield: OUMNH J.29881, BMNH RIIOO, BMNH R1101 and BMNH R283) no significant differences can be detected; BMNH 31811 can easily be accommodated in the OUMNH J.13560 morphotype. As a consequence, ilium morphology cannot be used to differentiate between large theropod taxa in the Stonesfield fauna, at least on the basis of current evidence (contra Allain & Chure, 2002). 4. DISCUSSION Differences in the morphology of the femur (and possibly of the scapulocoracoid) in Stonesfield Megalosaurus material demonstrate clearly the presence of two distinct morphs of large theropod dinosaurs in this series of deposits. These two morphs may represent distinct taxa, as suggested by Allain & Chure (2002), sexual dimorphs of a single taxon, or ontogenetic or individual variants. We recognize several other significant differences between the femora of the two morphs (see above) in addition to the overall variation in femoral shape noted by Allain & Chure (2002), further emphasizing the distinction between these two groups of specimens. The penecontemporaneous Chipping Norton Formation (Hook Limestone Member) of New Park Quarry, Gloucestershire (Zigzagiceras zigzag Zone, convergens Subzone, lower Bathonian; Torrens, 1980) has also yielded remains of large theropods that are usually referred to Megalosaurus (Reynolds, 1939; Benton & Spencer, 1995). Femora from this collection indicate that two morphs of large theropod, corresponding to those found at Stonesfield, were also present in this slightly older fauna (SDM 44.23 a right femur, morphotype B; and SDM 44.24, a right femur, morphotype A). Isolated coracoids and proximal portions of several scapulae were also collected from the same localities, but as with the Stonesfield material are generally too poorly preserved to yield useful information on whether the pectoral girdles were definitely unitary structures or not (Reynolds, 1939). It seems unlikely that ontogenetic variation can account for the observed differences in morphology because elements of both morphotypes occupy similar size ranges (see Table 1). For example, although fusion of the scapula and coracoid in the small coelophysoid theropod Syntarsus appears to be under ontogenetic control (Raath, 1990), the apparently unfused scapulae and coracoids from the Middle Jurassic of the UK are from individuals that are of similar size to, or are larger than, those that possessed fused scapulocoracoids (see above). Age-related variation is seen in the relative

proportions of the hindlimb segments of the allosauroid Allosaurus (Smith, 1998; Loewen & Sampson, 2000; Loewen et al., 2002); however, there are no reports of ontogenetic variation in the character states that vary in the Megalosaurus sample, such as in the possession (or not) of a ridge ventral to the anterior trochanter. Moreover, as the femora (and potentially the pectoral girdles) display two discrete, easily recognized morphotypes with no specimens of 'intermediate' morphology between them, individual variation does not seem able to account for the differences observed in the sample. The consistent morphological variation between the femora (and perhaps among the scapulocoracoids) therefore implies either possible sexual dimorphism or taxonomically significant differences. Raath (1990) suggested that some of the variation exhibited by a population of Syntarsus might be attributable to sexual dimorphism: in particular, differences in the positions of the femoral trochanters and muscle scars seem to indicate the presence of a gracile and robust morpho Some of the differences between the two morphs in the Megalosaurus sample are similar to those listed for Syntarsus; however, we suggest that sexual dimorphism cannot be responsible for the variable characteristics seen in Megalosaurus, especially as regards the wholesale difference in overall femoral shape. Both femoral morphs of Syntarsus are gently sigmoidal along their lengths: there is no discrepancy as marked as that between the 'sigmoidal' and 'straight' morphs seen in Megalosaurus (Raath, 1990). To our knowledge, no theropod taxon is known to encompass a population with such different femoral designs; consequently, we agree with Allain & Chure (2002) that morphotypes A and B represent two discrete taxa, although for rather different reasons. OUMNH J.l3561, the femur from the original syntype series of Megalosaurus bucklandii, belongs to morphotype B ('sigmoidal'); therefore we provisionally retain all type B femora in Megalosaurus bucklandii, pending a thorough taxonomic evaluation of this genus, and further suggest that morphotype A femora should not be regarded as representative for Megalosaurus as they do not share any features with the original syntype specimens. As the syntype series contained no pectoral girdle elements, the potential differences in scapulocoracoid morphology do not currently affect discussions of Megalosaurus taxonomy. Allain & Chure (2002) noted that the 'sigmoidal' femora were superficially similar to those of the ceratosaurian theropod Ceratosaurus from the Late Jurassic of the USA (see Madsen & Welles, 2000, plate 21). However, general phenetic resemblance aside, morphotype B femora do not exhibit any ceratosaurian synapomorphies (cf. Sereno, 1999; Holtz, 2000). Although many femoral character states appear to be homoplastic within Theropoda, the presence of a distinct extensor groove on the anterodistal femoral surface (Holtz, 2000) in morphotype B probably indicates referral to Tetanurae. The absence of

