Squamata M. Oppel 1811 [K. de Queiroz and J. A. Gauthier], converted clade name 
Registration Number: 101 2012). By contrast, analyses based on DNA-
sequence data place the root of the squamatan 
De!nition: !e largest crown clade containing tree within “Scleroglossa,” usually with either 
Lacerta agilis Linnaeus 1758 but not Sphenodon Dibamidae or Gekkota, or a clade composed 
(originally Hatteria) punctatus (Gray 1842). of both taxa, as the sister to all other squa-
!is is a maximum-crown-clade de"nition. matans (e.g., Townsend et al., 2004; Vidal and 
Abbreviated de"nition: max crown ∇ (Lacerta Hedges, 2005; Wiens et al., 2010; Pyron et 
agilis Linnaeus 1758 ~ Sphenodon punctatus al., 2013; Reeder et al., 2015). In the context 
(Gray 1842)). of these alternative hypotheses, the two pri-
mary extant subclades are di#erent and in one 
Etymology: Derived from the Latin squama case have been given the names Dibamia and 
(scale) + -ata (provided with), thus, “scaled.” Bifurcata (Vidal and Hedges, 2005). Squamata 
is composed of 10,078 currently recognized 
Reference Phylogeny: Figure 1 of Gauthier et extant species (Uetz, 2016), thus rivaling Aves 
al. (2012) is the primary reference phylogeny and surpassing its sister clade, Sphenodon (with 
(see also Estes et al. 1988: Fig. 6; Lee, 1998: only one currently recognized extant species), 
Figs. 1–2; Conrad, 2008: Fig. 56); however, by four orders of magnitude. Somewhat dated 
our de"nition has been formulated so that it lists of extinct species are available for Serpentes 
will apply to a clade of identical composition in (Rage, 1984) and for non-Serpentes, non-mosa-
the context of phylogenies based on molecular saurian Squamata (Estes, 1983). 
(e.g., Townsend et al., 2004: Fig. 8; Vidal and 
Hedges, 2005: Fig. 1; Wiens et al., 2010: Fig. 4; Diagnostic Apomorphies: Gauthier et al. 
Pyron et al., 2013: Fig. 1) and combined mor- (1988) listed 69 + 5 (addendum) putative syn-
phological and molecular data (Reeder et al., apomorphies of Squamata relative to other 
2015: Fig. 1; Simões et al., 2018: Fig. 2) that extant diapsids, which are also diagnostic rela-
di#er regarding relationships within Squamata tive to most other extinct diapsids given that so 
(see Composition and Comments). Although few potential stem squamatans have been iden-
Lacerta agilis is not included in the primary ti"ed (see Pan-Squamata in this volume). Estes 
reference phylogeny, it is most closely related to et al. (1988) added 18 potential synapomor-
Lacerta viridis of the taxa included in that tree phies that have reversed in some squamatans. 
(see Baeckens et al., 2015). Gauthier et al. (2012) listed only 59, includ-
ing 20 that are shared with the putative stem 
Composition: Squamata was hypothesized squamatan Huehuecuetzpalli mixtecus (see Pan-
by Estes et al. (1988; see also Gauthier, 1982) Squamata, this volume), but this lower num-
to be composed of two primary extant sub- ber re$ects the near-absence of soft anatomical 
clades, Iguania and Scleroglossa, and those sub- characters in their study. Some of the most obvi-
clades have been corroborated by subsequent ous diagnostic apomorphies are paired male 
analyses based on morphological data (e.g., intromittent organs (hemipenes), loss of car-
Lee, 1998; Conrad, 2008; Gauthier et al., uncle (false egg tooth), mobile fronto-parietal 
Squamata 
joint (modi"ed in fossorial forms and mosasau- (1820) and Haworth (1825), Sauri of Gray 
roids), embryonic fusion of parietals (reversed (1825), Pedata, Tetrapoda, and Communipedis 
in some gekkotans, some xantusiids, and of Haworth (1825), Saurae of Gray (1827) and 
Sineoamphisbaena hexatabularis), mobile quad- Wagler (1828), Lacertae of Wagler (1830) and oth-
