Prognathodon overtoni (Williston, 1897)

Conrad, J. L., 2008, Phylogeny And Systematics Of Squamata (Reptilia) Based On Morphology, Bulletin of the American Museum of Natural History 2008 (310), pp. 1-182 : 130-144

overtoni

(figs. 54F, 55F)

DIAGNOSIS: These two species of Prognathodon included in this analysis form a clade diagnosed by 178(0) dentary ventrally convex along its long axis and 219(1) striated tooth crowns.

SECONDARY ANALYSES OSTEOLOGY- ONLY ANALYSIS

Some recent analyses based on subsets of this data set have recovered somewhat different phylogenetic hypotheses for specific parts of the squamate tree. Among these are analyses of Iguania ( Conrad and Norell, 2007a; Conrad et al., 2007), Gekkonomorpha ( Conrad and Norell, 2006a), and Anguimorpha (Conrad, 2006b). Because these analyses each focused on the placement of specific fossil taxa, they relied heavily upon osteological characters. These differing phylogenetic hypotheses (when compared with the current analysis) are also important because the current phylogenetic hypothesis is based on data obtained, in part, while researching those studies. Additionally, the current analysis relies heavily upon fossil taxa and nonosteological characters cannot be scored for those fossils. Because of all these things, both individually and in concert, I performed an osteology-only analysis. The results of this analysis are presented as fig. 60 (note that, where the osteology analysis is identical with the full analysis, some taxa have been collapsed into larger clades in the figure). Areas of divergence between the osteological analysis and the full analysis will be highlighted below.

The osteology-only analysis was run exactly as was the full analysis of all the characters and taxa. A total of 3,973 equally short trees were recovered, each with a length of 3,034 steps. Each of these trees had a consistency index (excluding uninformative characters) of 0.137 1 and a retention index of 0.710 0. Note that because the character/ taxon ratio has decreased, a decrease in consistency index is also expected.

IGUANIA ( FIG. 60A): Most cladistic analyses (morphology-based and molecular) have suggested a basal dichotomy between non-acrodontan iguanians (5 Pleurodonta; 5 Iguanidae sensu lato) and Acrodonta (see figs. 2, 3, 6, 7, 13; see also Conrad et al., 2007, Conrad and Norell, 2007a), but analysis of this hypothesis has generally been relatively weak. However, the full analysis presented above suggests that Acrodonta is nested within non-acrodontan iguanians. The osteology-only analysis reproduces the hypothesis of a basal dichotomy between Acrodonta and Pleurodonta (sensu Schulte et al., 2003; Conrad and Norell, 2007a; Conrad et al., 2007), but still suggests that hoplocercids are close to acrodontans. Additionally, this analysis supports the presence of a Cretaceous radiation of iguanians from the Gobi ( Conrad and Norell, 2007a), a hypothesis that was neither supported nor denied by the full analysis (figs. 54–56).

Opluridae is problematic. Opluridae is the monophyletic sister-taxon to a monophyletic Tropiduridae sensu Frost and Etheridge (1989) in the full analysis. Recent analyses have questioned this hypothesis ( Conrad and Norell, 2007a; Conrad et al., 2007), as does the osteological data presented here (fig. 60A). Indeed, the osteology-only analysis suggests that Chalarodon madagascariensis is nested within Polychrotidae sensu Frost and Etheridge (1989) and that Oplurus is nested within Tropiduridae . Given the distribution of extant iguanians, this topology implies separate invasions of Madagascar by American clades of iguanians. However, many nested fossil ‘‘pleurodontans’’ are from Asia (e.g., Ctenomastax , Igua , Polrussia ), suggesting that the biogeography of the group is more complex than it might appear based on extant taxa alone. Although this cannot be considered an argument in favor of oplurid polyphyly, it does offer some plausibility to the hypothesis.

