Reptilia – Definition, Characteristics, Classification

What is Reptilia?

• Reptilia, also known as reptiles, are a diverse class of vertebrate animals belonging to the phylum Chordata. They were the first organisms to adapt to life on land and are believed to have evolved from amphibians millions of years ago. With approximately 10,000 different species, reptiles are found in various habitats across the globe.
• The name “Reptile” originates from a Latin phrase meaning “Creeping Creatures,” which accurately describes their characteristic mode of movement. The class Reptilia includes a wide range of creatures, such as snakes, lizards, crocodiles, caimans, alligators, turtles, geckos, and chameleons. Among these, lizards and snakes comprise the majority of reptile species. One defining feature of reptiles is their cold-blooded nature, which means they cannot regulate their own body temperature and rely on external sources to warm up or cool down.
• Reptiles trace their evolutionary lineage back to amphibians. Approximately 320 million years ago, during the Carboniferous period, reptiles emerged as sophisticated four-limbed vertebrates known as reptiliomorpha. These early reptiles gradually developed adaptations to thrive on land. They evolved various defense mechanisms, such as biting, hissing, camouflaging, and avoidance, to protect themselves from potential threats.
• In terms of classification, Reptilia is a paraphyletic group that traditionally comprises all sauropsids except birds. However, modern cladistic classification systems include birds within Reptilia due to their close evolutionary relationship with crocodilians. Alternatively, some classifications abandon the term “reptile” altogether in favor of the clade Sauropsida, which encompasses all amniotes more closely related to modern reptiles than to mammals.
• The earliest known proto-reptiles appeared around 312 million years ago, evolving from advanced reptiliomorph tetrapods that gradually adapted to terrestrial life. One of the earliest eureptiles, Hylonomus, resembled a small lizard-like animal.
• Genetic and fossil evidence suggests that the two major lineages of reptiles, Archosauromorpha (including crocodilians, birds, and their relatives) and Lepidosauromorpha (including lizards and their relatives), diverged near the end of the Permian period.
• Over time, various reptile groups became extinct, with mass extinction events like the Cretaceous-Paleogene event wiping out pterosaurs, non-avian dinosaurs, and other reptilian species.
• Reptiles exhibit a range of reproductive strategies. Most reptiles are oviparous, meaning they lay eggs, while some species of squamates are viviparous, giving birth to live young. Viviparous reptiles use a placenta-like structure to provide nourishment to developing embryos.
• Reptile eggs are surrounded by protective membranes that allow them to be laid and incubated on land. The size of reptiles varies greatly, from tiny geckos measuring a few centimeters to large saltwater crocodiles reaching over six meters in length and weighing more than a ton.
• Reptiles share several common anatomical characteristics. They are tetrapod vertebrates, either possessing four limbs or descended from four-limbed ancestors. Unlike amphibians, reptiles lack an aquatic larval stage. Their bodies are covered in scales or scutes, and they breathe using lungs.
• The skull of reptiles has undergone modifications, resulting in an efficient and powerful jaw action while keeping the skull relatively lightweight. One distinctive feature of reptile skulls is the presence of temporal vacuities or fossae, empty spaces in the temporal region behind the eyes.
• These fossae allowed the jaw muscles to extend onto the upper surface of the skull, providing greater leverage for a powerful bite. Different groups of reptiles developed these fossae in various locations, including parapsid, synapsid, and diapsid types.
• Reptiles have captivated human interest for centuries, both as fascinating creatures to observe and as subjects of scientific study. The field of herpetology focuses on the study of reptiles and amphibians, further enhancing our understanding of these remarkable animals.
• From their ancient origins to their diverse forms and adaptations, reptiles continue to thrive in ecosystems around the world, contributing to the rich biodiversity of our planet.

Reptilia Scientific Classification

• Kingdom: Animalia
• Phylum: Chordata
• Clade: Sauropsida
• Class: Reptilia
• Sub-classes: Anapsida, Parapsida, Diapsida

Characteristics of Reptilia

The characteristics of animals belonging to Class Reptilia are as follows:

