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Reptile

Reptilia

Reptiles are cold-blooded (ectothermic) vertebrates of the class Reptilia, which includes turtles, crocodilians, lizards, and snakes. They use their four limbs (tetrapods) for locomotion and reproduce by laying amniotic eggs, in which the embryo is suspended in amniotic fluid and covered by an amniotic sac to prevent desiccation. 

As of March 2024, about 12,162 known living species are listed in the Reptile Database.  

Description

Size

While the smallest reptile in the world, the chameleon Brookesia nana, measures 13.5 mm (0.53 inches) in body length (excluding tail), the largest living reptile, the Saltwater Crocodile (Crocodylus porosus), reaches more than 23 feet (7 meters) and weighs over 1,000 kg (2,200 lbs).

Some ancient reptile groups, like Sauropods, grew up to 30 meters (98 feet), the largest animals to have ever lived on Earth.

Body Plan

Like most vertebrates, they have a bony skull, a long vertebral column enclosing the spinal nerve cord, a protective rib cage around the viscera, and two pairs of limbs. Some lizards, like snakes, have lost all their limbs and may only retain a few vestiges, whereas marine reptiles have their limbs modified into flippers or fins.

The reptilian skull consists of an upper and a lower jaw, with the latter bearing a single bone called the dentary, which contains teeth.

Organ System

Integumentary 

The integument constitutes a horny, keratinized epidermis, followed by a thin dermal layer. In lepidosaurians, like lizards and snakes, the skin is covered with overlapping epidermal scales or scutes. Similarly, crocodilians have dermal scales with a bony base (osteoderms) that function as protective armor.

Circulatory 

All turtles, lepidosaurs, and most squamates have three-chambered hearts comprising two atria and one partially partitioned ventricle. The main components of the reptilian heart are the sinus venosus, the pacemaker, the left and right atrium, the atrioventricular valve, paired aortic arches, the cavum venosum, and arteriosum, the cavum pulmonale, the muscular and ventricular ridge, and the pulmonary veins.

Crocodilians are the only reptiles to possess anatomically four-chambered hearts; however, some squamates, such as pythons and monitor lizards, have three-chambered hearts that become functionally four-chambered during contraction.

Respiratory 

All reptiles respire through the lungs, but the ventilation mode varies among different groups. In squamate lungs, the axial musculature controls the ventilation, whereas, in tegu lizards, a proto-diaphragm segregates the pulmonary and visceral cavities, allowing space for greater lung inflation.

In crocodiles, a muscular diaphragm (hepatic piston) helps pull the movable pubis down, thus lowering the liver and creating enough room for the lung to expand.

Digestive 

The basic plan of the digestive system in modern-day reptiles is similar to those of all higher vertebrates, including the mouth and its salivary glands, the esophagus, the stomach, the intestine, and the cloaca.

Since most reptiles are insectivorous or carnivorous, their digestive tracts are relatively short as it is relatively easier to break down meat than plant matter.  

Excretory 

Reptiles eliminate nitrogenous waste through a pair of kidneys, usually in the form of uric acid (except turtles that excrete urea). However, unlike kidneys in birds and mammals, the reptilian kidney lacks the elongated Loop of Henle and sometimes reabsorbs water with the help of the colon.

Both urinogenital ducts and the anus empty into the cloaca, which, in some reptiles, like turtles and tortoises, may open into a urinary bladder. Sometimes, turtles possess two accessory bladders located dorsal to the pubis and lateral to the neck of the primary urinary bladder.

Nervous 

As in all vertebrates, reptiles have a brain, a spinal nerve cord, and twelve pairs of cranial nerves running from either the brain or the spinal cord to the different parts of the body. The cerebrum and cerebellum of the reptilian brain are larger than those of amphibians.

Crocodiles and turtles have eyes made up of three types of photoreceptors: rod cells, single-cone cells, and double-cone cells, which enable sharp vision across a range of ultraviolet wavelengths. Most reptiles also possess a translucent membrane called the nictitating membrane over their eyes, which protects and moistens them while maintaining vision, especially underwater.

Taxonomy

Between the 13th and 18th centuries, scientists like Carolus Linnaeus grouped reptiles with amphibians, and the terms ‘reptilian’ and ‘amphibian’ were used interchangeably. However, around the beginning of the 19th century, the French zoologist P.A. Latreille shed light on the differences between the two and divided tetrapods into four distinct classes: reptiles, amphibians, birds, and mammals.

In 1863, T.H. Huxley divided vertebrates into mammals, sauroids, and ichthyoids, the latter comprising fish and amphibians. To supplement his classification, Huxley also suggested the names Sauropsida and Ichthyopsida for sauroids and ichthyoids.

In 1916, the English zoologist E.S. Goodrich defined the two major groups within the animal kingdom: the Sauropsida, including lizards, birds, and their kin, and the Theropsida, including mammals and their extinct relatives. He believed that both groups originated from an earlier stem group called the Protosauria, or first lizards, in which he included some modern-day reptile-like amphibians along with some early reptiles.