The osteology-only analysis shows more complete resolution of the iguanian clades than the full analysis. For instance, it demonstrates the Cretaceous Gobi clade and resolves the tree supporting a hypothesis that Igua and Polrussia form a clade close to the tropidurid- Oplurus clade.

GEKKONOMORPHA ( FIG. 60B): The full analysis suggested that Parviraptor ( P. estesi and P. cf. estesi as described by Evans, 1994a) is a basal genus of gekkonomorph, falling between AMNH FR21444 and Gobekko cretacicus on the tree (figs. 54–56). However, Conrad and Norell (2006a) suggest that Parviraptor is a basal member of the lineage including Autarchoglossa and its stem taxa (Evansauria; see above). The osteologyonly analysis, instead, suggests that Parviraptor is a scincogekkonomorph basal to the gekkonomorph-evansaur split at the level of (and in a polytomy with) Scandensia ciervensis . Gobekko cretacicus and AMNH FR21444 are recovered as basal gekkonomorphs and the topology within Gekkota is identical to that of the full analysis.

Conrad and Norell (2006a) also suggest that Gobekko cretacicus is nested within Gekkota and that pulls the minimum divergence time of the primary gekkotan lineages fall in the Cretaceous. The placement of Gobekko cretacicus as a proximal outgroup to Gekkota does not refute that hypothesis, but it does remove all of the evidence supporting it.

SCINCOMORPHA ( FIG. 60B): The osteology-only analysis does not recover a monophyletic Scincomorpha. Instead, Tepexisaurus tepexii, Scincoidea (including snakes, amphisbaenians, dibamids, acontids, and feyliniids), and a clade composed of Lacertoidea, Cordyloidea, Pseudosaurillus becklesi , ‘‘ Pseudosaurillus ’’ sp. sensu Estes, 1983, and Anguimorpha form a polytomy. In this topology, Parmeosaurus scutatus is a scincoid, Slavoia darevskii is the outgroup to Scincophidia, and Bainguidae is a basal radiation of Lacertoidea. Inclusion of bainguids in Lacertoidea and of Parmeosaurus scutatus in Scincoidea based on osteology is more in line with the traditional views of these taxa.

CARUSIOIDEA AND ANGUIDAE ( FIG. 60C): Conrad (2006b) did not recover a monophyletic Carusioidea, but suggested that Carusia intermedia was a basal member of Anguimorpha (outside of the crown group). The current full analysis and the osteology-only analysis each recovers a Carusioidea, but the placement of that group varies between the two phylogenetic hypotheses. The full analysis places carusioids and anguids as a clade exclusive of Platynota, but the osteology-only analysis suggests that Carusioidea and the anguid clade (including glyptosaurs, see below) are successively more proximal outgroups to Platynota. The latter hypothesis is more similar to that of Gao and Norell (1998) (fig. 11) and Conrad (2005a, 2006b). Note that Shinisaurus is never a carusioid in the current analyses (figs. 54–56, 60) (contra Gao and Norell, 1998). The differences in topology between the present and previous analyses are probably related to the inclusion of numerous basal anguimorphs and/or scincomorphs (e.g., Becklesius , Paramacellodus , Parmeosaurus scutatus , Pseudosaurillus ) and their effects on character polarities near the base of the anguimorph tree.

The full and osteology-only analyses show numerous minor differences in the placements of fossil anguids (including glypto- saurs). The phylogenetic placements of Apodosauriscus and Parophisaurus are unresolved in the osteology-only analysis. Parophisaurus is recovered in a trichotomy with Anguidae sensu stricto and glyptosaurs. Numerous differences exist between the two trees in the placements of specific glyptosaurs. Notably, the osteology-only analysis suggests that the ‘‘melanosaurs’’ are more closely related to Glyptosaurus and Proglyptosaurus than the latter two taxa are to the Helodermoides - clade. The addition of data for Placosaurus and related, non–North American glyptosaurines, may help resolve this issue.