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several derived char acter sta tes for various tetanuran in-gro ups (Holtz, 2000; Rauhut, 2003), such as the presence of a trochanteric crest or the pro ximal expansion of the anterior trochanter, suggests that the possessor of morphotype B femora occupied a basal position within this clade, at least on the basis of femoral features. Therefore, the acqu isition of'sigmoidal' femo ral morphology in the latt er taxon probably represents a parallelism with Cera tosaurus. In cont rast, morphotype A femora exhibit no unamb iguous tetanur an synapomorphies. Separation of the anterior troch anter from the femoral head (as also seen in mor pho type B) occurs in severa l tetanuran in-groups, but is also seen in robust cerato saur ians (Ceratosauru s and abelisaurids) (Holtz, 2000); no other features of morphotype A femora indicate referral to any particular tetanuran clade. The presence of a groove in the crista tibiofibularis is usually regarded as a ceratosaurian synapomorphy (Sereno, 1999; Holtz, 2000; but see Rauh ut , 2003, for a different interpretation ); the occurrence of this feature in one of the morphotype A femora (BMNH 31806) may, ther efore, support referral of this theropod ta xon to Ceratosaur ia. However, no other ceratosaurian synapomorphie s are identifiable in this specimen. Con sequently, the phylogenetic position of morphotype A femora is ambiguous at present and we adopt a con servat ive approach, regarding this morph as Theropod a indet. Recognition of two large theropod taxa in the Stonesfield fauna leads us to advocate caution in the use of the various Mega losaurus species (e.g. M. bucklandii, M . nethercom bensis and AI . hesperis) as operational taxonomic units in phylogenetic analyses until a full re-evaluation of th is material is completed. ACKNOWLEDGEMENTS We ded icate this paper to H. Philip Powell on the occasion of his retirement from the O UMNH. Phil' s interest in the Middle Jurassic dinosau r material from the UK is both comprehensive and infectious. We tha nk him for continuing discussion, his generous sharing of unpublished work on this material , and for access to the collections at the OUMNH . M. Loewen and D . Smith are thanked for inform ation on A llosaurus hindlimb ontogeny, and R . Sadleir helped with measurements on the OUMNH mater ial. Helpful re-

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views of a previous draft were received from the Editor, O. Rauhut, P. Upchurch, D . Naish and an anonymous referee. Photographs were taken by Phil Hurst (NHM Photographic Unit ). 1.1.0. began this project while at the Department of Earth Sciences, University of Cambridge, funded by a Levcrhulme grant (F/09633A). 1.1.0. also acknowled ges the hospitalit y of Th e Na tural History Museum , Lond on . A. APPENDIX

Definitions for the taxonom ic terms (syntype, lectot ype and par alectot ype) used in the text follow tho se given in the International Code of Zool ogical Nomenclature (Intern ational Commission on Zool ogical Nomenclature, 1999). Syntyp e. A syntype is one of a series of type specimens for a par ticular species, where none of the specimens in the series has been designated as either a holotype or lectot ype (see below). For example, in the case of M egalosaurus the original description listed a number of different specimens, but did not explicitly identify one of these to be the holot ype. As a result, all of these specimens formed the syntype series for Megalosaurus, and all originally had a name-bearing function for M egalosaurus. Lectotype. Following taxonomic revision of species erected on the basis of a syntype series (such as M egalosaurus), one of the syntype specimens can be designated the sole name-bearin g specimen for the species in question. This designated syntype specimen is kno wn thereafter as the lectot ype. Paralectotyp e. Following taxonomi c revision of a species that was erected on the basis of a syntype series (such as Mega losaurusi, once one of the syntype specimens has been designated the sale name-bearing specimen for the species in quest ion (the lectoty pe), the remaining specimens from the original syntype series are designated as paralectotypes. Alth ough part of the original syntype series, paralectotypes have no formal name-b earing function. In the case of Mega losaurus, a left dent ary from the original syntype series was designated the lectotype (name-bearing specimen); the remaining specimens (including vertebrae and limb mat erial) are now paralectotypes (non -name-bearing specimens).

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Manuscript received I December 2003; revised typescript accepted 24 April 2004