By Leo Schwartz

Reptiles are amniotes, a clade that consists of the mammals, the birds, and the vertebrates referred to as reptiles. During the Mesozoic era, an evolutionary radiation of amniotes gave rise to three major groups. Modern reptiles are descended from the diapsid lineage of the split, along with birds. Reptiles were more widespread, numerous, and diverse during the Mesozoic era (251-65.5 million years ago) than they are today. The oldest reptile fossils are about 300 million
The Far Side Anthology by Gary Larson
years and were found in Kansas. Reptiles were the dominant terrestrial vertebrates for over 200 million years. During

the late Triassic times (about 200 million years ago), they diversified into two groups: the famous dinosaurs and the pterosaurs, or flying reptiles. At the end of the Cretaceous, the last period of the Mesozoic era, the dinosaurs became extinct. Although the greatest age of the reptiles is long gone, there are about 6,500 species of extant reptiles alive today.


1 Diagnostic Characteristics
2 Acquiring and Digesting Food
3 Sensing the Environment
4 Locomotion

5 Respiration
6 Metabolic Waste Removal
7 Circulation
8 Self Protection
10 Temperature Balance

Diagnostic Characteristics
American crocodile
American crocodile

Reptiles branched from an amphibian ancestor and possess several adaptations for terrestrial living not usually found in amphibians.

All reptiles have, or had an ancestor with certain common characteristics. These characteristics include scales, complementary limbs, 5 toes, lungs, and a 3-4 chambered heart. Additionally, the eggs of reptiles have a shell made out of calcium. (AK) Reptiles’ skin is covered with scales containing the protein keratin, which waterproof the reptile and prevent dehydration in dry air. Most reptiles lay amniotic eggs (shelled, water-retaining eggs) on land. The most distinguishing characteristic of reptiles is that they are cold-blooded, or ectotherms. This means that they do not use their metabolism to control body temperature but instead regulate body temperature by using behavioral adaptations. Reptiles today are classified into four groups: Testu

dines (turtles), Sphenodontia (tua

taras), Squamata (lizards and snakes), and Crocodilia (alligators and crocodiles). Depending on whether the traditional or alternative classification system is used, these groups are respectively either orders or classes within the class Reptilia. [The distinction between the 5 subclasses of these groups are based on skull anatomy (number/location of openings in the skull). Today's reptiles are only in 2 of the 5 subclasses. (BY)] Turtles can be characterized by their hard shell, which protects them against predators. However, not only does turtles' hard shells protect them, it also serves as a "calcium bank," as calcium and other cations are taken from the shell to buffer the blood during hibernation, a time during which metabolic acids are likely to build up. (SW) Lizards are the most numerous and diverse reptiles alive today. Snakes are limbless reptiles that live above the ground, although there is evidence that snakes evolved from reptiles with legs. Crocodiles and alligators spend most of their time in the water, but breathe water through their upturned nostrils. They are the most closely related reptiles to dinosaurs.
Reptiles have adapted to living (and reproducing on) land with the amniotic egg. With an external amniotic egg, reptiles can practice internal fertilization and still provide their offspring with an exchange of nutrients and physical protection. (DRM) [The majority of reptiles are oviparous(egg-laying) although certain species of squamates are capable of giving live birth. RS]
[Compared to mammals, reptilian skin is rather thin, and lack the thick dermal layer that produces leather in mammals.[[#cite_note-27|[28]]] Exposed parts of reptiles are protected by scales or scutes, sometimes with a bony base, forming armour.RS]



Acquiring and Digesting Food

There were two main dinosaur lineages: the ornithischians, which were mostly herbivores; and the saurischians, which included both herbivorous and carnivorous dinosaurs. All reptiles ingest their food through their mouths. Snakes are carnivorous and have a number of adaptations that aid them in hunting and eating prey. Snakes utilize acute chemical sensors and are sensitive to ground vibrations, helping them to detect the movements of prey. Certain snakes have heat-detecting organs between their eyes and nostrils, allowing them to sense minute temperature changes and locate warm animals in the night. Snakes use loosely articulated jaws in order to enable them to swallow prey larger than the diameter of the snake itself. Many snakes have slow metabolisms, and after "bulk feeding," will not feed again for long periods of time (AS), Most other reptiles are carnivorous. Digestion is slower than in animals and they have a lower metabolism. Certain herbivorous reptiles swallow rocks and pebbles to aid in digestion by helping to grind up plant matter in the stomach.

