Modern biologists arrange the more than 8,000 living species of reptiles into four main groups: Squamata, Crocodilia, Testunides and Sphenodons. These groups are only distantly related, with some reptiles being more closely related to birds than to other reptiles, and so they do not represent a true clade, or evolutionary family. Instead, reptiles as a group are regarded as a "paraphyletic" family of animals.
Alligators, crocodiles and caimans are all modern members of the group Crocodilia. The first members of this group appeared 84 million years ago, and their modern descendants have survived largely unchanged. Crocodilians are the closest reptilian relatives to birds and are, in fact, more closely related to birds than to turtles. The group Testunides first appeared during the late Triassic Era and includes all living turtles and tortoises. As of 2014, the Sphenodon group has dwindled to only two living species, both confined to a limited area in New Zealand. Sphenodons are lizardlike in appearance, though they do not have lizards' characteristic jointed skulls. Squamata is by far the largest and most diverse reptile group. The over-7,400 living squamates live in a wide variety of climates all over the world and include all modern lizards and snakes.