The full analysis suggests that glyptosaurs are deeply nested within Anguidae , the sister group to diploglossines (fig. 54D, 55D, 56D), but the osteology analysis suggests that glyptosaurs fall outside of the anguid crown group. If the latter is true, then taxonomy becomes an issue for this group. As described above, anguids traditionally are defined as a crown clade, but glyptosaurs are usually considered anguids. Camp (1923) considered Anguidae and Glyptosauridae to represent distinct ‘‘families’’ (fig. 1), but that distinction has not been widely followed since McDowell and Bogert (1954). Regardless, this semantic issue will be investigated further elsewhere.

PLATYNOTA ( FIG. 60D): The osteologybased analysis is generally quite similar to the full analysis for Platynota. Dorsetisaurus and shinisaurs remain platynotans in both hypotheses. Minor differences include the placement of some taxa and polytomies, such as the specific placement of Primaderma , within monstersaurs and of ‘‘ Saniwa ’’ feisti and Necrosaurus cayluxi within goannasaurs. Importantly, the osteology-only analysis does not recover a mosasauriform position for Eosaniwa and leaves the placement Paravaranus unresolved with respect to varaniforms and mosasauriforms. However, the placement of mosasaurs within Goannasauria is confirmed.

DEFORMATION COMPARISONS

Several previous analyses of squamate relationships are described above and major differences between the phylogenetic hypotheses are highlighted. The current phylogenetic hypothesis was further compared with two

Fig. 60.

morphological analyses ( Evans and Barbadillo, 1999; Lee and Caldwell, 2000) and two molecular analyses ( Townsend et al., 2004; Vidal and Hedges, 2005) through deformation analyses of tree topology and length. Branches of the tree were manipulated and changes in tree length given the current data matrix were reported in the computer program Mesquite ( Maddison and Maddison, 2006). Only the branches that were represented in each cited analysis (e.g., Evans and Barbadillo, 1999) were moved; the rest of the topology stayed as is within the current Adams consensus hypothesis (fig. 55).

MORPHOLOGY: The current phylogenetic hypothesis, reformed such that it is consistent with that of Evans and Barbadillo (1999), Continued.

lengthens the tree by 72 steps. To retrieve the topology presented by Lee and Caldwell (2000) requires an additional 106 steps. Making the limbed, Cretaceous, snakes form a basal clade outside of crown-group snakes (and changing nothing else) requires a tree 22 steps longer. Moving this clade into a position as the sister group to a polytomy with dolichosaurids, the Adriosaurus - Pontosaurus clade, and higher mosasauroids requires another 37 steps, for a total of 59 steps longer than the Adams tree presented here. Forcing shinisaurs into the more traditional position of being xenosaurids requires the addition of 10 steps.

MOLECULAR DATA: Only subtle differences are present between the studies of Town- Fig. 60.

send et al. (2004) and Vidal and Hedges (2005), but the taxonomic inclusiveness does vary. Forcing the current topology to reflect that of Townsend et al. (2004) requires an additional 175 steps. Deforming the Adams consensus of the current analysis to resemble that of Vidal and Hedges (2005) adds 171 steps to the tree.

Continued.

BREMER SUPPORT

Numerous fossil taxa (e.g., Chamops , Sakurasaurus shokawensis , Restes ) included in this analysis are so poorly known that only a few characters may be scored for them. These taxa were included if they were diagnostic from all other taxa in some way

Fig. 60.

for reasons described above in the materials and methods section. However, the incomplete nature of these fossils means that they may be somewhat volatile within the phylogenetic tree (as evidenced in comparing the strict and Adams consensuses; figs. 54, 55). In many cases only one additional step is necessary to change their position in the phylogenetic topology and, thus, collapse a number of nodes. One way to deal with this problem would be to delete these problematic taxa. However, again as described above, their deletion might be deleterious to the analysis as a whole and is not desirable. Bremer supports are listed for the strict consensus tree (fig. 55).