1. Creeping and burrowing terrestrial animals with scales on their body: Reptiles are primarily land-dwelling animals that move by creeping or burrowing. They have scales on their bodies, which provide protection and help reduce water loss.
2. Cold-blooded animals found in most of the warmer regions of the world: Reptiles are ectothermic, which means they rely on external sources of heat to regulate their body temperature. They are commonly found in warmer regions but can inhabit a variety of ecosystems, including deserts, forests, grasslands, and wetlands.
3. Dry and rough skin without glands: Reptiles have dry and rough skin that lacks glands. Unlike mammals, they do not have sweat glands, sebaceous glands, or mucous glands. Their skin plays a crucial role in preventing water loss and protecting their bodies.
4. Divided body structure: The body of a reptile is divided into distinct regions, including the head, neck, trunk, and tail. This division allows for specialized functions and movement.
5. Shedding of scales: Some reptiles, such as snakes and lizards, shed their scales periodically. This process, known as molting or skin casting, allows them to grow and replace old or damaged skin.
6. Respiration through lungs: Reptiles respire using lungs. They do not possess gills like fish or rely on cutaneous respiration like amphibians. Their lungs enable efficient exchange of gases and provide oxygen for cellular respiration.
7. Monocondylic skull: Reptiles have a monocondylic skull, which means they have a single occipital condyle connecting the skull to the vertebral column. This skull structure provides stability and allows for precise movement.
8. Limbs: Most reptiles have limbs, specifically two pairs of pentadactyl limbs (five-fingered or five-toed limbs) with claws. However, snakes do not have limbs and have evolved a unique mode of locomotion.
9. Three-chambered heart (except crocodiles): Reptiles have a three-chambered heart consisting of two auricles and a partially divided ventricle. However, crocodiles have a four-chambered heart, which is more similar to that of birds and mammals.
10. Twelve pairs of cranial nerves: Reptiles have twelve pairs of cranial nerves that play crucial roles in sensory perception, motor control, and other functions.
11. Absence of external ear openings: Reptiles do not have external ear openings. Instead, they have a tympanum, which serves as the hearing organ.
12. Cloaca: Reptiles possess a cloaca, a single opening that serves as the exit for the digestive, urinary, and reproductive systems. It is a common feature among reptiles, birds, and some amphibians.
13. Uricotelic excretion: Reptiles primarily excrete nitrogenous waste as uric acid, a relatively insoluble compound. This adaptation helps conserve water in their bodies and is more efficient in arid environments.
14. Internal fertilization: Most reptiles have internal fertilization, where the male deposits sperm inside the female’s body. This ensures higher chances of successful fertilization and protects the eggs from external environmental conditions.
15. Oviparous with yolky eggs: Reptiles are mostly oviparous, meaning they lay eggs. The eggs are usually large and yolky, providing nourishment for the developing embryos. Some reptiles are ovoviviparous, where the eggs develop internally, and the young are born live.

Some examples of reptiles include snakes, turtles, lizards, and crocodiles. These animals exhibit a wide range of adaptations and behaviors that have allowed them to thrive in diverse habitats across the globe.

Origin of the reptiles and Rise of the reptiles

• The origin of reptiles can be traced back to around 310-320 million years ago, during the late Carboniferous period. They evolved from advanced reptiliomorphs, which were a group of reptile-like amphibians. The transition from amphibians to reptiles marked a significant step in the evolution of vertebrates.
• Casineria is considered one of the oldest known animals that may have been an amniote, although it could potentially be a temnospondyl. Fossil footprints from Nova Scotia dating back to 315 million years ago show reptilian characteristics, including reptilian toes and scale imprints. These tracks are attributed to Hylonomus, which is regarded as the oldest unquestionable reptile known. Hylonomus was a small lizard-like creature, approximately 20 to 30 centimeters long, and had sharp teeth suggesting an insectivorous diet. Other examples of early reptiles include Westlothiana and Paleothyris, which had similar characteristics and likely similar habits.
• During the Carboniferous Rainforest Collapse, a significant event that led to the decline of many large groups of animals, primitive tetrapods, including amphibians, were heavily affected. However, stem-reptiles, which were better adapted to drier conditions and could lay their eggs on land due to the presence of amniotic eggs with shells, fared better. This ecological advantage allowed stem-reptiles to occupy new niches and diversify at a faster rate than primitive tetrapods. They developed new feeding strategies such as herbivory and carnivory, expanding their dietary range beyond insects and fish. This period marked the rise of reptiles, and they began to dominate communities, exhibiting greater diversity than their primitive tetrapod counterparts.
• Mesosaurus, a genus from the Early Permian, is one of the well-known early stem-reptiles. It had adapted to an aquatic lifestyle, feeding on fish. The success of reptiles during this time set the stage for the Mesozoic Era, often referred to as the Age of Reptiles.
• Recent studies have revealed that reptile diversity during the Carboniferous and Permian periods was much higher than previously thought, rivaling or even surpassing that of synapsids (mammal-like reptiles). This has led to the proposal of the “First Age of Reptiles,” highlighting the significant role reptiles played during this early period of their evolution.

Circulation system of Reptilia

• The circulation system of Reptilia, which includes lepidosaurs (lizards and snakes), turtles, and crocodilians, exhibits interesting adaptations to accommodate their unique physiological requirements. While reptiles generally have a three-chambered heart, there are variations in the degree of mixing of oxygenated and deoxygenated blood, allowing for different functions and capabilities.
• In most lepidosaurs and turtles, the heart consists of two atria, one variably partitioned ventricle, and two aortas that lead to the systemic circulation. The three-chambered heart of reptiles differs from the four-chambered hearts of mammals and birds. The mixing of oxygenated and deoxygenated blood can vary among species and can be influenced by the physiological state of the reptile. This mixing can occur in different ways, such as deoxygenated blood being shunted back to the body or oxygenated blood being redirected to the lungs.
• The arrangement of the heart structures in reptiles can vary as well. For instance, the hearts of squamates, including popular species like iguanas, are composed of the sinus venosus, pacemaker, left atrium, right atrium, atrioventricular valve, cavum venosum, cavum arteriosum, cavum pulmonale, muscular ridge, ventricular ridge, pulmonary veins, and paired aortic arches. The ventricle is shared by the two aortas, and the presence of cardiac involuntary muscles aids in the heart’s pumping action.
• Some squamate species, such as pythons and monitor lizards, possess hearts that functionally become four-chambered during contraction. This transformation occurs due to the presence of a muscular ridge in the ventricle, which divides it during ventricular systole (contraction). This adaptation enables these reptiles to generate ventricular pressure differentials similar to those observed in mammals and birds. It is worth noting that this is not the case for all reptiles, as the presence of a fully divided ventricle is not universal among squamates.
• Crocodilians, including crocodiles and alligators, possess anatomically four-chambered hearts, similar to birds and mammals. However, they also have two systemic aortas, allowing them to bypass their pulmonary circulation. This adaptation is unique to crocodilians and provides them with certain advantages. By bypassing the pulmonary circulation, these reptiles can maintain efficient blood flow to their vital organs even during prolonged dives, contributing to their ability to stay submerged for extended periods.
• Overall, the circulation system of reptiles showcases diverse adaptations to their specific needs. These adaptations allow reptiles to thrive in various habitats and fulfill their physiological requirements, highlighting the remarkable diversity and ingenuity of nature’s designs.

