XX fiS7/30,/ 



fii///. mtf. «/.s7. Mus. bond. (Zool.) 68(2): 131-142 



Issued 28 November 2002 



The Cretaceous marine squamate Mesoleptos 

 and the origin of snakes 



MICHAEL S. Y. LEE AND JOHN D. SCANLON 



Department of Palaeontology, The South Australian Museum, North Terrace, Adelaide SA 5000, Australia, e- 



mail Lee. Mike® saugov.sa.gov.au 



Department of Environmental Biology, The University of Adelaide, Adelaide SA 5005, Australia 



SYNOPSIS. The poorly known marine squamate Mesoleptos is reassessed based on two previously known specimens and a 

 newly referred specimen. The three specimens of Mesoleptos zendrinii share unique characters such as long, posteriorly tapering 

 centra and distally straight but non-pachyostotic ribs. Mesoleptos had a narrow neck (and presumably small head), long laterally 

 compressed body, and small fore- and hindlimbs. Phylogenetic analysis suggests that Mesoleptos is the nearest relative of snakes 

 this phylogenetic position is consistent with its morphology being intermediate between typical marine squamates (e.g 

 mosasauroids) and primitive marine snakes (pachyophiids). However, this interpretation remains tentative because Mesoleptos 

 is very poorly known, and many of the characters uniting it with mosasauroids and primitive snakes are correlates of marine habits 

 and/or limb reduction. 



INTRODUCTION 



Whereas sea snakes (Laticaudinae and Hydrophiinae) and marine 

 iguanas (Amblyrhyncluts) are the only truly marine squamates living 

 today, there was a more diverse and very different radiation of such 

 forms during the Cretaceous. These extinct marine squamates 

 included the large monitor-like aigialosaurs and mosasaurs, the 

 small, long-necked dolichosaurs, and the medium-sized limbed 

 snakes Pachyrhachis, Pachyophis, and Haasiophis. These forms 

 were suggested by workers in the late nineteenth and early twentieth 

 centuries to be closely related to each other and to modern snakes 

 (e.g. Cope, 1869; Boulenger, 1891; Gorjanovic-Kramberger, 1892; 

 Nopcsa, 1908, 1923), a view which has been supported by some 

 recent phylogenetic analyses (e.g. Scanlon, 1996; Caldwell 1999; 

 Lee and Scanlon 2002; but see Tchernov et al. 2000; Rieppel and 

 Zaher 2000). 



One poorly known form that has been associated with this radia- 

 tion is Mesoleptos zendrinii (Cornalia and Chiozza, 1852; 

 Gorjanovic-Kramberger, 1 892; Calligaris, 1988). M. zendrinii was a 

 marine squamate with a rather elongated body, long ribs, and well- 

 developed but rather small hindlimbs. It has been repeatedly 

 associated with other contemporary marine squamates, largely on 

 the basis of common habitat rather than any detailed analysis of 

 morphology. Cornalia and Chiozza (1852) suggested affinities with 

 i Raphiosaurus\ based on a specimen (BMNH R32268) figured 

 under this name by Owen ( 1 842) but later referred to Dolichosaurus 

 (Owen 1850a, 1851). Subsequent workers have commented on 

 errors in the original description, though a full redescription of the 

 type specimen has not appeared. Gorjanovic-Kramberger (1892) 

 referred an additional specimen to Mesoleptos cf. zendrinii, dis- 

 cussed below, and referred this genus to the Varanidae, although 

 acknowledging that it differed from other varanids in being highly 

 aquatic. Nopcsa (1903) referred it tentatively to Aigialosauridae, 

 and suggested that the moderate elongation of the trunk region 

 relative to other known aigialosaurs was analogous to the independ- 

 ent elongation of the body in some mosasaurs such as Clidastes. 

 Later, Nopcsa (1923) compared M. zendrinii with Eidolosaunts 

 trauthi, including both in a subfamily Mesoleptinae within his 

 broadly conceived Dolichosauridae (Mesoleptinae, Aigialosaurinae, 

 Dolichosaurinae). He regarded the Mesoleptinae as intermediate 



between two main lineages, one consisting of the Aigialosaurinae 

 plus their probable descendants the Mosasauridae, and the other 

 consisting of the Dolichosaurinae plus their probable nearest rela- 

 tives - though not direct descendants - the snakes. Nopcsa's ( 1 903, 

 1 923) classifications still represent the most complete discussion of 

 these forms to date, and are summarised by Calligaris (1988). 

 However, no unambiguous derived characters have been proposed 

 linking Mesoleptos with any of the other marine groups or with 

 snakes, and these interpretations need to be critically examined. 



Here, we identify a new specimen of Mesoleptos, compare it to 

 previously known specimens, and use the combined material to infer 

 the phylogenetic relationships and palaeoecology of Mesoleptos. 

 Mesoleptos emerges as on the stem lineage leading to snakes, lying 

 phylogenetically between marine lizards (mosasauroids, 

 dolichosaurs, Adriosaurus) and primitive limbed snakes 

 (Pachyrhachis. Pachyophis, Haasiophis). Garth Underwood's ear- 

 liest research interests included the origin and evolution of snakes, 

 and he has contributed to possibly the two most influential papers on 

 this topic (Bellairs and Underwood 1951; Underwood 1967). The 

 current paper is thus a small contribution to a field of inquiry that 

 Garth Underwood helped establish. 



Institutional abbreviations 



HUJ PAL, Hebrew University of Jerusalem Palaeontological Col- 

 lection; MCSNT, Museo Civico di Storia Naturale di Trieste; MNHN, 

 Musee Nationale d'Histoire Naturelle, Paris; SAM, South Austral- 

 ian Museum. 



DESCRIPTION OF NEW SPECIMEN 



Material and horizon 



The specimen consists of part and counterpart, but all morphological 

 information is preserved on the part (Fig. 1A). Anterior vertebral 

 column, ribs, shoulder girdle, and partial forelimbs. Locality: 'Ein 

 Jabrud (Ain Yabrud), 7 km north-east of Ramallah (West Bank, 

 Palestine) and 20 km north of Jerusalem. Stratigraphic horizon: Bet- 

 Meir Formation (Lower Cenomanian; earliest Upper Cretaceous). 

 Catalogued as HUJ-PAL EJ699. 



© The Natural History Museum, 2002 