DISCUSSION AND CONCLUSIONS PHYLOGENETIC HYPOTHESIS

Squamate relationships as identified from the present analysis differ somewhat from all previous analyses. Some of the major differences will be highlighted below.

BASIC TREE STRUCTURE: Importantly, if all extinct taxa are ignored, the basic structure of the tree (fig. 61) is similar to Continued.

that of Estes et al. (1988) (fig. 2) with similarly applied names to the major clades. Iguania, Scleroglossa, Gekkota, Autarchoglossa, Scincomorpha, Lacertoidea, Scincoidea, Amphisbaenia, Dibamidae, Serpentes, and Anguimorpha are recovered as monophyletic. However, the current analysis resolves the position of amphisbaenians, dibamids, and snakes whereas Estes et al. (1988) does not (fig. 2A). Moreover, several new clades are recognized, including the Bainguidae and the Scincophidia.

GEKKONOMORPHS, SCINCOMORPHS, AND SNAKE ORIGINS: In contrast to the hypotheses put forward by Evans and Barbadillo (1998, 1999), the amphisbaenian-dibamidsnake clade is not closely related to geckos in this analysis. Xantusiidae are basal members of the Lacertoidea, in contrast the findings of some recent analyses ( Presch, 1988; Evans and Barbadillo, 1998; Lee, 1998, 2000; Lee and Caldwell, 2000; Vicario et al., 2003).

Scincomorpha is found to be monophyletic, in contrast to some more recent analyses ( Lee, 1998, 2000; Lee and Caldwell, 2000; Townsend et al., 2004; Vidal and Hedges, 2005). According to the present study, Scincomorpha includes Dibamidae , Amphis- baenia, and Serpentes . Thus, the present analysis does not support a close relationship between snakes and mosasaurs as has been suggested by some morphological analyses ( Lee, 1997, 1998, 2000; Caldwell, 1999a; Lee and Caldwell, 2000; Lee and Scanlon, 2001; Scanlon and Lee, 2002; Caldwell and Dal Sasso, 2004) or with an anguimorph-iguanian group as suggested by molecular data ( Townsend et al., 2004; Vidal and Hedges, 2005). Instead, snakes are nested in a group of limbless and limb-reduced scincoids, including feyliniids, acontids, dibamids, and amphisbaenians. Numerous synapomorphies support this hypothesis, but the lack of a fossil record for most clades is somewhat worrisome. It is possible that future discovery of basal members of any of those clades ( Feyliniidae , Acontidae, or Dibamidae ) may show that the known extant taxa are convergent in their morphology. Additionally, more inclusive taxon sampling will be necessary to analyze the position of limbreduced gymnophthalmids and lacertids with regard to amphisbaenians, dibamids, and snakes.

ANGUIMORPHA: Among anguimorphs, Xenosauridae is decidedly distinct from shinisaurs. Shinisaurs are found to be basal platynotans.

Although Caldwell (1999a) suggested that mosasaurs might fall outside of Scleroglossa, the current analysis supports the more common placement of mosasaurs as derived varanoids (contra Caldwell, 1999a). Importantly, some ‘‘necrosaurid’’ taxa are more closely related to the mosasaur clade than to any extant radiation (e.g., Varanidae or Shinisauridae ).

WHY THE DIFFERENCES?: Differences in topology between this and other recent analyses of squamate phylogeny (e.g., Estes et al., 1988; Wu et al., 1996; Evans and Barbadillo, 1997, 1998; Lee, 1998, 2000; Caldwell, 1999a; Lee and Caldwell, 2000) probably result from more taxon sampling in this analysis and, perhaps to a lesser degree, from character selection. Character selection probably bears less of the impact on differences in the topological tree than does taxonomic selection for many reasons. First, the present analysis and all those listed above draw heavily from the data set of Estes et al. (1988). Second, in addition to the inclusion of several new characters, the present analysis has been designed with the intention of including all of the nonredundant, informative, characters used in the described earlier studies. Thus, there is extensive overlap between previous analyses and this analysis. Third, taxonomic selection has varied widely in the previous analyses described and the current analysis has been designed with the intention of including all of the previously analyzed taxa. Fourth, new taxa have been incorporated in this analysis (e.g., Lakumasaurus , Parmeosaurus scutatus , Temujinia ) that were not available to those researchers creating the earlier data matrices.

