Snake

**Snakes**, otherwise known as **serpentes**, are reptiles. They are part of the order Squamata. They are carnivores, with long narrow bodies and no legs. There are at least 20 families, about 500 genera and 3,400 species of snake.

## Evolution 
The earliest known snake fossils are from the Middle Jurassic to Lower Cretaceous periods. Scientists think snakes evolved from lizard-like ancestors around 100 million years ago.  A wide range of snakes appeared during the Paleocene period (*c* 66 to 56 million years ago).

Bone fossils show how snakes have adapted to their surroundings, enabling them to become successful predators in various ecosystems. Over time, the snake skull has evolved and become more flexible, which makes it possible to swallow prey whole. Snakes also lost their legs over time and developed elongated bodies, which helped them move more efficiently. 

### Not a clade 
The Squamata, ancestors of snakes and lizards, are a monophyletic group - descended from a single ancestor. It is a sister group to the Tuatara. 

Judged by their fossil record, squamates existed in the Mesozoic era, but they had a minor place in the land ecology. Three of the six lines are recorded first in the Upper Jurassic, the others in the Cretaceous. Probably all, certainly the lizards, arose earlier in the Jurassic. The Mosasaurs of the Upper Cretaceous were by far the most successful of all the lizards, becoming the top predator in their ecosystem.

Although snakes and lizards look very different, neither is a proper clade. Snakes did descend from early lizards, not once, but many different times.

There is a monophyletic clade within the Squamata. It is the Toxicofera. It includes all venomous snakes and lizards, and many related non-venomous species. The evidence for this is in recent molecular analyses.

### Fossil snakes 
Scientists have a fossil of a primitive snake from the Lower Cretaceous epoch. It lived about 113 million years ago. It had rather small front and rear legs. Several other fossil snakes have been found with small rear legs, but this is the first one with all four legs. 

The snake, *Tetrapodophis amplectus*, lived on land and was adapted to burrowing. The researcher said there were "a lot of very advanced snake features, including its hooked teeth, flexible jaw and spine – and even snake-like scales. And there's the gut contents – it's swallowed another vertebrate. It was preying on other animals, which is a snake feature". The snake came from the Crato Formation in Brazil, and lay in a private collection for many years. It was re-discovered in a museum at Solnhofen, Bavaria.

## Habitat 
Most snake species live in the tropics. Few live beyond the Tropic of Cancer or Tropic of Capricorn. Only one species, the common viper (*Vipera berus*), lives beyond the Arctic Circle.

Most snakes live on the ground, and in the trees. Others live in the water, and a few live under the soil. Like other reptiles, snakes are ectotherms. They control their body temperature by moving in and out of the direct sunshine. That is why they are rare in cold places. 

## Venom 
Most snakes are nonvenomous. Those that have venom use it mainly to kill and subdue prey rather than for self-defense. Some have venom potent enough to cause painful injury or death to humans. Nonvenomous snakes either swallow prey alive or kill by squeezing.

There are three major families of snakes:

# Elapids are all venomous. They include cobras, kraits, mambas, Australian copperheads, sea snakes, and coral snakes.
# Viperids are all venomous. They include vipers, rattlesnakes, copperheads/cottonmouths, and bushmasters.

## Anatomy 
 and joints enable them to swallow large prey]]
Snakes range in size from the tiny, 10.4&nbsp;cm (4&nbsp;inch)-long thread snake to the reticulated python of  in length. The extinct snake *Titanoboa* was  long.

### Bones and joints 
Many species have skulls with more joints than the skulls of their lizard ancestors. This allows the snakes to swallow prey much larger than their heads. The bones of the head and jaws can move apart to let large prey move into their body. The throat, stomach and intestines can also expand. In this way, a thin-looking snake can swallow and digest a larger animal.

### Long body 
Their long, slender body has some special features. 

Because snakes are cold-blooded, they do not have to eat as regularly as mammals. People who own pet snakes feed them as infrequently as once per month. Some snakes can go as long as six months without a good meal.

Snakes have a very flexible lower jaw; its two halves are not rigidly attached. Neither are many other joints in their skulls. This enables snakes to open their mouths wide enough to swallow their prey whole, even if the prey is larger in diameter than the snake itself.

## Movement 
Snakes can move even without arms and legs. They have developed several different ways of moving to deal with particular environments. Each type of snake movement is distinct from the others.

### Lateral undulation 
Lateral undulation is a snake's only way of moving in water, though snakes commonly use it on land. A snake moving this way burns as many calories per metre as a running lizard does.

#### Terrestrial 
Terrestrial lateral undulation is the most common mode of moving for most snake species.

#### Aquatic 
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Snakes move forward in water by moving their bodies in a wave-like motion. The waves become larger as they move down the snake's body, and the wave travels backwards faster than the snake moves forwards. They achieve thrust by pushing their body against the water. 

Studies show that the pattern of muscle activation is somewhat different in lateral undulation on land compared to in water.  All snakes can laterally undulate forward (with backward-moving waves), but only sea snakes have been observed reversing the motion (moving backwards with forward-moving waves). This mode of moving overcomes the slippery nature of sand or mud by pushing off with only static portions on the body, thereby minimizing slipping. In this mode, the belly scales are lifted and pulled forward before being placed down and the body pulled over them. Waves of movement and stasis pass posteriorly, resulting in a series of ripples in the skin. While on tree branches, snakes use several modes of moving depending on species and bark texture. These snakes can perform a controlled glide for hundreds of feet depending upon launch altitude and can even turn in midair.

## Other websites 
* [Snake (organism)](http://en.citizendium.org/wiki/Snake_(organism))  -Citizendium
* Snakes world  by Marie Curie