Metabolism of Reptilia

• The metabolism of reptiles is characterized by their cold-bloodedness, which encompasses a combination of poikilothermy, ectothermy, and bradymetabolism. Unlike warm-blooded animals, reptiles have limited physiological mechanisms to regulate their body temperature and often rely on external heat sources. This reliance on environmental conditions for maintaining body temperature affects the biochemistry of reptiles, requiring them to possess enzymes that remain efficient over a wider range of temperatures.
• The optimal body temperature range for reptiles varies among species but is generally lower than that of warm-blooded animals. For many lizards, the optimal physiological temperature falls within the range of 24°C to 35°C (75°F to 95°F). However, certain heat-adapted species, like the American desert iguana, can have optimal temperatures within the mammalian range of 35°C to 40°C (95°F to 104°F). Although reptiles often experience their optimal temperature while active, their low basal metabolism causes their body temperature to drop rapidly when they are inactive.
• Reptilian muscles generate heat through their action, which is utilized by large reptiles like leatherback turtles. These reptiles have a low surface-to-volume ratio, allowing the metabolically produced heat to keep them warmer than their environment, despite lacking a warm-blooded metabolism. This form of homeothermy, known as gigantothermy, is believed to have been common in large dinosaurs and other extinct large-bodied reptiles.
• The advantage of having a low resting metabolism is that reptiles require significantly less fuel to sustain basic bodily functions. By utilizing temperature variations in their surroundings and conserving heat when not in motion, reptiles can save substantial amounts of energy compared to animals of the same size that are endothermic. For instance, a crocodile needs only a fraction of the food required by a similarly sized lion and can survive for several months without eating. This lower food requirement and adaptive metabolism allow reptiles to thrive in regions where net calorie availability is insufficient to support large-bodied mammals and birds.
• It is often assumed that reptiles are incapable of producing sustained high energy output necessary for activities such as long-distance chases or flying. The evolution of warm-bloodedness in birds and mammals has been associated with increased energetic capacity. However, studies investigating the correlation between active capacity and thermophysiology in reptiles have shown a weak relationship. While most extant reptiles are carnivorous with a sit-and-wait feeding strategy, the link between reptilian cold-bloodedness and their ecological traits remains unclear. Some energetic studies have even demonstrated that certain reptiles possess active capacities equal to or greater than similarly sized warm-blooded animals.

Respiratory system of Reptilia

• The respiratory system of reptiles is adapted to their specific lifestyles and body structures. All reptiles breathe using lungs, although there are some variations and adaptations in different reptilian groups.
• In most reptiles, including squamates (lizards and snakes), lung ventilation is primarily achieved through the axial musculature. These muscles are also used for locomotion, which poses a challenge during intense runs, as reptiles are forced to hold their breath. However, some species, like varanids and certain lizards, have developed a method called buccal pumping. This complementary breathing technique allows them to completely fill their lungs during intense locomotion, enabling them to remain aerobically active for longer durations. Tegu lizards possess a proto-diaphragm that helps in lung inflation by taking the weight of the viscera off the lungs, although it does not have the capacity for movement like the mammalian diaphragm.
• Crocodilians, on the other hand, possess a muscular diaphragm analogous to that of mammals. The muscles of the crocodilian diaphragm pull back the pubis, which moves the liver down and creates space for the expansion of the lungs. This mechanism, known as the “hepatic piston,” allows for unidirectional airflow through the airways, similar to the respiratory system of birds, monitor lizards, and iguanas.
• Most reptiles lack a secondary palate, making it necessary for them to hold their breath while swallowing. However, crocodilians have evolved a bony secondary palate that allows them to continue breathing while remaining submerged, protecting their brains when dealing with struggling prey. Some skinks have also developed a bony secondary palate to varying degrees. Snakes have taken a different approach, extending their trachea. This tracheal extension acts like a fleshy straw, allowing snakes to swallow large prey without the risk of asphyxiation.
• Turtles and tortoises have unique adaptations in their respiratory system due to the rigid nature of their shells. The Indian flapshell turtle, for example, has a sheet of muscle surrounding the lungs, allowing for exhalation when the muscle contracts and inhalation when it relaxes. Turtles have a series of special muscles that enable them to push their viscera up and down, facilitating effective respiration. The attachment points of these muscles in conjunction with the forelimbs contribute to their breathing mechanism.
• Breathing patterns during locomotion differ among reptiles. Adult female green sea turtles, for instance, do not breathe while moving on land and hold their breath during terrestrial locomotion. North American box turtles breathe continuously during locomotion, using their abdominal muscles for respiration. Red-eared sliders breathe during locomotion but take smaller breaths while moving compared to small pauses between locomotor bouts, indicating possible mechanical interference between limb movements and breathing apparatus. Box turtles have also been observed to breathe while completely enclosed inside their shells.
• Overall, reptiles have evolved diverse respiratory adaptations to suit their unique anatomical features and lifestyles. From axial breathing to buccal pumping, hepatic piston mechanisms, and specialized structures in turtles, these adaptations ensure efficient respiration while accommodating the constraints and demands of their respective environments.