SCINCOPHIDIA, TAX. NOV.

According to this analysis, Feyliniidae and Acontidae, and the more problematic Dibamidae, Amphisbaenia, and Serpentes form a clade termed Scincophidia within Scincidae sensu lato. This topology seemingly represents a marriage of thought between the traditional anatomical studies suggesting that dibamids and amphisbaenians are scincomorphs and the recent cladistic studies identifying a potential relationship between dibamids, amphisbaenians, and snakes. It is unsurprising that the scincophidian clade has been previously unrecognized given the taxonomic sampling of earlier analyses.

Lee (1998, 2000) argues that the similarities between snakes, dibamids, and amphisbaenians is an example of convergence influenced by a fossorial lifestyle. He states that ‘‘nearly all of the characters supporting this arrangement are correlated with headfirst burrowing … and invariably co-occur in other tetrapods with similar habits’’ ( Lee, 1998: 369). It follows, then, that the unusual suite of characteristics associated with burrowing will cause unrelated fossorial forms to cluster together on a cladogram and that all fossorial squamates would share most or all of these character states, possibly recovering an erroneous topology. This is certainly a legitimate concern requiring analysis.

Importantly, the only specialized headfirst burrowers Lee (1998, 2000) includes in his analyses are Pygopodidae, Amphisbaenia , and Dibamidae . He codes scincids sensu lato, Anguidae , and Serpentes (exclusive of Pachyrhachis problematicus ) without breaking them into constituent clades. In doing so, he eliminates four major fossorial squamate radiations included here ( Feyliniidae , Acontidae, anguines, and Scolecophidia) that could be used to further analyze his hypothesis of a convergent ecomorph. He also constrains Pachyrhachis problematicus to fall outside of crown-group Serpentes and, through his character codings, indicates his a priori assessment that fossorial snakes are not basal. Finally, Lee’s (1998, 2000) answer to the perceived problem of the fossorial ecomorph is to downweight all of the characters he considers to represent fossorial adaptations. This requires an a priori judgment about which characters are associated with headfirst burrowing.

A NEW TEST FOR THE FOSSORIAL ECO- MORPH: The phylogenetic analysis above presents a hypothesis in which headfirst, limb-reduced burrowing appears no fewer than four times (within Gekkota, twice within Anguidae , and within Scincoidea), demonstrating that Lee’s (1998, 2000) ecomorph problem is not a major concern for the current data matrix; and even this hypothesis neglects to assess the placement of the limbless lacertoids and cordyliforms. A further analysis of the current data matrix was used here to determine the role of the fossorial ecomorph in the current analysis.

If Lee’s (1998, 2000) strategy of downweighting characters contributing to the fossorial ecomorph is accepted, then making assumptions about which character states to include and therefore which characters to downweight or delete is problematic. In the context of the present analysis, it is unnecessary to determine exactly which characters might contribute to a fossorial lifestyle because extant forms are readily recognized as fossorial or not. Nullifying the impact of the fossorial ecomorph may be accomplished by deleting all taxa in the current analysis except for the limb-reduced, fossorial forms (extinct or extant and fossorial; see below). This was accomplished in two analyses by deleting taxa of variable limb robustness and two analyses including only those taxa that are suspected of being closely related to snakes. The limbed outgroup Rhynchocephalia was always retained. All limbless snakes were always retained because even snake taxa that are not exclusively fossorial practice some headfirst burrowing. (The strictly marine hydrophiines do not burrow, but are universally considered derived colubroids and were not specifically considered here.) Four analyses were then run with taxonomic inclusions as listed below.