rate with peg-in-socket squamosal-quadrate ers, Lacertiformes of Bonaparte (1831), Lacertina 
articulation (lost in chameleons and modi"ed of Müller (1831), Squamati of Wiegmann 
in snakes), pterygoids and vomers separated by (1834), Saures of Swainson (1839), Lacertilia of 
palatines (reversed in some amphisbaenians and Owen (1842 [including Rhynchosaurus articeps], 
some polyglyphanodontians), very slender sta- 1866) and others, Saura of Gray (1845), Lacertia 
pes (reversed in some fossorial forms), slender of Owen (1845), Kionocrania of Stannius (1856) 
epipterygoid, subdivision of embryonic metotic and Huxley (1886), Lepidota of Jan (1857), 
"ssure into vagus (jugular) foramen posteriorly Autosaurii of Gegenbaur (1859), Autosauria of 
and recessus scalae tympani anteriorly, embry- Haeckel (1866), Lacertilia vera of Boulenger 
onic or early post-embryonic (dibamids and (1884, 1885–7), Eusauri of Gadow (1898), 
some xantusiids) fusion of exoccipitals and opis- Autosauri of Gadow (1898, 1901), Lacerti of 
thotics, coronoid eminence formed by coronoid Jaekel (1911), Lacertosauria of Tornier (1913), 
bone only (reversed in some snakes and some and Lacertidae of Nopcsa (1923) are partial 
amphisbaenians), keeled cervical intercentra, as well as approximate synonyms that refer to 
all ribs (including cervicals) single-headed, pro- taxa that exclude various highly modi"ed squa-
coelous vertebrae (reversed in some gekkotans), matans, such as snakes, amphisbaenians, other 
absence of trunk intercentra (reversed in some serpentiform species, and chameleons (in vari-
gekkotans and full-grown xantusiids), emargin- ous combinations). 
ated scapulocoracoid (reversed in Heloderma, Other partial (and approximate) synonyms 
amphisbaenians, chameleons, some dolicho- were used for a paraphyletic group made up of 
saurs, some dibamids, some pygopodids), and Iguania and Gekkota, which is associated with the 
absence of gastralia. same ancestor as Squamata on the reference phy-
logeny (but not on some of those based on molecu-
Synonyms: Numerous approximate synonyms lar data—see Reference Phylogeny). !ese include 
of Squamata match the modern concept to vari- Pachyglossi of Bonaparte (1840), Amblyglossae 
ous degrees in terms of composition. Squamosa of Fitzinger (1843), Pachyglossae of Gray (1845), 
of Latreille (1825, except for the inclusion of cae- Ascalabota of Camp (1923), and Iguaniformes of 
cilians), Saurophidia of Blainville (1835), Reptilia Hay (1930), although Gray’s (1845) Pachyglossae 
of Bonaparte (1840, 1841), Streptostylica of might alternatively be considered a partial synonym 
Stannius (1856) and Cope (1900), Lepidosauria of Lepidosauria given that it included Sphenodon 
and Diplophalli of Haeckel (1866, inclusion punctatus. Still other partial synonyms were used 
of Sphenodon unclear), Pholidota of Haeckel for a doubly paraphyletic group that excluded 
(1895), Sauria of Gadow (1898, 1901), and both Autarchoglossa and Chamaeleonidae; these 
Lyognathi and Lyognatha of Jaekel (1911) refer include Ascalabotae of Merrem (1820), Ascolabata 
to a taxon of more or less identical composition. of Gray (1825), Inextensilinguis of Haworth (1825), 
Sauria of MaCartney (1802) and others Pachyglossae of Weigmann (1834), and Pachyglossa 
(some of whom included crocodilians), Saurii of of Strauch (1887; who also included Xenosaurus 
Kuhl (1820) and others, Gradientia of Merrem and Heloderma). 
1094 
Squamata 
Comments: Many eighteenth and early nine- French vernacular name Bispeniens in refer-
teenth century naturalists based their pri- ence to the hemipenes, which are still consid-
mary groupings within ectothermic tetrapods ered diagnostic (see Diagnostic Apomorphies). 
