There are two Sub-orders under testudines:
- Family Chelydridae – Snapping Turtles
- Family Emydidae – Pond/ Box Turtles
- Family Testudinidae – Tortoises
- Family Geoemydidae – Asian River Turtles and Allies
- Family Carettochelyidae – Pignose Turtles
- Family Trionychidae – Softshell Turtles
- Family Dermatemydae – River Turtles
- Family Kinosternidae – Mud Turtles
- Family Cheloniidae – Sea Turtles
- Family Dermochelyidae – Leatherback Turtles
Sub Order Pleurodira:
- Family Chelidae – Austro American Sideneck Turtles
- Family Pelomedusidae Afro American Sideneck Turtles
- Family Podocnemididae – Madagascan Big Head Turtles
Turtles, Tortoises and Terrapins are reptiles in the order Testudines, whose bodies are shielded by a special bony or cartilagenous shell developed from their ribs.
About 300 Species of this Order are alive today.
Hylomonus (above) is the oldest-known reptile, and was about 8 to 12 inches (20 to 30 cm) long. Westlothiana has been suggested as the oldest reptile, but is for the moment considered to be more related to amphibians than amniotes. Petrolacosaurus and Mesosaurus are other examples. The earliest reptiles were found in the swamp forests of the Carboniferous, but were largely overshadowed by bigger labyrinthodont amphibians such as Proterogynrius. It was only after the small ice age at the end of the Carboniferous that the reptiles grew to big sizes, producing species such as Edaphosaurus and Dimetrodon.
The first true “reptiles” (Sauropsids) are categorized as Anapsids, having a solid skull with holes only for nose, eyes, spinal cord, etc. Turtles are believed by some to be surviving Anapsids, as they also share this skull structure; but this point has become contentious lately, with some arguing that turtles reverted to this primitive state in order to improve their armor. Both sides have strong evidence, and the conflict has yet to be resolved.
Shortly after the first reptiles, two branches split off, one leading to the Anapsids, which did not develop holes in their skulls. The other group, Diapsida, possessed a pair of holes in their skulls behind the eyes, along with a second pair located higher on the skull. The Diapsida split yet again into two lineages, the lepidosaurs (which contain modern snakes, lizards and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic) and the archosaurs (today represented by only crocodilians and birds, but also containing pterosaurs and dinosaurs).
The earliest, solid-skulled amniotes also gave rise to a separate line, the Synapsida. Synapsids developed a pair of holes in their skulls behind the eyes (similar to the diapsids), which were used to both lighten the skull and increase the space for jaw muscles. The synapsids eventually evolved into mammals, and are often referred to as mammal-like reptiles, though they are not true members of Sauropsida. (A preferable term is “stem-mammals”.)