MESOLEPTOS AND THE ORIGIN OF SNAKES 



137 



Fig. 2 Reconstruction of Mesoleptos in dorsal and lateral views. The head and tail are not known in any specimen and are thus conjectural. Note the 

 long neck, long laterally compressed body, and short webbed limbs. Scale bar = 10cm. 



unless the neck was unusually long. Short, curved ribs are present on 

 most of the cervical vertebrae, implying a narrow cylindrical neck, 

 which is similar to conditions in Eidolosaanis and some dolichosaurs, 

 rather than aigialosaurs which have longer ribs on most of the 

 cervicals. The cervical-thoracic boundary presumably lies around, 

 or immediately posterior to, the sharp increase in rib length. 



The anterior thoracic ribs are straight distally, implying a lateral 

 flattening of the trunk region. Allowing for apparent variation 

 between the three known specimens in the proportional length of the 

 ribs, the ribs remain long throughout the mid-trunk region, where the 

 largest vertebrae occur. Vertebral and rib dimensions increase stead- 

 ily up to at least the tenth thoracic vertebra, are highest in mid-trunk 

 and decrease, apparently more slowly, in the last ten or so presacrals. 

 These size gradients are stronger than seen in measured skeletons of 

 Varanus and Heloderma (Scanlon, unpublished data), and far more 

 conspicuous than in any other marine varanoids described. Unlike 

 some aigialosaurs and all mosasaurs, there is not a long series of 

 shortened posterior dorsal ribs. Rather, long, distally straight ribs 

 continue at least to within the last five presacral vertebrae (as 

 indicated by the MCSNT specimen; the most posterior ribs have 

 been damaged or lost in both this and the type). 



The cervical vertebrae bear prominent ventral keels and hypa- 

 pophyses, which are reduced on the first two thoracics and then 

 disappear. The centra of the following thoracic vertebrae are smooth 

 ventrally, but posterior trunk vertebrae apparently have laterally 

 paired keels defining a median trough, a feature that also occurs in 

 dolichosaurs. Eidolosaurus and some aigialosaurs (but often com- 

 mencing more anteriorly in the trunk). The sacral vertebrae (in the 

 MCSNT specimen at least) are shorter than the immediately preced- 

 ing trunk vertebrae, and are fused (or at least very tightly articulated) 

 together. Parts of the first few caudal vertebrae are present in the 

 type, indicating a laterally compressed tail with elongate but antero- 

 posteriorly narrow neural spines and chevrons. 



The trends in vertebral size (length and width) and rib length 

 indicate an animal with a relatively small head and narrow neck in 

 relation to its body, similar to dolichosaurs and Eidolosaurus. The 

 curved cervical ribs indicate that the cervical region of the animal 

 was approximately round in cross-section. However, the distally 

 straight dorsal ribs indicate that the trunk region of the animal was 

 laterally compressed and very deep. These long ribs projected only 

 a short distance laterally from the vertebrae before curving to extend 

 downward (and obliquely backward) for most of their length. The 

 girdles and limbs were rather small, although most elements were 

 probably present; compared to adjacent vertebrae, both the femur 



and humerus are relatively shorter than in aigialosaurs, but the 

 forelimb was not as reduced as in dolichosaurs or Eidolosaurus (Fig. 

 2). 



In comparison with the similar-sized aigialosaurs, Pontosaurus 

 and species of Varanus. trends in vertebral size within the column of 

 Mesoleptos are somewhat different. There is a local minimum of 

 centrum length in the posterior cervical region, but the elongation of 

 the anterior cervicals is much less pronounced than the condition in 

 most Varanus spp. (a derived condition within that genus). Gradi- 

 ents of vertebral length and width within the thoracic and dorsal 

 region are stronger than in any of the other taxa. The centrum is 

 narrower posteriorly than in aigialosaurs, Pontosaurus and Varanus, 

 indicating a condyle-cotyle joint of smaller diameter and surface 

 area. This in turn suggests weaker compressive forces within the 

 column, along with a less energetic style of locomotion and/or a 

 greater capacity for lateral flexion of the neck and trunk. On the 

 other hand, the combination of long transverse processes and long 

 narrow centra increases both leverage and space for muscles con- 

 necting successive transverse processes, such as the m. 

 interarticularis (cf. Mosauer, 1935; Gasc, 1974). These could then 

 be of increased importance in lateral undulation, perhaps taking over 

 in this role from longer muscles inserting on the ribs whose effec- 

 tiveness would be decreased by lateral compression of the trunk. If 

 the above interpretation of the affinities of the Novak specimen is 

 correct, the derived vertebral morphology evolved before the lateral 

 compression, so that this 'takeover' could happen via an intermedi- 

 ate where both sets of muscles were effective. Zygosphenes, 

 considered to be of biomechanical importance in limiting twisting 

 between adjacent vertebrae (Gasc, 1974), are well-developed (ex- 

 posed dorsally in the MCNST specimen) and articulate with zygantra 

 in the preceding neural arches as in other aquatic varanoids and all 

 snakes. 



Among living squamates, the only forms with distally straight 

 ribs (and thus laterally compressed bodies) are highly aquatic 

 caenophidian snakes, such as file snakes (acrochordids) and sea 

 snakes (laticaudine and hydrophiine elapids). This feature has rarely 

 been discussed in extant snakes; Hoffstetter and Gayrard (1965) do 

 not comment on any unusual features of the ribs in Acrochordus or 

 Enhydrina (Hydrophiinae), though it was described in 'Enhydris' 

 (=Lapemis) hardwickii (Hydrophiinae) by Janensch (1906: 22). In 

 Acrochordus arafurae (SAM R26956, R26966) the anterior ribs are 

 robust and strongly curved, while those of the posterior half of the 

 body are much more slender and only weakly curved except near the 

 base. The pachyophiids - primitive marine snakes - also had a 



