Caldwell, 1999a

Zoological Journal of the Linnean Society (1999), 125: 115–147. With 7 figures Article ID: zjls 1997.0144, available online at http://www.idealibrary.com on

Squamate phylogeny and the relationships of snakes and mosasauroids MICHAEL W. CALDWELL∗ Department of Geology, The Field Museum, Roosevelt Road at Lakeshore Drive, Chicago, IL 60605, U.S.A. Received March 1996; accepted for publication July 1997

Cladistic analysis of extant and fossil squamates (95 characters, 26 taxa) finds the fossil squamate, Coniasaurus Owen, 1850, to be the sister-group of the Mosasauroidea (mosasaurs and aigialosaurs). This clade is supported in all 18 shortest cladograms (464 steps; CI 0.677; HI 0.772) by nine characters of the dermatocranium, maxilla, and mandible. A Strict Consensus Tree of the 18 shortest trees collapses to a basal polytomy for most major squamate clades including the clade (Coniasaurus, Mosasauroidea). A Majority Rule Consensus Tree shows that, in 12 of 18 shortest cladograms, the clade Coniasaurus– Mosasauroidea is the sister-group to snakes (Scolecophidia (Alethinophidia, Dinilysia); this entire clade, referred to as the Pythonomorpha ([[Scolecophidia [Alethinophidia, Dinilysia]], [Coniasaurus, Mosasauroidea]]) is the sister-group to all other scleroglossans. Pythonomorpha is supported in these 12 cladograms by nine characters related to the lower jaw and cranial kinesis. In 6 of 18 shortest cladograms, snakes are the sister-group to the clade (Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae))). None of the cladograms support the hypothesis that coniasaurs and mosasauroids are derived varanoid anguimorphs.

Two additional analyses were conducted: (1) manipulation and movement of problematic squamate clades while constraining ‘accepted’ relationships; (2) additional cladistic analyses beginning with extant taxa, and sequentially adding fossil taxa. From Test I, at 467 steps, Pythonomorpha can be the sister-group to the Anguimorpha, Scincomorpha, ‘scinco-gekkonomorpha’ [scincomorphs, gekkotans, and amphibaenids-dibamids]. At 471 steps Pythonomorpha can be placed within Varanoidea. Treating only mosasauroids and coniasaurs as a monophyletic group: 469 steps, mosasauroids and coniasaurs as sistergroup to Anguimorpha; 479 steps, mosasauroids and coniasaurs nested within Varanoidea.

Test II finds snakes to nest within Anguimorpha in a data set of only Mosasauroidea+Extant Squamates; the sistergroup to snakes+anugimorphs is (Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae))). No one particular taxon is identified as a keystone taxon in this analysis, though it appears true that fossil taxa significantly alter the structure of squamate phylogenetic trees.  1999 The Linnean Society of London

ADDITIONAL KEY WORDS:—coniasaurs – mosasauroids – phylogeny – snakes – squamates.

∗ Present address: Paleobiology, Research Division, Canadian Museum of Nature, P.O. Box 3443, Stn. D, Ottawa, Ontario, Canada, K1P 6P4. Email: mcaldwell@mus-nature.ca 0024–4082/99/010115+33 $30.00/0

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 1999 The Linnean Society of London

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M. W. CALDWELL CONTENTS

Introduction . . . . . . . . . . . . . . . .

Methods . . . . . . . . . . . . . . . . .

Matrix structure and analysis . . . . . . . . .

Characters and character states . . . . . . . .

Extant taxa . . . . . . . . . . . . . .

Fossil taxa . . . . . . . . . . . . . . .

Polarity, rooting and outgroups . . . . . . . .

Polymorphic characters . . . . . . . . . .

Phylogenetic analysis . . . . . . . . . . . . .

Results . . . . . . . . . . . . . . . .

Relationships of Coniasaurus, mosasauroids and Serpentes Relationships of Coniasaurus . . . . . . . . .

(Serpentes (Mosasauroidea, Coniasaurus)) . . . . .

Global squamate relationships . . . . . . . . . .

Mosasauroids+Coniasaurs as anguimorphs: additional tests Test number I . . . . . . . . . . . . .

Test number II: sequential addition of fossil taxa . .

Inter-subjective consensus . . . . . . . . . .

Conclusions . . . . . . . . . . . . . . . .

Snakes, Mosasauroidea, Coniasaurus; Pythonomorpha .

Squamata . . . . . . . . . . . . . . .

Snake origins . . . . . . . . . . . . . .

Acknowledgements . . . . . . . . . . . . .

References . . . . . . . . . . . . . . . .

Appendix 1 . . . . . . . . . . . . . . . .

Appendix 2 . . . . . . . . . . . . . . . .

Appendix 3 . . . . . . . . . . . . . . . .

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INTRODUCTION

Recent cladistic analysis of morphological characters from mosasaurs, aigialosaurs, and fossil and extant varanoids (Caldwell, Carroll, & Kaiser, 1995), found that the accepted phylogenetic scheme finding mosasaurs to be derived varanoids, and more specifically, the sister-taxon to varanids (McDowell & Bogert, 1954; Carroll & deBraga, 1992; deBraga & Carroll, 1993), was potentially inaccurate. Among varanoids, Varanus has usually been identified as the sister-group of mosasauroids (Carroll & deBraga, 1992; deBraga & Carroll, 1993), though Lee (1997) suggested that a mosasauroid–snake clade might be the sister-taxon of a varanoid clade composed of varanids, lanthanotids, and related fossil forms.

The retracted narial opening and elongate head, common to both mosasauroids and varanids, but absent or less exaggerated in Heloderma and Lanthanotus, have traditionally been important characters supporting ‘ancestor-descendant’ relations between mosasauroids and varanid lizards. The possible polyphyly of a mosasauroidvaranid grouping, identified by Caldwell et al. (1995), was based on the re-characterization of the structure of the bony nares and palate of mosasauroids. Caldwell et al. (1995) argued that the bony elements of the elongate snout and retracted nares were not similar in form or topological relation between varanids and mosasauroids, and when tested for congruence in a cladistic analysis of all characters, failed that test of congruence (Patterson, 1982). Synapomorphies were not found, homology was not inferred, and monophyly of a taxon describing mosasauroids in some close relationship to varanids was not supported.

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Based on these findings, a research program designed to investigate ingroup and outgroup relations of mosasauroids was initiated. The first step was consideration of the composition of Mosasauroidea. Previous schemes have included the Aigialosauridae and Mosasauridae. DeBraga & Carroll (1993) consider the taxon monophyletic despite the fact that they did not test aigialosaur monophyly in the context of all Mosasauroidea. In contrast, Bell (1993) concluded that the Aigialosauridae was likely paraphyletic. Caldwell (1996) tested Bell’s conclusions using data from various aigialosaurs and mosasaurs and could find no support for aigialosaurian monophyly within Mosasauroidea (in a strict consensus of all trees, aigialosaurian taxa collapsed to a polytomous node nested within the clade Mosasauridae). Paraphyly of Aigialosauridae was further supported by reference to a majority-rule consensus tree (see fig. 2B, Caldwell [1996]). The consensus derived from these studies is that aigialosaurs are paraphyletic.

The search for the monophyletic squamate group that includes mosasauroids, and to resolve mosasauroid ingroup relationships, has since led to the re-examination of other putative fossil varanoids that have been considered ‘related’ to mosasaurs, i.e. dolichosaurs, coniasaurs (Caldwell & Cooper, 1998), and other aigialosaurs. I present the results of a cladistic analysis of fossil and extant squamates and discuss them in terms of the sister-group relationships of coniasaurs, mosasauroids, and snakes. The results presented here are preliminary in the sense that the data set is continually being expanded by the addition of new data. This data is derived from the ongoing descriptions of new and poorly known coniasaur-like squamates.

METHODS

Matrix structure and analysis A data matrix composed of 25 taxa of fossil and extant squamate reptiles, and 95 characters (Appendices 1, 2), was analysed using Heuristic algorithms in the software program PAUP Version 3.1.1 for the Macintosh (Swofford, 1993). Specific Heuristic Search Options were: Random Addition Sequence (100 Replications); Tree-Bisection-Reconnection (branch swapping); and Steepest Descent Off. It is important to recognize that Heuristic Searches do not perform maximum parsimony analyses and that the ‘shortest’ trees produced by any one analysis are the most locally Optimal Trees; it also is not known if these resultant cladograms are the Global Optimum or Local Optimum type (Swofford, 1993). Therefore, cladograms discussed and consensus trees presented will not be referred to as Most Parsimonious Trees (MPTs), or the product of MPTs, but rather as ‘shortest trees’ found.

Multistate characters were unordered and character distributions optimized by ACCTRAN. ACCTRAN optimizes characters by accelerating character transformations. The effect is that changes appear lower in the tree, thereby identifying synapomorphies (unequivocal and equivocal characters) for more inclusive clades, rather than being optimized as apomorphies of more exclusive clades or even terminal taxa (DELTRAN).

Characters and character states Eighty-nine of the 95 characters described (Appendix 1) and coded in the data matrix (Appendix 2) were derived from Estes et al. (1988); in early stages of this

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study, the 89 characters were coded as given by Estes et al. During my own examination of squamate osteological materials (see below, and Appendix 3), six new characters were created, and more than half of the state codings for the 89 characters of Estes et al. (1988) were modified.

The character set created by Estes et al. (1988) described 130 osteological and 18 soft tissue characters for 19 extant squamate taxa. Soft tissue characters were excluded from this study because of their absence in fossil material, the small number included in the data set of Estes et al. (1988), and the colossal size of the task required to rescore old and new characters for so many extant taxa. Another 41 of the characters described by Estes et al. (1988) either were not scored or were subsumed in the character descriptions of the 89 characters I included in my own data matrix (Appendices 1, 2). These excluded characters were judged redundant, clearly dependent on other characters, or did not characterize the intended homologue as I interpreted it.

Extant taxa The squamate taxa included in this study as terminal taxa are derived in part from Estes et al. (1988). Changes and additions were made to the composition of the ingroup by the inclusion and distinction of Shinisaurus, Xenosaurus, Eublepharidae, Gekkonoidea, alethinophidian and scolecophidian snakes, and ‘other iguanids’ [see below].

Sub-division of the Xenosauridae is based on the recognition of poor support for xenosaurid monophyly (Rieppel, 1980a; Caldwell, pers. observ.). Character states for Shinisaurus were taken from Hecht & Costelli (1969), Costelli & Hecht (1971), Rieppel (1980a) and from observation of skeletonized materials (Caldwell, pers. observ.).

Division of the Gekkota into Eublepharidae and Gekkonoidea is derived from Kluge (1987); states for eublepharids and other gekkonoids are from Kluge (1967, 1987). Character states of the Pygopodidae (Kluge, 1976) are included in the state codings for Gekkonoidea.

Division of Serpentes into Alethinophidia and Scolecophidia is based on Rieppel (1988) and Kluge (1991), and was done in order to avoid creating a paraphyletic Serpentes by the subsequent inclusion of the fossil taxon Dinilysia (see below).

Iguanian codings were taken from Etheridge & deQueiroz (1988), Estes et al.