1. The first analysis included the ingroup taxa Acontidae, Amphisbaenidae, Anilioidea , Blanus , Dibamidae ( Anelytropsis papillosus and Dibamus ), Dinilysia patagonica , Feyliniidae , limbless anguids ( Anniella , Anguis , Dopasia , Ophiodes , ‘‘ Ophisaurus ’’ attenuatus , Ophisaurus ventralis , and Pseudopus ), limbless macrostomatans, Pygopodinae ( Aprasia , Delma , Pletholax , and Pygopus ), Rhineuroidea, Scelotidae, Trogonophidae , and Wonambi naracoortensis .

2. The second analysis included all the taxa from the first analysis and the bipodal taxa Bipes and pachyrhachids.

3. The third analysis was used to analyze the position of mosasaurs when most limbed forms are deleted. All taxa were deleted except for those listed in the first two analyses and all of the Mosasauria.

4. The final analyze excluded the limbless anguids, but included all of the other taxa included above and added the limbed gekkotans ( Diplodactylinae , Eublepharidae , and Gekkonidae ), Scelotidae, and Scincidae .

Snakes, amphisbaenians, and dibamids formed a clade in all of these analyses. Successively more distant outgroups in the first three analyses were pygopodines and a clade including Acontidae, Feyliniidae , and Scelotidae. The fourth analysis recovered a clade containing Scincidae sensu lato as the sister group to the snake-amphisbaeniandibamid clade with a monophyletic Gekkota as the next outgroup. Limbless anguids were invariably monophyletic, but were found to be closer to the other limbless taxa than to the mosasaurs in the third analysis. Mosasaurs were monophyletic in both analyses in which they were included and always represented the basalmost ingroup lineage.

These results demonstrate the cohesiveness of the amphisbaenian-dibamid-snake clade even after nullification of fossorial/limbless characters. Snakes are not mosasaurs or even anguimorphs in any iteration of these analyses. Although the topology of the tree is somewhat different from that of the full analysis, this is not unexpected given the number of deleted taxa.

Certainly, being fossorial is a contributing factor to the morphology of scincophidians, but ecology is expected to be represented in morphology and phylogeny. Varanoids are usually predators of relatively large prey, chameleons are specialized for an arboreal existence, and gekkonids are typically crepuscular or nocturnal predators. These animals show heritable morphological adaptations for these behavioral and ecological aspects of their biology. The same should not be surprising in the fossorial clade Scincophidia.

BASAL SCINCOGEKKONOMORPHS AND EVANSAURS

The current phylogenetic hypothesis invites re-interpretation of some known squamate radiations pursuant to more precise understandings of geckos, scincomorphs, and necrosaurs. Based on current evidence, the traditional understanding of the diagnostic characters of these groups is insufficient for referral of many fossil taxa. That is to say, for example, that many taxa that have been described as scincomorphs do not represent members of a monophyletic Scincomorpha. Instead, these misidentified taxa are important transitional forms representing intermediate morphologies between the major squamate clades. These misidentified taxa demonstrate the incremental acquisition of character states along a much broader span of squamate phylogeny than to which they are usually attributed. For example, Ardeosaurus and Eichstaettisaurus represent not true geckos, but basal scincogekkonomorphs possessing some characteristics usually attributed to the Gekkota.

Ardeosaurus , and Eichstaettisaurus are representative examples of many taxa close to the main trunk of the squamate family tree whose morphologies show a mosaic pattern of primitive and derived character states when placed in the context of Scincomorpha proper and Gekkota proper.

