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Reptiles

Reptiles are characterized by breathing air, laying shelled eggs (except for some vipers and constrictor snakes that give live birth), and having skin covered in scales and/or scutes. Reptiles are classically viewed as having a "cold-blooded" metabolism. They are tetrapods (either having four limbs or being descended from four-limbed ancestors). Modern reptiles inhabit every continent, with the exception of Antarctica, and four living orders are currently recognized: Crocodilia (crocodiles, gavials, caimans, and alligators), Sphenodontia (tuataras from New Zealand), Squamata (lizards, snakes, and worm lizards), and Testudines (turtles and tortoises). 

 

Reproduction

Unlike amphibians, reptiles do not have an aquatic larval stage. As a rule, reptiles are oviparous[IC69] (egg-laying), although certain species of squamates are capable of giving live birth. This is achieved by either ovoviviparity (egg retention) or viviparity (birth of offspring without the development of calcified eggs). Many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings.

 

Regulating Body Temperature

All reptiles exhibit some form of cold-bloodedness. This means that most reptiles have limited physiological means of keeping the body temperature-constant, and often rely on external sources of heat. Due to a less-stable core temperature than endothermic birds and mammals, reptilian biochemistry requires enzymes capable of maintaining efficiency over a greater range of temperatures than warm-blooded animals. The optimum body temperature range varies with species, but is typically below that of warm-blooded animals, in the 24-35°C range for many lizards, while extreme heat adapted species like the American desert iguana, Dipsosaurus dorsalis, can have optimal physiological temperatures in the mammalian range, between 35 and 40°C.

 

Crocodilia

 

Crocodile head

Figure 10. Crocodile head
"Crocodile head" by Steve Hillebrand, U.S. Fish and Wildlife Service, Wikimedia Commons is in the Public Domain

 

Crocodilia (or Crocodylia) is an order of large reptiles that appeared about 84 million years ago in the late Cretaceous Period (Campanian stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosauria. Members of the crocodilian total group, the clade Crurotarsi, appeared about 220 million years ago in the Triassic Period and exhibited a wide diversity of forms during the Mesozoic Era. A crocodile is any species belonging to the family Crocodylidae (sometimes classified instead as the subfamily Crocodylinae). The term can also be used more loosely to include all extant members of the order Crocodilia: i.e. the true crocodiles, the alligators and caimans (family Alligatoridae) and the gharials (family Gavialidae), as well as the Crocodylomorpha which includes prehistoric crocodile relatives and ancestors.

 

Sphenodontia

 

Tuatara

Figure 11. Tuatura
"Tuatara" by Andrew McMillan, Wikimedia Commons is in the Public Domain

 

The tuatara is a reptile endemic to New Zealand which, though it resembles most lizards, is actually part of a distinct lineage, order Sphenodontia. The two species of tuatara are the only surviving members of its order, which flourished around 200 million years ago. Their most recent common ancestor with any other extant group is with the squamates (lizards and snakes). For this reason, tuatara are of great interest in the study of the evolution of lizards and snakes, and for the reconstruction of the appearance and habits of the earliest diapsids (the group that also includes birds, dinosaurs, and crocodiles).

 

Squamata

Figure 65. Achalinus_spilanus_01_juv_dorsal.jpg

Figure 12. Achalinus spilanus
"Juvenile Japanese Odd-scaled Snake" by Unknown, Wikimedia Commons is in the Public Domain

 


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