Taxon
Leptocleidus sp.
Diagnosis
Emended diagnosis
Leptocleidus can be defined as a small genus of pliosauroid possessing cranial and post cranial characters of a conservative kind, being in many ways no more derived than the Liassic (Early Jurassic) genera Eurycleidus and Rhomaleosaurus (Andrews 1922; Cruickshank 1994, 1996a; Taylor 1992). Skull triangular in outline, with a prominent midnasal ridge which merges with the parasagittal crest, flanked by deep grooves or depressions which in turn cause the orbital rims to stand up from the general profile of the skull. Compared with Rhomaleosaurus the tooth count is reduced, to 21 positions on each side of the upper jaw (5 in each premaxilla + 16 in each maxilla), compared with at least 27 in the upper jaw of R. megacephalus (Cruickshank 1994), and a similar number in R. zetlandicus (Taylor 1992; Cruickshank 1996a). No complete jaw is known for the genus, but an estimated tooth count is 35 (L. capensis), at the lower end of the known range for pliosaurids. A spatulate lower jaw symphysis with five pairs of teeth is assumed for L.capensis (Cruickshank 1997). Leptocleidus possesses a dorsomedially directed trough on the prearticular and adjacent bones of the lower jaw, similar to Rhomaleosaurus. Pectoral girdle primitive, having large clavicles and interclavicles and small scapulae (Andrews 1922); humerus has a very much more symmetrical (fan- shaped) distal end, in contrast to Rhomaleosaurus which has the humerus gently curved posteriorly (Cruickshank 1996a; in press). None of the vertebrae are compressed, cervicals being spool- shaped with the neural arches relatively large when compared with, for example, Pliosaurus brachyspondylus (Taylor and Cruickshank 1993). Cervical vertebral count in excess of 13. Leptocleidus differs from early forms in one other feature in possessing a forwardly pointing expansion ('cockscomb') on the squamosal mid-line, at the rear of the parasagital crest, very similar to that seen in Late Cretaceous Polycotylidae (Welles 1962; Thurmond 1968)
Leptocleidus can be defined as a small genus of pliosauroid possessing cranial and post cranial characters of a conservative kind, being in many ways no more derived than the Liassic (Early Jurassic) genera Eurycleidus and Rhomaleosaurus (Andrews 1922; Cruickshank 1994, 1996a; Taylor 1992). Skull triangular in outline, with a prominent midnasal ridge which merges with the parasagittal crest, flanked by deep grooves or depressions which in turn cause the orbital rims to stand up from the general profile of the skull. Compared with Rhomaleosaurus the tooth count is reduced, to 21 positions on each side of the upper jaw (5 in each premaxilla + 16 in each maxilla), compared with at least 27 in the upper jaw of R. megacephalus (Cruickshank 1994), and a similar number in R. zetlandicus (Taylor 1992; Cruickshank 1996a). No complete jaw is known for the genus, but an estimated tooth count is 35 (L. capensis), at the lower end of the known range for pliosaurids. A spatulate lower jaw symphysis with five pairs of teeth is assumed for L.capensis (Cruickshank 1997). Leptocleidus possesses a dorsomedially directed trough on the prearticular and adjacent bones of the lower jaw, similar to Rhomaleosaurus. Pectoral girdle primitive, having large clavicles and interclavicles and small scapulae (Andrews 1922); humerus has a very much more symmetrical (fan- shaped) distal end, in contrast to Rhomaleosaurus which has the humerus gently curved posteriorly (Cruickshank 1996a; in press). None of the vertebrae are compressed, cervicals being spool- shaped with the neural arches relatively large when compared with, for example, Pliosaurus brachyspondylus (Taylor and Cruickshank 1993). Cervical vertebral count in excess of 13. Leptocleidus differs from early forms in one other feature in possessing a forwardly pointing expansion ('cockscomb') on the squamosal mid-line, at the rear of the parasagital crest, very similar to that seen in Late Cretaceous Polycotylidae (Welles 1962; Thurmond 1968)
Etymology
Greek, leptos, 'slender', Greek, kleid- (kleis), 'key, clavicle': Slender clavicle.
Species