BASAL GEKKONOMORPHA?: Identification of ‘‘ardeosaurs’’ and ‘‘bavarisaurs’’ as basal scincogekkonomorphs rather than as stemgeckos demonstrates the poor quality of the gecko fossil record. Gobekko cretacicus remains the only well-preserved basal gekkonomorph described to date, but even this taxon seems very like modern geckos (see Borsuk- Białynicka, 1990; Conrad and Norell, 2006a) and offers little in the way of a transitional form between the basal scleroglossan condition and Gekkota. However, a new taxon from the Aptian-Albian of Mongolia (AMNH FR21444) was included in the current analysis and helps to polarize character states for Gekkonomorpha. This currently unnamed taxon possesses primitive characteristics such as a complete supratemporal arch and a toothed pterygoid, but also possesses characteristics shared with geckos and Gobekko cretacicus . Intermediate taxa such as this reduce the number of character states that may be used to diagnose a given clade by expanding the distribution of some characters and helping to bridge morphological gaps between previously known taxa. They also offer important insights into the relative timing of synapomorphy acquisition for clades and character evolution.

It is worth noting that Sereno (2006) does not consider AMNH FR 21444 (fig. 62) to represent a basal gekkonomorph, but suggests it as a possible basal squamate. He bases his assertion on ‘‘the narrow width of the nasals, the simple transverse frontoparietal suture, broad pyriform recess, and absence of [a] pterygoid-vomer contact’’ ( Sereno, 2006:124 A). Because Sereno (2006) is the only other study currently addressing this specimen, I will discuss this hypothesis and the characters used to support it here.

Nasal width is difficult to assess outside the context of some comparison (that is, narrow relative to what?). Evans and Barbadillo (1998) compared nasal width to the width of the external nares, but it is unclear if Sereno (2006) is also making that comparison. Regardless, no other group or species otherwise hypothesized to be near the basal squamate condition (by this or other studies) has particularly narrow nasals (e.g., iguanians, Bavarisaurus , Eichstaettisaurus (fig. 29), dibamids (fig. 31), or basal rhynchocephalians (fig. 32)); indeed, narrow nasals seem to be a varanoid characteristic. Moreover, the nasals are not preserved in AMNH FR 21444.

The suggestion that AMNH FR 21444 has a transverse frontoparietal suture ( Sereno, 2006) is erroneous (see illustrations and CT data in Conrad and Norell, 2006a, 2007b). Instead, this animal possesses a gently anteriorly arched frontoparietal suture.

A broad pyriform recess (as defined above, character 123) is plesiomorphic for iguanomorphs, gekkonomorphs, scincomorphs, and anguimorphs, with reversals in most of these groups. Presence of this character state in AMNH FR 21444 does not suggest that it is close to the basal squamate.

The specimen AMNH FR 21444 lacks as vomer-pterygoid contact as described by Sereno (2006). However, this character state is present in the majority of squamates (reversals within chamaeleontiforms and amphisbaenians, and in polyglyphanodontids and Shinisaurus crocodilurus ).

Thus, none of the character states suggest- ed by Sereno to place AMNH FR 21444 ‘‘just outside Squamata …’’ or ‘‘…at a basal position within Squamata ,’’ ( Sereno, 2006: 124 A) actually support that hypothesis. Indeed, these character states would not be useful for placing any taxon at the base of Squamata . Given the six unambiguous synapomorphies uniting AMNH FR 21444 with other gekkonomorphs listed above, it currently is most prudent to consider AMNH FR 21444 a basal gekkonomorph.

TAXONOMIC CONSIDERATIONS

STRINGENCY: Taxonomy is a tool for communicating about groups of things. Phylogenetic taxonomy has been, and will continue to be, an important tool with which to discuss organisms in a phylogenetic framework. Ideally, a single taxonomic scheme would be used for every given taxonomic group and that taxonomy would be based on the one true phylogeny of the group. Unfortunately, we are unlikely ever to know the one true phylogeny for any group with more than a few species. Therefore, taxonomists must be careful to make their nomenclatural schemes strict enough to be meaningful, but not so rigid that they are useless if some taxonomic content changes based on new discoveries and/or analyses.

Lee (1998) presents a taxonomic scheme in which any shifting of taxa between groups invalidates the meaning of two or more taxon names. This, or any similar stringency in taxonomy, makes taxonomy less useful as a tool for discussion of ideas or phylogeny.