It was initially thought that class Reptilia represents a paraphyletic group (including recent common ancestors but not all descendants) and did not include birds and mammals, despite these vertebrates that evolved from lineages once considered reptiles. This view demanded attention since birds were found to be more closely related to crocodilians than crocodilians were to other reptiles.

In 1988, Jacques Gauthier proposed reptiles as a monophyletic group, including their most recent common ancestor and all its descendants, namely turtles, lizards, snakes, crocodilians, and birds.

Here is the list of all the extinct and living reptiles with their major groups. 

Reptiles (Reptilia)

Initially, turtles were considered relatives of primitive anapsid reptiles; however, their placement in the phylogenetic classification has remained uncertain. Currently, turtles are considered a sister clade to the archosaurs, including crocodiles, non-avian dinosaurs, and birds.

Evolution

Distribution and Habitat

They are mostly found in the temperate and tropical regions, especially between 30o N and 30o S latitudes. However, the European viper and the viviparous lizard have been spotted in the Arctic region at around 66o N.

While crocodiles and a few snakes are aquatic, occupying freshwater habitats like lakes, rivers, and mountain streams, terrestrial reptiles, such as turtles and lizards, inhabit forests, grasslands, and deserts.

Diet

Most reptiles are carnivores, feeding on insects, mollusks, amphibians, birds, fish, small mammals, and other reptiles. However, a few, such as tortoises, the green iguana, and spiny-tailed agamids, consume grass and other plant parts.

Behavior

Locomotion

Defense

When in danger, most reptiles resort to avoidance by slipping away from the eyes of their enemies. The best way to avoid a predator’s eye is to camouflage against the background using cryptic coloration through shades of mottled grays, greens, and browns. When camouflaging fails, reptiles, like blue-tongued skinks, flash their blue tongues at the predators, while frill-necked lizards display their vibrant frills. When facing a similar situation, rattlesnakes rapidly vibrate the tips of their tails, which consist of a series of hollow beads that fend off their enemies.

Lizards of the genus Heloderma, coral snakes, and a few non-venomous North American snake species have high-contrast color patterns on their bodies, which act as warning coloration against potential predators.

When caught by an enemy, snakes elevate their heads and try to appear dangerous by spreading out the skin of their neck. When the attacker does not recede, the venomous varieties, such as cobras and vipers, use their fangs to inject toxic venom into the enemy’s body. In contrast, non-venomous snakes, like Hognose snakes and European grass snakes, either feign death or exude foul-smelling liquids at the attacker.

When facing danger, crocodilians inflate their bodies and flash their teeth and tongues as a warning; however, they also bite their enemies with full force in the first place. Some use their heads as sledgehammers to thrash the opponent, while others chase them onto land and gallop after them.

If attacked, lizards, like geckos and skinks, can shed their tails immediately (autotomy). The detached tail wriggles and distracts the enemy, helping these reptiles escape the grip easily. In bobtail lizards, the tail is broad enough to resemble the head, thus deceiving their enemies, who try to attack the tail instead of the head.

Vocalization

In general, reptiles are relatively less vocal than amphibians, birds, and mammals, but some, like geckos and turtles, vocalize by vibrating the fold-like structures in their larynx or glottis. Although snakes communicate through hissing, it is not considered a true vocalization as it is generated by forcing air into a partly closed glottis.

Reproduction and Life Cycle

Although most reptiles reproduce sexually, some, like New Mexico whiptail lizards, geckos, and lacertids, resort to asexual reproduction through parthenogenesis (direct growth from egg without fertilization).

In sexual reproduction, all copulatory actions occur through the opening at the base of the tail (cloaca). While most reptiles have specific copulatory organs that are usually retracted and stored inside their bodies, a few, like tuataras, lack these organs and press their cloacas together during copulation.

Almost all reptiles are oviparous, laying amniotic eggs covered by a leathery, calcareous shell. The three embryonic membranes, the amnion, the chorion, and the allantois, protect the embryo from external pressures and enable the exchange of gases and waste products between the inside and outside of the egg.

The embryo derives nutrition from the protein and fat-rich yolk sac surrounding the egg yolk. 

Some modern-day reptiles, like the Eurasian Lizard, are viviparous, giving birth to live young ones, whereas all boas and most vipers are ovoviviparous, producing eggs that develop within the mother’s body. In all crocodilians, most turtles, and some lizards, the sex of the young depends on the incubation temperature of the embryo (Temperature-dependent sex determination).

Young reptiles shed their old cuticles and replace them with new ones through ecdysis or molting once every 5 to 6 weeks. Once they reach adulthood, the molting frequency reduces to 3 or 4 times a year.

Adaptations

Interesting Facts

References Article last updated on 27th April 2024
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