Registration Number: 61
Definition: The smallest crown clade contain- ing Lacerta agilis Linnaeus 1758 (Squamata) and Sphenodon (originally Hatteria) punctatus (Gray 1842) (Rhynchocephalia). This is a minimum- crown-clade definition. Abbreviated definition:
min crown ∇ (Lacerta agilis Linnaeus 1758 &
Sphenodon punctatus (Gray 1842)).
Etymology: Derived from the Greek lepidos, scale, plus sauros, lizard, reptile.
Reference Phylogeny: Gauthier et al.
(1988a: Fig. 13), where Lacerta agilis is part of Squamata and Sphenodon punctatus is part of Rhynchocephalia. See also Gauthier (1984: Figs.
32–33), Evans (1984: Fig. 2, 1988: Figs. 6.1–
6.2), Rest et al. (2003: Fig. 3), Hill (2005: Fig. 3), Evans and Jones (2010: Fig. 2.1), Crawford et al.
(2012: Fig. 2), Jones et al. (2013: Figs. 3–4), and Simões et al. (2018: Fig. 2).
Composition: Lepidosauria is composed of two primary crown clades: the New Zealand endemic Sphenodon with one currently recog- nized extant species (Hay et al., 2010) and the globally distributed Squamata with approxi- mately 10,078 currently recognized extant species (Uetz, 2016). See Squamata and Pan- Squamata (this volume) for references regarding extinct species in those clades. Reviews of dis- parate and diverse fossil rhynchocephalians can be found in Jones et al. (2013), Apesteguía et al.
(2014), and Bever and Norell (2017).
Diagnostic Apomorphies: According to Gauthier et al. (1988a), Lepidosauria has at
least 35 apomorphies relative to other extant amniotes (only some of which are diagnostic relative to different stem lepidosaurs). These apomorphies derive from disparate anatomical systems, are apparently unrelated functionally and developmentally, and have persisted for hundreds of millions of years among lepido- saurs with remarkably divergent ecologies. They include the following apomorphies (those lack- ing citations are from Gauthier et al., 1988a):
(1) transversely oriented external opening of cloaca and loss of amniote penis (hemipenes of squamatans are neomorphic); (2) kidney in tail base and adrenal gland suspended in gonadal mesentery (Gabe, 1970); (3) lingual prey pre- hension; (4) sagittal crest of scales projecting from neck, body and tail; (5) scales composed of superimposed, rather than juxtaposed, amni- ote alpha keratin and reptilian phi keratin layers (Maderson, 1985); (6) skin shed regularly in its entirety; (7) prefrontal braces skull roof on pal- ate (Gauthier, 1994); (8) lacrimal bone largely confined to orbital rim; (9) maxilla broadly contributes to ventral orbital margin (Gauthier, 1994); (10) marginal teeth attached superficially to lingual surface of jaw (rather than in shal- low sockets; further modified to more apical attachment in some taxa); (11) teeth lost from transverse process of pterygoid and from sphe- noid bones (Gauthier et al., 1988b); (12) zygos- phene-zygantrum accessory intervertebral joints formed from dorsal extensions of zygapophysial surfaces (see Petermann and Gauthier, 2018);
(13) autotomic and regenerable tail (autotomy, but not regeneration, has also been reported in some captorhinids; LeBlanc et al., 2018); (14) all non-ossifying cartilaginous parts of skeleton cal- cify during postnatal ontogeny; (15) neomorphic
ossification centers in limb bone epiphyses that
fuse to diaphyses near maximum adult size;
(16) medial centrale larger than lateral central in wrist (Gauthier et al., 1988b); (17) radiale contacts 1st distal carpal or 1st metacarpal in hand (Gauthier et al., 2012); (18) 4th metacar- pal shorter than 3rd (symmetrical metacarpals);
(19) fenestrate pelvic girdle; (20) posterodorsally sloping ilium; (21) embryonic fusion between anlage of lateral centrale and astragalus in tar- sus; (22) astragalus and calcaneum fused in adult (and lack a perforating foramen between them as neurovascular system passes between tibia and fibula proximal to tarsus; Rieppel and Reisz, 1999); (23) absence of separate 1st distal tarsal in foot; (24) hooked 5th metatarsal—and absence of discrete 5th distal tarsal presumably incorporated into it—modified to act as both a
‘heel’ and a grasping ‘thumb’ enabling the 5th toe to rotate 90° with respect to rest of the foot according to Robinson (1975).