(1988), and Frost & Etheridge (1989). The taxon ‘other iguanids’ contains Frost & Etheridge’s (1989) families Corytophanidae, Crotaphytidae, Hoplocercidae, Iguanidae, Opluridae, Phrynosomatidae, Polychridae and Tropiduridae. These taxa were excluded as independent terminal units due to the required memory increase to analyse more than 34 taxa (as per Swofford [1993]), and because the emphasis in this analysis is on relationships in squamate clades other than Iguania sensu Frost & Etheridge (1989).

Fossil taxa The phylogenetic relationships of mosasauroids, dolichosaurs and coniasaurs are a central theme of the investigation presented here. The hypothesis that some or

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all of these taxa might be derived varanoid lizards, and that the characters of fossil varanoids might be essential to place mosasauroids, etc., within Varanoidea, prompted the inclusion of the Cretaceous fossil varanoid Estesia mongoliensis; character states for Estesia are from Norell et al. (1992).

Other fossil taxa include Dinilysia patagonica (Estes, Frazetta & Williams, 1970), which was included in the analysis to test characters from a primitive fossil snake relative to mosasauroids, dolichosaurs, coniasaurs and living snakes (alethinophidians and scolecophidians). States for the Cretaceous fossil snake Dinilysia patagonica are from Estes et al. (1970), Hecht (1982), and Rage & Albino (1989).

Characters and character states for the Mosasauridae are derived from examination of clidastine, platecarpine and halisaurine mosasaurs (Bell, 1993; Caldwell, 1996), and from various aigialosaur taxa (Carroll & deBraga, 1992; deBraga & Carroll, 1993; Caldwell et al., 1995). The probable paraphyly of Aigialosauridae is recognized here (Bell, 1993), but for the purposes of this analysis the terms aigialosaur(s)/ aigialosaurid(s) will be used in reference to mosasauroids that still retain limbs that are not specialized as paddle-like appendages (see Caldwell, 1996). States for Coniasaurus were obtained from a new and undescribed species of Coniasaurus (manuscript submitted), and from a redescription of the type species, Coniasaurus crassidens (Caldwell & Cooper, 1998).

Polarity, rooting and outgroups The extant lepidosaur Sphenodon was used as the outgroup in this analysis for rooting the tree and establishing character polarities. In comparison, Estes et al.

(1988) determined polarity using an outgroup code (the Outgroup Comparison Method of Maddison, Donoughue & Maddison [1984]) derived from eight taxa (Sphenodon, Rhyncocephalia, Archosauromorpha, Younginiformes, Palaeagama, Saurosternon, Paliguana and Kuehneosaurus). The rationale for limiting the number of outgroup taxa to only Sphenodon is based on a series of analyses using three different outgroups or outgroup codes.

Three tests of polarity and rooting were constructed to determine the most viable and informative outgroup:(1) using the outgroup code of Estes et al. (1988); (2) using all eight taxa as independent terminals in a global analysis of the ingroup and outgroup taxa; (3) using only Sphenodon. The intention of these three tests of outgroup structure was to determine the necessity and importance of using more than one outgroup taxon (see Nixon & Carpenter, 1993) In all three tests the ingroup tree topologies were identical. Using all eight taxa as independent terminals simply produced a large number of trees, of greater length than the trees found using only Sphenodon (the relationship of outgroup taxa to themselves and to Squamata was not resolved). The only uncertainty introduced by using only Sphenodon, was that some character polarities could not be determined because the character was missing or unidentified in that taxon.

In conclusion, using only Sphenodon produced the same ingroup topologies as did the use of all eight taxa, and the use of the Estes et al. (1988) outgroup code.

Exclusive use of Sphenodon produced ‘better’ polarity determination for almost all state changes because Sphenodon is an extant taxon and is therefore more readily observed in all its detail. Exclusive use of Sphenodon also reduced the number of shortest trees without altering the topology of ingroup relations.

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M. W. CALDWELL Agamidae Chamaeleonidae Other Iguanians Anguidae Shinisaurus Xenosaurus Heloderma Lanthanotus Estesia Varanus Cordylidae Scincidae Gymnophthalmidae Teiidae Lacertidae Xantusiidae Dibamidae Gekkonoidea Eublepharinae Amphisbaenia Alethinophidia Dinilysia Scolecophidia Mosasauroidea Coniasaurus Sphenodon

B

Agamidae Chamaeleonidae Other Iguanians Anguidae 100 Shinisaurus 100 Xenosaurus Heloderma 100 100 100 Lanthanotus 100 Estesia 100 Varanus Cordylidae Scincidae 100 Gymnophthalmidae 100 67 100 Teiidae 100 100 Lacertidae Xantusiidae 83 Dibamidae 50 100 Gekkonoidea 67 Eublepharinae Amphisbaenia Alethinophidia 100 100 Dinilysia Scolecophidia 100 Mosasauroidea Coniasaurus Sphenodon 33

67 100

Figure 1. Consensus trees of 18 cladograms (464 steps) showing ingroup relationships of 21 extant and fossil squamate taxa using morphological data (95 osteological characters). A, Strict Consensus Tree.

B, Majority Rule Consensus Tree.

Polymorphic characters At higher taxonomic levels of investigation, in any diverse group, large numbers of polymorphic states are observed in terminal taxa. However difficult to study, polymorphisms are data, and are therefore included in this analysis. Admittedly, many polymorphisms may exist that have not been coded in this matrix; in most cases, this is omission through absence of information rather than exclusion by choice.

However, some polymorphisms were excluded by choice. Where a polymorphism was noted in only one individual, or one species of a diverse family-level terminal taxon, it was not coded. For such states it was assumed that the polymorphism is ‘autapomorphic’ to that individual or species, and not informative in the sense of determining cladistic branching order.

PHYLOGENETIC ANALYSIS

Results Cladistic analysis of the data matrix (Appendix 2) found 18 shortest cladograms (464 steps) with a Consistency Index (CI) of 0.677, a Homoplasy Index (HI) of 0.772, and a Retention Index (RI) of 0.604. The Strict Consensus Tree (Fig. 1A)

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shows Coniasaurus in the sister-group position to the Mosasauridea (Mosasauridae and the paraphyletic aigialosaurs). The topology supports previous hypotheses of relationship for most squamate crown groups (Estes et al., 1988), but provides no resolution on more problematic relationships at internal nodes. Two major groups of squamates are identified: ((Other Iguanians, Chamaeleonidae, Agamidae) + (all other taxa)). Relationships between non-iguanian squamates collapse to a single internal node that defines the Scleroglossa. However, a number of clades are resolved even though the sister-group relationships of those clades at internal nodes are not (Fig. 1A).

The Majority Rule Consensus Tree (Fig. 1B) shows that in 67% of the cladograms (12 of 18) Coniasaurus and Mosasauroidea are the sister-group to ophidians (Scolecophidia, (Dinilysia, Alethinophidia)); this clade lies outside Scleroglossa (all other squamates excluding iguanians). Coniasaurus and the taxon Mosasauroidea are never nested within Varanoidea. The Majority Rule Consensus also shows at least 67% support for the relationships shown in (Fig. 1B).

In six of 18 cladograms a monophyletic Serpentes is found to be the sister-group to an incompletely resolved clade composed of Dibamidae, Amphisbaenia, and gekkotans (in all six cladograms gekkotan monophyly is maintained). The uncertainty with the Dibamidae-Amphisbaenia–Gekkotan clade results from the instability of dibamids and amphisbaenids, and whether or not these two taxa form a clade, or are successive sister-groups along the branch leading to gekkotans. In all six of these cladograms the basal scleroglossan position of the Mosasuroidea+Coniasaurus clade is maintained.

In 15 of 18 cladograms, the clade containing dibamids, amphisbaenids and gekkotans is found to be monophyletic. In 9 of 18 cladograms the relationships of this group are resolved as follows: ((Amphisbaenia) + (Dibamidae (Gekkonoidea, Eublepharidae))). The clade (Amphisbaenia (Dibamidae (Gekkonidae, Eublepharidae))) is the sister to the conventional Scincomorpha (Xantusiidae (Lacertidae (Teiidae, Gymnophthalmidae))) + (Cordylidae, Scincidae)))).

The conventional structure of the clade Anguimorpha, i.e. Anguoidea and Varanoidea, is found in all cladograms and consensus trees (Fig. 1A, B). However, the relationships of several crown-group taxa differ compared to conventional phylogenies. The Xenosauridae of McDowell & Bogert (1954) and Estes et al. (1988) is found to be paraphyletic: Shinisaurus and Xenosaurus are successive sister-taxa to Anguidae. Within Varanoidea, Varanus is found as the sister-taxon to all other varanoids. Estesia is the sister to Lanthanotus and Heloderma. The clade (Estesia (Lanthanotus, Heloderma))) is informally referred to here as the ‘estesioids’.

In only six of 18 trees are the iguanian crown group taxa conventionally resolved (Other Iguanians (Chamaeleonidae, Agamidae)).

RELATIONSHIPS OF CONIASAURUS, MOSASAUROIDS AND SERPENTES

The following discussion examines the relationships of Serpentes as reconstructed in 12 of the 18 shortest cladograms (Fig. 1B). Character state distributions are taken from the cladogram with a topology identical to that of the Majority Rule Consensus Tree (Fig. 1B). The characters and characters states as optimized on the selected tree serve as the template for the discussion of characters and synapomorphies diagnosing clades presented in the remainder of this paper.

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Of the possible relationships of snakes, mosasauroids and Coniasaurus, only two variations are found among 18 shortest cladograms: (1) (Serpentes (Mosasauroidea, Coniasaurus)) as the sister-group to all other scleroglossans (Fig. 1B); (2) Serpentes as the sister-group to a clade composed of amphisbaenids, dibamids and gekkotans, with (Mosasauroidea, Coniasaurus) as the sister-group to all scleroglossans; this second consensus topology (snake-amphisbaenid relationships) was found in six of 18 cladograms.

Because the relationship and characters of dibamids, amphisbaenids and snakes have been examined recently (Wu, Brinkman & Russell, 1996), the phylogenetic hypothesis as reconstructed in Figure 1B is explored more fully here. This is not to suggest that a snake–amphisbaenid–dibamid clade is poorly supported, but rather that the hypothesis of a snake-mosasuroid clade has never been so well supported (12 of 18 cladograms, and a large number of characters). Therefore, a detailed examination of this hypothesis is justified by the paucity of such investigations.

Relationships of Coniasaurus The clade (Mosasauroidea, Coniasaurus) is supported by nine characters. This finding is in agreement with earlier results presented by Polcyn and Bell (1994) and with untested suggestions made by Caldwell & Cooper (in press). Six characters are optimized unequivocally: 4[0–1], frontals fused; 9[0–1], frontal tabs project over parietal; 17[0–1], parietal tabs are present; 20[0–1], maxilla does not extend posteriorly below orbit; 47[0–1], subdivision of intramandibular septum of Meckel’s canal occurs near posterior end of tooth row with well developed septum; 55[0–1], elongate anterior extension of coronoid. Three characters are optimized equivocally: 2[0–1], no contact between nasals and prefrontals; 7[1–0], weakly developed descending process of frontals, prefrontal participates widely in orbitonasal fenestra; 44[0–1], Vidian canal posterior opening at basisphenoid prootic suture.

It is significant that a number of characters shared by mosasauroids and coniasaurs (see Caldwell & Cooper, in press) were not included in the character descriptions or data matrix (Appendices 1, 2). This omission was intentional. The goal was to avoid biasing the result of a global analysis of squamates to insure that the ‘expected’ result would be obtained. Previous authors (Owen, 1850; Nopcsa, 1923; McDowell & Bogert, 1954; Bell et al., 1982) had provided such a range of possible options for the relationships of Coniasaurus that it was clear that a more global test needed to be procedurally sound.

