Snake

Snakes are elongated, limbless reptiles belonging to the suborder Serpentes. As members of the clade Squamata, they are ectothermic vertebrates, meaning they rely on external heat sources to regulate their body temperature.

Most snakes are covered in overlapping scales, a characteristic shared with many other reptiles. They are found on every continent except Antarctica. They inhabit most small land masses, though they are absent from large islands like Ireland, Iceland, and Greenland.

Sea snakes are also widespread throughout the Indian and Pacific oceans.

The internal anatomy of a snake is highly specialized to accommodate its narrow body. In most vertebrates, paired organs like the kidneys are positioned side by side. In snakes, these organs appear one in front of the other.

Furthermore, most species possess only one functional lung. A major anatomical advantage is cranial kinesis, which refers to the many joints in the skull. This flexibility allows snakes to swallow prey that is much larger than their own heads.

This mechanism is vital for their survival as predators.

Snake skeletons differ significantly from those of their lizard ancestors. This change is driven by the evolution of Hox genes, which control limb morphogenesis, or the development of limbs. In snakes, the expression of genes responsible for the thorax, or chest area, became dominant. As a result, most of a snake's skeleton is an extremely extended thorax.

While most reptiles have distinct neck, chest, lower back, and tail sections, snakes have a highly reduced number of lumbar and pelvic vertebrae. They typically possess only 2 to 10 lumbar or pelvic vertebrae. Ribs are found exclusively on the thoracic vertebrae. Some primitive groups, like boas and pythons, still retain vestigial hind limbs known as anal spurs.

Taxonomically, modern snakes are divided into two main infraorders: Alethinophidia and Scolecophidia. This classification is based on morphological characteristics and similarities in mitochondrial DNA sequences.

Alethinophidia is a large group that includes various families like Colubridae, Viperidae, and Elapidae. Scolecophidia consists of smaller, often blind snakes. Within the Alethinophidia, some scientists further divide the group into Henophidia and Caenophidia. This complex system helps scientists organize the immense variety of species found globally.

The evolutionary history of snakes is linked to lizards. Genomic analysis and comparative anatomy confirm that snakes descended from lizards, perhaps during the Jurassic period.

The fossil record is often difficult to study because snake skeletons are small and fragile. However, the earliest known true snake fossils, such as the marine simoliophid *Haasiophis terrasanctus*, date to the Late Cretaceous, between 112 and 94 million years ago. The modern diversity of snakes expanded during the Paleocene epoch, roughly 66 to 56 million years ago. This followed the Cretaceous–Paleogene extinction event.

Today, there is incredible biological diversity among snakes. Scientists recognize approximately 30 families, which include about 520 genera and more than 4,170 species.

Sizes vary drastically across the group. The Barbados threadsnake is among the tiniest, while the reticulated python is among the largest. Most species are nonvenomous and use constriction to kill prey.

Venomous species use toxins to subdue prey, though some venoms are potent enough to cause serious injury to humans. This diversity allows snakes to occupy many different ecological niches.

It is important to distinguish snakes from legless lizards. Some lizards have independently evolved long, limb-less bodies through a process called convergent evolution.

This means they developed similar traits to snakes even though they are not closely related. You can often tell them apart by looking for specific features. Many common groups of legless lizards possess eyelids and external ears, whereas snakes lack these structures. By studying these physical differences, biologists can accurately classify the vast array of creeping, crawling reptiles in our world.