Sound production of Reptilia

Sound production in reptiles:

1. Limited vocalization: Compared to frogs, birds, and mammals, reptiles are generally less vocal.
2. Hissing: Reptiles often produce hissing sounds, which are created by forcing air through a partially closed glottis. However, this is not considered a true vocalization.
3. Crocodilians: Some crocodilians possess the ability to vocalize. They produce sounds by vibrating fold-like structures in their larynx or glottis.
4. Lizards: Certain lizard species can vocalize using vibrating structures in their larynx or glottis. Some geckos and turtles also have true vocal cords that contain elastin-rich connective tissue.
5. Hearing in snakes: Snakes have a unique hearing mechanism. They lack an outer ear, middle ear, and tympanum. Instead, their inner ear structure, including the cochleas, is directly connected to their jawbone.
6. Vibrations and mechanoreceptors: Snakes can feel vibrations generated by sound waves through their jaw as they move on the ground. Mechanoreceptors, sensory nerves running along their body, transmit these vibrations to the brain via spinal nerves.
7. Directional perception: Despite not relying on traditional hearing mechanisms, snakes have a sensitive auditory perception. They can determine the direction of sound sources, allowing them to sense the presence of prey or predators.
8. Sensitivity to airborne sound waves: The sensitivity of snakes to sound waves traveling through the air is still not fully understood. Further research is needed to determine the extent of their auditory capabilities in this regard.

Skin of Reptilia

1. Horny epidermis: Reptilian skin is covered in a horny epidermis, which makes it watertight and enables reptiles to live on dry land.
2. Thin and lacking dermal layer: Compared to mammalian skin, reptilian skin is relatively thin and lacks the thick dermal layer that produces leather in mammals.
3. Scales and scutes: Exposed parts of reptiles are protected by scales or scutes. Lepidosaurians, such as lizards and snakes, have overlapping epidermal scales. Turtles and crocodiles have scutes of dermal origin, forming their shells.
4. Leather production: Reptilian leather, particularly crocodile skin, is used for decorative purposes in products like shoes, belts, and handbags. However, it is not as strong as mammalian leather due to the thinner dermis.
5. Shedding (ecdysis): Reptiles undergo shedding throughout their lifetime, a process called ecdysis.
6. Shedding frequency: Younger reptiles shed more frequently, about once every 5-6 weeks, due to their rapid growth. Adults shed 3-4 times a year.
7. Shedding process: Reptiles form a new layer of skin under the old one, with proteolytic enzymes and lymphatic fluid secreted between the layers. This lifts the old skin, allowing shedding to occur.
8. Shedding patterns: Snakes shed from head to tail, while lizards shed in a “patchy pattern.”
9. Dysecdysis: Dysecdysis is a common skin disease in snakes and lizards when shedding fails. It can be caused by factors like inadequate humidity and temperature, nutritional deficiencies, dehydration, and traumatic injuries.
10. Reasons for shedding failure: Nutritional deficiencies reduce proteolytic enzymes, dehydration decreases lymphatic fluids, and traumatic injuries disrupt the shedding process by forming scars that prevent new scales from forming.

Excretion of Reptilia

1. Kidneys: Reptiles have two small kidneys that are responsible for excretion.
2. Nitrogenous waste: Diapsids, including most reptiles, excrete uric acid as the main nitrogenous waste product. However, turtles excrete mainly urea, similar to mammals.
3. Concentration of urine: Reptile kidneys cannot produce urine more concentrated than their body fluid due to the absence of a specialized structure called the loop of Henle, found in the nephrons of birds and mammals.
4. Water reabsorption: Reptiles utilize the colon to aid in the reabsorption of water since their kidneys cannot produce concentrated urine. Some reptiles can also take up water stored in the bladder.
5. Salt excretion: Excess salts are excreted by nasal and lingual salt glands present in some reptiles.
6. Cloaca: In all reptiles, the urinogenital ducts and the anus empty into a common organ called a cloaca.
7. Bladder presence: Some reptiles, including turtles and most lizards, have a midventral wall in the cloaca that opens into a urinary bladder. However, this structure is absent in monitor lizards, legless lizards, snakes, alligators, and crocodiles.
8. Large bladder adaptations: Many turtles, tortoises, and lizards have proportionally large bladders. They can store a significant amount of urine, with adaptations seen in species like the Galapagos tortoise, which can store up to 20% of its body weight in its bladder.
9. Desert adaptations: Desert-dwelling reptiles have large bladders that serve as a long-term reservoir of water, aiding in survival in water-scarce environments and osmoregulation.
10. Accessory urinary bladders in turtles: Turtles have two or more accessory urinary bladders located in their body cavity. These bladders are often bilobed, with the right section located under the liver, preventing the retention of large stones, while the left section is more prone to the formation of calculi.