(Serpentes (Mosasauroidea, Coniasaurus)) The clade (Serpentes (Mosasauroidea, Coniasaurus)) is supported by nine characters: five unequivocal, four equivocal. Most of these characters reflect on specializations of the lower jaw and postcranial skeleton, i.e. feeding and locomotory specializations.

Because the debate on the phylogenetic relationships of mosasaurs and snakes as independent lineages, and as a possible clade (Cope, 1869), has been so extensive and long-lived, the characters synapomorphic for (Serpentes (Mosasauroidea, Coniasaurus)) are discussed in detail.

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Character 33[0–1], long posterior processes of the septomaxilla. This character is difficult to code in many taxa due to a lack of disarticulated skulls. However, in mosasaurs, where this character can be observed in Plotosaurus (Camp, 1942), the medial flanges of the septomaxillae extend posteriorly to contact, or come very close to contacting the medial wall of the prefrontal. In Coniasaurus this feature cannot be scored. The septomaxilla of mosasaurs is autapomorphic as compared to other squamates. The exception, in the context of the posteriorly directed medial processes, is within snakes. The condition observed in Cylindrophis, Anilius and Boa shows that the septomaxilla extends posteriorly to a point below the nasofrontal suture and may contact the prefrontal or decensus frontalis (pers. observ.).

Character 52[0–1], vertical articulation between the splenial and angular. (Fig. 2A–D). The vertical articulation of the angular and splenial in mosasauroids, coniasaurs, and snakes is unique among squamates. McDowell & Bogert (1954:60) pointed out the similarities of the angular-splenial articulation between these taxa referring to the joint as the ‘aigialosaurian hinge’ (mosasaurs and aigialosaurs). They also considered Lanthanotus, a rare varanoid from Borneo, to share this condition and intended their character complex to support Lanthanotus as a ‘structural ancestor’ to snakes (they also regarded Lanthanotus as a possible extant aigialosaur). They also noted that absence of a lateral exposure of the angular was a shared character shared by snakes, mosasauroids, and Lanthanotus.

Even though a vertical articulation of the angular and splenial is common to Lanthanotus, Mosasauroidea + Coniasaurus, and snakes, differences exist in the way the joint is constructed. Close inspection reveals that Lanthanotus does not show the same structure as snakes and Mosasauroidea+ Coniasaurus, but rather shares two of three characters with other varanoids. In Lanthanotus, Rieppel (1983) observed that there is a lateral, internal flange of the angular that crosses the intramandibular hinge thereby overlapping the splenial laterally. Examination of varanids and helodermatids shows a lateral and internal overlapping process of the angular extending between the splenial and dentary, and a medial process of the splenial that overlaps the angular ventromedially. However, Varanus and Heloderma do not have a medially vertical contact of the angular-splenial.

A functional intramandibular hinge is present in a large number of squamates (Estes et al., 1988). As such, intramandibular kinesis is a functional characteristic, like flight, and has been achieved in numerous diapsids employing a variety of different morphologies between the dentary, splenial, and postdentary bones.

Congruence of function is meaningless unless characterization of the primary homologues, the elements performing the function, i.e. the bones at that joint, also pass the tests of similarity and topological relationship (Patterson, 1982).

Character 53[0–1], reduced overlap of the postdentary-dentary bones. This character is shared with varanoids, in which it is an unequivocal apomorphy of that clade. The principal difficulty is to decide during the primary analysis of homologues if the joint is the homologue for which congruence must be tested, or if specific elements associated with that joint are of relevance to discovery of a pattern of relationship. The latter perspective is taken here. As such, mobility in the middle of the lower jaw is seen as convergent.

Character 54[0–1], reduced splenial-dentary suture. The distribution of this character is identical to the previous character. Unlike the previous two characters, the nature

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Figure 2. Details of the angular and splenial. A, angular-splenial of the holotype specimen of Dolichosaurus longicollis, BMNH R49002, in lateral view; B, mandible and angular-splenial of Coniasaurus cf. C. crassidens, BMNH R3421, in medial view; C, reconstruction of the mandible of an aigialosaur (deBraga & Carroll, 1993); D, detail of the mandible and angular-splenial of Elaphe obsoletus, RTMP 90.7.190, in medial view. Abbreviations: A, angular; C, coronoid; Comp, compound bone of snakes; D, dentary; P, prearticular; Sa, surangular; S, splenial.

of this feature, a reduced splenial, leading to a very open Meckel’s groove, need not be directly related to an intramandibular joint. Exposure of the groove and a reduction of the splenial to a thin splint of bone covering the lower portion of the

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groove is not obviously related to the hinge/joint area. Of primary concern to the mechanics of the joint is the nature of the splenial at the hinge, and the degree to which it extends over the angular, prearticular, and coronoid. A more likely scenario is that reduction of the splenial is paedomorphic and is linked to exposure of Meckel’s groove and cartilage. Whether or not this character is synapomorphic for snakes and (Mosasauroidea, Coniasaurus), to the exclusion of varanoids, or some combination of this three-taxon problem, can only be solved by congruence with other characters. The current hypothesis predicts convergence between pythonomorphs and varanoids; this character could also be found apomorphic for any combination, or in fact for all three, in the context of global squamate phylogeny.

Character 56[0–1], anterior end of coronoid meets dentary directly. A similar contact of the coronoid with the dentary is observed in some iguanians, amphisbaenids, and Lanthanotus. This character shows a possible reversal within the type species of Coniasaurus where the coronoid is immediately opposite to, but does not contact, the dentary.

Character 62[0–1], mandibular symphysis absent. Absence of an intermandibular symphysis (Character 67), i.e. lack of a bony symphyseal surface rigidly connecting the right and left mandibular rami. This character is only observed among pythonomorphs. In association with other characters linked to mobility and gape in the lower jaw, the absence of a symphysis has promoted feeding strategies that have been widely adaptable throughout pythonomorph evolution. Snakes have continued the evolutionary trend of increasing jaw mobility to its extremes. Mosasauroids appear to have limited their experiments with jaw mobility to the elements of the lower jaw.

Character 73[1–0], zygosphene-zygantra present. Zygosphenes and zygantra are present in the known dorsal vertebrae of Coniasaurus (Caldwell & Cooper, 1998; see above).

They are known to be present throughout the vertebral column in aigialosaurs (Carroll & deBraga, 1992; pers. observ.) and in anterior vertebrae in mosasaurs (Russell, 1967). Snakes uniformly possess zygosphenes and zygantra throughout the vertebral column. The principal difference between the snake and mosasauroid/ Coniasaurus condition is the shape of the neural arch lamina separating the zygosphenes. In mosasauroids and Coniasaurus, and other limbed squamates in which zygosphenes and zygantra are found, the laminar arch is notched (lacertids, teiids, gymnophthalmids, some large iguanids, and some cordyline cordylids [(Estes et al., 1988]). This notch is not present in snakes, though a notch has been reported in the putative oldest snake (Rage & Richter, 1994).

Character 64[1–0], presence of pterygoid teeth (equivocal). Pterygoid teeth are found in Heloderma, Lanthanotus, mosasauroids, Coniasaurus (at least one species), Dinilysia, Shinisaurus, and in some iguanians, anguids, cordylids, scincids, gymnophtalmids, teiids, lacertids and alethinophidian snakes. Pterygoid teeth are not present in scolecophidians, varanids, Estesia, agamids, or chamaeleonids. The distribution of this character is so plastic that as a presence/absence character it is uninformative.

GLOBAL SQUAMATE RELATIONSHIPS

A number of important and intriguing alternative relationships for other squamate groups were also discovered in this analysis. They merit some discussion of supporting

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characters, and in some cases indicate that the included taxa require further systematic examination. Characters that are optimized unequivocally are noted by bold font (e.g. 4[0–1]) and are given before those characters optimized equivocally; numbers in parantheses, following the character number, indicate the direction of character state transformation.

Iguanians. Six characters support the monophyly of Iguania (Other Iguanids (Chamaeleonidae, Agamidae)): 4[0–1], frontals fused; 6[0–1], frontal shelf broad; 14[0–1], jugal-squamosal contact on supratemporal arch; 5[0–1], frontals constricted between orbits; 11[0–1], postfrontal forked; 39[0–1], epipterygoid absent.

Scleroglossa, exclusive of (Serpentes (Mosasauroidea, Coniasaurus)). Six characters support this clade: 25[0–1], dorsal process of squamosal absent; 85[0–1], clavicle angled and curved anteriorly; 89[0–1], interclavicle cruciform with large anterior process; 95[0–1], pubis long with narrow symphysis, ventrally directed, pubic tubercle anteroventral; 50[0–1], dorsal extension of dentary coronoid process contacts coronoid; 83[1–0], epicoracoid cartilage does not contact meso- and suprascapular cartilages.

(Scleroglossa (Serpentes (Mosasauroidea, Coniasaurus))). Fifteen characters support the monophyly of this clade: 12[0–1], postorbital absent as a separate element; 30[0–1], vomer longer than half of the maxillary tooth row; 31[0–1], septomaxillae meet or nearly meet on midline in raised crest; 32[0–1], septomaxillae expanded and convex; 36[0–1], choanal fossae of palatines large; 40[0–1], alar process of prootic elongate; 75[0–1], cervical intercentra underly posterior part of preceding centra; 79[0–1], number of presacrals greater than 26; 7[0–1], descending processes of frontals well developed and exclude or nearly exclude prefrontals from margins of orbitonasal fenestra; 13[0–1], postorbital less than half of orbit length and has reduced ventral process; 49[0–1], subdental shelf large; 59[0–1], retroarticular process absent; 60[0–1], retroarticular process twisted; 66[0–1], marginal tooth replacement posterolingual, small pits; 94[0–1], epiphyses fusion of diaphysis at same time as braincase elements.

Anguoidea and a paraphyletic Xenosauridae. Three characters support the Anguoidea: 4[0–1], frontals fused; 5[0–1], frontals constricted between orbits; 14[0–1], jugalsquamosal contact on supratemporal arch.

Four characters support the clade Shinisaurus-Anguidae as distinct from Xenosaurus: 27[0–1], palpebral ossifications present; 44[0–1], posterior opening of vidian canal at basisphenoid-prootic suture; 64[1–0], pterygoid teeth present; 78[1–0], autotomy septa in caudal vertebrae present.

Varanoids and estesioids. Thirteen characters support the Varanoidea: 20[0–1], posterior extent of maxilla just anterior to orbit; 34[0–1], neochoanate; 37[0–1], ectopterygoid contacts palatine; 53[0–1], reduced articulation of dentary-postdentary bones; 54[0–1], reduced bone-bone contact at splenial-dentary suture; 66[1–2], marginal tooth replacement posterolingual, no pits; 67[0–1], plicidentine present; 71[0–1], condyle-cotyle orientation strongly oblique; 38[0–1], ectopterygoid expanded and fenestra restricted; 50[1–0], dorsal extension of dentary coronoid process absent; 51[0–1], surangular expanded anterodorsally and nearly vertical at anterior margin; 72[0–1], centrum constricted anterior to condyles; 83[0–1], no contact of epicoracoid cartilage with meso- or suprascapula.

