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Snake internal organs. (Illustration by Marguette Dongvillo)

manipulation of agile prey. Such jaws bend and better conform to prey, enhancing feeding success.

This combination of skull and chemosensory modifications gave scleroglossans access to microhabitats and prey previously unavailable to iguanians and predisposed them to higher activity levels. For example, an ability to detect and discriminate prey chemically gave scleroglossans access to prey that could not be detected visually. No longer limited to prey moving across their field of vision, squamates could now find highly cryptic invertebrates and vertebrates living in crevices, in the ground, or in water. Remaining in one place for long periods of time has a low-energy payoff compared with searching actively through the habitat for hidden and seden tary prey. Active or wide foraging provided these lizards with a competitive advantage and selected for higher levels of activity. Moving about searching for prey is energetically costly and also increases the risk of exposure to potential predators. Alert behavior and rapid response to predators evolved to enable increased activity levels. Widely foraging scleroglossans find and consume more prey calories per unit time than do iguanians. Gekkota evade both competition and predation by being nocturnal, whereas Autarchoglossa evade potential diurnal predators by being exceedingly alert and agile. Elongation of the body and increased jaw flexibility permitted varanoid lizards to swallow large prey and set the stage for the evolution of snakes.

Lizard Cranial Kinesis

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Snake skull. (Illustration by Marguette Dongvillo)

Snakes carried cranial kinesis to an even higher level than did their lizard ancestors, evolving numerous flexible joints in their skulls. Liberation of the mandibular symphysis (the tendons connecting the two lower jaws) set off snake evolution. Unlike lizards, most snakes also have independent movement of bones on the left and right sides of their skulls. Coupled with streptostyly, these adaptations allow snakes to swallow exceedingly large prey. Snake skulls have diversified widely. Snakes lost both temporal arches and apertures, permitting greater independent movements of the head bones. Many snakes have highly flexible jaws and snouts with many joints and considerable cranial kinesis. The musculature of a snake's head is quite complex, allowing for independent movements of cranial bones. When swallowing large prey, snakes "walk" their way down a prey item, first opening one side of their jaws, extending the jawbones forward, biting down, and then repeating the process on the other side.

Owing to lack of limbs, snake diversity is restricted by morphologic features. Nevertheless, snakes have accomplished some rather spectacular things. Some snakes (Dipsas) pull snails out of their shells. Snail-eating lizards (Dracaena) crush snail shells with molariform teeth. Many lizards are termite specialists. Some, such as certain geckos, catch termites at night when they are active above ground. Others, such as lac-ertids and teiids, break into termite tunnels during the day. Still others, such as some fossorial skinks, find termites in tunnels and termitaria below ground. All termite-specialized snakes find termites below ground, and many actually spend most of their lives inside termitaria.

Skulls of burrowing snakes are secondarily compacted. Two major sister clades of snakes are Scolecophidia (blind-snakes in the families Leptotyphlopidae, Typhlopidae, and Anomalepididae) and Alethinophidia (all others). Blindsnakes have solid, blunt, and nearly toothless skulls. Considerable variation exists in scolecophidian skulls. Leptotyphlopids manipulate and transport prey with their mandibles (lower jaw), whereas typhlopids and, presumably, anomalepidids rake prey into their mouths with teeth in the upper jaw by rapidly protracting and retracting their maxillae. Mouths of other snakes (Alethinophidia) are filled with dozens of sharp recurved teeth arrayed along several different bones. Snake maxillae vary widely and are movable: hollow hypodermic fangs attached to these bones in viperids swing through almost a full 90° from the folded back, closed-mouth position to the fully erect, stabbing position.

To understand the origin of snakes, one must examine snakelike lizards. Burrowing lizards have small appendages or no limbs at all. They also have no external ear openings, and their eyes often are capped over with a clear spectacle. Ancestral snakes probably were fossorial. Snake eyes have been rebuilt after degenerating during an extensive subterranean existence. All other tetrapods focus by changing the lens curvature using muscles within the eye, but snakes have no such muscles and focus instead by moving the lens back and forth with another set of muscles in the iris.

A rare autarchoglossan lizard from Borneo known as the earless monitor (Lanthanotus) has been identified as a likely candidate for the position of sister group to snakes. Lanthanotus are cylindrical, long-tailed lizards with long necks and short legs. Like snakes, they have a hinge in the lower jaw and no external ear opening. They have forked tongues and tails that do not regenerate, and they shed their skins in one piece, just like snakes. Lanthanotus is the only anguimorphan lizard with a clear brille in the lower eyelid, which could be a precursor to the spectacle of snakes. Other snakelike traits of Lanthanotus include a solidly encased brain, loss of the upper temporal arch, and teeth on the palatine and pterygoid bones.

