MESOLEPTOS AND THE ORIGIN OF SNAKES 



133 



Table 1. Measurements of'HUJ-PAL EJ699: midline length between rims 

 of cotyle and condyle; width across transverse processes; straight-line 

 length of rib. The vertebrae are numbered from the first preserved 

 centrum. 



Vertebra no. 



Centrum length 



Greatest width 



Rib length 



1 



22 



16 



_ 



2 



20.5 



19 



- 



3 



22 



20 



- 



4 



- 



- 



19.5 



5 



19 



20 



20 



6 



20 



20.5 



41 



7 



21 



20 



49 



8 



(16+) 



23 



49 



9 



22 



23 



59 



10 



23.5 



24 



73 



II 



(23+) 



(24+) 



89 



12 



25 



31 



(92+) 



13 



26 



39 



121 



- 



- 



- 



132 



- 



- 



- 



139 



14 



23 



34.5 



- 



articulated series. It is too large to fit next to vertebra 1, and 

 furthermore could not be a cervical as it lacks the mid-ventral keel 

 and hypapophysis. However, it is too small to fit next to vertebra 1 3. 

 As in most squamates, after reaching maximum size (at or past 

 vertebra 13), the centra must have again gradually decreased in size 

 towards the posterior end of the dorsal region. The isolated vertebra 

 appears to belong to this region. Its surface is worn in a manner that 

 suggests there were laterally paired ventral mounds or processes 

 defining a median longitudinal trough on the posterior part of the 

 centrum. 



Ribs 



Ribs are preserved in association with vertebrae 4 to 1 3. Only the left 

 rib (right in ventral view) of vertebra 4 is preserved. Both ribs are 

 preserved in association with vertebrae 5 to 8. Only the left ribs are 

 associated with vertebrae 9 to 11. Both ribs are associated with 

 vertebrae 12 and 13, but the right ribs are displaced so that they 

 overlie the left ribs and point anteriorly. Three additional ribs 

 belonging to the next three (missing) dorsal vertebrae are also 

 preserved; these are presumably right ribs based on their similar 

 orientation to the right ribs of the last two preserved vertebrae. 



The anteriormost preserved rib is associated with the 4th vertebra. 

 It is short (only as long as the centrum) and smoothly curved. The 

 shaft is oval in cross-section and uniformly thick throughout its 

 length. Slightly longer ribs, of similar shape, are associated with the 

 5th vertebra. The next pair of ribs, associated with the 6th vertebra, 

 are much longer and quite different in shape. The distal end of the 

 left rib (right in ventral view) is weathered away; the right rib is 

 complete and its proximal half is smoothly curved, but the distal half 

 is nearly straight. The more posterior ribs are similar in shape, 

 except that the curved proximal portion occupies progressively less 

 and less of the shaft. By vertebra 13, the curved portion only 

 occupies the proximal one-fifth of the shaft. 



The articular surfaces are visible on the left ribs associated with 

 vertebrae 7, 8, 10 and 13, and on the second of the three isolated ribs. 

 The ribs are all single-headed. The anterior ribs are flared at the 

 proximal end and then nearly uniformly thick thoughout their 

 length, while more posterior ones have a distinct neck proximally 

 before becoming thickened in the region of greatest curvature, then 

 gradually tapering distally in the straight part of the shaft. The distal 

 ends are truncated squarely where they joined the costal cartilages. 



which are not preserved. 



Approximate measurements of the vertebrae and ribs (Table 1) 

 show a more or less steady increase in dimensions from vertebra 1 to 

 1 3, continued in the ribs belonging to the next two missing vertebrae 

 (both ends of the last known rib are incomplete or obscured and its 

 length is therefore not measurable). As noted above, the posteriormost 

 preserved ribs cannot belong to vertebra 14, which is from the 

 posterior trunk (abdominal) region. 



Shoulder girdle and forelimb 



Both scapulocoracoids are preserved in medial view. The right is 

 complete except for the dorsal scapular blade, while the left is partly 

 covered by a rib and is missing the distal (anterior) end of the 

 procoracoid process. A curved strip of bone adjacent to the left 

 scapulocoracoid is probably the left clavicle. The left humerus is 

 preserved in proximal dorsal view. All appendicular elements are 

 very small in proportion to the axial elements. 



The scapula is a simple, rectangular plate; the scapular blade is 

 short. Its anterior margin is weakly concave; a scapulocoracoid 

 emargination was thus present. The coracoid is single and bears two 

 processes, and two emarginations. The more dorsal process is much 

 longer and extends anterodorsally. forming the ventral margin of the 

 scapulocoracoid foramen and the dorsal margin of the coracoid 

 emargination that represents the anterior coracoid foramen. The 

 ventral process is shorter and expanded distally. It forms the ventral 

 border of the anterior coracoid foramen and the dorsal border of the 

 emargination representing the posterior coracoid foramen. The ven- 

 tral margin of the coracoid is smoothly convex, and the posterior 

 margin is drawn out into a posteroventral spur. The probable clavicle 

 is a tiny curved rod, tapered at each end. There is no ventromedial 

 expansion or foramen. The humerus is relatively large compared to 

 the shoulder elements, though still small compared to the axial 

 elements. The proximal end is expanded and flattened. The entire 

 articular surface is occupied by a large, semilunar epiphysis which 

 caps the humerus. The distal end of the humerus is weathered. 



COMPARISONS WITH SIMILAR TAXA 



The specimen is clearly a squamate, as it possesses all the 

 synapomorphies of squamates (Estes etal., 1988) for which it can be 

 coded: single-headed ribs, cervical vertebrae with hypapophyses, 

 procoelous vertebrae, presence of anterior coracoid emargination. 

 Admittedly, these are relatively few because the specimen is very 

 incomplete, but still sufficient to make a firm identification. Among 

 squamates, it is clearly different from most groups in possessing 

 distally straight ribs. The only taxa that possess such ribs are 

 Mesoleptos, Adriosaiirus,Acteosaurus, and various groups of aquatic 

 snakes. The specimen here is compared to these forms, and to some 

 other superficially similar taxa to which it might be related. 



Mesoleptos zendrinii 



HUJ-PAL EJ699 is extremely similar to Mesoleptos, which is 

 known from two specimens. The type of Mesoloptos zendrinii, from 

 the Upper Cretaceous of Comen, Slovenia, is an articulated series of 

 dorsal, sacral and anterior caudal vertebrae with ribs and a partial 

 hindlimb. The specimen has been illustrated as a lithographic plate 

 (Cornalia and Chiozza, 1852: pi. 3) and an interpretive line drawing 

 (Calligaris, 1988: fig. 2). Cornalia (in Cornalia and Chiozza, 1852) 

 considered the specimen to be exposed in dorsal view, while 

 Gorjanovic-Kramberger ( 1 892) maintained it was exposed ventrally, 