Synonyms: All synonyms are approximate (not phylogenetically defined). The names that fol- low were used after Sphenodon punctatus was first recognized (Gray, 1831, 1842) for taxa that explicitly included Sphenodon (sometimes as Hatteria or Rhynchocephalus) and squamatans:
Squamata of Gray (1845), partial (amphisbae- nians excluded); Saura of Gray (1845), partial (amphisbaenians and snakes excluded); Lacertia of Owen (1845), partial (snakes excluded);
Squamata of Cope (1864) and Günther (1867);
Lacertilia of Cope (1864) and Huxley (1886), partial (snakes excluded); Saurii of Gegenbaur (1874, 1878), partial (snakes excluded). In most of these cases (except for Günther, 1867), Sphenodon punctatus was considered nested within the taxon corresponding to Squamata.
Comments: Changing ideas about the rela- tionship between Sphenodon punctatus and Squamata have come nearly full circle. Originally
described as an agamid “lizard” (Gray, 1831, 1842), S. punctatus was later separated from aga- mids as Hatteriidae (Cope, 1864), and later still from “lizards” (and snakes) as Rhynchocephalia within Squamata (Günther, 1867). Cope (1875) furthered the separation, first, by not recog- nizing Squamata, second, by including the extinct protorosaurs (i.e., Protorosaurus speneri) and rhynchosaurs (i.e., Rhynchosaurus articeps) along with extant Sphenodon punctatus in his Rhynchocephalia, and third, by interposing tur- tles (Testudines) between Rhynchocephalia and
“lizards” (and snakes) in his taxonomy. Cope (1889) later continued this trend by separating Rhynchocephalia and Squamata as taxa of equal rank (although the taxa were adjacent in his list). Distancing them further still, Cope (1900) placed Rhynchocephalia and Squamata in sepa- rate higher taxa, Archosauria and Streptostylica, respectively, although his phylogenetic diagram (p. 160) had them closely related. In general, many late nineteenth and early to mid twen- tieth century authors treated Sphenodon and Squamata (not always using that name) as rel- atively distantly related among reptiles (e.g., Cope, 1875; Haeckel, 1895; Williston, 1917, 1925), or at least considered Sphenodon closer to rhynchosaurs (and sometimes also to choris- toderans) than to Squamata (e.g., Gadow, 1898, 1901; Jaekel, 1911; Nopcsa, 1923; Romer, 1933, 1945, 1956, 1966; Underwood, 1957; Kuhn, 1966). Dissolution of Cope’s idea that rhyn- chosaurs and “protorosaurs” are closely related to Sphenodon began with Hughes’ (1968) study of the rhynchocephalian tarsus and culminated in Carroll’s (1975) argument that characters traditionally used to unite Sphenodon punc- tatus (and its legitimate fossil relatives) with rhynchosaurs and “protorosaurs” are erroneous.
Subsequent authors working in an explicitly phylogenetic framework (Benton, 1982; Evans, 1984; Gauthier, 1984; Carroll, 1985) presented evidence that rhynchosaurs and “protorosaurs”
(including Protorosaurus speneri and Prolacerta broomi) are related to archosaurs while Sphenodon punctatus is closer to squamatans. Gauthier et al. (1988a) provided extensive morphologi- cal evidence for a close relationship between Sphenodon punctatus and Squamata based on a computer-assisted analysis of 171 characters in 13 taxa, and this result has been corroborated by subsequent studies based on morphology (e.g., Evans, 1988; Hill, 2005; Evans and Jones, 2010;
Gauthier et al., 2012), molecules (e.g., Rest et al., 2003; Crawford et al., 2012; Pyron et al., 2013), and combined morphological and molecular data (e.g., Jones et al., 2013; Reeder et al., 2015;
Simões et al., 2018).