Digestion of Reptilia

1. Diet: Most reptiles are insectivorous or carnivorous, feeding on meat. This is reflected in their digestive system, which is adapted for breaking down and digesting meat.
2. Simple digestive tract: Reptiles have relatively simple and short digestive tracts compared to mammals. Meat is easier to break down and digest, requiring less complex digestive processes.
3. Slower digestion: Digestion in reptiles is slower than in mammals, as their lower resting metabolism and inability to divide and masticate their food contribute to a slower digestive process.
4. Energy requirements: Reptiles have low energy requirements due to their poikilotherm metabolism, allowing them to survive for extended periods with a single large meal. Large reptiles like crocodiles and constrictors can digest a large meal slowly over several months.
5. Herbivorous reptiles: While most reptiles are carnivorous, there are herbivorous reptiles as well. Turtles are predominantly herbivorous, and some lines of agamas and iguanas have evolved to live on plants either wholly or partly.
6. Mastication challenges: Herbivorous reptiles face challenges in masticating plant matter due to the lack of complex teeth like those found in mammals. To aid in digestion, many species swallow rocks and pebbles known as gastroliths. These rocks move around in the stomach, helping to grind up plant matter.
7. Gastroliths: Gastroliths are also used by reptiles like saltwater crocodiles as ballast, providing stability in the water or assisting in diving. Fossil gastroliths have been found associated with ornithopods, sauropods, and plesiosaurs, serving potential functions as gastric mills or stabilizing ballast.

Nerves of Reptilia

1. Basic structure: The nervous system of reptiles shares similarities with that of amphibians, consisting of the same basic parts. However, the reptile cerebrum and cerebellum are slightly larger in size.
2. Sense organs: Most typical sense organs in reptiles are well developed. However, there are exceptions, such as snakes lacking external ears. Despite the absence of external ears, middle and inner ears are present in snakes.
3. Cranial nerves: Reptiles possess twelve pairs of cranial nerves, which are responsible for various sensory and motor functions in the head and neck regions.
4. Hearing adaptations: Reptiles have adaptations in their auditory system. Due to their short cochlea, they use electrical tuning to expand their range of audible frequencies, allowing them to detect a wider range of sounds. This adaptation helps compensate for the lack of external ears in some reptiles.

Vision of Reptilia

1. Diurnal adaptation: Most reptiles are diurnal animals, meaning they are active during the day. Their vision is well-adapted to daylight conditions, providing them with color vision and advanced visual depth perception.
2. Long-distance detection: Reptiles have excellent vision that allows them to detect shapes and movements at long distances. However, they often have a limited number of rod cells, resulting in poor vision in low-light conditions.
3. Color vision and ultraviolet detection: Reptiles possess “double cones” cells that provide sharp color vision and enable them to see ultraviolet wavelengths. This ability to perceive ultraviolet light is crucial for various behaviors and functions.
4. Parietal eye: Many lepidosaurs, such as lizards and tuataras, have a photosensory organ called the parietal eye or third eye. Although it cannot form images like a normal eye, it can detect changes in light and dark and detect movement.
5. Heat-sensitive pits: Some snakes, including pit vipers, boas, and pythons, have specialized pits sensitive to infrared radiation (heat). These pits allow them to sense the body heat of birds and mammals, aiding in hunting prey in the dark.
6. Nictitating membrane: Most reptiles, including birds, possess a nictitating membrane, which is a translucent third eyelid. It can be drawn over the eye from the inner corner, providing protection and allowing vision underwater in aquatic reptiles like crocodilians.
7. Eyecap or brille: Many squamates, particularly snakes and geckos, lack eyelids. Instead, they have a transparent scale called the brille or eyecap that protects the eyes from dust and dirt. The brille becomes visible during the molting process.

Reproduction of Reptilia

1. Sexual reproduction: Most reptiles reproduce sexually, with reproductive activities occurring through the cloaca, the single exit/entrance at the base of the tail. Copulatory organs are present in many reptiles, such as a single median penis in turtles and crocodilians, and a pair of hemipenes in squamates (snakes and lizards).
2. Asexual reproduction: Some reptiles are capable of asexual reproduction through a process called parthenogenesis. This occurs in certain lizard species, where a population of females can produce a diploid clone of the mother without fertilization.
3. Amniotic eggs: Most reptiles lay amniotic eggs covered with leathery or calcareous shells. These eggs protect the developing embryo and provide essential nutrients and gas exchange. The amnion, chorion, and allantois are present during embryonic life.
4. Viviparity and ovoviviparity: Viviparity (giving live birth) and ovoviviparity (retaining eggs until just before hatching) have evolved in some reptile groups. Some species provide maternal nourishment or lack yolk and rely on structures similar to mammalian placenta for nutrient exchange.
5. Extinct reptiles: Viviparity has been observed in various extinct reptile groups, including mesosaurs, mosasaurs, ichthyosaurs, and plesiosaurs.
6. Temperature-dependent sex determination (TDSD): In some reptiles, incubation temperature determines the sex of the offspring. This phenomenon is most common in turtles and crocodiles, but also occurs in lizards and tuatara. The role of TDSD in snakes is yet to be confirmed.