(Amphibia of some authors, Reptilia of others) !e taxon was recognized inconsistently under 
on mode of locomotion, separating the slither- a variety of names by nineteenth century 
ers with small or no limbs (Serpentes, Ophidia, authors (e.g., Blainville, 1822, 1835; Latreille, 
and variants of those names) from the limbed- 1825; Haworth, 1825; Fitzinger, 1826; Gray, 
propelled walkers and crawlers (Gradientia, 1831a; Bonaparte, 1840, 1841, 1850; Stannius, 
Reptilia, and variants of those names). !us, 1856; Zittel, 1887–1890; Cope, 1889; Haeckel, 
squamatans with well-developed limbs (“typi- 1895), but during the twentieth century it 
cal lizards”) were commonly grouped with was recognized more consistently under the 
crocodilians and sometimes with turtles and name Squamata (e.g., Cope, 1900; Hay, 1902; 
even salamanders and frogs, rather than with Williston, 1917, 1925; Camp, 1923; Nopcsa, 
snakes, while those with long bodies and 1923; Romer, 1933, 1945, 1956, 1966; Gans, 
reduced limbs (e.g., Anguis, Amphisbaena, and 1978; Estes, 1983; Estes et al., 1988; Lee, 1998), 
sometimes even the amphibian Caecilia) were a use that continues in the present century (e.g., 
considered “snakes” (e.g., Linnaeus, 1758; Conrad, 2008; Wiens et al., 2010; Gauthier et 
Laurenti, 1768; Scopoli, 1777). !e “lizards” al., 2012). Several early explicit phylogenetic 
(under various names) were "rst separated from analyses based on morphology supported squa-
turtles (e.g., Brongniart, 1800; Cuvier, 1800; matan monophyly (e.g., Rage, 1982; Benton, 
Daudin, 1802–1803) and later from crocodil- 1985; Gauthier et al., 1988; Evans, 1988), a 
ians (e.g., Blainville, 1816, 1822; Merrem, 1820; result that has been corroborated by subsequent 
Gray, 1825). Derivation of snakes from within molecular and combined morphological and 
“lizards” was inferred by several authors (e.g., molecular studies (e.g., Townsend et al., 2004; 
Haeckel, 1866; Camp, 1923; McDowell and Hugall et al., 2007; Albert el al., 2009; Reeder 
Bogert, 1954), and the formal abandonment of et al., 2015; Simões et al., 2018). 
the paraphyletic group “lizards”, by that time !e name Squamata was "rst used by Oppel 
usually referred to as either Sauria or Lacertilia, (1811), who applied it to a group composed of 
was proposed by Estes et al. (1988). Although “lizards” (including crocodilians) and snakes. 
the paraphyly of “lizards” (relative to both Crocodilians (Loricata), were removed from 
snakes and amphisbaenians) is now highly cor- Squamata by Merrem (1820; see also Blainville, 
roborated and widely acknowledged, the group 1816), thus more closely approximating the 
continues to be treated as a taxonomic unit, modern composition of the taxon, and this pro-
although no longer given a formal name (e.g., posal was followed by most subsequent authors 
Vitt and Caldwell, 2014; Pough et al., 2016; who recognized a taxon designated by that 
Crother, 2017). name (e.g., Haworth, 1825; Fitzinger, 1826; 
Despite the long history of treating “lizards” Gray, 1831a; Bonaparte, 1841 [as a synonym]; 
and snakes as separate taxa, a close relation- Gravenhorst, 1843 [though he excluded snakes]; 
ship between them has also been recognized for Gray, 1844, 1845 [though he excluded amphis-
a long time. !e "rst author to recognize and baenians]; Bonaparte, 1850). When Sphenodon 
name a taxon composed exclusively of “lizards” punctatus (tuatara) was "rst described (Gray, 
(including amphisbaenians) and snakes appears 1831b), it was considered an agamid “liz-
to have been Blainville (1816), who used the ard” and thus implicitly part of Squamata. 