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Four characters support the clade referred to here as ‘estesioids’: 3[0–1], prefrontal contacts postorbital, or postorbitofrontal, above orbit; 19[0–1], pineal foramen absent; 44[0–1], opening of vidian canal at basisphenoid-prootic suture; 51[1–2], anterior end of surangular terminates closer to coronoid eminence on surangular.

Four characters support the clade Lanthanotus–Heloderma: 10[1–0], postfrontal present as separate element; 15[0–1], supratemporal fenestra open and no arch; 64[1–0] pterygoid teeth present; 63[1–0], palatine teeth present.

Amphisbaenids, dibamids and gekkotans (A-D-G). Eight characters support the clade (Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae))): 2[0–1], no contact of nasal and prefrontal bones; 26[0–1], supratemporal absent; 8[0–1], contact of descending processes of frontals; 23[0–1], jugal reduced or absent, postorbital bar incomplete; 37[0–1], ectopterygoid contacts palatine; 46[0–2], Meckel’s canal enclosed, bone fused; 57[0–1], angular absent; 80[0–3], two or fewer rib attachments on sternum.

Two characters support the clade (Dibamidae (Gekkonoidea, Eublepharidae)): 75[2–0], cervical intercentra under posterior part of preceding centrum; 82[0–1], scapula emarginated.

Gekkota is supported by 12 characters: 10[1–0], postfrontal absent as separate element; 45[0–1], jaw adductors originate on ventral surface of parietal; 55[0–1], elongate anterior extension of coronoid; 66[1–0], lingual tooth replacement with large resorption pits; 4[0–1], frontals fused; 17[1–0], parietal tabs absent; 43[0–1], lateral head vein enclosed in bony canal formed by crista prootica; 50[1–0], dentary coronoid process no contact with coronoid; 69[1–0], more than 14 scleral ossicles; 70[1–0], second ceratobranchials present; 80[3–1], four rib attachments on sternum; 92[0–1], postcloacal bones present.

Scincomorphs+(Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae))). Four characters support this clade: 19[0–1], pineal foramen absent; 78[1–0], autotomic septa present; 15[0–1], supratemporal fenestra open and no arch; 17[0–1], parietal tabs present.

Scincomorphs. Seven characters support this clade: 18[0–1], parietal downgrowths present and extend to, or almost to, epipterygoids; 45[0–1], jaw adductors originate on ventral surface of parietal; 66[1–0], lingual tooth replacement, large resorption pits; 95[1–2], pubis long, narrow, ventrally directed, symphysial process elongate; 15[1–2], supratemporal arch present, fenestra closed; 73[1–0], zygosphenes and zygantra present; 75[2–1], cervical intercentra under anterior part of preceding centrum.

Scincoidea. Three characters support this clade: 10[1–0], postfrontal present as separate element; 93[0–1], osteoderms present; 12[1–0], postorbital present as separate element.

Lacertoidea. Seven characters support this clade: 58[0–1], prearticular crest present; 59[1–0], sulcus/pit on retroarticular process absent; 60[1–0], retroarticular process not twisted; 70[1–0], second ceratobranchials present; 77[0–1], two pairs of diverging transverse processes of caudal vertebrae; 79[1–0], more than 26 presacrals; 90[0–1], sternal fontanelle present.

Serpentes. Serpentes is supported by ten characters: 8[0–1], median contact of descending process of frontals; 15[0–1], supratemporal fenestra open and no arch; 19[0–1], pineal foramen absent; 22[1–0], anteroventral border of orbit formed by

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maxilla with jugal confined to medial surface of maxilla; 23[0–1], jugal reduced or absent, postorbital bar incomplete; 24[0–1], squamosal absent; 34[0–2], posterior border of opening for Jacobson’s organ closed by contact of vomer and septomaxilla; 39[0–1], epipterygoid absent; 66[1–3], marginal tooth replacement posterolingual, no pit, tooth rotates into position; 13[0–1], Postorbital less than one half of posterior border of orbit.

The clade Alethinophidia–Dinilysia is supported by two characters: 12[1–0], postorbital present as separate element; 63[1–0], palatine teeth present.

MOSASAUROIDS+CONIASAURS AS ANGUIMORPHS: ADDITIONAL TESTS

Commonly accepted phylogenetic relationships for mosasauroids and coniasaurs find them to be either ingroup anguimorphs, closely related to Varanus or Lanthanotus, or the sister-taxon of Anguimorpha (McDowell & Bogert, 1954; deBraga & Carroll, 1993; Lee, 1997). Similar hypotheses have been made that find snakes as the sistergroup to anguimorphs, or more specifically one of the extant varanoid genera (for reviews see Rieppel, 1988; Lee, 1997).

As stated earlier, Caldwell et al. (1995) found no support for mosasauroids as derived varanid lizards, a conclusion supported in this analysis (Fig. 1A, B). It is also clear from this analysis that any classification placing mosasauroids within anguimorphs as either derived varanoids or the sister-group to varanoids, would be polyphyletic.

Therefore, in the context of this data set, it is important to further test the possibility that (Mosasauroidea, Coniasaurus) are nested within Anguimorpha either with or without close relationship to snakes. Two different tests were constructed: (1) using MacClade 3.1 (Maddison & Maddison, 1992), the manipulation various clades while constraining other ‘accepted’ squamate relationships, and noting the number of steps required to produce alternate phylogenies; (2) additional PAUP analyses using a restricted ingroup set, beginning with only extant taxa, and adding sequentially, the fossil taxa listed in Appendix 2.

Test number I The topologies and lengths of cladograms n-steps removed from the shortest trees (464+n) were examined. Results are tabulated for tree lengths found when (Serpentes (Mosasauroidea, Coniasaurus)) was moved into various sister-group relations with other squamate taxa and clades (Table 1), when the clade (Serpentes (Mosasauroidea, Coniasaurus)) was broken apart and Serpentes moved into various sister-group positions (Table 2), and when Serpentes was placed in the sister-group position to Anguimorpha and (Mosasauroidea, Coniasaurus) moved into various sister-group positions within Anguimorpha (Table 3).

(Serpentes (Mosasauroidea, Coniasaurus)) Monophyletic (Table 1).

The next-shortest alternative topology to Figure 1B has a tree length (TL) of 467 steps, and places (Serpentes (Mosasauroidea, Coniasaurus)) in the sister-group position

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T 1: Other possible sister-group relationships of the Pythonomorpha+Serpentes (Fig. 15) and the number of steps from the shortest tree required to demonstrate them Number of steps

Pythonomorpha as sister to:

464 467 468 460 471 472 473 474 475 469–472 471–473 471–476 471–481 473–476 471

scleroglossan clades scincogekkonomorphs, Varanoidea, Scinomorpha Anguimorpha, Amphisbaenia dibamogekkota Varanus estesioids (Estesia + Lanthanotus + Heloderma) Dibamidae Lanthanotus, Heloderma, Lanthanotus+Heloderma Estesia Anguoidea, or with various anguoid taxa Scincoidea, or with various scincoid taxa Iguania, or with various iguanian taxa Lacertoidea, or with various lacertoid taxa Gekkota, or with various gekkotan taxa Sister to Squamata

T 2: Other possible sister-group relationships of Serpentes (Alethinophidia+Dinilysia +Scolecophidia) and the number of steps from the shortest tree required to examine them.

The clade Mosasauroidea+Coniasaurus is fixed outside Scleroglossa and the relationships of snakes are tested against this topology by comparison to all other squamates. Emphasis is placed on the number of steps required to find sister-group relationships of Serpentes with Amphisbaenia+Dibamidae+Gekkota, and Anguimorpha, or with various anguimorph taxa Number of steps

Snakes as sister-group to:

464 465 466 468 469 470 470 473 473 476 469–480 469–471 472–477 473–480 ∗473

Mosasauridae+Coniasaurus; Amphisbaenia+Dibamidae+Gekkota scincogekkonomorphs Amphisbaenia, Scincomorpha, dibamogekkota Anguimorpha, Scleroglossa Dibamus Gekkonoidea, Eublepharidae Varanoidea Varanus estesioids (Estesia+Lanthanotus+Heloderma) Lanthanotus, Heloderma, or Estesia Lacertoidea, or with various lacertoid taxa Scincoidea, or with various scincoid taxa Anguoidea or with various anguoid taxa Iguania, or with various iguanian taxa Sister to Squamata

to either Varanoidea, scincogekkonomorphs, or Scincomorpha. At 468 steps (Serpentes (Mosasauroidea, Coniasaurus)) is the sister-group to either Anguimorpha, Amphisbaenia, or Amphisbaenia+Dibamids+gekkotans. Finding (Serpentes (Mosasauroidea, Coniasaurus)) to be within Varanoidea requires an increase of 7 steps (TL=471) to be the sister-taxon to Varanus, and between 8 to 10 steps to be the sister to any of the estesioid taxa.

While tempting to argue that three steps (as the sister-taxon to Varanoidea) is minimal, it is noted that this same number of steps finds (Serpentes (Mosasauroidea, Coniasaurus)) to be the sister-group to scincomorphs or amphisbaenids, dibamids,

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T 3: Number of steps required to demonstrate other possible sister-group relationships of Mosasauroidea+Coniasaurus. The monophyly of other major squamate clades is constrained as found in the 12 shortest trees, and Serpentes (Alethinophidia+Dinilysia+ Scolecophidia) is fixed in the sistergroup position to Anguimorpha. Mosasauroidea+Coniasaurus is examined within Anguimorpha. This test examines the view that mosasauroids and coniasaurs are derived varanoid squamates Number of steps

Serpentes outside Anguimorpha and M+C as sister to:

470 471 472 473 474 475 476 478 479 470–478

Anguimorpha Serpentes+Anguimorpha, Varanoidea Anguoidea Varanus, Shinisaurus Xenosaurus, Shinisaurus+Anguidae Anguidae estesioids Heloderma+Lanthanotus Lanthanotus, Estesia, Heloderma Serpentes outside scleroglossans and M+C as sister-group to various anguimorphs

and/or gekkotans. Nesting (Serpentes (Mosasauroidea, Coniasaurus)) within varanoids is so many steps removed from the shortest tree that any relationship within Squamata is equally well supported (Table 1). The most important alternative relationship to that indicated in the Majority Rule Consensus Tree (Fig. 1B) is the sister-group relationship to amphisbaenids and dibamids.

(Serpentes (Mosasauroidea, Coniasaurus)) Polyphyletic: I (Table 2) In this test, the clade including Serpentes + (Mosasauroidea, Coniasaurus) is considered to be a polyphyletic assemblage, Serpentes is removed, (Fig. 1B), and (Mosasauroidea + Coniasaurus) is constrained to the node Scleroglossa. Constraining relationships in this manner finds Serpentes to be the sister to Anguimorpha at 468 steps. Placing Serpentes in the sister-group position to any or all varanoid taxa is 4 to 11 steps longer than the shortest tree. As noted above, there are a great number of equally parsimonious topologies supporting relationships with other squamate taxa at this number of steps removed from the shortest tree.

However, an alternative and potentially important sister-group relationship for Serpentes (assuming a polyphyletic (Serpentes (Mosasauroidea, Coniasaurus))) exists at 464 steps: Serpentes (Amphisbaenia (Dibamidae (Gekkonoidea, Eublepharidae).

