The origin of lizards

1.The origin of lizards

Generally speaking, the evolution of lizards is probably the most notable example of natural selection amongst vertebrates. A series of key transformations resulted in multiple drastic adaptive expansions of vertebrates, and from them emerged animals we now know as lizards and snakes. These reptiles originated in an animal similar to present day lizards, the Lepidosauria, followed by the emergence of the Sphenodontia. Although the male tuataras lack sexual organs, they developed many traits that were carried over and flourished in the order of Squamata. The basic morphology of lizards gradually took shape, including traits such as well developed optical nerve systems for tracking prey movement, tongues evolved to catch prey, and the incomplete but useful chemical sensory system, designed for olfactory, gustatory and food detection. 

At the same time, the invertebrates began to rapidly evolve and diversify, beginning with plant life. The gymnosperms were the first to flourish, then came the vascular plants, and finally the evolution branching of the invertebrate animals. For example, spiders emerged from arthropods early in evolution, which allowed their own adaptive expansions and enabled them to prey on other arthropods and small invertebrates today.
Early lizards were thus faced with a world full of potential arthropods prey, and as such have developed special characteristics to fully utilize these resources. It is known that at least five revolutionary events occurred during the diversification of lizards: (1) the diversification of iguanas, (2) the evolution of Scleroglossa, (3) the branching of the Gekkonidae, (4) the diversification of the suborder Diploglossa, and (5) the evolution of snakes from the diploglossa suborder, possibly the Anguimorpha. At each of the nodes during evolution, the morphology of lizards underwent drastic revisions to adapt to different survival requirements: limbs were constantly changing or disappearing altogether; tongues underwent function specific modifications; eyes and ears either reduced in size or disappeared altogether as new signal detection mechanisms, such as chemical or temperature sensing, replaced that of the conventional visual/hearing senses. Some lizards have even evolved to sense and communicate socially via ultraviolet radiation. Snakes, on the other hand, adapted to underground environments by extending their bodies, and simultaneously losing their limbs, ears, and eyesight. However, when they re-emerged above ground again, they became one of the most advantageous terrestrial vertebrates by regaining and refining their visual acuity. A contributing factor of the return of snakes to terrestrial life is probably their discovery of the hierarchical insect hives in the depths of the earth. This discovery allowed snake ancestors to side-step the physical limits imposed by digging through collapsible dirt layers, instead taking advantages of the ready-made insect tunnels and immediate food sources. Some snakes returned to the desert where their lizard ancestors once roamed and foraged on lizard or lizard eggs; others have developed a “sixth sense” like ability of infrared thermal detection, which is linked to the optical nerve system and enables snakes to locate the body heat response of prey in total darkness. Other types of snakes reverted back to the stationary foraging mode akin to the iguanas and use an enhanced chemical detection system to set up the perfect ambush site.
2. Fossil records
To fully understand the evolution history of lizards, we start by first examining the fossil records. We reviewed the morphology, behavior, physiology and ecology backgrounds on lizard evolution, examined the main branching of the evolutionary tree of lizards, and evaluated the diversification of the different types of lizards. Finally, we drafted and presented a hypothetical scenario for the evolutionary diversity of lizards.
Due to their smaller stature, fragility and terrestrial nature, few lizard fossils remained, consisting mostly of scattered pieces or fragments. However, a small number of intact fossils have been excavated. There are at least five excavated fossil records that possibly represent the ancient extinct species of lizards, dated around the late Jurassic period (about 150 million years ago). Since these fossils are spread over different classification trees, we theorized that the main evolutionary branches of lizards (iguana, gecko, tuatara, and anguidae) were established by the late Jurassic period. The common ancestors of snakes and lizards must have existed in the period between early Triassic to late Jurassic (about 200 million years ago), however no fossils of such species have been found to date. Two similar species from the main branch – the Paliguanidae and the Kuehneosaurus, existed during late Triassic in Australia and Africa, respectively. Their existence could be explained by the migration of lizard ancestors from Pangaea to the Gondwanaland via continental drift. When Pangaea was breaking up 200 million years ago, some ancestral lizard species also settled on the Laurasia, a super continent formed by the plate tectonic. However, species that resemble present day lizards have yet to appear during this period. 
A fossil specimen dated approximately 125 million years ago was discovered in China. This fossil is believed to be the remains of Yabeinosaurus tenuis, an organism thought to represent an ancestral branch of the Gekkonidae order. A Mongolian fossil site dated to the late Cretaceous period contained many magnificent complete specimens of dinosaurs and lizard fossils (probably preserved by sandstorms), among them is a 800 thousand year old skeletal fossil of a Varanidae genus lizard (named Estesia in honor of Richard Estes, an American artist), which greatly resembles the present day descendants. This suggests that the evolution of Varanidae lizards is so successful that the body structures remained relatively unchanged throughout the years. The Estesia is thought to be poisonous because it possesses grooved teeth like those of the Heloderm lizards, which are used to transmit venom; however, the teeth on the Estesia are wider and may have been used for other purposes. Aside from its usual diet, the Estesia lizards may raid dinosaur nests and prey on eggs and infant dinosaurs.
The fossil records of the Scleroglossa lizards revealed that a possible major bloom of diversification occurred at least once during the Cretaceous period. Although the Teiidae lizards are widely distributed and flourished in North America, the lizards of this genus in Asia seem to be limited to one Mongolian species that dates back to the early Cretaceous periods. From an evolutionary viewpoint, the dentition of Teiidae lizards seem to originate from simple structures formed by single canine teeth, which resembles the cone-shaped teeth of geckos. A few other types of Teiidae lizards appear during the mid-Cretaceous periods, some have kept the cone-shaped single canine configuration, while some evolved to the typical front-and-back configuration of double canine teeth; furthermore, some have developed triple-crowned canine teeth. There were also species that have horizontal double-crowned canine teeth configurations. Although these Teiidae lizards were all extinct in North America, some have survived in South America, but they no longer retain the diverse teeth configurations seen on the North American Teiidae lizards. During the late-Cretaceous periods, the South American Teiidae lizards migrated north and replaced the extinct species on the North American continent. However, only two species have made the invasion, and only one has reached the temperate zones of the northern latitude areas. Therefore, we can discern that the diversity of teeth configuration in South American Teiidae lizards must have happened in recent times.
Other types of Scleroglossa lizards were also common during the Cretaceous periods. For example, the well-known girdle-tailed (Codylus) lizards on the late-Cretaceous island of Madagasgar, and a type of Varadiae lizards, similar to Heloderms that existed in the early-Cretaceous state of Utah. Because these two species are high derived, we can speculate that the evolution of the Scleroglossa lizards has been proceeding smoothly from the start. The mosasaur, a type of ancient marine lizard (Megalania), is the most famous lizard of the Cretaceous period, partly because many magnificent fossil records of this species have remained. The mosasaurs were extinct by the end of Cretaceous period. 
The diversification of snakes began during the mid-Cretaceous periods. Recently a Cretaceous snake fossil was discovered in Utah, which suggests that snakes might have originated from the Gondwanaland super continent, before North America drifted to its present day north-west location.
Another interesting lizard fossil from the Tertiary-Miocene period was discovered in Africa. It is a skull of a chameleon, encased in calcite crystals. Also, a complete fossil of a Lacertidae lizard was found preserved in a piece of Baltic Sea amber, which also contained specimens dating back to mid-Eocene periods, 40 million years ago.

National Museum of Natural Science