140 



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



clades of immediate relevance to Mesoleptos are listed below. The 

 character changes diagnosing these clades in the 'ordered' analysis 

 under delayed transformation optimisation are listed in Appendix 2; 

 the changes in the 'unordered' analysis are very similar, except that 

 some clades collapse (compare Figs 3 A and B ). Unequivocal changes, 

 i.e. those which occur under both delayed and accelerated 

 optimisation, are indicated with an asterisk (*). Note that most 

 characters diagnosing snakes (Ophidia) are equivocal because 

 Mesoleptos, the sister group of snakes, is poorly known and nearly 

 all the characters could apply to a more inclusive clade that also 

 contains Mesoleptos (clade C). As discussed below, the clades are 

 not strongly corroborated due to missing data and possible correla- 

 tion of the supporting characters, and are thus not yet named 

 formally. 



EVOLUTIONARY IMPLICATIONS 



The phylogenetic results imply that snakes arose from within a 

 plexus of marine varanoids, an idea suggested initially by Nopcsa 

 (1908, 1923) and later by Haas (1980). The aquatic hypothesis is 

 often ascribed to Cope (1869), but Cope never suggested that the 

 aquatic mosasaurs were ancestral to snakes: rather, he suggested that 

 both had a close common ancestor, which might even have been 

 terrestrial. However, critics subsequently misquoted Cope as sug- 

 gesting that snakes evolved directly from mosasaurs and thus had 

 marine ancestors, and then proceeded to argue that as snakes could 

 not have evolved from mosasaurs (which possess numerous 

 specialisations), they could not have had marine ancestors (e.g. 

 Owen, 1877; Dollo, 1903, 1904; Janensch, 1906; McDowell and 

 Bogert, 1954; Zaher and Rieppel, 1999). Nopcsa (1908, 1923) 

 recognised and addressed the erroneous arguments of Owen and 

 Janensch, and put forward a rigorous case for a marine stage in snake 

 ancestry. More recently, by interpreting aigialosaurs as probable 

 ancestors of snakes, McDowell and Bogert (1954) implicitly pro- 

 posed a marine ancestry. In describing the second specimen of 

 Pachyrhachis (=Ophiomorphus), Haas (1980: 191) stated that the 

 fossil 'points to the fact that the snakelike body and loss of limbs did 

 develop in a marine surrounding'. Despite this, the aquatic theory 

 has in recent times been largely rejected in favour of the 'fossorial 

 theory', i.e. that snakes evolved from small elongate burrowing 

 lizards (e.g. Janensch, 1906; Walls, 1940; Bellairs and Underwood, 

 1951; Underwood, 1967; Rieppel, 1988; Greene, 1997). Thus, few 

 modern studies rigorously surveyed marine varanoids and marine 

 ophiomorphs for possible relationships with modern snakes. 



This analysis indicates that the closest four to eight outgroups to 

 modem (terrestrial) snakes are marine; the exact number varies 

 depending on how the polytomies are resolved. The most parsimo- 

 nious interpretation is that marine or at least semi-aquatic habits 

 were primitive for pythonomorphs, and that snakes evolved in a 

 marine or semi-aquatic environment and are secondarily terrestrial 

 (Nopcsa, 1908, 1923; McDowell and Bogert, 1954; Haas, 1980). 

 In order to maintain that the snake stem lineage was always terres- 

 trial, between four and eight convergent invasions of marine 

 habitats must be assumed to have occurred in mosasauroids, 

 dolichosaur-like taxa, and basal snakes. The analysis further sug- 

 gests that, of all the marine varanoids, Mesoleptos occupies a 

 crucial phylogenetic position, as the nearest relative of snakes 

 (Ophidia). If this is true, the similarities between Mesoleptos and 

 primitive snakes are not convergent; these include such traits as a 

 proportionally small head, long body, limb reduction, and lateral 

 body compression. In these features, Mesoleptos appears inter- 



mediate between the typical lizard-like marine varanoids (e.g. mosa- 

 saurs) and primitive marine snakes. 



Two substantial caveats must be added to this interpretation. 

 Apart from mosasaurs and aigialosaurs, all the marine varanoids are 

 very imperfectly known. For instance, Mesoleptos can be scored for 

 only 13% of characters, dolichosaurs for 35% and Adriosaurus for 

 38%. Such large amounts of missing information suggest that their 

 positions cannot be very robust, a view confirmed by low bootstrap 

 and Bremer supports. This missing information also reduces support 

 throughout the tree, as the poorly known taxa can fit into many 

 different places with only slight loss in parsimony. Additionally, 

 many of the characters that unite dolichosaurs, Adriosaurus and 

 Mesoleptos with mosasauroids and snakes, to the exclusion of other 

 varanoids, are correlates of marine adaptation. Within this group 

 (Pythonomorpha), many of the characters uniting dolichosaurs, 

 Aphanizocnemus, Adriosaurus and Mesoleptos with snakes to the 

 exclusion of mosasauroids are correlates of body elongation and 

 limb reduction. Thus, the position of these poorly known taxa close 

 to snakes might reflect a false signal caused by marine adaptation 

 and body elongation, both features found in basal snakes. More 

 complete fossil finds, and thus, information on characters not obvi- 

 ously correlated with habitat and body form, are required before 

 their phylogenetic relationships can be conclusively ascertained and 

 the early evolution of snakes clearly understood. The fundamental 

 questions investigated by Bellairs and Underwood (1951) and 

 Underwood (1967) regarding the affinities and ecological origins of 

 snakes still await convincing answers. 



ACKNOWLEDGEMENTS. ML thanks Garth Underwood for assistance, 

 friendship, and inspiration during his visits to the Natural History Museum. 

 We also thank Harry Greene. David Cundall. Michael Caldwell, Jenny Clack 

 and Susan Evans for comments and/or discussion, and Eitan Tchernov 

 (Hebrew University, Jerusalem) for hospitality in Israel and allowing study 

 and description of their specimen of Mesoleptos. Supported by an Australian 

 Research Council Senior (QEII) Fellowship and Research Grant to ML. 



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