If snake ancestors were subterranean, ancestors of snakes were the most successful among many scleroglossans that experimented with fossoriality. Considering the many times limblessness has arisen in autarchoglossans, why did evolution of limblessness in varanoids set off such an extensive adaptive radiation as that seen in snakes? Varanoid lizards share a combination of characteristics that opened up a unique opportunity for them, compared with other subterranean lizards. Possession of a forked tongue allowed for keen chemosensory discrimination of prey and detection of airborne chemical signals as well as the ability to follow chemical trails by using the deeply forked tongue as an edge detector. Because fossorial lizards tend to be relatively small, a fossorial varanoid (ancestral snake) would probably be small as well. A fossorial vara-noid encountering termites could determine what they were and feed on them, and it could trace their chemical trail back to the colony. Other fossorial autarchoglossans might be able to identify termites, but the lack of forked tongues would inhibit their ability to trace prey to the nest.

Evolution of a body small enough to allow movement through termite passageways, along with a correspondingly small head, would permit access to a rich food resource base. Extreme elongation of the trunk is restricted primarily to subterranean autarchoglossans, but none has taken it to the extremes that snakes did. Limbless or nearly limbless terrestrial scleroglossans (Ophisaurus and pygopodids) have relatively truncated bodies compared with most snakes. Locomotion through existing passageways would favor a concertina-like movement, which in turn would select for longer bodies (as opposed to longer tails) in these snake ancestors. This set of traits describes fairly accurately the three primitive snake families Typhlopidae, Leptotyphlopidae, and Anomalepididae. Elongation of the body most likely preadapted these reptiles for a return to the surface, where a banquet of large vertebrate prey (amphibians, lizards, birds, and mammals) had diversified. Once on the surface, these snake ancestors underwent selection for increased ability to ingest large prey and evolved larger body sizes. They also evolved a loose mandibular symphysis, which allows the two lower jawbones to spread apart, facilitating ingestion of large prey.

Two evolutionary innovations contributed to the success of snakes above ground, efficient locomotion and their highly derived feeding mechanism. An elongated and very flexible body provides much more trunk control over locomotion. Limbed tetrapods expend considerable energy working against gravity to move their own body mass up and down with each step. Across lizard species, the net cost of locomotion (per gram) decreases linearly with increased body mass. The energetic cost of snake locomotion is much more variable. Snakes using concertina locomotion expend more energy than similarly sized lizards, whereas others, who use sidewinding locomotion, expend much less. Snakes can move quite rapidly, and, using an S-shaped loop in the neck, they can strike quickly to capture prey. After returning to the surface, not only could snakes eat large prey relative to their body and head diameter, they also could move their highly flexible bodies around in a manner that few elongated lizards could. Any crevice, hole, or passageway into which they could get their heads was accessible. Increased numbers of vertebrae and associated musculature facilitated swimming, climbing, and other types of locomotion that were either poorly developed or nonexistent in lizards: rectilinear, concertina, sidewinding, and lateral undulation.

Size

Snakes and lizards vary widely in size, from diminutive to gigantic. The smallest lizards, such as the Australian skink Menetia, are among the smallest of terrestrial vertebrates. Neonates have a snout-vent length of only 0.4 in (10 mm) and weigh less than 0.0035 oz (0.1 g), and adults have a snout-vent length of 1 in (25 mm) and a weight of 0.01 oz (0.3 g). Contrast these tiny skinks with Komodo dragons (Varanus komo-doensis), at 5 ft (1.5 m) in snout-vent length with a weight of up to 154 lb (70 kg). The largest living squamate is the South American green anaconda (Eunectes murinus), with a snout-vent length of over 30 ft (10 m) and a weight of more than 330 lb (150 kg). Reticulated pythons are almost as large but not as massive. Both constrictors kill and swallow extremely large prey.

General body form

The ancestral condition was that of a tetrapod with four limbs, each with five toes. Reduced limbs and leglessness have arisen repeatedly among squamates, especially in skinks. Except for some pythons and boas, which possess rudimentary vestigial remnants of hind limbs, all snakes are completely limbless. Elongation of the body or tail generally accompanies limb reduction, as it facilitates locomotion without limbs.

Fossoriality has arisen independently many times among scleroglossans. Chemosensory abilities and narrowing of the skull through loss of the temporal arches preadapted scle-roglossan clades to burrowing. Chemoreception allowed them to find and pursue prey underground and also to eliminate potentially noxious prey from their diets, opening up yet another adaptive zone. Ultimately, species in nine scleroglossan families (Pygopodidae, Scincidae, Dibamidae, Amphisbae-nidae, Trogonophidae, Rhineuridae, Bipedidae, Gymnop-

Snakes With Vestigial Limbs
The elegant-eyed lizard (Prionodactylus argulus) lives on low vegetation and vines in undisturbed Amazon rainforest where it feeds on small spiders and insects. (Photo by Laurie J. Vitt. Reproduced by permission.)

thalmidae, and Anguidae), and snakes in several families, took maximum advantage of a new underground world. Sclero-glossans with strikingly similar body plans swim through sand, burrow in tropical soils, and haunt the nests of social insects.

Coloration

Lizards come in a wide variety of colors, including red, orange, yellow, green, blue, indigo, and violet. Most match the color of substrates on which they live, offering camouflage, which confers some degree of protection from predators. Snakes are equally colorful, with some, such as coral snakes (Micrurus), being warningly colored with bright bands of red, yellow, and black.

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