Gekkota - geckoes and pygopodids

The Gekkota is a monophyletic clade containing the familiar geckoes, as well as the pygopodids - a group of limbless lizards commonly referred to as Australasian legless lizards. To learn more about their place in squamate phylogeny, please return to the Squamata main page.

Diversity and Lower Taxonomy

Traditionally (e.g. Estes et al. 1988), the geckoes and pygopodids have been divided into two distinct families, as follows:

• Family Gekkonidae (geckoes) - comprising approximately 1180 species of gecko divided between five subfamilies.
  • Subfamily Gekkoninae - containing 75 genera.
  • Subfamily Teratoscincinae - containing a single genus, Teratoscincus.
  • Subfamily Diplodactylinae - containing 18 genera.
  • Subfamily Eublepharinae - containing 5 genera.
  • Subfamily Aeluroscalabotinae - containing one genus, Aeluroscalabotes.
• Family Pygopodidae - 39 species of pygopodid divided between seven genera in two subfamilies.
  • Subfamily Pygopodinae - conatining 23 species divided between 3 genera.
  • Subfamily Lialisinae - containing 16 species in 4 genera.

This Gekkonidae-Pygopodidae dichotomy proposed the monophyly of the two groups, and Gekkota was therefore defined as encompassing the last common ancestor of these two families, plus all its descendents. Both morphological (e.g. Kluge 1987) and molecular (e.g. Saint et al. 1998) phylogenetic studies have, however, demonstrated that Pygopodidae is nested within Gekkonidae, more closely related to the diplodactylines (subfamily Diplodactylinae) than to other gekkotans. Under these conditions, the term Gekkonidae becomes phylogenetically redundant, as if Pygopodidae is excluded from its definition, then Gekkonidae is paraphyletic, and if Pygopodidae is included, then Gekkonidae becomes equivalent to the higher Gekkota (including geckoes and pygopodids).

Nonetheless, Gekkotan interrelationships are by no means fully resolved, and not all researchers agree with a nested position for Pygopodidae. More recently, a molecular study conducted by Jonniaux & Kumazawa (2008) proposed that the subfamilies Eublepharinae and Aeluroscalabotinae form a distinct family, Eublepharidae, that is siter to Gekonidae (containing the three remaining traditional gekkonid genera), and in turn this clade is sister to Pygopodidae. This molecular-based grouping is interesting as it corroborates certain morphological differences between these groups (see Descriptions section below).

Description

It is useful to describe the appearance and morphology of geckoes and pygopodids separately, as they are clearly distinct. Bear in mind, however, that it is possible that the pygopodids evolved within the geckoes. If this is the case, then pygopodids are in fact geckoes themselves, and may have secondarily lost many gekkotan features in favour of specialisation for a burrowing mode of life. Consequently, the term 'gecko' below refers to an informal (i.e., not phylogenetic) grouping meaning 'any non-pygopodid gekkotan'.

Geckoes are a familiar group of often strikingly coloured small to average-sized lizards, ranging from 30 mm (Sphaerodactylus ariasae) to 350 mm (e.g. Rhacodactylus leachianus, Gekko gecko), with a tail that is similar in length to the snout-vent length (s-v; distance between the snout and the cloacal opening, or vent, at the base of the tail). Most are nocturnal (these species have vertically slitted pupils), but some genera are diurnal (with rounded pupils, e.g. Phelsuma), and a few even show both diurnal and nocturnal activity. The majority are insectivorous, but some will eat small reptiles, and some larger species have been known to consume small rodents. They are oviparous, with the exception of members of the following three live-bearing genera: Hoplodactylus, Naultinus and Rhacodactylus.

Geckoes can be divided into two main forms: those with moveable eyelids (the eublepharines & aeleuroscalabotines, corresponding to the potential family Eublepharidae, as above), and those with fixed eyelids (the gekkonines, teratoscincines, and diplodactylines, corresponding to the remaining members of the traditional family Gekkonidae). Within the latter grouping, there is a further division based on the presence (arboreal species) or absence (usually terrestrial species) of adhesive toe pads - a specialisation for arboreal locomotion. These pads consist of a set of overlapping expanded scales on the base of the toes, possessing millions of microscopic hair-like protrusions (called setae), each of which branches into hundreds of 200 nm wide tips (called spatulae). This microstructure acts as an extremely strong adhesive (each hair can resist 200 µN of force), allowing these geckoes to walk up smooth vertical surfaces, and even upsidedown. In fact, these hairs are so sticky that a gecko can hang from a ceiling by just a single toepad, and a single hair (seta) could lift an ant!

Pygopodids are a small group of average sized (7-25 cm s-v) limbless lizards, feeding mainly on insects and some lizards. They are slender and elongate, with no traces of pectoral skeleton. The pelvic girdle is, however, still present in part, and the hindlimbs persist as vestigial scaly flaps. The majority of pygopodids are diurnal, but Paradelma orientalis, as well as subsepecies of Pygopus nigriceps are nocturnal. Like most geckoes, they are oviparous.

Distribution and Habitat

Geckoes are distributed worldwide, and are most speciose in the tropics, subtropics, and deserts. They are either terrestrial or arboreal.

Pygopodids are found in Australia, with Lialis also present in Indonesia. While two genera, Aprasia and Ophidiocephalus, are burrowing, the majority inhabit grass and litter.

Conservation Status (IUCN)

Geckoes - Of the 91 species of gecko listed in the IUCN Red List, the majority (52 species) have been assessed as Least Concern (LC). Two species (Lepidoblepharis montecanoensis and Phelsuma antanosy) are Critically Endangered (CR), while four are Endangered (EN), ten are Vulnerable (VU), and eleven are Near Threatened (NT). The remainder (10 species) are Data Deficient (DD).

Pygopodids - the conservation status of seven pygopodid species has been assessed by the IUCN. All but one are Vulnerable (VU), and the other is Near Threatened (NT).

Synapomorphies of the Gekkota

• Upper temporal bar absent.
• Incomplete post-orbital bar.
• Absence of the lacrimal - the anterior-most bone in the medial wall of the orbit.