Lizards are a very large and widespread group of squamate reptiles, with nearly 3,800 species, ranging across all continents except Antarctica as well as most oceanic island chains. The group, traditionally recognized as the suborder Lacertilia, is defined as all extant members of the Lepidosauria (reptiles with overlapping scales) which are neither sphenodonts (i.e., Tuatara) nor snakes. While the snakes are recognized as falling phylogenetically within the anguimorph lizards from which they evolved, the sphenodonts are the sister group to the squamates, the larger monophyletic group which includes both the lizards and the snakes.

Lizards typically have limbs and external ears, while snakes lack both these characteristics. However, because they are defined negatively as excluding snakes, lizards have no unique distinguishing characteristic as a group. Lizards and snakes share a movable quadrate bone, distinguishing them from the sphenodonts which have a more primitive and solid diapsid skull. Many lizards can detach their tails in order to escape from predators, an act called autotomy, but this trait is not shared by all lizards. Vision, including color vision, is particularly well developed in most lizards, and most communicate with body language or bright colors on their bodies as well as with pheromones. The adult length of species within the suborder ranges from a few centimeters for some chameleons and geckos to nearly three meters (9 feet, 6 inches) in the case of the largest living varanid lizard, the Komodo Dragon. Some extinct varanids reached great size. The extinct aquatic mosasaurs reached 17 meters, and the giant monitor Megalania prisca is estimated to have reached perhaps seven meters.

Physiology

Sight is quite important for most lizards, both for locating prey and for communication, and as such, many lizards have highly acute color vision. Most lizards rely heavily on body language, using specific postures, gestures and movements to define territory, resolve disputes, and entice mates. Some species of lizard also utilize bright colors, such as the iridescent patches on the belly of Sceloporus. These colors would be highly visible to predators, so are often hidden on the underside or between scales and only revealed when necessary.

A particular innovation in this respect is the dewlap, a brightly colored patch of skin on the throat, usually hidden between scales. When a display is needed, the lizards erect the hyoid bone of their throat, resulting in a large vertical flap of brightly colored skin beneath the head which can be then used for communication. Anoles are particularly famous for this display, with each species having specific colors, including patterns only visible under ultraviolet light, as lizards can often see UV.

Evolution and relationships

The retention of the basic 'reptilian' amniote body form by lizards makes it tempting to assume any similar animal, alive or extinct, is also a lizard. However, this is not the case, and lizards as squamates are part of a well-defined group.

The earliest "lizard" was superficially lizard-like, but had a solid, box-like skull, with openings only for eyes, nostrils, etc (termed Anapsid). Turtles retain this skull form. Early anapsids later gave rise to two new groups with additional holes in the skull to make room for and anchor larger jaw muscles. Those with a single hole, the Synapsids, gave rise to the superficially lizard-like Pelycosaurs which include Dimetrodon and the Therapsids, including the Cynodonts, from which would evolve the modern mammals.

The Diapsids, possessing one temporal fenestra before the eye and one behind it, continued to diversify. One branch, the Archosaurs, retained the basic Diapsid skull, and gave rise to a bewildering array of animals, most famous being the crocodilians, the pterosaurs, the dinosaurs and their descendants, birds. The Ichthyosaurs and Plesiosaurs radiated from the same basal Diapsid group.

The smaller Lepidosaurs which would give rise to the lizards began to reduce the skull bones, making the skull lighter and more flexible. The modern Tuatara retains the basic Lepidosaur skull, distinguishing it from true lizards in spite of superficial similarities. Squamates, including snakes and all true lizards, further lightened the skull by eliminating the lower margin of the lower skull opening.

Lizard diversification

Within the Lacertilia are found four generally recognized suborders, Iguania, Gekkota, Amphisbaenia and Autarchoglossa, with the "blind skinks" in the family Dibamidae having an uncertain position. While traditionally excluded from the lizards, the snakes are usually classified as a clade with a similar subordinal rank.