Defense mechanisms of Reptilia

1. Camouflage: Reptiles use cryptic coloration to blend into their natural environment, making it difficult for predators to spot them. They often have plain or mottled gray, green, or brown skin tones that match their surroundings.
2. Warning displays: Some reptiles, like blue-tongued skinks and frill-necked lizards, display their bright colors or expand their frills as a warning to potential predators. These displays are also used during territorial disputes and courtship.
3. Hissing and rattling: When confronted by an enemy, crocodiles, turtles, certain lizards, and some snakes hiss loudly or rapidly vibrate their tails (in the case of rattlesnakes) to deter approaching danger.
4. Warning coloration: Some venomous reptiles, such as the Gila monster, beaded lizard, and coral snakes, have high-contrast warning coloration to signal their venomous nature to potential predators.
5. Defensive behaviors: Snakes adopt defensive tactics when caught out, such as elevating their heads, spreading their neck skin, or playing dead (seen in non-venomous snakes like hognose snakes and grass snakes).
6. Venomous bites: Certain snake species, including cobras and vipers, use venom as a defensive mechanism. They deliver modified saliva through fangs to immobilize or deter predators.
7. Aggressive postures: Crocodilians, when concerned about their safety, may expose their teeth and tongue by gaping their mouths. They can also change their posture to appear more intimidating, inflating their bodies to increase apparent size.
8. Biting and attacking: Crocodilians have powerful bites and may rush, swim, or gallop after threats. They can use their heads as sledgehammers or attack opponents with aggression.
9. Tail shedding and regeneration: Some reptiles, like geckos and skinks, have the ability to shed part of their tail (autotomy) when captured. The detached tail continues to move, distracting predators while the reptile escapes. The tail can partially regenerate over time, although it may be shorter and different in appearance compared to the original tail.
10. Mimicry: Certain non-venomous snake species mimic the warning coloration of venomous snakes (e.g., coral snake mimicry), deterring potential predators.
11. Intense coloration: In some reptiles, the tail is brightly colored or resembles the head to trick predators into attacking the less vulnerable part of the body.
12. Biting and tooth use: Crocodilians possess strong bites and canine-like teeth that they use for seizing prey, fighting, and display.

These defense mechanisms help reptiles avoid predation and increase their chances of survival in their respective habitats.

Classification of Reptilia

There are around 7000 living and extinct reptile species. The class reptilia is split into five primary groupings or subclasses based on the presence or absence of specific apertures through the posterolateral or temporal area of the skull.

Subclass I: Anapsida

1. Solid skull roof: Anapsida reptiles have a skull with a solid roof, lacking any openings or fenestrae. Unlike other reptile subclasses, such as Synapsida and Diapsida, Anapsida reptiles do not have temporal openings in their skulls.
2. Absence of fossae behind the eyes: The skull of Anapsida reptiles is devoid of fossae or depressions located behind the eyes. This characteristic further distinguishes them from other reptile subclasses.
3. Primitive reptiles: Anapsida represents a group of primitive reptiles. They exhibit a more primitive cranial structure compared to other reptile subclasses, reflecting their evolutionary position.
4. No temporal openings: Anapsida reptiles lack temporal openings, which are openings or fenestrae located on the sides of the skull. These temporal fenestrae are found in other reptile subclasses and play a role in jaw muscle attachment and other functions.

Overall, Anapsida reptiles have a unique skull structure characterized by a solid roof and the absence of temporal openings or fossae behind the eyes. This subclass represents a group of primitive reptiles that exhibit distinct cranial features in the reptilian evolutionary lineage.

Order 1: Chelonia or Testudinata (Gr; chelone= turtle, L; testudo= turtle)

1. Dorsoventrally flattened body: Chelonia or Testudinata, commonly known as turtles or tortoises, have a body shape that is flattened from top to bottom. Their bodies are more or less elliptical in shape.
2. Shell covering: The body of Chelonia or Testudinata is covered dorsally by a shield-like structure called the carapace. The ventral side is covered by a plate called the plastron. The shell is externally protected by either polygonal scutes or leathery scales.
3. Retractile neck, limbs, and short tail: Turtles have retractile necks, limbs, and tails. They can withdraw their neck and limbs into their protective shell. The tail is usually very short in comparison to the body size.
4. Pentadactyl weak limbs: Turtles have weak limbs that are pentadactyl, meaning they possess five digits. In some marine turtle species, the limbs are modified into paddle-like structures for efficient swimming.
5. Absence of teeth and presence of horny plates: Adult turtles lack teeth. Instead, their jaws are covered by sharp horny plates, which aid in biting and crushing food.
6. Longitudinal cloacal opening and copulatory organ: Turtles have a longitudinal cloacal opening, which serves as the common opening for excretion and reproduction. Males possess a copulatory organ that remains attached to the ventral wall of the cloaca.
7. Oviparous reproduction: Turtles are oviparous, meaning they lay eggs. They deposit their eggs in nests on land, where they are incubated until hatching.
8. Hibernation in temperate regions: Turtles in temperate regions undergo regular hibernation during the colder months of the year. They enter a state of dormancy to conserve energy and survive harsh environmental conditions.
9. Immovable quadrate bone in the skull: The quadrate bone in the turtle’s skull is immovably articulated, known as monimostylic. This characteristic distinguishes turtles from other reptiles.
10. Fusion of thoracic vertebrae and ribs with the carapace: The thoracic vertebrae and ribs of turtles are usually fused with the carapace, providing additional protection and structural support to the shell.
11. Pectoral and pelvic girdle composition: The pectoral girdle of turtles consists of a scapula, a long procoracoid, and a coracoid. The pelvic girdle is composed of ilia, ischia, and pubis.

