Squamates are the most variably sized living reptiles, ranging from the 16 mm (0.63 in) dwarf gecko (Sphaerodactylus ariasae) to the 6.5 m (21 ft) reticulated python (Malayopython reticulatus).

Among other reptiles, squamates are most closely related to the tuatara, the last surviving member of the once diverse Rhynchocephalia, with both groups being placed in the clade Lepidosauria.

Squamata and Rhynchocephalia form the subclass Lepidosauria, which is the sister group to the Archosauria, the clade that contains crocodiles and birds, and their extinct relatives.

[4][5] A study in 2018 found that Megachirella, an extinct genus of lepidosaurs that lived about 240 million years ago during the Middle Triassic, was a stem-squamate, making it the oldest known squamate.

The phylogenetic analysis was conducted by performing high-resolution microfocus X-ray computed tomography (micro-CT) scans on the fossil specimen of Megachirella to gather detailed data about its anatomy.

The presence of an essentially modern crown group squamate so far back in time was unexpected, as their diversification was previously thought to have occurred during the Jurassic and Cretaceous.

[11] The oldest unambiguous fossils of Squamata date to the Bathonian age of the Middle Jurassic of the Northern Hemisphere,[1] with the first appearance of many modern groups, including snakes, during this period.

[1] Squamate morphological and ecological diversity substantially increased over the course of the Cretaceous,[12] including the appeance of groups like iguanians and varanoids, and true snakes.

Although many of the groups originally recognized on the basis of morphology are still accepted, understanding of their relationships to each other has changed radically as a result of studying their genomes.

Genetic data also suggest that the various limbless groups – snakes, amphisbaenians, and dibamids – are unrelated, and instead arose independently from lizards.

The male members of the group Squamata have hemipenes, which are usually held inverted within their bodies, and are everted for reproduction via erectile tissue like that in the mammalian penis.

Due to being everted and inverted, hemipenes do not have a completely enclosed channel for the conduction of sperm, but rather a seminal groove that seals as the erectile tissue expands.

The only exception is found in blind lizards and three families of geckos (Gekkonidae, Phyllodactylidae and Sphaerodactylidae), where many lay rigid and calcified eggs.

However, the Colombian rainbow boa, Epicrates maurus, can also reproduce by facultative parthenogenesis, resulting in production of WW female progeny.

The Komodo dragon has been known to kill people due to its size, and recent studies show it may have a passive envenomation system.

Named Toxicofera, it encompasses a majority (nearly 60%) of squamate species and includes Serpentes (snakes), Iguania (agamids, chameleons, iguanids, etc.