LIZARDS

Does the variation within lizard groups support evolution, creationism, or intelligent design?

EVOLUTIONARY MODEL

If evolution is correct, then separate groups of lizards could continue to develop significant variations within their groups. Similar adaptations, such as leglessness, might develop independent lineages.

CREATIONISM MODEL

If the creationism model is correct, it is not expected that significant variations would develop in groups which share a common ancestry.

INTELLIGENT DESIGN

If intelligent design is true, then a complex adaptation--such as the adaptations of snakes or the live birth of mammals--could not occur through a series of intermediate steps because only the full complement of an irreducibly complex system would benefit an individual.



Do lizards give evidence to an intelligent design? One of the problems in answering that question stems from the fact that lizards can be quite diverse no accepted classification system exists among creationists to identify those groups which have a common ancestry.

lizards

The general lizard body plan is not as specialized as that of many other reptiles. A number of groups bear a superficial resemblance to lizards such as the earliest anapsid reptiles, the earliest diapsid reptiles, and even advanced amphibians. Because of this similarity, the first lizards do not seem to have required a supernatural design but rather a modification of ancestral reptiles which were already lizard-like in their overall form. The first lizards were small, less than 10 cm in length, and were probably insectivorous. Upper Permian and Lower Triassic lizards are more primitive than later forms in that they possessed unfused proximal tarsal bones in the foot, poorly developed fenestration of limb girdles, and primitive vertebrae.


Lizards are quite diverse. Modern lizards range from the equator to New Zealand and north of the Arctic circle and inhabit ecosystems from sea level to 5,000 meters in elevation. The sizes of chameleons can vary from less than 2.5 cm to more than 50 cm. The modern Komodo dragon reaches 3 meters and the extinct Megalania that reached twice this size. Australia's fossil fauna included a lizard (goanna) which reached 4 to 5 meters in length (Dawson, 1995). Extinct mosasaurs reached 10 meters.


LOCOMOTION
Although most lizards move on four limbs, some groups have significantly modified this ancestral "design". Some lizards have reduced their limbs to the point that they are vestigial stubs or are completely absent. This is true of all species of the limbless family Anniellidae, the family Pygopodidae (which have lost their front legs and whose hind legs are vestigial), and Dibamidae (in which males retain vestiges of hind legs). In other families, only some members of the family have legs which are reduced or absent. This is true in the family Teiidae (in which some South American species of the genus Bachia possess small arms and vestigial leg stubs), Scincidae (in which some species have reduced legs, some are missing front legs, and some are completely legless), Cordylidae (in which some species possess small arms and vestigial leg stubs), and Anguidae (in which limbs can be reduced or absent in some species). (Mattison, 1989; Halliday, 1987; Pough, 1998).
Ophiodes intermedius
lizard
There are 4 families of lizards known as amphisbaenids. In this group, the species of three families lack any trace of limbs or limb girdles. The fourth family has not only retained its arms, but the hands have grown to considerable size.

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The fossil record indicates that one early group of lizards significantly reduced its limbs.
Other significant variations exist as well. Some iguanids, igamids, and monitor lizards are capable of bipedal locomotion (even running across the surface of water in the South American basilisk) and standing. Some species of Crotaphytidae are bipedal while jumping. Species of the family Corytophanidae can run bipedally. When threatened, Gould's monitor stands on its hind legs (Halliday, 1987; Pough, 1998).
Some geckos can glide through the air using skin flaps between their limbs and webbed digits. In the agamid genus Draco, elongated ribs allow a skin flap between the limbs to be used in gliding (Mattison, 1989). A separate lineage of fossil lizards (Kuehnosaurus) also possessed expanded their ribs for gliding.
The lizard genus Helodermoides possessed armor (Carroll, 1988).


OTHER VARIATIONS
Within the family Agamidae, individual species can possess neck frills, throat fans, spines along their backs, and modified head structures. Some chameleons have horns. Extensive skull casquing in known in the family Polychrotidae. Many groups developed a firmer tooth attachment and one extinct group possessed deep sockets for its teeth. The Gila Monster is poisonous (Pough, 1998).
Some lizard scales are thick enough to serve as armor while others (such as those of gecko feet) are covered with microscopic bristles which allow them to climb smooth surfaces. Some defend themselves through bluffing, inflating their body, hissing, extending a neck ruff to appear larger, and even squirting blood out of their eyes (in Phrynosoma; blood can be squirted up to four feet). Some lizards can change their coloration in response to the background of the environment, a threat from a predator, or in courtship/territorial displays (Mattison, 1989).

Most lizards depend primarily on vision, although eyes are often reduced (or even vestigial) in burrowing lizards. Some lizards (geckos, pygopodids, and xantusids) have fused their eyelids to form a protective brille over the eye, just as in snakes. Chameleon eyes are capable of independent movement. Most lizards possess an external ear opening and some make sounds which serve in intra-specific communication. The Bornean earless monitor lacks both an outer and middle ear cavity. (Mattison, 1989).

Lizard tails can be used for a variety of purposes ranging from balance in bipedal locomotion, a fat reserve, a prehensile grasping structure, and the source of noxious secretions which deter predators. A number of lizards have modified their tail so that it is prehensile, including species of the families Chamaeleonidae, Agamidae, Gekkonidae, Polychrotidae, and Scincidae (Halliday, 1987; Pough, 1998). Tails are adapated to break in at least some species of 11 of the 16 families of modern lizards (Halliday, 1987).

REPRODUCTION
Many lizards reproduce through live birth and this adaptation has arisen many times within different groups. Both egg-laying and live-bearing species are known in the families Agamidae, Chamaeleonidae, Phrynosomatidae, Tropiduridae, Gekkonidae, Eublepharidae, Lacertidae, and Scincidae. Some species brood over their eggs and others (such as the Great Plains skink, the desert night lizard, and the California alligator lizard) offer maternal care after birth. Live birth seems to have evolved separately in skinks more than 20 times. Parthenogenesis (all-female populations which reproduce by giving birth to clones of themselves) is known in some in rock lizards, geckos, whiptails, and racerunners. The number of eggs laid per clutch can vary significantly in a family. For example, clutch size can vary from 4 to 70 in diverse chameleon species.

LIFESTYLE
The lizard body form has been modified for a diversity of habitats. While most lizards are terrestrial, adaptations for arboreal life have evolved in some species of the families Iguanidae, Phrynosomatidae, Tropiduridae, and Teiidae. A number of groups have adapted to burrowing such as glass lizards, blind lizards, snake lizards, and many skinks. Amphisbaenans are unusual in that the group is exclusively subterranean in its habits.
Some Teiid lizards are semi-aquatic and the extinct mosasaurs were highly specialized for marine life. Fossil lizards of the family Aigialosauridae were semiaquatic and were transitional between varanoids and mosasaurs.

Most lizards prey on other animals. While many feed on a variety of prey, some have become more specialized such as the Caiman lizard whose teeth are modified to crush snail shells and the Komodo dragon whose teeth are modified to cause lethal infections and can bring down water buffaloes. About two percent of lizard species are herbivores including iguanas, the Egyptian spiny-tailed lizard (an agamid), some girdle-tailed lizards, and some Australian skinks.

Not only can lizards in different groups vary in their habits, lifestyle can vary significantly within a family. Many iguanas are terrestrial, many are arboreal, and the marine iguana have become adapted for swimming in the coastal ocean. While many iguanas feed primarily on insects, others are herbivores, and the marine iguana feeds on algae. Diet can vary considerably in the family Tropiduridae. While most species are generalists which prey on insects, some are herbivores and others eat only ants.