Evolution and classification

Squamates, along with the Rhynchocephalia (a group whose only living representative is the tuatara [Sphenodon]), are the two orders that make up a clade of reptiles known as Lepidosauria. Lepidosauria contains the last common ancestor shared between order Squamata and order Rhynchocephalia, along with all of their descendants. The tuatara is lizardlike in overall appearance. However, it differs from squamates in important ways. Male tuatara have no paired copulatory organ; they have saclike structures from which the paired hemipenes of squamates were likely derived. In addition, the tuatara does not have a movable quadrate bone in the jaw; this characteristic is present in all squamates.

Relationships between the major groups of squamates remain in flux. One hypothesis posits that early in the evolutionary history of lizards an important split occurred that not only influenced the disposition of taxonomic units but had cascading effects on the ecology of lizards and led to the diversification of snakes. This first split produced the iguanians (infraorder Iguania) and the scleroglossans (infraorder Scleroglossa), two large groups within order Squamata that were fundamentally different from each other. The ancestors of all lizards possessed an ability to capture and manipulate prey with the tongue (lingual prehension). Iguania retained the ability; however, this likely precluded the development of the tongue into an organ that transmitted chemical signals to a rudimentary vomeronasal system in this group (see Jacobson’s organ).

Another hypothesis, based largely on unclear gene sequence data, posits that the group containing geckos (and possibly the dibamids) diverged from all other squamates and that the iguanians are nested deeply within the stock of remaining lizards (that is, the autarchoglossans). If additional data support this hypothesis, then the presence of many of the ecological, morphological, and behavioral traits in iguanians suggests that the iguanians evolved independently within a group of squamates that had already diverged considerably from its squamate ancestors. At present, herpetologists operate under the assumption that the first hypothesis is better supported.

For the most part, Iguania is composed of foraging lizards that use cryptic coloration, morphology, and behaviour to escape detection by predators. Typically, they can be described as territorial, sit-and-wait predators that rely on visual cues to detect prey (usually by movement) and in social interactions. Within Iguania, one family, Iguanidae, retained the ancestral pleurodont dentition similar to the tuatara, in which teeth are essentially cemented to the inside of the jawbones. In contrast, the ancestor of two other families within Iguania, Agamidae and Chamaeleonidae, developed acrodont teeth, which are characterized by their attachment to the surfaces of jawbones. The most striking deviation from the typical Iguanian consumptive pattern is the evolution of herbivory in lizards, which occurred independently in several lineages. Herbivory evolved in the ancestor to the Leiolepinae (family Agamidae), the ancestor to the Iguaninae (family Iguanidae), and in some members of the Liolaeminae (family Iguanidae).

In some of these groups, the tongue is used less for prey capture and more for chemoreception. Scleroglossan ancestors retained pleurodont dentition, but use of the tongue in prey capture was replaced by grasping jaws (jaw prehension). Freed from its direct involvement in feeding, the scleroglossan tongue was allowed to develop into a carrier of chemical information. Although the actual sequence of events will never be known with certainty, the vomeronasal system grew in importance, and lizard evolution proceeded in an entirely new direction.

An early split within Scleroglossa produced the Gekkota (geckos) and the Autarchoglossa (snakes, skinks, and their relatives). Use of the vomerolfaction system did not develop within Gekkota to the extent that it did within Autarchoglossa; however, the tongue was increasingly used as a tool for cleaning the spectacle, a transparent scale covering the eye. A nasal chemosensory system became enhanced in the Gekkota, but many retained (or redeveloped) the sit-and-wait foraging mode of their ancestors.

Autarchoglossans took maximum advantage of the vomeronasal chemosensory system. It dominates social behaviour in extant groups. The vomeronasal system allows members of this group to discriminate species, sex, and sexual receptivity on the basis of chemical cues; it also allows lizards to discriminate among prey types. Autarchoglossans can select particularly energy-rich prey and avoid those containing noxious chemicals. They also can seek out prey that may not be visible on the surface, such as colonies of social insects and insect larvae in the soil and vegetation. Moreover, cryptic-coloured invertebrates that might never be detected visually by iguanian lizards are detectable chemically by autarchoglossans; by searching the habitat, autarchoglossans can locate prey not otherwise available. In contrast to the sit-and-wait foraging mode of the iguanians, autarchoglossans developed an active foraging mode where much time is spent searching for energy-rich prey items. An important trade-off must have existed in which the benefits of searching large areas for prey outweighed the benefits of maintaining defended territories.

The development of the vomeronasal system caused a major shift in social systems. Autarchoglossans developed a sort of sequential polygyny mating system in which a single male guards a number of females as they move about. This is quite different from the territorial systems in the Iguania in which males defend particular places where females reside. Maintaining all of this activity required autarchoglossans to make some major physiological adjustments. They have higher activity levels, appear more alert, use more energy while foraging, take in more energy, and often have higher body temperatures than iguanians.

An active, energy-intensive lifestyle, combined with keen chemosensory abilities, sets the stage for the evolution of limblessness and the use of subterranean habitats. Limblessness evolved independently in several groups of autarchoglossans, but it did not evolve within the Iguania. The extreme development of the tongue and vomeronasal system in superfamily Varanoidea (a group made up of monitor lizards, snakes, and their relatives) set the stage for the evolution of snakes, which are thought to have evolved from terrestrial lizards during the Early Cretaceous Epoch (145.5 million to 99.6 million years ago). The oldest known fossil snake, Coniophis precedens, lived in North America some 65–70 million years ago.