Iguania

The suborder Iguania, found in Africa, south Asia, Australia, the New World, and with iguanas colonizing the islands of the west Pacific, form the sister group to the remainder of the squamata. They are largely arboreal, and have primitively fleshy, non-prehensile tongues, but this condition is obviously highly modified in the chameleons. This clade includes the following families:

• Family Agamidae – Agamid Lizards, Old World Arboreal Lizards
• Family Chamaeleonidae – Chameleons
• Family Corytophanidae – Helmet Lizards
• Family Crotaphytidae – Collared Lizards, Leopard Lizards
• Family Hoplocercidae – Dwarf and Spiny Tail Iguanas
• Family Iguanidae – American Arboreal Lizards, Chuckwallas, Iguanas, Iguanids
• Family Opluridae – Malagasy Iguanas
• Family Phrynosomatidae – North American Spiny Lizards
• Family Polychrotidae – Anoles and kin
• Family Tropiduridae – Tropidurid Lizards

Gekkota

Active hunters, the Gekkota includes three families comprising the distinctive cosmopolitan geckos and the legless flap-footed lizards of Australia and New Zealand. Like snakes, the geckos and the flap-footed lizards lack eyelids. Unlike snakes, they use their tongues to clean their often highly developed eyes. While gecko feet have unique surfaces which allow them to cling to glass and run on ceilings, the flapfoot has lost its limbs. The three families of this suborder are:

• Family Eublepharidae – Eublepharid Geckos
• Family Gekkonidae – Geckos
• Family Pygopodidae – Flap-footed Lizards

Relationship with humans

Most lizard species are harmless to humans. Only the very largest lizard species pose threat of death; the Komodo dragon, for example, has been known to stalk, attack, and kill humans. The venom of the Gila monster and beaded lizard is not usually deadly but they can inflict extremely painful bites due to powerful jaws. Numerous species of lizard are kept as pets.

Lizard symbolism plays important, though rarely predominant roles in some cultures (e.g. Tarrotarro in Australian Aboriginal mythology). The Moche people of ancient Peru worshiped animals and often depicted lizards in their art. According to a popular legend in Maharashtra, a Common Indian Monitor, with ropes attached, was used to scale the walls of the Sinhagad fort in the Battle of Sinhagad.

Green Iguanas are eaten in Central America and Uromastyx in Africa. In North Africa, Uromastyx are considered dhaab or 'fish of the desert' and eaten by nomadic tribes.

Classification

Suborder Lacertilia (Sauria) - (Lizards)

• Family †Bavarisauridae
• Family †Eichstaettisauridae
• Infraorder Iguania
  • Family †Arretosauridae
  • Family †Euposauridae
  • Family Corytophanidae (casquehead lizards)
  • Family Iguanidae (iguanas and spinytail iguanas)
  • Family Phrynosomatidae (earless, spiny, tree, side-blotched and horned lizards)
  • Family Polychrotidae (anoles)
    • Family Leiosauridae (see Polychrotinae)
  • Family Tropiduridae (neotropical ground lizards)
    • Family Liolaemidae (see Tropidurinae)
    • Family Leiocephalidae (see Tropidurinae)
  • Family Crotaphytidae (collared and leopard lizards)
  • Family Opluridae (Madagascar iguanids)
  • Family Hoplocercidae (wood lizards, clubtails)
  • Family †Priscagamidae
  • Family †Isodontosauridae
  • Family Agamidae (agamas)
  • Family Chamaeleonidae (chameleons)
• Infraorder Gekkota
  • Family Gekkonidae (geckos)
  • Family Pygopodidae (legless lizards)
  • Family Dibamidae (blind lizards)
• Infraorder Scincomorpha
  • Family †Paramacellodidae
  • Family †Slavoiidae
  • Family Scincidae (skinks)
  • Family Cordylidae (spinytail lizards)
  • Family Gerrhosauridae (plated lizards)
  • Family Xantusiidae (night lizards)
  • Family Lacertidae (wall lizards or true lizards)
  • Family †Mongolochamopidae
  • Family †Adamisauridae
  • Family Teiidae (tegus and whiptails)
  • Family Gymnophthalmidae (spectacled lizards)
• Infraorder Diploglossa
  • Family Anguidae (glass lizards)
  • Family Anniellidae (American legless lizards)
  • Family Xenosauridae (knob-scaled lizards)
• Infraorder Platynota (Varanoidea)
  • Family Varanidae (monitor lizards)
  • Family Lanthanotidae (earless monitor lizards)
  • Family Helodermatidae (gila monsters & beaded lizards)
  • Family †Mosasauridae (marine lizards)

References

1. Lizards at eduscape.com
2. ITIS http://www.itis.gov/servlet/SingleRpt/SingleRpt?search_topic=TSN&search_value=173861
3. Berrin, Katherine & Larco Museum. The Spirit of Ancient Peru:Treasures from the Museo Arqueológico Rafael Larco Herrera. New York: Thames and Hudson, 1997.
4. pg 48, Grzimek,Bernhard. Grzimek’s Animal Life Encyclopedia (Second Edition) Vol 7 - Reptiles. (2003) Thomson - Gale. Farmington Hills, Minnesota. Vol Editor - Neil Schlager. ISBN 0-7876-5783-2 (for vol.7)

General references