This topology was discussed above and contributes to the polytomy resolved in the Strict Consensus Tree (Fig. 1A). At 465 steps, Serpentes can be reconstructed in the sister-group position to scincogekkonomorphs. At 466 steps, Serpentes can be reconstructed as the sister-group to Amphisbaenia, Scincomorpha, or dibamogekkotans.

(Serpentes (Mosasauroidea, Coniasaurus)) Polyphyletic: II (Table 3) Serpentes is fixed in the sister-group position to Anguimorpha (Estes et al. 1988; Schwenk, 1988, 1994) and (Mosasauroidea, Coniasaurus) tested relative to this clade.

At 470 steps (Mosasauroidea, Coniasaurus) is the sister-group to Anguimorpha with Serpentes as the sister-taxon to both. At 471 steps (Mosasauroidea, Coniasaurus) is

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the sister-group to Serpentes+Anguimorpha. For (Mosasauroidea, Coniasaurus) to be a sister-group to any other anguimorph taxon, tree lengths must increase by 7 to 15 steps beyond the tree length (TL=464) of the 18 shortest trees (Fig. 1A, B).

Hypotheses of varanoid affinities of (Mosasauroidea, Coniasaurus) requires the longest tree lengths of any of the alternative trees tested. If Serpentes is fixed outside of Scleroglossa or Squamata, consistent with suggestions by Underwood (1970) and Rieppel (1988), than finding (Mosasauroidea, Coniasaurus) to be within Anguimorpha requires tree lengths of 470–478 steps.

Test number II: sequential analysis of fossil taxa It has been argued that certain taxa, fossil and/or extant, act as ‘keystone’ taxa that significantly affect tree topology by altering internal nodes (Gauthier et al., 1988; Donoghue et al., 1989). Data sets excluding fossil taxa are likely to be inadequate as many fossil taxa demonstrate characters that are informative regarding ambiguous apomorphies present in crown-groups (i.e. extant taxa only). The question asked here, in terms of this data set, is whether any or all of the fossil taxa included in this study act as keystone taxa in the cladistic analysis (Fig. 1A, B), i.e. whether any taxa are essential to the tree topology as reconstructed. To test this question, and to further examine the problem of (Mosasauroidea, Coniasaurus) as varanoids or anugimorphs, fossil taxa were added to a data matrix composed only of extant squamate taxa. Each new and larger matrix was then analysed cladistically using the same protocol as described in Methods for the entire matrix (Appendix 2).

Extant taxa: the baseline (Fig. 3 A, B). Analysis found 160 cladograms of 441 steps each (CI 0.707, HI 0.760, RI 0.589). A Strict Consensus Tree of these cladograms finds little resolution of squamate relationships that is comparable to previous phylogenies (Estes et al., 1988; Schwenk, 1988, 1994). Iguanian monophyly is not supported, though there is support for the distinction between Iguanian taxa and all other squamates. The monophyly of Scleroglossa is supported. However, Anguimorpha and Anguoidea (Anguidae, Shinisaurus, Xenosaurus) are found to be paraphyletic. Anguidae is found to be the sister to all non-anguid scleroglossans, while Shinisaurus and Xenosaurus are successive outgroups to Anguidae + all other scleroglossans. Varanoid monophyly is supported though ingroup relationships are unresolved. Scincomorpha is not recognized: Scincidae is unresolved within Scleroglossa, though Cordylidae + Lacertoidea (xantusiids, lacertids, gymnophthalmids, teiids) is found to be a monophyletic group. Gekkotan monophyly is supported. Amphisbaenids and dibamids are unresolved within Scleroglossa.

Alethinophidians and scolecophidians form a monophyletic ophidian clade that is also unresolved within scleroglossans.

The Majority Rule Consensus Tree (Fig. 3B) supports, in more than 50% of the 160 cladograms, most of the previously hypothesized squamate crown groups Camp, 1923; Estes et al., 1988). The major exceptions are a polyphyletic Anguimorpha, a paraphyletic Anguoidea, and a paraphyletic Scincomorpha. A new clade is found that places varanoids in the sister-group position to a gekkotan– snake–amphisbaenid–dibamid clade. This entire clade shares a common ancestor with the cordylid–lacertoid clade.

None of the 160 cladograms of extant squamates has a similar topology to any

132 A

M. W. CALDWELL Agamidae

B

Agamidae 51

Anguidae

Chamaeleonidae

Cordylidae

Other Iguanians

Gymnophthalmidae

Anguidae

Teiidae

Cordylidae

Lacertidae

Gymnophthalmidae

51

100 100 100

Xantusiidae

Gekkonoidea

Xantusiidae

100

Dibamidae

Eublepharinae 46

Heloderma Lanthanotus Varanus

100 78 100

Amphisbaenia

66 100 66

100

76 80 100

Scolecophidia

Eublepharinae

100

Heloderma

Alethinophidia Scolecophidia

Alethinophidia

Gekkonoidea

Scincidae Amphisbaenia

Teiidae Lacertidae

Dibamidae

98

Lanthanotus

Xenosaurus

Varanus

Shinisaurus

Scincidae

Chamaeleonidae

Xenosaurus

Other Iguanians

Shinisaurus

Sphenodon

Sphenodon

Figure 3. Consensus trees derived from 160 cladograms (441 steps; CI 0.707; HI 0.760) showing ingroup relationships of 21 extant squamate taxa using morphological data (95 osteological characters).

A, Strict Consensus Tree. B, Majority Rule Consensus Tree.

of the 18 cladograms found using the complete data set. Distinction of the node differentiating between scleroglossans and iguanians appears to be a constant at any level of observation (Camp, 1923; Estes et al., 1988; this study in its several parts).

Apart from this well-supported dichotomy, previous hypotheses of monophyly for Lacertoidea, Varanoidea, Ophidia, Gekkota, and Iguania are supported in both the analysis of fossil and extant squamates (Fig. 1A, B) and by the analysis of extant squamates only (Fig. 3A, B). The basal relationships of these and other taxa are uncertain in both studies but are clearly better resolved in the analysis including fossil taxa.

Sequential addition of fossil taxa (1) Extant taxa + Dinilysia (Fig. 4A, B). Analysis of this matrix finds 8 trees of 443 steps each (CI 0.704, HI 0.761, RI 0.602) in which Dinilysia forms a monophyletic group with Alethinophidia; Scolecophidia is the sister to that clade. Characters of Dinilysia are most congruent with those of alethinophidians, i.e. as crown-group Serpentes.

The Strict Consensus Tree (Fig. 4A) shows that the addition of Dinilysia has an important effect on the resolution of more basal scleroglossan relationships; where

SQUAMATE PHYLOGENY A

Agamidae

133 Agamidae

B

Anguidae

Anguidae

Cordylidae

Cordylidae

Gymnophthalmidae

Gymnophthalmidae 100 100 100

Teiidae Lacertidae

Lacertidae

Xantusiidae

Xantusiidae

100

Dibamidae

Dibamidae 71

Gekkonoidea Eublepharinae Heloderma

50

64 79 100 100 100 43

Amphisbaenia 100

71

93

Heloderma 50

93

Lanthanotus

100

Scincidae Amphisbaenia

Gekkonoidea Eublepharinae

Lanthanotus Varanus

Teiidae

Varanus 100 100

Alethinophidia

Alethinophidia

Dinilysia

Dinilysia

Scolecophidia

Scolecophidia

Scincidae

Xenosaurus

Xenosaurus

Shinisaurus

Shinisaurus

Chamaeleonidae

Chamaeleonidae

Other Iguanians

Other Iguanians

Sphenodon

Sphenodon

Figure 4. Consensus trees derived from 8 cladograms (443 steps; CI 0.704; HI 0.761) of 21 extant squamates and the fossil taxon Dinilysia using morphological data (95 osteological characters). A, Strict Consensus Tree. B, Majority Rule Consensus Tree.

derived characters of snakes are unresolved with respect to amphisbaenids and dibamids versus varanoids, dinilysid characters resolve these problems. Addition of Dinilysia therefore influences the number of trees found (8 versus 160 for extant taxa only). This results from the redistribution of characters found apomorphic for the snake–dibamid–amphisbaenid clade (from the analysis of extant taxa) that no longer support that grouping to the exclusion of a snake–varanoid clade.

(2) Extant taxa + Dinilysia + Estesia (Fig. 5 A, B). Analysis found 8 trees of 445 steps each (CI 0.701, HI 0.762, RI 0.609). The addition of Estesia is important to the resolution of characters within Varanoidea as constituted by this data set.

Resolving character incongruities within varanoids, in association with congruence added by characters of Dinilysia, reduces the number of cladograms for extant taxa only (i.e. 160) to only 8. This small number of trees is associated with only a 4 step increase in treelength (441 to 445).

Varanoids are found to be monophyletic, though with some restructuring of relationships within the crown-groups. The clade Heloderma–Lanthanotus is differentiated within Varanoidea, as are the ‘estesioids’, when Estesia is included.

Snakes are resolved in a clade with dibamids and amphisbaenids, but cannot be resolved as the sister-taxon to varanoids specifically, nor anguimorphs generally.

The monophyly of Serpentes is not altered and Dinilysia remains as the sister to

134 A

M. W. CALDWELL Agamidae

B

Agamidae 33

Chamaeleonidae

Chamaeleonidae

Other Iguanians

Other Iguanians

Anguidae

Anguidae 100

Shinisaurus

Shinisaurus 100

Xenosaurus

Xenosaurus

100

Heloderma

100 100 100

Lanthanotus Varanus

Varanus

100

Cordylidae

Cordylidae

Scincidae

Scincidae

100 100 100 100

Teiidae Lacertidae

100 100

100

Dibamidae 100

Amphisbaenia

100 100

100

Gekkonoidea Eublepharinae

Eublepharinae Alethinophidia

Teiidae Xantusiidae

Dibamidae Gekkonoidea

Gymnophthalmidae Lacertidae

Xantusiidae Amphisbaenia

Lanthanotus Estesia

Estesia

Gymnophthalmidae

Heloderma

100 100

Alethinophidia

Dinilysia

Dinilysia

Scolecophidia

Scolecophidia

Sphenodon

Sphenodon

Figure 5. Consensus trees derived from 8 cladograms (445 steps; CI 0.701; HI 0.762) of 21 extant squamates and the fossil taxa Dinilysia and Estesia using morphological data (95 osteological characters).

A, Strict Consensus Tree. B, Majority Rule Consensus Tree.

Alethinophidia (Fig. 5A, B), while the sister-group relationship of dibamids, gekkotans, and amphisbaenids are with snakes. Scincomorpha is monophyletic, and iguanian interrelationships remain unresolved though this clade appears to be monophyletic (Fig. 5A, B).

(3) Extant taxa + Dinilysia + Estesia + Coniasaurus (Fig. 6A, B). Analysis found 8 trees of 453 steps each (CI 0.689; HI 0.766; RI 0.599). Based on this data set, Coniasaurus is found to be outside Anguimorpha, with snakes as the sister-group to amphisbaenid–dibamid clade. Scleroglossans are distinct from iguanians, and Anguimorpha is paraphyletic.

The addition of Coniasaurus to the analysis introduces character states that identify unsuspected incongruence within the data set; significant changes seem to consistently influence the monophyly of the Anguimorpha by destabilizing characters that normally support anguoid monophyly; a similar set of incongruent characters affect the scincomorpha and ingroup structure of varanoids.

