The four orders of living (extant) reptiles – snakes and lizards, turtles, crocodilians, and sphenodons (Tuatara) – possess a distinctive suite of characteristics that allows them to function as true land animals. Unlike their amphibian kin, reptiles are able to live and reproduce entirely on land.
What is a Reptile?
Zoologically speaking, a reptile is defined to be any vertebrate animal that breathes air and whose body is ectothermic (cold-blooded) and covered with scales and scutes as opposed to hair and feathers. In addition, reptiles are tetrapods (possessing four limbs) that lay amniotic eggs (eggs whose embryos are covered by membranes and in the case of reptiles, shells) on land.
How Many Kinds of Reptiles Exist?
There are four groups of living reptiles on the planet today:
Crocodilia – 23 species of crocodiles, alligators, gavials, and caimans
Sphenodontia – 2 species of tuatara
Squamata – approximately 7,900 species of lizards, snakes, and amphisbaenids (worm lizards)
Testudinata – approximately 300 species of turtles, tortoises, and terrapins
What are the General Characteristics of Reptiles?
- A tough, dry, scaly skin that is heavily keratinized. Few cutaneous (skin) glands are present. Keratin is a resistant protein found in the epidermis of amniotes. It is protective and prevents water loss across body surfaces. Human hair and nails are made of keratin.
- The body is covered and protected by scales and scutes. Scales are made of keratin and are formed from the epidermis. Scutes are structurally similar to scales and serve the same function.
- Two sets of paired limbs (tetrapods), usually with five clawed toes. Limbs are heavily muscled and angled downward from the body allowing for more efficient and rapid locomotion than is possible with the leg plan of amphibians. Limbs are vestigial or absent in snakes, some lizards, and amphisbaenians (worm lizards).
- A strong, sturdy skeleton with a well-developed rib cage.
- Large well-developed lungs. There are no gills and cutaneous (skin) respiration is negligible.
- A circulatory system functionally divided into pulmonary (heart and lungs) and systemic (body) circulation. The reptilian heart has three chambers: two atria for receiving blood and one partially divided ventricle for pumping blood. The crocodile is an exception in reptiles in regards to heart structure as it has a four-chambered heart – 2 atria and 2 ventricles – similar to those of the birds and mammals.
- Although ectothermic (cold-blooded), reptiles maintain a relatively high body temperature during periods of activity and are more active than amphibians.
- Paired metanephric kidneys that eliminate considerable nitrogen as uric acid thus conserving water.
- Internal fertilization with sperm introduced directly into the female reproductive tract with a copulatory organ.
- A shelled amniotic egg that is laid on land, not in water. Even the sea turtles which spend their entire lives in the ocean crawl ashore to lay their eggs.
The characteristics of reptiles are so unique and distinct that even the most casual observer would find it relatively easy to identify the reptiles in a group of pictures of different types of vertebrate animals.
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