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Sensing the Environment

Snakes use acute chemical sensors and are sensitive to ground vibrations, although they lack eardrums. The flicking tongue common in snakes is used to fan odors toward olfactory organs on the roof of the mouth. Some snakes have heat-detecting organs. Most reptiles have adapted their vision to the daytime and have color vision and advanced visual depth perception compared to amphibians and most mammals. Crocodiles and alligators have adapted to seeing underwater, although they cannot focus, meaning their other senses are more important when they are submerged.
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Pit organs can sense body heat of the snakes prey (LD)


Most turtles walk on four limbs and others are able to swim, since many are water dwelling. Sea turtles have flippers instead of legs. Turtles are famous for walking slowly, mostly due to their shells. Crocodiles and alligators have similar methods of locomotion. Snakes are probably descendents of lizards that adapted to a burrowing lifestyle and have retained a limbless condition. Vestigial pelvic and
Sidewinder Photo 1
limb bones in primitive snakes offer evidence that snakes evolved from reptiles with legs. Snakes use lateral undulation for moving exclusively in the water and mostly on land. Lateral undulation uses wave-like movement patterns to propel the snakes
[The turtle on the left uses its four flippers to move around, while the turtle on the right uses its four legs. (DG)]
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Reptiles cannot breathe through their keratinized dry skin and must obtain all of their oxygen with lungs. Many turtles use the moist surfaces of their cloaca (posterior openings) for gas exchange. Practically all reptiles have a three chambered heart. The one exception is the chrocodile, which has a four chambered heart. Three chambered hearts contain two atria (chamber that recieves blood) and one ventricle (chamber that pumps blood out of the heart). A four chambered heart has two atria and one ventricle.(JAS)

Metabolic Waste Removal

Excretion in reptiles is facilitated using two small kidneys. Most reptiles excrete their waste in the form of uric acid, while turtles excrete urea. Reptile kidneys are unable to produce liquid urine more concentrated than their body fluid and must use their colon to aid the reabsorption of water. Others use their bladder to take up water.


Most reptiles have a three-chamber heart with two atria, one ventricle, and two aorta. Depending on the species and physiological state, the degree of mixing oxygenated and deoxygenated blood in the three-chamber heart changes. During different conditions, deoxygenated blood can be shunted back to the body and oxygenated blood can be shunted back to the lungs.
This diagram compares the three-chambered reptilian heart to the two-chambered fish heart and the four-chambered mammalian heart.
This diagram compares the three-chambered reptilian heart to the two-chambered fish heart and the four-chambered mammalian heart.

Self Protection

Reptiles have a number of adaptations that protect them. Reptiles have scales, which are able to
Chameleon Photo 2
protect them from predators. Turtles have hard shells, which also protects them from predators. Many snakes have venom both as protection and for hunting. They also have a number of warning mechanisms such as the rattle of a rattlesnake. Other reptiles camouflage themselves with their environment, most famously seen in chameleons, which are able to alter their color in order to match their surroundings. The Australian frillneck lizard spreads its frill when threatened, a response that discourages many predators.

Australian frillneck lizard (KD)
Australian frillneck lizard (KD)

Osmotic Balance

Since many reptiles live in dry air, they use their scales to prevent dehydration. Their scales contain the protein keratin, which waterproofs their skin. Reptiles use osmoregulation, meaning that they must keep active regulation of the osmotic pressure of their fluids in order to maintain the homeostasis of their water content.

Temperature Balance

Reptiles are ectotherms, meaning they do not use their metabolism to control body temperature, but instead regulate body temperature by using behavioral adaptations. Lizards regulate their internal temperature by basking in the sun when the air is cold and seeking shade when the air is too warm. Reptiles absorb external heat instead of generating their own. By heating directly with solar energy rather that through the metabolic breakdown of food, they can survive on less than 10% of the calories required by a mammal of the same size. There is debate about whether dinosaurs were endothermic, or capable of keeping their bodies warm through metabolism.
a lizard warming itself in the sun (JL

Review Questions:
1) How do reptiles prevent dehydration in their dry environments?(APS)
2) Why might crocodiles, above all other reptiles, have four chambered hearts? What benefits come with a four chambered heart as opposed to a three chambered heart? (KNS)
3)Explain three methods reptiles use for self-protection. (Walker K.)
4) What is the function of the protein keratin and why is it essential to reptiles? (JAF)
5) Explain some of the behavioral adaptations used by reptiles to regulate their body temperatures. (JM)

Campbell, Neil A., and Jane B. Reece. Biology. Sixth Edition. Boston: Benjamin-Cummings Company, 2002.
Photo 1
Photo 2 (Ali Kirsch) (Jake Schwartz) (Sara Waugh) (Sarah Fleming) (AG) (Donna McDermott) (JJF) (KD) (Alyssa Zisk) (Liz Daley) (Jesse Landy) (DG) (DG)