Reptile
| Reptiles (traditional classification) | ||||||
|---|---|---|---|---|---|---|
| ||||||
| Orders | ||||||
|
Order Crocodilia (Crocodilians) | ||||||
The reptiles are a group of vertebrate animals, today represented with four orders:
- Order Crocodilia (crocodiles and alligators): 23 species
- Order Rhynchocephalia (tuataras from New Zealand): 2 species
- Order Squamata (lizards and snakes): approximately 7,600 species
- Order Testudines (turtles): approximately 300 species
Reptiles are found on all continents except for Antarctica, although their main distribution comprises the tropics and subtropics. Reptiles don't have a constant body temperature. They are only able to a limited extent to actively regulate their body temperature, which is largely dependent on the environmental temperature. Most reptile species are carnivorous and oviparous (egg-laying). Some species are ovoviviparous, and a few species are truly viviparous.
Classification of reptiles
Reptiles classically included all the amniotes except birds and mammals. They were grouped together in the class Reptilia, as biologists had observed common features among them. However, in recent years biologists have tended to emphasize that groups should be monophyletic, i.e. include all descendants of a particular form. Since birds originated from a particular group of reptiles, members of that group (including Crocodilia) are closer related to them than to other reptiles. Similarly certain extinct forms are closer to mammals than to any extant reptiles. As such, the Reptilia are a paraphyletic group.
Newer systems abandon or alter the composition of the Reptilia. The synapsids, comprising mammals and their close relatives, are typically excluded. Included is the common ancestor of living saurians (birds, crocodiles, lizards, snakes and turtles) and all their descendents.
Evolution of the reptiles
Several thousand fossil species showing a clear smooth transition from the ancestors of reptiles to present-day reptiles exist.
In addition the transition from synapsid reptiles to mammals is one of the best detailed transitions, with in many cases the lineage being traced down to the genus level, from paleothyris to climolestes. One offshoot branch is still alive today (monotremata).