Examples of turtles and tortoises include Testudo, Emys, Trionyx (Indian softshell turtle), Chelone (green turtle), Dermochelys (leatherback turtle), Chelys, and Lepidochelys olivacea (olive ridley turtle). These species showcase the diversity within the order Chelonia or Testudinata.

Subclass II: Euryapsida (Extinct)

Characteristics of Euryapsida in reference to a single dorso-lateral temporal opening on either side, bounded below by postorbital and squamosal bones:

1. Skull with a unique temporal opening: Euryapsida, an extinct subclass of reptiles, is characterized by a distinct feature in the skull. It possesses a single dorso-lateral temporal opening on either side of the skull.
2. Boundaries of the temporal opening: The temporal opening is bounded below by two specific bones known as the postorbital and squamosal bones. These bones define the lower boundary of the temporal opening.
3. Unique skull structure: The presence of a single dorso-lateral temporal opening, along with the arrangement of the postorbital and squamosal bones, sets Euryapsida apart from other reptilian subclasses.
4. Extinct nature: It is important to note that Euryapsida is an extinct subclass of reptiles. These reptiles existed in the past but are no longer present in the present-day animal kingdom.

Euryapsida represents an interesting group of reptiles with a distinctive skull structure characterized by the presence of a single dorso-lateral temporal opening on either side, bounded below by the postorbital and squamosal bones.

Subclass III: Parapsida (Extinct)

The Parapsida, an extinct subclass of reptiles, possessed unique skull characteristics that set them apart from other reptilian subclasses. Here are some key features of the Parapsida subclass based on the presence of a single dorsolateral temporal opening on either side, bounded below by the supratemporal and postfrontal bones:

1. Skull structure: The Parapsida reptiles had a skull with a single dorsolateral temporal opening on each side. This temporal opening was bounded below by the supratemporal and postfrontal bones.
2. High position of temporal fossa: These reptiles exhibited a unique feature where the temporal fossa, a depression in the skull, was located high up on the skull rather than in the lower region.
3. Reptile groups: The Parapsida subclass included several reptile groups such as Protosaurs, Nothosaurs, and Placodonts. These reptiles exhibited the characteristic skull structure with a single dorsolateral temporal opening.
4. Dominant groups: Among the Parapsida reptiles, the two largest and most notable groups were Ichthyosaurs and Plesiosaurs. These groups thrived during their existence but eventually became extinct at the end of the Cretaceous period, along with several other reptiles including dinosaurs.

The Parapsida subclass represents an intriguing group of reptiles characterized by a unique skull structure with a single dorsolateral temporal opening bounded below by the supratemporal and postfrontal bones. With the dominance of Ichthyosaurs and Plesiosaurs, the Parapsida reptiles played a significant role in prehistoric ecosystems before their extinction at the end of the Cretaceous period.

Subclass IV: Synapsida (Extinct)

The Synapsida subclass, an extinct group of reptiles, possessed distinctive skull characteristics that set them apart from other reptilian subclasses. Here are some key features of the Synapsida subclass based on the presence of a single lateral temporal opening on either side bounded above by the postorbital and squamosal bones:

1. Skull structure: The Synapsida reptiles had a skull with a single lateral temporal opening on each side, which was bounded above by the postorbital and squamosal bones. This characteristic skull structure distinguished them from other reptilian subclasses.
2. Temporal fossa: These reptiles had one temporal fossa on the lower side of the skull. The presence of this depression in the skull indicated the attachment and development of jaw muscles, a significant evolutionary adaptation.
3. Dominance during the Permian period: The Synapsida reptiles were the most dominant group of reptiles during the Permian period, a geological era that spanned from approximately 298 to 252 million years ago. They exhibited a wide range of ecological adaptations and occupied various niches in diverse environments.
4. Transition to mammals: The surviving members of the Synapsida subclass in the Mesozoic era, which followed the Permian period, gave rise to mammals through evolutionary processes. These reptiles gradually evolved into early mammalian forms, eventually leading to the diverse group of mammals that inhabit the Earth today.
5. Replacement by dinosaurs: While some Synapsida reptiles gave rise to mammals, the majority of the subclass was eventually replaced by dinosaurs. The emergence and dominance of dinosaurs in the Mesozoic era led to the decline and extinction of many Synapsida reptile lineages.

Example: Plesiosaurus is an example of a reptile belonging to the Synapsida subclass. It was a marine reptile with a unique body structure and lived during the Mesozoic era.

The Synapsida subclass represents an important group of reptiles with a distinct skull structure characterized by a single lateral temporal opening. Their dominance during the Permian period and subsequent transition to early mammals highlight their evolutionary significance. Although most Synapsida reptiles were replaced by dinosaurs, their legacy lives on through their contribution to the origin and development of mammalian life forms.

