The Squamata are the most successful order of Cenozoic reptiles, with some six thousands recent species (more species than there are mammals!). They are found on every continent except Antarctica, in such diverse modes of life as climbers, burrowers, crawlers, aquatic forms, and even gliding types.
Technically, the Squamata are lepidosaurs, distinguished from other related and similar forms by a flexible and powerful jaw structure. This came about through the loss of the lower temporal bar in the skull, which gave more room for the jaw muscles to develop, and the development of a special moveable hinge, the quadratojugal hinge, between the quadrate and squamosal bones in the skull. (This is termed streptostyly). This hinge gives greater flexibility and allows the pterygoideus (jaw) muscle to exert more force. It is clearly this dual adaptation that has been key to the success of the Squamates.
The Squamata were traditionally divided into two suborders - Lacertilia (lizards) and Ophidia (snakes). This division is rather simplistic, being based on superficial features. It is now believed that the Iguania represent a lineage separate from the others. In addition to iguanas, the Iguania include agamids, chameleons and a few lesser known groups. The rest of the modern lizards, as well as the snakes, are classified as Scleroglossa.
Proto-lizards evolved perhaps during the Late Triassic (the earliest putative squamate is the poorly known Tikiguana, from the Late Carnian of India) or Early Jurassic from lepidosaur ancestors sphenodont-like ancestors. They underwent a great evolutionary radiation during the Middle and late Jurassic, and by the Early Cretaceous had replaced the sphenodonts as the dominant "lizard" ecomorph. These reptiles are often ornamented with a crest of scales along the back, as well as assorted spines, frills, and throat fans; all this gives them a remarkably prehistoric appearance. Primitive forms are small and insectivorous, but where there is less competition or predation they may develop into larger herbivores, as represented by the famous marine iguana of the Galapagos.
MAK000207, MAK990716, MAK101106.