LIZARDS (and SNAKES)
Today's
lizards are descendants of a major group of amniote tetrapods, all of which share
the characteristic of having two openings in the side of the skull, marking
them as Diapsids (a group which includes snakes, lizards, crocodiles --
and also dinosaurs and birds, all of whom are now classed as "reptiles").
Diapsids display the widest array of species of all amniotes, and have invaded
all major habitats from polar regions to deserts to the oceans and oceanic islands.
Diapsids in turn are classified into "Archosaurs" ["ruling
lizards"] -- crocodilians, birds, and their extinct relatives (dinosaurs,
pterosaurs, etc.) -- and "Lepidosaurs" [reptiles with overlapping
scales] -- lizards, snakes, sphenodon (the living Tuatara of New Zealand),
and their extinct relatives. The Lepidosaurs are by far the largest non-avian
group of reptiles, with some 4,000 species of lizards and 2,700 of snakes. Strictly
speaking, snakes are lizards who have lost their legs (though other kinds of lizards
have also lost their legs).
Lizards (and snakes) share a series of distinctive features as members of the Order "Squamata" -- "the scaly ones": determinate, limited-period of growth; light skull-bone construction; mobile-jointed skulls (especially the squamosal-quadrate joint); unique male reproductive organ ("hemipenes"); repeated evolutionary limb-reduction (every major group has at least one species with some degree of it); and "voluntary" tail loss (a capacity lost in some species, but definitely an ancestral trait of the group). The earliest known Squamata date from the Triassic Era (248-206 MY).
Sub-groupings of Squamata are based in part on the degree of mobility of the jaws, differences that correlate with differing patterns of predation. The Geckos, for example, have just two flexible joints in the middle of the skull, whereas the most specialized of lizards -- the snakes -- have a series of flexible skull joints as well as ligaments that allow the jawbones to move well apart (enabling them to swallow comparatively large prey).
We will list Squamata -- lizards -- of our region as we encounter and photograph them:
Click on: Banded Gecko (Coleonyx variegatus)
Click on: Whiptail Lizards (Cnemidophorus spp)
Click on: Desert Regal Horned Lizard (Phrenosoma solare)
Click on: Spiny Lizard (Sceloporus spp)
Click on: Madrean Alligator Lizard (Gerrhonotus kingii)
Click on: Greater Earless Lizard (Cophosaurus texanus)
Click on: Zebra-tailed Lizard (Callisaurus draconoides)
Click on: Gila Monster (Heloderma suspectum)
Click on: Snakes