132 



M.S.Y. LEE AND J.D. SCANLON 





p.vert 



Vertebrae 



An articulated series of thirteen vertebrae (here referred to as 

 vertebrae 1-13) is preserved, along with an isolated element on the 

 lower left (vertebra 14). All vertebrae are exposed ventrally only; 

 the surfaces of vertebrae 1-7 are weathered, while that of vertebra 1 1 

 is broken. The series 1-13 represents the anterior presacral part of 

 the column. Vertebra 1 , the anteriormost, is the smallest; size then 

 increases gradually along the series such that the last is approxi- 

 mately twice the dimension of the first. The cervical-dorsal boundary 

 cannot be precisely determined because the cartilaginous sternal 

 contacts are not preserved. However, in typical lizards (anonymous 

 referee, pers. comm.), the cervical-dorsal boundary lies slightly 

 behind an abrupt increase in rib length. There is an abrupt change in 

 the size and shape of the ribs between preserved vertebrae 5 and 6 

 (see below), suggesting the cervical-dorsal boundary was slightly 

 behind this region, perhaps between vertebrae 7 and 8. Both shoulder 

 girdles, however, are preserved around the level of vertebra 5, 

 suggesting a slightly more anterior cervical-dorsal boundary. 



The centra are all elongate, the length being approximately three 

 times the width across the middle of each vertebra. They narrow 

 sharply behind the transverse processes, and then more gradually 

 posteriorly. All centra are procoelous; the anterior cotyle is deeply 

 concave and the posterior condyle strongly convex. The articular 

 surfaces of the condyles face posteriorly; part of the surface is 

 sometimes exposed in ventral view, so they were at most only 

 slightly inclined dorsally. 



Subcentral foramina are visible on the ventral surface of most 

 vertebrae: two are present on vertebrae 6 to 9, and one is present on 

 vertebrae 10 and 12. They were presumably present on the other 

 vertebrae but are not visible due to weathering and/or damage. 

 Where two foramina are present on a single vertebrae, they are never 

 bilaterally symmetrical and are often both on the same side of the 

 midline. 



A sagittal keel, extending along the posterior half of the centrum, 

 is present on vertebrae 1 to 7. The keel terminates posteriorly in a 

 prominent knob-shaped hypapophysis, which is, however, partly 

 weathered away on all except vertebrae 6 and 7. The keels and 

 (where preserved) the hypapophyses are more prominent on the 

 anteriormost vertebrae and gradually decrease in size posteriorly. 

 On vertebra 8, there is no keel. A weak hypapophysis may have been 

 present, but this cannot be confirmed due to breakage. Both the keel 

 and hypapophysis are absent from vertebrae 9 to 13, and the ventral 

 surface is completely smooth. 



A pair of transverse processes extend laterally from the anterior 

 end of each centrum. These processes extend proportionally further 

 laterally in the more posterior vertebrae: the diameter across the 

 transverse processes is slightly less than the length of the centrum in 

 the anteriormost vertebra, but slightly more in the posteriormost 

 vertebra (Table 1). Most of the tranverse processes on the anterior 

 vertebrae are weathered ventrally, but at least one is complete on 

 most of the posterior vertebrae. The articular surfaces of the proc- 

 esses are not fully exposed, but appear to have been single based on 

 the morphology of the proximal ends of the ribs. 



The isolated vertebra '14' does not fit onto either end of the 



Fig. 1 (A) Photograph of the third known individual of Mesoleptos (HUJ- 

 PAL 699). (B) Specimen drawing. The anterior end of the specimen is to 

 the top right. Unstippled areas represent areas repesent broken bone. 

 Scale bar = 2cm. Abbreviations: cor, coracoid; sea, scapula; cla, 

 clavicle; hum. humerus; ep, epiphyseal ossification; vl first 

 (anteriormost) preserved vertebra; p.vert, isolated posterior vertebra: r5. 

 rib of fifth preserved vertebra: hyp. hypapophysis. 



