The class of Reptilia is nothing but a pale reflection of the many species that inhabited the Earth during the Mesozoic era, which was supposedly 65 to 230 million years ago via the scientists' calculations albeit I've never really put too much faith into carbon dating - especially when covering such a broad scale of time. To me, there are too many factors and variables that could effect the results of such tests, but to save all the poppycock, we'll just say the Mesozoic era was a very long freakin' time ago, to say the least; ha!
Anyway, the study of fossils has allowed a subdivision of reptiles into about twenty orders, in place of the four orders still existent.
Reptiles, such as crocodiles, turtles, snakes, lizards and many more, are distributed throughout the entire world, with the exception of the cold polar regions and high mountainous areas. Most species are terrestrial, but there are a good number living at least partly in an aquatic environment. There are also tree-dwelling reptiles, and others that burrow and remain hidden in the earth. The body is covered by scales or scutes - patches of horny, sometimes bony, skin. Limbs are usually short or absent; feet show many variations in form, in response to different lifestyles.
Most reptiles are oviparous; the eggs, rich in nutritionally valuable yolk, are covered with a strong shell. Incubation is ensured by the warmth of the earth on which they are laid, or in which they are buried. Some species of lizards and snakes are what is termed ovoviviparous - eggs are retained in the mother's body until they hatch, whereupon the young emerge under their own steam. There is no larval stage, as the young are mini replicas of their parents.
When it comes to how many species of reptiles there are in the world, I can't give you a solid answer. More and more are being discovered and added to the list all the time. I have read that there or 5 or 6,ooo different species all the way up to 9,000, so who knows, really... A lot of the data is according to whether the "experts" regard some forms as separate species or subspecies within one species.
There are four orders: Chelonia (turtles and tortoises), Crocodilia (alligators, caymans, crocodiles, and gavial), Rhynchocephalia (sphenodon or tuatara), and Squamata which is divided into two suborders, Sauria (lizards) and Serpentes or Ophidia (snakes).
Chelonia - Tortoises & Turtles - which have aptly been described as "living fossils," are identifiable by their characteristic bony carapace which is covered with big horny scutes; this protective shell consists of two main parts: the top or dorsal carapace, and the plastron which protects the underside of the body.
Crocodilia - alligators, crocodiles, etc. - are relics of prehistoric times and are the biggest of reptiles. They have thick scales, reinforced on the back by bony plates that stand out from the skin; their jaws are edged with a large number of conical teeth. They live in marshland, by running water, in estuaries within warm regions, and some can travel far out to sea.
Rhynchocephalia - is another "living fossil" and sole representative of an order which flourished during the Mesozoic era. It's not large, but its body is thick and its head is enormous (Hmm, I've seen humans like that; ha!). Its metabolism is slow... Its eggs stay in the earth for 12 to 15 months before hatching, and the young cannot procreate for another 20 years!
Sauria - lizards - There are many various forms, often adorned in brilliant colors, with crests, spiky ruffs, etc. Many species are almost or entirely limbless and look like worms or snakes. The Komodo dragon is the biggest while the smallest is the Antillan gecko.
Ophidia - snakes - A snake's spinal column can consist of up to 400 vertebrae or more. Some species, the Boas for example, have vestiges of limbs. The scaly skin is often decorated with magnificent patterns. Like Sauria, Ophidia have to molt as they get bigger; the old skin, which has become too small, splits and the animal extracts itself from it, dressed in its new clothes, so to speak...
Additional Link: "Big Green Reptilian Aliens and their Physical Characteristics"
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