136 



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



Aigialosauridae. However, it has narrower condyles and cotyles, 

 relatively longer centra and more prominent transverse processes 

 than known aigialosaurs. This suggests it should be compared with 

 HUJ-PAL EJ699, which it resembles in size. The Pachyvaranus 

 specimens are from marine phosphate deposits, and differ from 

 HUJ-PAL EJ699 in the thick and compact ossification of the verte- 

 brae (pachyostosis). The vertebrae also differ in that the centrum of 

 Pachyvaranus is triangular, tapering rather than nearly parallel- 

 sided posteriorly, but this could be a result of pachyostosis; in other 

 pachyostotic reptiles the centra are further expanded posterolaterally, 

 and nearly rectangular. Further, the reported 'zygosphene' is only a 

 small triangular projection comparable to that of Varamis, which 

 does not bear facets for articulation with a zygantrum on the 

 preceding vertebra. Arambourg and Signeux considered possible 

 affinities with dolichosaurs (ruled out by the lack of true zygosphenal 

 articulations in Pachyvaranus) as well as aigialosaurs (noting differ- 

 ences including the narrower condyles). The lack of zygosphenes in 

 Pachyvaranus also rules out affinities with aigialosaurs, since recent 

 studies (Carroll and DeBraga, 1992) have demonstrated the pres- 

 ence of well-developed zygosphenes in aigialosaurs. However, 

 affinities with Mesoleptos were not considered. No material other 

 than trunk vertebrae (and doubtfully associated osteoderms) has 

 been described for Pachyvaranus, and conversely the vertebrae of 

 Mesoleptos are not fully known 'in the round', so that it is not yet 

 possible to make detailed comparisons. 



Pachyophiidae 



Three long-bodied, limb reduced Cretaceous marine squamates 

 have been referred to Pachyophiidae: Pachyophis woodwardi 

 Nopcsa, 1923 (Lee et al., 1999), Mesophis nopcsai Bolkay, 1925, 

 and Pachyrhachis problematicus Haas, 1979 (Haas, 1980; Lee and 

 Caldwell, 1998; Zaher and Rieppel, 1999). Haas (1979) originally 

 included Pachyrhachis in Simoliophiidae, as did McDowell (1987) 

 who also added Pachyophis; but of the two family-group names 

 proposed by Nopcsa ( 1 923), Pachyophiidae has page priority. There 

 is now agreement that pachyophiids are snakes but their exact 

 position within snakes remains debated (Zaher and Rieppel, 1999; 

 Tchernov et al. 2000; Lee and Scanlon, 2002). These three taxa are 

 extremely similar, and possess small heads, heavily pachyostotic 

 mid-body vertebrae and ribs, and distally straight ribs indicating 

 lateral compression of the trunk. Radovanovic (1935: 411) postu- 

 lated that Mesophis was a terrestrial snake in which the very slender 

 distal parts of the ribs had been straightened by pressure during 

 fossilization. However, this hypothesis is very unlikely because ribs 

 of similar shape occur consistently in otherwise undistorted speci- 

 mens of larger pachyophiids, namely Pachyophis and Pachyrhachis, 

 as well as in other marine taxa (see below). 



The specimen described here is clearly not a pachyophiid because 

 in all known pachyophiids the forelimbs and shoulder girdle are 

 completely absent, and the mid-trunk ribs are heavily swollen 

 (pachyostotic). Also, the centra are long and taper posteriorly, unlike 

 the pachyophiid condition of short centra that are of constant width 

 throughout. The transverse processes also extend much further 

 laterally than they do in pachyophiids. 



Haasiophis 



A new limbed Cretaceous marine snake, Haasiophis, has been 

 described and interpreted to have affinities with Pachyrhachis 

 (Tchernov et al., 2000) and by implication with pachyophiids as a 

 group. However, certain cranial elements were apparently 

 misidentified, and a reassessment of the morphology suggests that 

 these taxa are not closely related, but are successive outgroups to 



crown-clade snakes (Lee and Scanlon 2002). The postcranial ele- 

 ments of Haasiophis have yet to be properly described, making 

 comparisions with Mesoleptos difficult. However, Haasiophis dif- 

 fers from HUJ-PAL EJ699 in possessing heavy pachyostosis of the 

 vertebrae, many more trunk vertebrae, and in completely lacking a 

 shoulder girdle and forelimb. 



Palaeogene Marine Snakes 



HUJ-PAL EJ699 can be confidently excluded from the following 

 groups of Tertiary snakes with distally straight ribs based on pres- 

 ence of forelimbs and very different vertebrae. Archaeophis 

 (Archaeophiinae) has long, proximally curved but distally straight 

 ribs (Janensch, 1906), and the ribs of Palaeophis share this morphol- 

 ogy (Owen, 1850b). However, the neural arch is narrow and high, 

 the centrum approximately cylindrical and the transverse processes 

 relatively small (Rage, 1984). In the complete skeleton of 

 Archaeophis proavus there are over 450 trunk vertebrae and no 

 traces of limbs or girdles (Janensch, 1 906), and there is no indication 

 of their presence in other less completely known species. 

 Anomalophis (Anomalophiidae) has similar ribs (Janensch, 1906; 

 Auffenberg, 1959) and also small transverse processes. However, 

 the centra are long and gradually tapering, and the neural arches are 

 narrow and depressed, except for a backsloping neural spine. Verte- 

 brae of other early aquatic snakes (Nigerophiidae and 

 Russellophiidae; Rage, 1984, Averianov, 1997) have features re- 

 sembling the palaeophiids, acrochordids and colubroids to a varying 

 extent, but no ribs or articulated skeletons are known and their 

 relationships remain obscure. 



Thus, the specimen HUJ-PAL EJ699 can be associated most 

 closely with Mesoleptos. However, it differs from the type of M. 

 zendrinii (as described by Cornalia and Chiozza, 1852; compare 

 Tables 1 and 2) in the ribs of the anterior thoracic region being 

 considerably shorter relative to vertebral length or width: the ribs are 

 also thick in the curved middle portion of the shaft rather than 

 uniformly slender. This region of the body is not preserved in the 

 other referred (MCSNT) specimen. If confirmed, these differences 

 would indicate a considerable variation in body shape (analogous to 

 the differences among known specimens of aigialosaurids) which 

 might justify erection of a new species. However, the location and 

 condition of the type and some other important specimens are 

 currently unknown, and the putative differences cannot be directly 

 confirmed. There remains a possibility that the description and 

 figure of the type are inaccurate, as they seem questionable in a 

 number of details, and that the two specimens are identical. Thus, we 

 have refrained from any formal taxonomic decisions pending a more 

 comprehensive search for the type, and simply refer the current 

 specimen to Mesoleptos sp. indet. 



RECONSTRUCTION AND PALAEOECOLOGY 



Based on all three specimens, Mesoleptos can be reconstructed as 

 follows (Figs. 1 and 2). Depending on where one draws the cervical- 

 dorsal boundary, there are five to seven cervical vertebrae preserved 

 in HUJ-PAL EJ699, and as these do not include the atlas or axis there 

 must have been at least seven to nine cervicals, and possibly several 

 more. Seven to nine cervicals are plesiomorphic for squamates and 

 occur in most terrestrial varanoids, aigialosaurs and some mosasaurs, 

 while dolichosaurs, Eidolosaurus and some mosasaurs have increased 

 from this number (Nopcsa, 1908, 1923; Caldwell. 2000). There are 

 23 trunk vertebrae in the type and thus at least 30 to 32 presacrals 

 altogether (cf. 34 in Eidolosaurus), but not many more than this 



