Squamata - lizards and snakes


Vertebrata; Gnathostomata; Osteichthyes; Sarcopterygii; Tetrapoda; Amniota; Sauropsida;
Reptilia; Diapsida; Sauria; Lepidosauromorpha; Lepidosauria; Squamata

Squamates are a highly successful and familiar group of reptilies including lizards (e.g. gekkotans, skinks, chamaeleons), snakes, and amphisbaenians. With over 7000 species, they are present on every continent except Antarctica, and have invaded marine environments (e.g. sea snakes, mosasaurs), as well as diversifying into many different specialised terrestrial forms, including burrowers (e.g. amphisbaenians), gliders (e.g. Draco), bipedal runners (e.g. Basiliscus), climbers (e.g. chameleons), and active predators (e.g. Varanus).

In addition to the plesiomorphic reptilian condition of egg-laying (oviparity), some groups give birth to live young (viviparity). More surprisingly, at least eight groups contain a species with only female members, reproducing by a process called parthenogenesis - the development of unfertilised eggs into functional (female) offspring.

Squamates range in size from approximately 3 cm (pygmy chamaeleons, e.g. Brookesia minima) to several metres (e.g. komodo dragon, anaconda, extinct marine mosasaurs). Limblessness is common in squamates (incluing snakes, amphisbaenians, dibamids, and members of each major lizard group), and is thought to have evolved as many as five times independently. For more information on squamate diversity, please visit the squamate subgroup pages visible in the table of contents to the left.

The Squamata is the sister group to the Rhynchocephalia (the Tuatara and extinct relatives), sharing a common ancestor around 250 million years ago at the start of the Mesozoic. Together, they form the monophyletic Lepidosauria. For lepidosaurian synapomorphies and phylogeny, see the information on the rhynchocephalian page.

Synapomorphies of the Squamata

  • Cranial kinesis - a high degree of flexibility between the bones of the back of the skull, allowing relative movements between them.
  • Paired hemipenes (present in all lepidosaurs) are fully eversible.
  • Pleurodont dentition - teeth set into the side of the inner surfaces of the jaws, and periodically replaced.
  • Loss of gastralia (ventral belly ribs).
  • Double-hooked fifth metatarsal, functionally analagous to the mammalian heel.
  • Further complexity to the mesotarsal joint (in comparison to non-squamate lepidosaurs).

Phylogenetic relationships of the Squamata

Researchers in squamate phylogeny have yet to reach a consensus on the evolutionary affinities of the group's different lineages. Traditionally, the crown-group Squamata was divided into Lacertilia (lizards and amphisbaenians) and Ophidia (snakes). Modern cladistic analysis, however, has demonstrated that both snakes and amphisbaenians can be grouped with certain lizards to the exclusion of other lizard groups (e.g. Estes et al. 1988), and thus the term 'lizard' is not a natural grouping (i.e. not monophyletic). Therefore, formal terms like Lacertilia should be avoided, as they are phylogenetically misleading. The term lizard can be used informally to indicate any squamate that is not a snake or amphisbaenian.

Studies based on cladistic analysis of the morphological characteristics of both extant and fossil taxa assert that the primary split in the crown-group Squamata was between the Iguania and the Scleroglossa around 206 million years ago, at the Triassic-Jurassic boundary. The Iguania contains the familiar iguanas and chamaeleons, as well as the agamids, and its monophyly is well supported. For more information see the Iguania page.

The first major phylogenetic analysis of the Squamata (Estes et al. 1988) defined three major scleroglossan clades- Gekkota, Anguimorpha, and Scincomorpha. The phylogeny of Squamata as proposed by Estes et al. (1998) is illustrated in the cladogram below (note the colour-coding of proposed clades, for comparison with conflicting theories below):












While the monophyly of Gekkota and Anguimorpha is usually supported by morphological studies, the monophyly of the Scincomorpha is not always reconciled (e.g. Lee et al.1998), and the interrelationships of these clades has been difficult to resolve. In addition to the three principle lineages, there are three other groups of limbless scleroglossans that have been difficult to place into the phylogeny (see cladogram above). These are the snakes (Serpentes), amphisbaenians (Amphisbaenia), and dibamids (Dibamidae). Snakes are usually placed within the anguimorphs (e.g. Lee et al. 1998), although their origins are a major subject of scientific dispute. Amphisbaenians and dibamids are, however, something of a phylogenetic mystery. Often grouped as a clade (Amphisbaenia + Dibamidae), some authors place them as sister to the Gekkota (e.g. Lee et al. 1998), while others suggest a close affinity with snakes (e.g. Rieppel & Zaher 2000).

Lee et al. (1998) suggested that the apparent grouping of these three problematic taxa as a clade within the anguimorphs seen in many phylogenetic studies is a misleading result, as it is almost exclusively supported by specialisations for a burrowing mode of life, such as cranial consolidation, loss of limbs and elongation of the body, which are known to convergently evolve in other tetrapods (e.g. caecilians). They argue that the addition of fossil taxa is crucial to understanding the evolutionary transition of these groups to leglessness, in order to detect any convergence, or homoplasy. Indeed, their inclusion of certain fossil taxa (namely the large marine mosasauroids, the basal snake Pachyrhachis, and the limbed amphisbaenian-like Sineoamphisbaenia) produced an amphisbaenian-dibamid clade as sister group to the Gekkota, away from snakes and anguimorphs. Snakes, however, remained as anguimorphs, forming the clade Pythonomorpha with the large marine mosasaurs of the Cretaceous, which is then sister group to the monitor lizards (family Varanidae) in the Varanoidea.

In addition, Lee et al. (1998) questioned the monophyly of the Scincomorpha, defined by (Estes et al. 1988). First, they proposed that scincids and cordylids are more closely related to anguimorphs than to other "scincomorphs". Second, they placed the xantusiids in a new clade, Nyctisaura, as sister group to the Gekkota-Amphisbaenia-Dibamida clade. The remaining "scincomorphs" persisted in the clade Lacertiformes.

The phylogeny of squamates proposed by Lee et al. (1998) is shown below (note the polyphyly of Scincomorpha, as well as the positions of Serpentes, Amphisbaenia, and Dibamidae, and the addition of mosasaurs):












More recently, the monophyly of the Scleroglossa has been questioned by researchers studying squamate DNA sequence data. For example, Townsend et al.(2004), who compared around 4600 DNA base pairs in 69 squamate species, placed the Iguania as well-nested within a consequently paraphyletic Scleroglossa - contrary to the usual Iguania-Scleroglossa dichotomy. Instead, they proposed that geckoes (plus relatives) and dibamids were the first squamate groups to diverge. The monophyly of the traditional Anguimorpha was supported, although it did not include snakes, and refuted the monophyly of Xenosauridae by proposing separate origins for the genera Xenosaurus and Shinisaurus. Again, the monophyly of Scincomorpha was refuted, albeit in a different manner (see below) to that proposed by Lee et al. (1998). Amphisbaenians were placed in yet another position, this time embedded within the Lacertiformes as sister group to the family Lacertidae. The authors showed that amphisbaenians share almost identical multicodon deletions in a gene called c-mos with members of a particular lacertid genus (Gallotia). This provided additional support to their sequence comparison results that demonstrated that dibamids are not closely related to snakes or dibamids, but to lacertids. Finally, the relationships between Anguimorpha, Lacertiformes, Serpentes, and Iguania were not resolved. The cladogram below illustrates the squamate molecular phylogeny sensuTownsend et al.(2004):












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