Other clades for which robust apomorphies appear to be absent are scincids. It is also clear that characters of Coniasaurus are congruent with anguimorph states when no other mosasauroids are included. The sister-group relationships of Serpentes indicate that Serpentes and dibamids–amphisbaenids are synapomorphic for a

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B

Agamidae 100

Chamaeleonidae

Chamaeleonidae

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Xenosaurus

Xenosaurus

Cordylidae

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Gymnophthalmidae

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Teiidae

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Lacertidae

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Scincidae Dibamidae Gekkonoidea

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Eublepharinae

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Amphisbaenia

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Lanthanotus

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Scolecophidia Heloderma

Gekkonoidea Eublepharinae

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Scolecophidia 100 100

Heloderma Lanthanotus

Estesia

Estesia

Varanus

Varanus

Shinisaurus

Shinisaurus

Coniasaurus

Coniasaurus

Sphenodon

Sphenodon

Figure 6. Consensus trees derived from 8 cladograms (453 steps; CI 0.689; HI 0.766) of 21 extant squamates and the fossil taxa Dinilysia, Estesia, and Coniasaurus, using morphological data (95 osteological characters). A, Strict Consensus Tree. B, Majority Rule Consensus Tree.

number of character states. In all cladograms an Amphisbaenia + ‘dibamogekkota’ clade is the sistergroup to Serpentes. This clade is supported by characters of limblessness.

(4) Extant taxa + Mosasauroidea (Fig. 7A, B). Analysis found 4 trees of 461 steps each (CI 0.681; HI 0.770; RI 0.568). Clade structure in these four cladograms is most similar to the that found in Test II, Number 3, above. Coniasaurus might have the same effect if its morphology was as completely known as that of mosasauroids (e.g. Russell, 1967; Bell, 1993).

Inter-subjective consensus Test II indicates the difficulty of congruence and consensus between subjective/ qualitative phylogenetic analyses. What taxa should be included? What characters become redundant or uninformative in one analysis compared to a second or third study where such characters are informative? And finally, how do we compare the results of such studies when they can vary so widely between analyses that vary by only a single taxon?

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Agamidae

Anguidae

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Teiidae

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Varanus

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Alethinophidia Scolecophidia

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100 100 100

Alethinophidia

Mosasauroidea

Scolecophidia

Amphisbaenia

Mosasauroidea

Xenosaurus

Amphisbaenia

Shinisaurus

Shinisaurus

Chamaeleonidae

Chamaeleonidae

Other Iguanians

Other Iguanians

Sphenodon

Sphenodon

Figure 7. Consensus trees derived from 4 cladograms (461 steps; CI 0.681; HI 0.770) of 21 extant squamates and the fossil taxon Mosasauroidea using morphological data (95 osteological characters).

A, Strict Consensus Tree. B, Majority Rule Consensus Tree.

The discovery of monophyletic groups is the goal of cladistic analysis. The technique used in such heuristic processes is intended to recover the order of branching/cladogenic events. The data set supporting this technique is derived from the primary analysis of homologues and the secondary discovery of synapomorphies/ homology by congruence of characters. In the context of any one character matrix, the monophyletic ‘signal’, i.e. the strength of support for a clade through the congruence of characters, is the strongest signal of relationship that persists in any character matrix. This is evidenced by the stability of several clades found in this study (e.g. Serpentes, Varanoidea, Iguania) throughout the various series of matrices analysed and discussed above. In other words, these groups are ‘real’ and the data sets will continually support their reconstruction.

This does not mean that monophyletic groups persist despite our best efforts to obscure them. On the contrary, monophyletic groups can easily be made paraphyletic or polyphyletic by experimental inaccuracies. Tests of similarity, topological relation, and congruence will present synapomorphies, and by phylogenetic interpretation, homologies, only when total evidence in both characters and taxa is considered, and only when the foundation of phylogenetic analysis rests on rigorous characterization of primary homologues. On this last point the subjectivity of analysis can be minimized but not extinguished; errors in characterizing primary homologues will continue. It

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is unrealistic to expect that characters and their states can be rigorously defined the first, second, or tenth time that they are characterized for cladistic analysis. In other words, satisfactory means must be explored to find inter-subjective consensus between disparate data sets and disparate phylogenetic hypotheses.

CONCLUSIONS

Snakes, Mosasauroidea, Coniasaurus; Pythonomorpha The prevalent opinion on mosasaurian relationships, held since Cuvier (1802), was that mosasaurs and related forms were aquatic varanoid lizards. The controversy and debate on mosasaurian phylogeny began when Cope (1869) identified characteristics of mosasaurs that he felt merited recognition of a closer relationship between mosasaurs and snakes, than between mosasaurs and any other lizards.

Further, Cope’s hypothesis did not recognize an any close relationship between mosasaurs and varanoid lizards.

A series of papers and responses, arguing points of morphology and phylogeny, were exchanged between Cope and Baur (see Cope 1869, 1895a, b, 1896a, b, and Baur, 1895, 1896). Baur took the position that mosasaurs were derived, aquatic varanoid lizards, and that there were no important similarities between ophidians and mosasaurs. Owen (1877), Boulenger (1891), and Osborn (1899) also extended opinions on this question of squamate phylogeny; Owen criticized Cope, while Boulenger and Osborn recognized some merit in Cope’s arguments. The debate has been long-lived as successive generations of systematists have continued to examine the relationships of aigialosaurs, mosasauroids, snakes and dolichosaurs (Feje´rva´ry, 1918; Nopsca, 1903, 1908, 1923; McDowell & Bogert, 1954; Rieppel, 1988; Carroll & deBraga, 1992; deBraga & Carroll 1993; Caldwell et al., 1995; Caldwell, 1996; Lee, 1997; Caldwell & Lee, 1997).

The hypothesis of squamate phylogeny presented here (Fig. 1A, B) is derived from a research program that originated with the re-characterization of the external bony nares of varanoids and mosasauroids (Caldwell et al., 1995). The ‘posteriorly retracted’ nares of Varanus, and to a far lesser degree Lanthanotus, had for a very long time been ‘homologized’ with the ‘retracted nares’ of mosasauroids (aigialosaurs and mosasaurs). Caldwell et al. (1995) showed that the only ‘homologue’ shared by varanids and mosasauroids is the presence of a large, empty space on the dorsal surface of the muzzle (the structure of the soft tissues in mosasaurs is unknown); nine other characters, derived from the bony elements framing this space, were found to have different states that were not synapomorphic for mosasauroids and varanoids. Therefore, if mosasauroids and Coniasaurus were indeed derived varanoids, or anguimorphs/platynotans of some sort, it seems logical that this relationship would have been supported in at least some of the shortest trees. The introduction of the Cretaceous lizard Estesius into the data matrix found this taxon to be a varanoid. If mosasauroids and Coniasaurus were varanoids they too would have been nested within that clade. This was not found to be the case. Other characters of mosasaurs, considered homologous with varanoids, were found by Caldwell et al.

(1995) to be plesiomorphic for all included anguimorphs. This paper details a similar suite of characters that are synapomorphic for mosasaurs and other squamates at the level of Scleroglossa, not Anguimorpha, and certainly not Varanoidea.

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Historical limitations on ascertaining the relationships of mosasaurs, and similarly snakes, have been due to a priori assumptions of crown-group relationship with other squamates. Where the ingroup examined by Caldwell et al. (1995) and Lee (1997) was limited to varanoids and some anguids, the ingroup tested here is not. Where the ingroup examined by Estes et al. (1988) was limited to extant squamates, the ingroup tested here is not. The taxonomic scope of this study is the logical test of hypotheses generated from all previous studies and therefore relies heavily on the data and hypotheses they produced. As phylogeny is historically contingent, so is this analysis.

Despite awareness of Cope’s (1869, 1895a, b, 1896a, b) hypothesis of a close relationship of mosasaurs with snakes, the initial intention of this study was to examine the phylogenetic relationships of coniasaurs and dolichosaurs relative to mosasaurs and aigialosaurs, and not to address snake-lizard relationships, nor to investigate Pythonomorpha sensu stricto Cope (1869). However, the synapomorphies found supporting a clade composed of Serpentes, Mosasauroidea, and Coniasaurus merit serious consideration of the included taxa as a ‘real’ clade. Detailed characterization of the morphology of more poorly known, but putatively closely related squamates such as adriosaurs, acteosaurs, dolichosaurs, and aigialosaurs, no matter how incomplete the material may be, might alter the understanding of their phylogeny, and by extension, squamate phylogeny. To this end, redescription of the holotype and referred material of Coniasaurus crassidens has been completed (Caldwell & Cooper, 1998), as has the description of a new species of Coniasaurus (Caldwell, in press).

Squamata As indicated by previous studies (Gauthier et al., 1988; Donoghue et al., 1989), and as discussed above, this study shows that the addition of fossil taxa to phylogenetic analyses can significantly alter our understanding of the interrelationships of organisms. Fossil taxa, despite their acknowledged incompleteness, offer data that is critical to the effective application of the principle of total evidence in phylogeny reconstruction (Kluge, 1989). Limiting data to specific subsets (e.g. extant vs. fossil) is logically inconsistent not only for characters but also for taxa. It is also evident that fossils need not be treated in any particularly unique manner relative to extant taxa, nor in relation to missing data (Kluge, 1990; Rieppel, 1994).

Squamata as a whole appears to be a robust group, though admittedly, this analysis did not specifically address this question. Two major clades of squamates are recognized: one clade composed of all iguanian taxa, and the other composed of all non-iguanian squamates including pythonomorphs. Scleroglossa requires no modification of taxic composition (Estes et al., 1988) and is retained to define the node differentiating pythonomorphs and all other non-iguanians. Assertions that Lacertilia or ‘lizards’ are paraphyletic (Estes et al., 1988) and that snakes were derived from within “lizards” (Camp, 1923), are supported by the re-structuring of Squamata regardless of the presence or absence of fossil taxa.

The basic composition of Anguimorpha (Anguoidea + Varanoidea) and Scincomorpha (Scincoidea + Lacertoidea) is by no means well supported using only osteological data (this data set). At this level among crown-groups, clade structure is re-organized for a number of taxa. The lack of resolution for Anguoidea found

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in the analysis of only extant taxa (Fig. 3) is resolved and paraphyly of extant Xenosauridae is recognized in the full data set (Fig. 1A, B). Addition of the fossil Estesia mongoliensis (Norell et al., 1992) alters varanoid ingroup structure by finding Lanthanotus be the sister-group to Heloderma, and Varanus the sister to all three (Figs 1 and 4).

An important and well supported clade are the ‘Dibamogekkota’ (Fig. 1). Dibamids share eight characters with gekkotans. Amphisbaenians share nine characters with ‘Scincogekkonomorphs’ and are the sister-group to ‘Dibamogekkota’. Only one character supporting relationships for either taxon concerns features of limbs or girdles. Based on these results, new relationships are found that support previous hypotheses of relationship between ‘scincogekkonomorphs’ and ‘dibamogekkotans’ (Rieppel, 1981, Rieppel, 1984; Greer, 1985). Following suggestions by Rieppel (1984) and Greer (1985), subsequent investigations of the relationships proposed here would benefit by closer examination of dibamids, amphisbaenids, Anelytropsis, feylinine and acontine scincomorphs, and gekkonids.