The name Lepidosauria was proposed by Haeckel (1866) for what is now known as Squamata—that is, “lizards” (a paraphyletic group) and snakes. Although Sphenodon punc- tatus was not explicitly included, members of this taxon were originally thought to be “liz- ards” (e.g., Gray, 1831, 1842). After Günther’s (1867) study demonstrating major anatomical differences between Sphenodon (as Hatteria) and “lizards”, and the subsequent increas- ing taxonomic separation of these groups (see previous paragraph), the name Lepidosauria was seldom used, or it was used for the taxon now called Squamata (e.g., Zittel, 1887–1890;
Williston, 1904; Jaekel, 1911). Romer (1933) resurrected the name Lepidosauria for a taxon designated “validity doubtful” composed of Eosuchia (also considered of doubtful valid- ity), Rhynchocephalia (choristoderans, rhyn- chosaurs, and sphenodontians), and Squamata.
This arrangement became more solidly estab- lished in Romer’s subsequent publications (e.g., 1945, 1956, 1966), although choristoder- ans (= champsosaurids) were transferred from Rhynchocephalia to Eosuchia. Despite Sphenodon having been considered more closely related to (extinct) rhynchosaurs than to squamatans, it should be noted that Sphenodon and Squamata
were considered most closely related among extant taxa. Therefore, when rhynchosaurs were allied to Archosauria and evidence was presented for an exclusive relationship between Rhynchocephalia and Squamata, the name Lepidosauria was applied to the group includ- ing the latter two taxa (e.g., Benton, 1982, 1983, 1985; Gardiner, 1982; Evans, 1984; Gauthier, 1984; Gauthier et al., 1988a; Pritchard and Nesbitt, 2017).
Selection of the name Lepidosauria for the Sphenodon + Squamata clade is relatively straight- forward. Other names that have been applied to a taxon composed of Sphenodon and squa- matans (see Synonyms) either have been little used (Saura, Lacertia, Saurii) or have been more commonly associated with a less inclusive clade (Squamata) or a paraphyletic group originating in the same ancestor (Lacertilia as well as the seldom-used names Saura, Lacertia, and Saurii).
The name Lepidosauria was first defined phylogenetically by Gauthier et al. (1988a:
34) as “the most recent common ancestor of Sphenodon and squamates and all of its descen- dants.” We have updated that definition by using species as specifiers. In the context of phy- logenies in which Sphenodon punctatus is more closely related to turtles, crocodilians, and birds than to Squamata (e.g., Hedges and Polling, 1999; Zardoya and Meyer, 2000), the names Lepidosauria and Reptilia (this volume) are syn- onyms, and Reptilia is to be granted precedence.
Until very recently, the earliest known unambiguous crown lepidosaurs were extinct relatives of Sphenodon punctatus extending back at least ~220 Ma (late Carnian; early Late Triassic; e.g., Pritchard and Nesbitt, 2017, and references therein). Simões et al. (2018) have, however, recently concluded that Megachirella wachtleri represents a stem squamatan (~240 Ma; Anisian; Middle Triassic), making it the only pan-squamatan currently known from the Triassic or Early Jurassic (Evans, 2003; see
Pan-Squamata, this volume). Moreover, some
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Authors
Kevin de Queiroz; Department of Vertebrate Zoology; National Museum of Natural History; Smithsonian Institution; Washington, DC 20560-0162, USA. Email: dequeirozk@
si.edu.
Jacques A. Gauthier; Department of Geology and Geophysics; Yale University; 210 Whitney Avenue, New Haven, CT 06511, USA. Email:
Date Accepted: 11 August 2018 Primary Editor: Philip Cantino
