Lizards are a rare family of reptiles, confined to rocky islands off the coast of New Zealand. Today, monitor lizards are among the least diverse reptile groups, with only one extant species: *Sphenodon punctatus*. However, they were once much more widespread and diverse, spanning Europe, Africa, South America, and Madagascar. There were once as many as 24 different genera of monitor lizards, but most of them disappeared around 100 million years ago during the Middle Cretaceous period, undoubtedly succumbing to competition from more adaptable dinosaurs, crocodiles, and other lizards.
Lizards are nocturnal burrowing reptiles that inhabit coastal forests. They forage within limited ranges, feeding on bird eggs, chicks, invertebrates, amphibians, and small reptiles. Because these reptiles are cold-blooded and live in cool climates, lizards have extremely low metabolic rates, grow slowly, and have impressive lifespans. Surprisingly, female monitor lizards are known to reproduce before the age of 60, and some experts speculate that healthy adult lizards can live up to 200 years (roughly equivalent to the lifespan of some large sea turtle species). Like some other reptiles, the sex of lizard larvae depends on ambient temperature. Exceptionally warm climates result in more males, while unusually cool climates result in more females.
The strangest feature of lizards is their "third eye" : a light-sensitive spot located on the top of the reptile's head, thought to play a role in regulating their circadian rhythm (i.e., their metabolic response during the day and night). As some mistakenly believe, this structure is not merely a piece of sunlight-sensitive skin; it actually contains the lens, cornea, and primitive retina, although their connections to the brain are loose. One possibility is that the final ancestor of Late Triassic lizards actually possessed three functional eyes, with the third eye gradually degenerating over millions of years into the acropanthes of modern lizards.
Where do lizards stand on the reptile evolutionary tree? Paleontologists believe these vertebrates trace back to an ancient split between lepidosaurs (reptiles with overlapping scales) and apexosaurs, a family of reptiles that evolved into crocodiles, pterosaurs, and dinosaurs during the Triassic period. Lizards are called "living fossils" because they are among the simplest amniotes (vertebrates that lay eggs on land or incubate them inside the female). Compared to turtles, snakes, and lizards, these reptiles have extremely primitive hearts, and their brain structure and posture can be traced back to the ultimate ancestor of all reptiles—amphibians.
Main characteristics of lizards
It grows extremely slowly and has a low reproduction rate.
Sexual maturity is reached between the ages of 10 and 20.
Diaporous skulls have two temporal foramina.
The protruding parietal lobe "eye" on top of the head.
Classification of monitor lizards
Lizards are classified according to the following hierarchical classification:
Animals > Chordata > Vertebrates > Tetrapods > Reptiles > Lizards