1095 
Squamata 
Cope (1864) separated Sphenodon (= Hatteria) the internal relationships in phylogenies based 
from Agamidae as Hatteriidae, and Günther on morphological (e.g., Estes et al., 1988; Lee, 
(1867) further separated it from all “lizards” 1998; Conrad, 2008; Gauthier et al., 2012; but 
(“Lacertilia”) as Rhynchocephalia, though both see Simões et al., 2018) versus molecular (e.g., 
authors retained Sphenodon within Squamata. Townsend et al., 2004; Vidal and Hedges, 2005; 
Cope (1875) increased the separation further Wiens et al., 2010; Pyron et al., 2013; see also 
still by including extinct taxa (Protorosauridae Reeder et al., 2015) data (see Composition). 
and Rhynchosauridae) within Rhynchocephalia, Despite these striking disagreements, data 
not recognizing Squamata, and placing turtles from both DNA sequences and phenotypic 
between Rhynchocephalia and “lizards” (and sources strongly support many of the same crown 
snakes) in his taxonomy. However, he later squamatan subclades: traditional Anguimorpha 
(Cope, 1889) recognized Squamata as a group plus Serpentes, Dibamidae, Gekkota, Iguania, 
including “lizards” and snakes but excluding Lacertoidea (including Amphisbaenia), and 
Sphenodon, and this arrangement has been Scincoidea. Estimating the timing of their 
adopted by most authors since the beginning divergence is complicated because their inter-
of the twentieth century (e.g., Cope, 1900; relationships are in dispute and their early 
Williston, 1917, 1925; Nopcsa, 1923; Romer, fossil records are sparse. A single partial, but 
1933, 1945, 1956, 1966; McDowell and Bogert, articulated, specimen from the mid-Triassic 
1954; Gans, 1978; Estes, 1983; Estes et al., (~240 Ma), Megachirella wachtleri (Renesto 
1988; Lee, 1998, 2005; Conrad, 2008). and Posenato, 2003), has recently been inferred 
Squamata is clearly the most appropriate name to be a stem squamatan (Simões et al., 2018). 
for the clade in question. Other candidate names It is the only potential pan-squamatan known 
(see Synonyms) either have seldom been used from the Triassic or Early Jurassic. Otherwise, 
since the eighteenth century (e.g., Saurophidia, the oldest pan-squamatan fossils that might 
Streptostylica, Lyognathi), are more appropri- be referrable to the total clades of any of the 
ately applied to di#erent clades (e.g., Pholidota, crown squamatan subclades are mainly partial 
Reptilia, Lepidosauria), or were used (most com- jawbones known from ~167 Ma (Bathonian, 
monly) for a paraphyletic group associated with Middle Jurassic; e.g., Caldwell et al., 2015). 
the same ancestor (e.g., Sauria, Lacertilia) and Relatively intact, or at least associated, remains 
thus do not correspond as closely in terms of of stem-members of three of them (Gekkota, 
composition (Squamata itself was used in this Scincoidea, Anguimorpha) are known from ~150  
way by Gravenhorst, 1843, and Haeckel, 1866). Ma (Kimmeridgian, Late Jurassic; e.g., Evans, 
Moreover, the name Squamata has previously 2003). Apart from Dibamidae, which has no fos-
been de"ned phylogenetically as designating the sil record, most of the rest are diverse and dispa-
same (crown) clade to which it is applied here rate by ~75 Ma (Campanian, Late Cretaceous; 
(Estes et al., 1988; Lee, 1998), although it has e.g., Gao and Norell, 2000). Crown squamatans 
sometimes been applied implicitly to a more appear to have been the dominant lepidosaurs in 
inclusive clade (see Conrad, 2008; Simões et terrestrial ecosystems since the Early Cretaceous 
al., 2018). We have de"ned the name using a (e.g., Evans and Matsumoto, 2015; and references 
maximum crown clade de"nition in the interest therein), while rhynchocephalians predominated 
of presenting a simple de"nition (one with few during the early Mesozoic (e.g., Cocude-Michel, 
speci"ers) that will nevertheless apply to the same 1963; Hsiou et al., 2015; and references therein; 
clade in the context of major di#erences between see also Lepidosauria, this volume). 
1096 
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Authors Primary Editor: Philip Cantino 
Kevin de Queiroz; Department of Vertebrate 
Zoology; National Museum of Natural 
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