The sister group to squamates is the Rhynchocephalia (the Tuatara and extinct relatives) which share a common ancestor around 250 million years ago at the start of the Mesozoic.
Together, they form the monophyletic Lepidosauria.
Most squamates have a transparent and rigid structure called brille or spectacle over their eyes.
The thing that all squamates have in common are movable quadrate bones that make it possible for the squamates to move their upper jaw relative to their braincase.
The movable quadrate bones are most visible in snakes that are able to open their mouth very wide to accomodate large prey.
The lepidosauria is the ancestor of Squamata.
The Lepidosauria is a subclass or superorder of reptiles that contains the orders Squamata and Rhynchocephalia.
Squamatas includes snakes, lizards, and amphisbaenians. Squamata contains over 9,000 species, making it by far the most species-rich and diverse order of non-avian reptiles in the present day.
Iguanians were long thought to be the earliest crown group squamates based on morphological data, although genetic data suggest that geckoes are the earliest crown group squamates.
Iguanians are now united with snakes and anguimorphs in a clade called Toxicofera.
The earliest unambiguously identified squamate fossils date from the Middle and Late Jurassic (174–145 Ma), and among them are forms that can be assigned to major modern clades of squamates, including both lizards and snakes (Evans, 1998; Evans, 2003; Caldwell et al., 2015), but many are isolated jaws and skull bones
Squamata is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians, which are collectively known as squamates or scaled reptiles. With over 10,900 species, it is also the second-largest order of extant vertebrates, after the perciform fish.
Class Reptilia includes many diverse species that are classified into four living clades.
Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).
There are more than 8,200 living species of reptiles, and they are placed in four orders: Crocodilia, which includes crocodiles and alligators; Sphenodontia, or tuataras; Squamata, which includes lizards and snakes; and Testudines, such as turtles and tortoises.
Gekkota (geckos and pygopods) constitute a diverse and early diverging clade of squamates, but their highly fragmentary fossil record allows only limited insights into their evolutionary history.