Squamata

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Squamata

The dominant order of living reptiles composed of the lizards and snakes. The group first appeared in Jurassic times and today is found in all but the coldest regions. Various forms are adapted for arboreal, burrowing, or aquatic lives, but most squamates are fundamentally terrestrial. There are about 4700 Recent species: 2200 lizards and 2500 snakes.

The order is readily distinguished from all known reptiles by its highly modified skull; an enlarged and movable quadrate; and a temporal opening that is lost or reduced in many forms. No other reptiles show these modifications, which allow for great kinesis in the lower jaw since it articulates with the quadrate. In addition, the order is distinct from other living reptile groups because its members have no shells or secondary palates and the males possess paired penes.

Traditionally the Squamata have been divided into two major subgroups, the lizards, suborder Sauria, and the snakes, suborder Serpentes. The latter group is basically a series of limbless lizards, and it is certain that snakes are derived from some saurian ancestor. There are many different legless lizards, and it has been suggested that more than one line has evolved to produce those species currently grouped together as snakes.

Sauria

The majority of saurians are insectivorous, but a few feed on plants while others, notably the Varanidae and allies, feed on larger prey including birds and mammals. The largest living lizard is the Komodo dragon (Varanus komodoensis).

The majority of lizards are quadrupedal in locomotion and are usually ambulatory scamperers or scansorial. Some forms are bipedal, at least when in haste. The coloration of each species of lizard is characteristic. Most forms exhibit marked differences in coloration between the sexes, at least during the breeding season, and frequently the young are markedly different from the parents. Color changes occur in rapid fashion among some species, and all are capable of metachrosis or changing color to a certain extent. See Sexual dimorphism

There are but two species of venomous lizards, both members of the genus Heloderma, in the family Helodermatidae: the Gila monster (H. suspectum) and the beaded lizard (H. horridum).

Serpentes

Snakes are basically specialized, limbless lizards which probably evolved from burrowing forms but have now returned from subterranean habitats to occupy terrestrial, arboreal, and aquatic situations. The following characteristics are typical of all serpents. There is no temporal arch so that the lower jaw and quadrate are very loosely attached to the skull. This gives the jaw even greater motility than is the case in lizards. The body is elongate with 100–200 or more vertebrae, and the internal organs are elongate and reduced. A spectacle covers the eye.

The largest living snake is the Indian python (Python reticulatus), which reaches 30 ft (9 m) in length and a weight of 250 lb (113 kg). The largest venomous snake is the king cobra (Ophiophagus hannah), of southern Asia, which is known to attain a length of 18 ft (5.5 m).

The senses of snakes are fundamentally similar to those of all terrestrial vertebrates. Great dependence is placed upon olfaction and the Jacobson's organs (olfactory canals in the nasal mucosa). The tongue of all snakes is elongate and deeply bifurcated. When not in use it can be retracted into a sheath located just anterior to the glottis, but it is protrusible and is constantly being projected to pick up samples for the Jacobson's organs from the surrounding environment. Snakes are deaf to airborne sounds and receive auditory stimuli only through the substratum via the bones of the head. The eyes are greatly modified from those in lizards, and there is no color vision. Some groups are totally blind and have vestigial eyes covered by scales or skin.

Four basic patterns of locomotion are found in snakes, and several may be used by a particular individual at different times. The most familiar type is curvilinear. Snakes using rectilinear locomotion move forward in a straight line, without any lateral undulations, by producing wavelike movements in the belly plates. Laterolinear locomotion, or sidewinding, is used primarily on smooth or yielding surfaces and is very complex. Concertina locomotion movement resembles the expansion and contraction of that musical instrument.

The vast majority of living snakes are harmless to humans, although a number are capable of inflicting serious injury with their venomous bites. The venom apparatus has evolved principally as a method of obtaining food, but it is also advantageous as a defense against attackers. Fangs are teeth modified for the injection of venom into the victim, and the venom glands are modified salivary glands connected to the grooved fangs by a duct. Special muscles are present in all proglyphous snakes to force the venom into the wound. The venom itself is a complex substance containing a number of enzymes. Certain of these enzymes attack the blood, others in the nervous system, and some are spreaders.

Squamata

a subclass or order of Reptilia. The Squamata include three orders or suborders: Sauria (lizards), Amphisbaenidae, and Serpentes (snakes). The characteristic feature U the movable joint between the quadrate bone of the upper jaw and the skull. The upper body is covered with horny scales, plates, and squamae.

Squamata

[skwə′mäd·ə] (vertebrate zoology) An order of reptiles, composed of the lizards and snakes, distinguished by a highly modified skull that has only a single temporal opening, or none, by the lack of shells or secondary palates, and by possession of paired penes on the males.

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Sauria

Serpentes

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Synonyms for Squamata

noun diapsid reptiles: snakes and lizards

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