CONTINUITY AND SUPERFLUOUS TAXON- OMY: New phylogenetic hypotheses sometimes require revisions in taxonomy, but the taxonomy of the squamate ‘‘families’’ has been relatively stable for well over 100 years.

Wallace (1876 a, 1876b) supplied a thencomprehensive list of squamate taxa including 2,256 species in 52 families. There is a remarkable correlation between that family list and the current understanding of squamate families, despite the fact that around 8,000 species of squamates are currently named ( Uetz, 2007) (fig. 63). Of course, there are some differences, but most of these include further subdivision of currently recognized families by Wallace (1876 a, 1876b), or vice versa rather than substantive differences in the included taxa. Other major differences include the recognition of the families Anguidae and Dibamidae , but Wallace’s (1876 a, 1876b) system remains useful even now. Similarly, Camp’s (1923) suprafamilial taxonomy remains useful (see part of it in fig. 1).

Vidal and Hedges (2005) recently proposed a radically different phylogenetic hypothesis for squamate interrelationships (fig. 12B), and applied new taxonomy to some groups. The phylogenetic hypothesis of Vidal and Hedges (2005), similar to that of Townsend et al. (2004), is important and intriguing given the dissimilarities between those hypotheses and the usual ideas of squamate interrelationships (for example, compare with figs. 2– 9, 53–56). However, much of the new +

the separation of approximately 130 years and since more than three times as many squamate species are now recognized. The major differences are mostly the result of identifying new squamate clades (in many cases, through the discovery of species) and subdivision of ‘‘families’’ or lumping them together. Clearly, taxonomy may be relatively constant and remain informative.

taxonomy presented by Vidal and Hedges (fig. 64) is unhelpful and gratuitous. Moreover, Vidal and Hedges (2005) offer no explanation for most of this taxonomy, leading to several problematic situations. Vidal and Hedges (2005) sampled within Dibamus , but applied to that branch the name Dibamidae (a name used to describe both Dibamus and Anelytropsis papillosus ) and Dibamia. Thus, the name Dibamia becomes an apparent synonym of Dibamidae or, conceivably, Dibamus . Lacertidae is also labeled Lacertiformata. The clade containing teiids and Gymnophthalmus underwoodi is labeled both Teiioidea and Teiformata. The clade formed by Teiioidea, Lacertidae , and Amphisbaenia in their tree is labeled Laterata ( Vidal and Hedges, 2005), even though this clade is essentially the same as the traditional idea of Lacertoidea (minus xantusiids; but see the usage of Lee, 1998 and Vicario et al., 2003). The Scinciformata of Vidal and Hedges (2005) is essentially the Scincoidea of Vicario et al. (2003) and is almost exactly the Scincoidea of Townsend et al. (2004).

CONCLUSIONS

The current study was undertaken with the intentions of supplying an extensive morphological phylogenetic data matrix for squamates, offering a phylogenetic hypothesis based on that matrix, and providing an updated and useful taxonomy. The data matrix provided here is the most taxonomically inclusive so far offered and it may be useful for morphologists as well as for systematists. The provided phylogenetic hypothesis will not be the last word on the subject of squamate phylogeny. Indeed, the matrix is already being expanded both in taxa and characters. Others may well analyze the provided data matrix differently, obtaining a different result. The phylogenetic hypothesis provided herein is no more than an accurate description of the data as it was analyzed. The taxonomy I provide reflects the usage of taxon names as I perceive them to be most often used. The definitions of existing taxon names in this study are offered only as a tool; a reference for the taxonomy as a whole. Taxonomy is most useful as a tool for discussing groups of animals, phylogenetic hypotheses, and ideas about evolutionary history.

Squamata is a clade of extraordinary diversity now and throughout its 210 million - year history. The wide geographic distribution of squamates, their prominence in modern and fossil ecosystems, and their remarkable morphological diversity must rank them as one of the most important vertebrate clades for continued scientific study.