SAUROPTERYGIA OWEN, 1860

Kear, Benjamin P. & Barrett, Paul M., 2011, Reassessment of the Lower Cretaceous (Barremian) pliosauroid Leptocleidus superstes Andrews, 1922 and other plesiosaur remains from the nonmarine Wealden succession of southern England, Zoological Journal of the Linnean Society 161 (3), pp. 663-691 : 666

SAUROPTERYGIA OWEN, 1860 PLESIOSAURIA DE BLAINVILLE, 1835 PLIOSAUROIDEA WELLES, 1943 LEPTOCLEIDUS SUPERSTES ANDREWS, 1922A

Holotype: NHM R4828 . Incomplete skull (distorted on the left-hand side and missing the anterior orbital margin plus rostrum) and articulated posterior cervical, pectoral, and anterior dorsal vertebral column, incomplete left and right clavicles, scapulae and coracoids, left and right humeri.

Type occurrence: Barremian (Lower Cretaceous) upper Weald Clay Formation, Wealden Clay Group; this unit forms the upper-most stratigraphical section of the UK mainland Weald Sub-basin sequence (see Allen & Wimbledon, 1991; Radley, 2006a). The holotype specimen was recovered from a brick pit in the vicinity of Berwick, East Sussex, England (see Andrews, 1922a).

Differential diagnosis: Character definitions follow O’Keefe (2001, 2004a), Druckenmiller & Russell (2008b), and Smith & Dyke (2008) with state distributions determined using the present rescored analyses. Features differentiating L. superstes from longnecked Plesiosauroidea: presence of a prominent ectopterygoid flange; possession of anterior cervical centra that are shorter than high; width of the cervical zygapophyses subequal to that of the centrum; and cervical neural spines lacking posterior articulations but displaying a distinct backwards slope. Leptocleidus superstes differs from Pliosauridae sensu stricto in: possession of a hemispherical occipital condyle with distinct basal groove; absence of a posterior bulb on the squamosal arch; absence of a ‘rolled’ ectopterygoid-pterygoid flange projecting ventrolaterally from the edge of the posterior pterygoid vacuity; parasphenoid being keeled along its entire length; a small, gracile tooth form; and cervical zygapophyses with width subequal to that of the centrum. It differs from Jurassic ‘rhomaleosaurids’ ( Rhomaleosaurus spp. , Simolestes Andrews, 1909 , Macroplata Swinton, 1930 ) in the following respects: lack of a dorsomedian foramen; presence of a hemispherical occipital condyle with a basal groove; a long, slender paraoccipital process; a reduced dorsal wing on the epipterygoid; the epipterygoid dorsal process failing to contact the parietal; absence of lateral palatal fenestrations bordered by the palatine and pterygoid; a small, gracile tooth form; striated ornamentation restricted to the lingual surface of the teeth; single-headed cervical ribs; and distal end of the humerus with two discrete facets for articulation with the epipodials. It can be distinguished from other Cretaceous Leptocleidus -like taxa in several ways: from Umoonasaurus and Nichollssaura in the possession of a hemispherical occipital condyle with distinct basal groove, the presence of a ‘cockscomb-like’ pointed vertex on the midline of the skull, the absence of a posterior ‘bulb’ on the squamosal arch, the absence of squared pterygoid lappets underlying the quadrate flange of the pterygoid, and neural spines of the cervical and dorsal vertebrae not laterally compressed, it differs from L. capensis in lacking a dorsomedian foramen in the frontal region, and no distinct change in the zygapophyseal angle along the vertebral column; and finally it potentially differs from L. clemai in possessing epipodials that are longer than broad (based on aff. Leptocleidus remains: see Conclusions).

DESCRIPTION AND COMPARISONS

NHM R4828 ( Figs 1–4 ) was reportedly recovered within a single ironstone nodule ( Andrews, 1922a: 285) that was distorted by intrusive calcite seams. Remnants of this hard matrix still encase the articulated posterior cervical-dorsal vertebral series and infest cavities within the skull. A number of posterior dorsals, anterior cervicals, and a caudal vertebra have been prepared out as isolated elements, together with neural arch fragments, ribs, gastralia, and an indeterminate limb girdle fragment, possibly part of the pelvis. Andrews (1922a) concluded that no trace of the pelvic elements remained. A mass of jumbled dorsal vertebrae and ribs are preserved on a separate ironstone slab. Andrews (1922a: 285) reported that at least part of the specimen was found disarticulated; the articulated sections may have been protected from currents and/or scavengers by burial in sediment. NHM R4828 represents an individual of around 3 m snout–tail length based on comparisons with the most complete known skeletons of other Leptocleidus -like pliosauroids: Umoonasaurus (AMF 99374: Kear et al., 2006); and Nichollssaura (TMP 94.122.01: Druckenmiller & Russell, 2008a). NHM R4828 appears to have been close to osteological maturity at the time of death, as suggested by the fused basioccipitalbasisphenoid complex and well-ossified articular surfaces on the girdle elements/humeri (sensu Brown, 1981). Nevertheless, many centrum-neural arch/rib contacts remain incompletely ossified and distinct sutures are visible throughout the vertebral column.

Figures

Figure 1. Skull of Leptocleidus superstes (NHM R4828). A, dorsal view. B, right lateral view. Scale bar = 50 mm. Abbreviations: bo, basioccipital; ept, epipterygoid; fr, frontal; hyo?, possible hyoid; jug, jugal; max, maxilla; orb, orbit; pa, parietal; pif, pineal foramen; pmx-st, premaxillary suture; pof, postfrontal; qr-pt, quadrate ramus of the pterygoid; qu, quadrate; sag, sagittal crest; sq, squamosal; sqap, squamosal apex; tmpf, temporal fenestra.

Figure 2. Skull, exoccipital-opisthotic, and tooth of Leptocleidus superstes (NHM R4828). A, skull in palatal view. B, skull in posterior view. C, exoccipital-opisthotic in dorsal view. D, exoccipital-opisthotic in lateral view. E, tooth in anterior view. Scale bars = 50 mm in A, B; 30 mm in C–E. Abbreviations: bo, basioccipital; ecfl, ectopterygoid flange; jf, jugular foramen; max, maxilla; pal, palatine; par, parasphenoid; ppr, paraoccipital process; pptv, posterior pterygoid vacuity; pt, pterygoid; qr-pt, quadrate ramus of the pterygoid; qu, quadrate; sof, sub-orbital fenestra; sq, squamosal; stf, sub-temporal fenestra.

Figure 3. Axial elements of Leptocleidus superstes (NHM R4828). A, cervical centrum in articular and lateral views. B, dorsal centrum in articular and lateral views. C, articulated vertebral series in lateral view. D, dorsal rib in posterior view. E, Gastral ribs in ventral view. Scale bars = 30 mm in A, B, D, E; 100 mm in C. Abbreviations: arg, articular groove; cerv, cervical series; crib, cervical rib; dor, dorsal series; epc, excavated posterior cavity; pec, pectoral series.

Figure 4. Appendicular elements of Leptocleidus superstes (NHM R4828). A, interclavicle and clavicles in dorsal view views. B, right clavicle in ventral view. C, left clavicle in ventral view. D, right scapula in lateral view. E, right scapula in posterior view. F, left humerus in posterior view. G, left humerus in ventral view. H, right humerus in posterior view. I, right humerus in ventral view. Scale bars = 50 mm in A, D–I; 100 mm in B, C. Abbreviations: cl, clavicle; icf, interclavicle foramen; incl, interclavicle; icn, intercoracoid notch; mas, muscle attachment scar; scs, scapular shelf.