Lizard - Evolution and Relationships

Evolution and Relationships

The retention of the basic 'reptilian' amniote body form by lizards makes it tempting to assume any similar animal, alive or extinct, is also a lizard. However, this is not the case, and lizards as squamates are part of a well-defined group.

The earliest amniotes were superficially lizard-like, but had solid, box-like skulls, with openings only for eyes and nostrils, termed the anapsid condition. Turtles retain this skull form. Early anapsids later gave rise to two new groups with additional holes in their skulls to make room for and anchor larger jaw muscles. The synapsids, with a single fenestra, gave rise to the superficially lizard-like pelycosaurs, which include Dimetrodon and the therapsids, including the cynodonts, from which the modern mammals would evolve.

The modern Tuatara retains the basic lepidosaur skull, distinguishing it from true lizards in spite of superficial similarities. Squamates, including snakes and all true lizards, further lightened the skull by eliminating the lower margin of the lower skull opening.

The earliest known fossil remains of a lizard belong to the iguanian species Tikiguania estesi, found in the Tiki Formation of India, which dates to the Carnian stage of the Triassic period, about 220 million years ago. However, doubt has been raised over the age of Tikiguania because it is almost indistinguishable from modern agamid lizards. The Tikiguania remains may instead be late Tertiary or Quaternary in age, having been washed into much older Triassic sediments. Lizards are most closely related to a group called Rhynchocephalia, which includes the tuatara. Rhynchocephalians first appeared in the Late Triassic, so it can be inferred that the lizard-rhynchocephalian divergence occurred at this time and that the earliest lizards appeared in the Triassic.

Mitochondrial phylogenetics suggest the first lizards evolved in the late Permian. Most evolutionary relationships within the squamates are not yet completely worked out, with the relationship of snakes to other groups being most problematic. From morphological data, iguanid lizards have been thought to have diverged from other squamates very early, but recent molecular phylogenies, both from mitochondrial and nuclear DNA, do not support this early divergence. Because snakes have a faster molecular clock than other squamates, and few early snake and snake ancestor fossils have been found, resolving the relationship between snakes and other squamate groups is difficult.