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The Three Largest Groups of Reptiles

The reptiles group was originally thought to consist of turtles, snakes, lizards, crocodiles, alligators and tuataras. But then DNA analysis shed light on the evolutionary history for reptiles. As a result, this group now includes birds. Some disagreement exists in the scientific community about the exact evolutionary history and taxonomy of the various groups. But there's general consensus on five reptile subgroups with varying degrees of relatedness: Reptilia (lizards and snakes), Chelonia (turtles), Eusochia (Crocodiles), tuatara and Aves (birds). The three largest of those are Reptilia, Chelonia and Aves.

Birds (Class: Aves)
- Crocodiles and dinosaurs are more closely related to this barn owl and other birds than to other reptile groups.
  
  Birds are identified by the following characteristics: feathers, a bill or beak, air sacs in their lungs, hollow bones, hard-shell eggs, homeothermy, a keeled sternum, nucleated blood cells and cloaca. None of these characteristics is unique to birds. But together they make up a unique organism capable of flight, a trait that influences the morphology and physiology of every living bird, whether it is the fastest organism on the planet (e.g., the peregrine falcon) or it has secondarily lost the power of flight through evolution (e.g., the ostrich). With more than 8,700 species, birds are the largest taxon within the reptile group.
Lizards and Snakes (Class: Reptilia)
- This crested gecko lizard is one example of the diverse class of Reptilia.
  
  The class Reptilia includes 7,750 species of snakes, lizards and amphisbaenians and are sometimes referred to by the suborder Squamata. This group has more than 70 anatomical characteristics, including a kinetic skull, a musculoskeletal system and the presence of a hemipenis. Repitilia's range is nearly worldwide, and as a result they are extremely diverse in their morphology.
Turtles (Class: Chelonia)
- This Eastern box turtle is one species within the class Chelonia.
  
  The turtles class Chelonia includes 300 species. The shell, composed of ribs, vertebrae, pectoral girdle and keratin or skin, is the defining characteristic for this group. As a result of the shell, turtles are unique in that they cannot breathe by expanding the rib cage. Turtles are among the most long-lived vertebrates, with documented captive life spans ranging up to 50 years.
Other Reptiles
- This saltwater crocodile is a member of one of the smallest taxonomic groups of "reptiles."
  
  With 23 species, crocodilians (Class: Eusuchia), which includes crocodiles and alligators, is the fourth-largest taxon traditionally considered a reptile and are characterized by their heavy armor, long bodies, long snouts, dorsally placed nostrils for life underwater and powerful tails. The smallest reptile taxon is the tuatara, with two species. Tuataras are small, lizardlike animals with large teeth that live only in New Zealand. Their name is derived from the Maori expression for "spines on the back," a characteristic especially true for male tuataras.

Animals & Science