Subclass V: Diapsida

The Diapsida subclass comprises a diverse group of reptiles distinguished by their skull structure, which features two temporal openings on either side separated by the bar of postorbital and squamosal bones. Here are some key characteristics of the Diapsida subclass:

1. Skull structure: The Diapsida reptiles possess a skull with two temporal openings, also known as temporal vacuities, on either side. These openings are separated by the bar formed by the postorbital and squamosal bones. This unique skull structure allows for the attachment of powerful jaw muscles and provides a significant advantage in terms of feeding and locomotion.
2. Diverse group: The Diapsida reptiles are the most diverse of all reptiles, exhibiting a wide range of forms and adaptations. They occupy various ecological niches and habitats, showcasing remarkable diversity in their body shapes, sizes, and lifestyles.
3. Inclusion of dinosaurs and pterosaurs: The Diapsida subclass includes iconic reptilian groups such as dinosaurs and pterosaurs. Dinosaurs, including the well-known Tyrannosaurus rex and Triceratops, were a group of terrestrial reptiles that dominated the Earth for millions of years. Pterosaurs were flying reptiles with membranous wings, often referred to as “pterodactyls.”
4. Division into major groups: The Diapsida subclass is further divided into two major groups based on their evolutionary relationships and characteristics:a. Archosauria: This group includes dinosaurs, crocodiles, birds, and their extinct relatives. Archosaurs were primarily terrestrial or semi-aquatic reptiles characterized by upright postures, diverse feeding strategies, and a wide range of sizes.b. Lepidosauria: This group comprises reptiles such as lizards, snakes, and tuataras. Lepidosaurs are primarily terrestrial or semi-aquatic reptiles known for their diverse body forms, specialized adaptations, and widespread distribution across various habitats.

Examples: Crocodilus (crocodile) and Chameleon are examples of reptiles belonging to the Diapsida subclass. Crocodiles are semi-aquatic reptiles with a characteristic elongated snout and powerful jaws. Chameleons are known for their unique ability to change color and their specialized climbing adaptations.

The Diapsida subclass encompasses reptiles with a distinctive skull structure characterized by two temporal openings. As the most diverse group of reptiles, it includes dinosaurs, pterosaurs, crocodiles, and various other reptilian lineages. The division into Archosauria and Lepidosauria further highlights the evolutionary and ecological diversity within this subclass.

Order of Reptilia

The class Reptilia encompasses a diverse group of reptiles, which can be classified into four main orders: Chelonia, Rhynchocephalia, Squamata, and Crocodilia. Each order has unique characteristics and includes various species. Let’s explore the features of reptiles belonging to these orders:

1. Chelonia (Order Chelonia): Chelonia includes turtles, tortoises, and terrapins. Some notable features of this order are:

• Four-legged locomotion: Chelonians move using four legs adapted for walking and swimming.
• Protective shell: They possess a hard shell composed of the carapace (dorsal part) and plastron (ventral part), providing them with excellent protection.
• Three-chambered heart: Chelonians have a three-chambered heart, which is a characteristic shared by reptiles in general.

2. Rhynchocephalia (Order Rhynchocephalia): The sole surviving species in this order is the Tuatara (Sphenodon). Key features include:

• Ancient lineage: Tuataras are considered living fossils, as they have ancient reptilian characteristics and a lineage dating back to the time of dinosaurs.
• Unique skull structure: They possess a distinct skull structure with two rows of teeth in their upper jaw and a single row in the lower jaw.
• Three-chambered heart: Similar to other reptiles, tuataras have a three-chambered heart.

3. Squamata (Order Squamata): The Squamata order includes lizards and snakes. Here are some characteristics of this order:

• Lizards: Lizards have four legs, allowing them to sprint, climb, and in some cases, swim. They exhibit a wide range of adaptations, including the ability to change color as a defense mechanism. Lizards typically have a three-chambered heart.
• Snakes: Snakes are legless reptiles that evolved from tetrapod ancestors. They have a highly flexible jaw that allows them to swallow prey whole. Some snakes are venomous, injecting poison through specialized fangs. Like lizards, they generally have a three-chambered heart.

4. Crocodilia (Order Crocodilia): The Crocodilia order includes crocodiles, alligators, caimans, and gharials. Notable features of this order are:

• Tooth renewal: Crocodilians continuously renew their teeth throughout their lives, allowing them to maintain a formidable bite.
• Quadrupedal locomotion: They have four well-developed legs adapted for various forms of locomotion, including galloping and swimming.
• Powerful jaws: Crocodilians possess strong jaws with a forceful bite, which they use for capturing and subduing prey.
• High intelligence: Among reptiles, crocodilians are known for their relatively advanced intelligence.
• Four-chambered heart: They have a four-chambered heart, which is more advanced compared to the three-chambered hearts of other reptiles.

These are the four orders of reptiles within the class Reptilia, each with its own unique characteristics and adaptations. From the diverse turtles and lizards to the formidable crocodilians, reptiles exhibit a wide array of traits that have allowed them to thrive in various environments throughout their evolutionary history.

FAQ

References

• https://collegedunia.com/exams/reptilia-science-articleid-3071
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