Snake origins The problem of snake origins is now the focus of an extended examination of other taxa including the very poorly known ‘ophiomorphs’ described by Kornhuber (1873) and Haas (1979, 1980a, b; see also Calligaris [1988]). Though no prediction is possible regarding the information and resolution that new data will provide, it is already evident that the phylogenetic relationships of snakes as reconstructed here drastically affect hypotheses of snake origins and hypotheses on the sister-group relationships of snakes (Caldwell & Lee, 1997; Lee & Caldwell, 1998).

Problems of snake relationship within Squamata affect adaptationist hypotheses of a fossorial versus marine origin for snakes (Bellairs & Underwood, 1951; Underwood, 1967; Bellairs, 1972; for a review see Rieppel, 1988). If the sister-group of snakes is a group of aquatic squamates that show a marked evolution towards highly modified limbs (Caldwell, 1996), then a reasonable alternative exists for the conventional view of a fossorial origin for snakes and snake characters. Mosasaur limb and girdle structure suggests that these animals were obligatorily aquatic (Russell, 1967; Caldwell, 1996), though this is not so readily apparent for aigialosaurs (Caldwell et al., 1995), dolichosaurs and coniasaurs (Owen, 1850; Bell et al., 1982; Polcyn & Bell, 1994; Caldwell & Cooper, 1998).

There is no doubt that subsequent snake evolution has involved adaptations among various taxa for fossorial habits. However, the question remains regarding the origin of the basic body plan of snakes: elongate, limbless, squamates with small heads and highly mobile skull bones and a bony cranium. All of these characters, with the exception of complete limblessness, are present in Pachyrhachis problematicus (Caldwell & Lee, 1997), the earliest and most complete snake known. This early snake shows numerous adaptations to marine environments (Lee & Caldwell, 1998) that could have been co-opted in later snake evolution for adaptations to terrestrial, and specifically, fossorial habits.

Later snake evolution towards burrowing habits has certainly produced numerous cranial specializations that characterize the highly fossorial scolecophidians versus the more adaptively diverse alethinophidians (i.e. fossorial, aquatic, arboreal, terrestrial). If fossoriality is primitive for snakes, as judged against the extreme adaptations

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of scolecophidians, then numerous features of alethinophidians and Pachyrhachis must be explained as very complex reversals.

Invoking complex processes to defend untested polarities for multiple characters is unwarranted, especially if the common ancestor of Serpentes was less fossorial than either descendant lineage. As hypothesized in the analysis given in this paper, snakes and mosasauroids/coniasaurs share a limbed common ancestor that was likely aquatic, not fossorial (Fig. 1A, B). Therefore, a ‘propensity’ for altering limb development was common to both descendent lineages. Serpentes evolved complete loss of the limbs. Coniasaurs and dolichosaurs reduced the overall size of their limbs, making the front limb smaller than the rear limb. Mosasaurs on the other hand, evolved the most highly modified limbs of the entire clade, producing paddle-like limbs (Caldwell, 1996). Among pythonomorphs, one clade retained limbs and adopted aquatic habits while the other lost its limbs and adapted to a wide range of terrestrial habitats.

ACKNOWLEDGEMENTS

For assistance while gathering data, I thank S. Chapman, J. Cooper, A. Currant, K. deQuieroz, J. Evans, D. Frost, C. Price, and V. Sowiak. I thank M. Wilson and J. Clark for use of lab space and equipment. I thank J. Clark and O. Rieppel for discussion and criticism of both the manuscript and my ideas. Research was supported by a Natural Sciences and Engineering Research Council of Canada (NSERC) Postdoctoral Fellowship.

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Schwenk K. 1988. Comparative morphology of the lepidosaur tongue and its relevance to squamate phylogeny. In: Estes R, Pregill G, eds. Phylogenetic Relationships of the Lizard Families. Stanford: Stanford University Press, 569–598.

Schwenk K. 1994. Why snakes have forked tongues. Science 263: 1573–1577.

Swofford DL. 1993. PAUP: Phylogenetic Analysis Using Parsimony, Version 3.1.1.. Laboratory of Molecular Systematics, Smithsonian Institution, Washington D.C. Underwood G. 1957. Lanthanotus and the anguimorphan lizards: A critical review. Copeia 1957: 20–30.

Underwood G. 1967. A contribution to the classification of snakes. Publication of the British Museum (Natural History) 653: 1–179.

Wu X-C, Brinkman DB, Russell AP. 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia.

Canadian Journal of Earth Sciences 33:, 541–577.

APPENDIX 1

Character and character state descriptions Numbers in parentheses indicate the character number of Estes et al. (1988), from which the character was derived. Numbers in boldface indicate new characters, or highly altered and rewritten characters taken from Estes et al. (1988).

1 (3). Nasals: paired (0); fused (1).

2 (4). Nasal and prefrontal bones contact: present (0); absent (1).

3 (5). Prefrontal contact with posterior orbital bones above orbit: no contact (0); contact (1).

4 (6). Frontals: paired (0); fused (1).

5 (7). Lateral borders of frontals: more or less parallel (0); constricted between orbits (1).

6 (8). Frontal Shelf: lacking broad shelf below nasals (0); broad shelf present (1).

7 (9). Descending processes of frontals, participation in orbitonasal fenestra: weakly developed and prefrontal participating in wide orbitonasal fenestra (0); prominently developed and prefrontals narrowly or not at all in margins of narrow orbitonasal fenestra (1).

8 (10). Median contact of descending process of frontals: not in contact (0); in contact (1).

9 (11). Frontal tabs projection posteriorly over parietal: absent (0); present (1).

10 (12). Postfrontal: present as separate element (0); absent as separate element (1).

11 (13). Postfrontal forking: sub-triangular, not forked medially (0); semilunate, forked medially, clasping frontoparietal suture (1).

12 (16). Postorbital: present as separate element (0); absent as separate element (1).

13 (17). Postorbital contribution to posterior border of orbit: one half of posterior orbital border and has strong ventral process (0); less than one half of orbit and is temporal bone with reduced ventral process (1).

14 (18). Jugal-squamosal contact on supratemporal arch: no contact (0); contact (1).

15. Supratemporal fenestra: widely open and supratemporal arch present (0); open and no arch (1); closed, arch present (2).

16 (21). Parietals: paired (0); fused (1).

17 (22). Parietal tabs: absent (0); present (1).

18 (23). Parietal down growths anterior to epipterygoid: absent (0); present, extending to, or almost to the epipterygoids (1).

19 (26). Pineal foramen: present (0); absent (1).

20 (27). Posterior extent of maxilla: beneath orbits (0); just beyond anterior extent of orbits (1).

21 (30). Lacrimal foramen number: single (0); double (1).

22 (31). Anteroventral border of orbit: formed by maxilla with jugal confined to medial surface of maxilla (0); formed by jugal (1).

144

M. W. CALDWELL

23 (32). Jugal-postorbital bar: jugal large, postorbital bar complete (0); jugal reduced or absent, postorbital bar incomplete (1).

24 (33). Squamosal: present (0); absent (1).

25 (34). Dorsal process of squamosal: present (0); absent (1).

26 (35). Supratemporal: present (0); absent (1).

27 (36). Palpebral ossifications: absent (0); absent (1).

28 (37). Pterygoid lappet of quadrate: present (0); absent (1).

29 (38). Vomer fusion: absent (0); present (1).

30 (39). Vomer size: short, less than one half length maxillary tooth row (0); elongate, more than half of maxillary tooth row (1).

31 (40). Median contact of septomaxillae: widely separated (0); meet or nearly meet on midline in raised crest (1).

32 (41). Septomaxillae: flat or concave (0); expanded and convex (1).

33. Septomaxilla posterior extension: short posterior processes (0); long posterior processes contacting or close to prefrontals (1).

34 (42). Posterior border of opening for Jacobsen’s organ: not closed by bone (Palaeochoanate) (0); closed by contact of maxilla and vomer (neochoanate) (1); closed by contact of vomer and septomaxilla (2).

35 (43). Medial extensions of palatine forming air passages for bony secondary palate: absent (0); present (1).

36 (44). Choanal fossae of palatines: small in relation to palatine size (0); large (1).

37 (45). Ectopterygoid contact with palatine: no contact (0); contact (1).

38 (46). Ectopterygoid size and suborbital fenestra: slender and fenestra wide (0); expanded and fenestra restricted (1).

39 (47). Epipterygoid: present (0); absent (1).

40 (49). Alar process of prootic: short a(0); elongate (1).

41 (50). Supratrigeminal process of prootic: absent or weak (0); finger-like above notch (1).

42 (51). Opisthotic-exoccipital fusion: bone separate (0); fused (1).

43 (52). Enclosure of lateral head vein in bony canal formed by crista prootica: no enclosure (0); enclosure (1).

44 (53). Posterior opening of vidian canal: within basisphenoid (0); at basisphenoid-prootic suture (1); within prootic (2).

45 (54). Origin of jaw adductor musculature: dorsal surface of parietal (0); ventral surface of parietal (1).

46 (55). Meckel’s canal enclosure: dentary forms open groove (0); bony dentary tube formed by union of upper and lower borders of canal (1); dentary tube closed and fused (2).

47 (56). Intramandibular septum of Meckel’s canal: subdivision anterior to posterior end of tooth row, intramandibular septum poorly developed (0); subdivision occurs near posterior end of tooth row with well developed septum (1).

48 (57). Meckel’s canal exposure: medial (0); ventral (1).

49 (58). Sub-dental shelf size: small or absent (0); large (1).

50 (60). Dorsal extension of dentary coronoid process contact with coronoid: absent (0); present (1).

51 (61). Lateral view of disarticulated surangular: tapers anteriorly, pointed distally (0); expanded anterodorsally and nearly vertical at anterior margin (1); similar to (1) but anterior end of surangular terminates closer to level of coronoid eminence on surangular (2).

52. Contact between angular and splenial: overlapping-interdigitating (0); no overlap, vertical articular faces (1); vertical faces, with lateral lappet from angular behind splenial [Lanthanotus condition] (2).

53 (64). Dentary-Postdentary articulation, in medial view: extensive overlap, tongue and groove (0): reduced overlap (1).

54 (67). Splenial-dentary suture: extensive bone-bone contact (0); reduced bone-bone contact (1).

55 (69). Anterior extension of coronoid: short (0); elongate (1).

56 (70). Anterior end of coronoid: clasps dentary laterally and medially (0); meets dentary directly, no overlap (1); only clasps dentary medially (2).

57 (72). Angular: present (0); absent (1).

58 (73). Prearticular crest: absent (0); present (1).

59 (74). Retroarticular process: sulcus or pit present (0); absent (1).

60 (79). Retroarticular process torsion: not twisted (0); twisted (1).

61 (81). Adductor fossa size: small or moderate (0); expanded, inflated, widely open (1).

SQUAMATE PHYLOGENY

145

62. Mandibular symphysis: bony joint formed (0); no bony joint (1).

63 (82). Palatine teeth: present (0); absent (1).

64 (83). Pterygoid teeth: present (0); absent (1).

65 (84). Marginal tooth implantation: pleurodont (0); acrodont (1).

66 (85). Marginal tooth replacement: lingual, large resorption pits (iguanid type)(0); posterolingual, small pits (1); posterolingual no pit (2); posterolingual, no pit, tooth rotates from horizontal to vertical (3).

67 (86). Basal infolding of marginal teeth: non-plicidentine (0); plicidentine (1).

68 (87). Step or offset in tooth margin of maxilla: absent (0); present (1).

69 (88). Scleral ossicle number: more than 14 (0); less than 14 (1).

70 (91). Second ceratobranchials: present (0); absent (1).

71 (92). Condyle-cotyle orientation: little or slight obliquity (0); strongly oblique (1).

72 (94). Centrum constriction anterior to condyles: not constricted (0); constricted (1).

73 (96). Zygosphenes and zygantra: present (0); absent (1).

74. Number of cervical vertebrae: 7–9 (0); 6 or less (1).

75 (97). Cervical intercentra position: intervertebral (0); under anterior part of following centrum (1); under posterior part of preceding centrum (2).

76 (99). Posterior trunk intercentra: present (0); absent (1).

77 (100). Transverse processes of caudal vertebrae: single pair or two pairs converging (0); two pairs diverging transverse processes; anterior pair of transverse processes absent (1); anterior pair of transverse processes absent (2).

78 (103). Autotomy septa in caudal vertebrae: present (0); absent (1).

79 (106). Number of presacrals: less than 26 (0); greater than 26 (1).

80 (109). Rib attachments on sternum: five (0); four (1); three (2); two or less (3).

81 (110). Postxiphisternal ribs: none continuous (0); some continuous (1).

82 (111). Scapular emargination: absent (0); present (1).

83 (114). Epicoracoid cartilage extent: contacts mesoscapula and suprascapula (0); no contact (1).

84 (115). Clavicle: present (0); absent (1).

85 (116). Clavicle shape: simple curved rod (0); angulated curving anteriorly (1).

86 (117). Dorsal articulation of clavicle with scapula: present (0); absent (1).

87 (118). Interclavicle: present (0); absent (1).

88 (11). Interclavicle lateral process: present (0); absent (1).

89 (120). Interclavicle shape and size of anterior process: t or anchor-shaped, anterior process small or absent (0); cruciform, large anterior process (1).

90 (121). Sternal fontanelle: absent (0); present (1).

91 (122). Ectepicondylar foramen: present (0); absent (1).

92 (125). Postcloacal bones: absent (0); present (1).

93. Osteoderms: absent (0); present (1).

94 (130). Long bone epiphyses: present (0); absent (1).

95. Pubis: short, symphysial process short, ventrally directed, pubic tubercle posterodorsally placed (0); long, symphysis narrower, ventrally directed, pubic tubercle anteroventral (1); as 1 but symphsial process elongate and anteriorly directed.

146

M. W. CALDWELL APPENDIX 2

Question marks indicate missing character states (usually for fossil taxa). Hyphens indicate character states where the character is not applicable. For polymorphic characters, letters are used in the data matrix to represent the various state combinations: A=(0,1); B=(0,1,2); C=(0,2); D=(1,2); E=(1,3); F=(0,1,2,3)

Agamidae Anguidae Chamaeleonidae Cordylidae Dibamidae Gekkonoidea Eublepharinae Gymnophthalmidae Heloderma Other Iguanians Lacertidae Lanthanotus Scincidae Teiidae Varanus Estesia Xantusiidae Xenosaurus Shinisaurus Amphisbaenia Alethinophidia Scolecophidia Dinilysia Mosasauroidea Coniasaurus Sphenodon

Agamidae Anguidae Chamaeleonidae Cordylidae Dibamidae Gekkonoidea Eublepharinae Gymnophthalmidae Heloderma Other Iguanians Lacertidae Lanthanotus Scincidae Teiidae Varanus Estesia Xantusiidae Xenosaurus Shinisaurus Amphisbaenia Alethinophidia Scolecophidia Dinilysia Mosasauroidea Coniasaurus Sphenodon

5

10

15

20

25

30

35

40

00011 0AA1A 10A11 0A010 01000 0AAA0 01010 0A01A 00100 0001A 0100A 10100 00A00 0A010 11000 00100 00000 00011 00011 01100 00A00 A0000 00000 01010 ??010 00000

10001 01A00 100A1 01000 01001 01A00 01100 01A10 01100 1A000 01001 01000 01000 00000 01101 01001 01A01 01001 00001 01101 01101 01101 01100 00011 0001?

00000

-0010 10100 -1010 10102 11--1 11101 11101 10100 11101 00010 11102 11101 1A112 00110 11100 1?100 11102 11110 11100 11--1 -00-1 -1--1 10001 11100 ????0 10000

1A000 1000A 1A0A0 1A1A0 11011 00010 10010 11110 10011 1A0A0 110A0 10011 1AAA0 111A0 10001 10011 A11A1 10000 10000 1?0A0 10010 10010 1??10 11001 11??1 00000

0A000 01AA1 0A000 0A001 --1A1 001A1 -1101 01001 01001 01000 0A001 11001 0AA01 0100A 11101 01001 00001 01000 01001 01A01 0011-011001101000 ???0?

00000

A0100 A1101 00110 0A101 10101 101A1 A0101 000A1 00001 00A00 01001 00101 0A1A1 00001 01101 0??01 00111 00101 01100 10101 00101 10101 00101 00101 ???01 10000

00000 11000 00000 11000 11021 11000 11000 11000 11000 00000 11000 11010 11001 11000 11010 11010 11000 11000 10000 11010 11120 11120 11??0 11100 ???0?

00000

000A0 10001 00010 10001 110A0 1A001 11001 10001 11101 000AA 10001 11101 10001 11101 11A01 111??

11101 10001 10001 1A0A1 10011--11100110001 ?????

00000

45

50

55

60

65

70

75

80

A100A 0101A 0100A 010A1 00020 01A01 01101 01A0A 01011 A100A 01011 01010 010A1 01000 0100A ????0 011A1 01001 01010 010?0 01000 01000 01?00 01010 ?????

00000

00000 0110A 0AA00 A0011 2?-11 2?-10 20-10 C00A0 01100 B0000 00010 01100 B001A A0010 01100 ???00 20-11 01101 01101 B?-0A 00010 00010 00010 01010 01000 0?000

00000 00000 000-0 00000 000-0 00001 00001 00001 20110 00000 00000 22111 00000 00000 10111 2?10 000-0 00000 00000 00000 01110 01110 0111?

01111 01111 000-0

00000 00011 000-0 -00A1 -1011 0101A 0A011 002A0 00011 AA0A0 00100 10011 -A011 00200 00011 000?1 -1100 00011 00010 AA011 1A011 1A011 ?00??

10011 00?1?

000--

00111 00AA0 00111 001A0 00110 00110 00110 101A0 00A00 00AA0 101A0 00000 A01A0 101A0 00110 00110 00110 00110 00100 0011A 010A0 01110 01000 01100 0??00 00011

-001A 10011 -0011 0001A 10011 0000A 000A0 A011A 21011 00A1A 00110 21011 A0011 A011A 21001 210??

00010 10011 10011 D001A 30011 30011 ?0???

1001? ?00??

-0000

0A101 001A2 00110 00A0A 00110 0010C 00100 00001 10102 00A0A 0000A 11102 000A2 0A001 11102 ?????

00101 00102 00102 001-2 0A0-2 000-2 000??

00002 000??

--000

1010B 10A11 10103 1A0A0 10013 100AE 100A1 110A1 10111 1AA0F 110A0 10113 10A11 110A1 10112 ?????

AA001 10111 10000 10A13 10111011????10110 ?????

00001

SQUAMATE PHYLOGENY

147

APPENDIX 2 contd

Agamidae Anguidae Chamaeleonidae Cordylidae Dibamidae Gekkonoidea Eublepharinae Gymnophthalmidae Heloderma Other Iguanians Lacertidae Lanthanotus Scincidae Teiidae Varanus Estesia Xantusiidae Xenosaurus Shinisaurus Amphisbaenia Alethinophidia Scolecophidia Dinilysia Mosasauroidea Coniasaurus Sphenodon

85

90

95

00A00 0A001 1111 A000A ----AAA01 A1001 A0001 00101 AAA00 A0101 00101 AA001 0A001 0010A ?????

00001 00001 00001 001A--------?????

0A1A0 ?0???

00000

A00AA 1A010 -1--A 10010 ----AAA1A 10010 10A11 101-0 A000A 1001A 10010 1A01A 10011 100AA ?????

1001A 10000 10010 11--0 --------????? ?0000 ?????

00000

00000 00112 10000 00112 -00-0AA11 0AA11 10012 00111 00A00 00112 00111 00112 10011 00A10 ??1??

00012 00111 00111 1001-00--00-?????

000?0 ??0??

00000

APPENDIX 3

Osteological characters of squamate taxa were obtained by examination of specimens and from available literature. Museum abbreviations: (AMNH) American Museum of Natural History, New York, New York; (BMB) Booth Museum of Natural History, Brighton, Sussex, England; (BMNH R), The Natural History Museum (British Museum), London, England; (FMNH) Field Museum of Natural History, Chicago, Illinois; (USNM) National Museum of Natural History, Smithsonian Institution, Washington, D.C. S: typhlopids (Typhlops AMNH 3001, 11633) (Rieppel, 1978; 1979; 1980b; Estes et al., 1988); Anomochilus (Cundall & Rossman, 1993; Cundall, Wallach & Rossman, 1993); Dinilysia (Estes et al., 1970). A: acrochordids: (Acrochordus USNM 347549); anilioids (Anilius scytale FMNH 11175); (Cylindrophis rufus FMNH 13100; Cylindrophis USNM 297456), colubrids (Elaphe obsoleta RTMP 90.7.190); hydrophiids, (Aypisurus laevis AMNH 86176, Laticauda colubrina AMNH 81880), booids (Python sp., USNM 220308; Boa sp. AMNH 57476; USNM 220299; 220300). A: Elgaria multicarinatus (USNM 11298; 292548); Ophisaurus apodus (FMNH 22088); Shinisaurus crocodilurus (AMNH 44928); Xenosaurus grandis (USNM 111531). V: Varanus komodoensis (NMNH 228163); Varanus salvator (FMNH 31358); Varanus rudicollis FMNH 145710; Heloderma suspectum (FMNH 218077; NMNH 228171); Lanthanotus borneensis (FMNH 134711). T: Dracaena guianensis (FMNH 207657;22365); Tupinambis tequixin (FMNH 98759). G: Cnemidophorus sexlineatus (FMNH 98505–98507).

C: Cordylus sp. (RTMP T–20); Gerrhosaurus flavigularis (USNM KdQ 134). S: Eumeces obsoletus (USNM 313463); Corucia zebrata (USNM 120164). L: Lacerta lepida (USNM 279861).

X: Xantusia riversiana (USNM 313463). O : Anolis carolensis (RTMP T160); Dipsosaurus dorsalis (RTMP, T343); Phrynosoma douglassi (RTMP T17); Ctenosaura similis (FMNH 211849); Conolophus subcristatus (FMNH 22406); Amblyrhynchus cristatus (FMNH 15072). A: Uromastyx aegypticus (FMNH 63961); Physignathus draconoides (RTMP 90.7.347). C: Chamaeleo oweni (FMNH 25408); Chamaeleo jacksoni (FMNH 206753). G: Gekko gekko (FMNH 14448); Hemitheconyx sp.

(RTMP T352); Pygopus nigriceps (USNM 292076). D: Dibamus novaeguineae (USNM 305914).

A: Amphisbaenia caeca (USNM 129269); Rhineura floridana (USNM 220289).