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What are Reptiles?

Reptiles are a Class of Animals which are Cold Blooded and lay eggs. Most of them are 'Tetrapods', meaning that they have four legs or leg-like appendages. They have a Bilateral Symmetry. Reptiles started evolving around 330 million years ago and dominated the Earth for over 200 million years. Reptiles include Lizards, Snakes, Crocodiles, Tortoise etc. There are about 8,200 extant Speciess of Reptiles of which almost half are Snakes. Lizards and Snakes are 'The Largest Group of Reptiles'. Of the 1,670 Reptile Species that have been evaluated on the IUCN Red List, almost 1 in 3 are threatened with extinction.

Orders of Reptiles

Crocodilia - These include Crocodiles, Gavials, Caimans and Alligators. There are about 23 Species of Crocodilia.

Sphenodontia - These include Tuataras from New Zealand. There are only 2 Species of Sphenodontia.

Squamata - These include Lizards, Snakes and Worm Lizards. There are approximately 7,900 Species of Squamata.

Testudines - These include Turtles and Tortoises. There are approximately 300 Species of Testudines.

Subclasses of Reptiles

Subclasses of Reptiles is based on the basis of Temporal Openings.

Anapsida - These are characterised with having no Temporal Opening like the Primitive Reptiles and Turtles.

Diapsida - These are charcterised by 2 Temporal openings. These include Snakes, Lizards, Crocodilians, Dinosaurs, Pterosaurs, Tuatara, Nothosaurs, Plesiosaurs and Icthyosaurs etc.

Euryapsida - These are characterised with one high Temporal Opening placed above the 'Postorbital' and 'Squamosal' (some of the bones in vertebrate skull. These include Protorosaurs (small, early lizard-like reptiles) and Sauropterygia (plesiosaurs and their relatives).

Synapsida - These are characterised by One low Temporal opening. These include Pelycosaurs, Therapsid Mammals such as Dicynodonts, Cynodonts and the ancestors of Mammals.

Parapsida - These are characterised by One Temporal Opening placed high up on the skull. These include extinct Species like Protosaurs, Nothosaurs and Placodonts.

Today’s Reptiles fall into only 2 of the 5 Subclasses.

History and Evolution of Reptiles

The origin of the Reptiles lies about 320–310 million years ago, in the steaming swamps of the late Carboniferous Period, when the first Reptiles evolved from advanced 'Reptiliomorph labyrinthodonts'. The oldest trace of Reptiles is a series of footprints from the fossil strata of 'Nova Scotia', dated to 315 million years ago. The tracks are attributed to 'Hylonomus', the oldest known Reptile in biological sense. It was a small, Lizard-like Animal, about 20 to 30 cm (8–12 in) long, with numerous sharp teeth indicating an Insectivorous diet. Other examples include 'Westlothiana' and 'Paleothyris', both of similar build and presumably similar habit. One of the best known early Reptiles is Mesosaurus, a genus from the early Permian Period that had returned to water, feeding on Fish. The earliest Reptiles were largely overshadowed by bigger Labyrinthodont Amphibians such as Cochleosaurus, and remained a small, inconspicuous part of the Fauna until after the small Ice Age at the end of the Carboniferous Period.

Ancient Reptiles

The first Reptiles were Anapsids, i.e. they had a solid Skull with holes for only nose, eyes, spinal cord, etc.

Very soon after the first Reptiles appeared, they split into two branches.

1. Synapsida - These type of Reptiles had two openings in the Skull roof behind the eyes. These included 'Mammal-like Reptiles' or 'Stem Mammals'

2. Diapsida - These type of Reptiles possessed a pair of holes in their Skulls behind the eyes, along with a second pair located higher on the Skull. The function of the holes in both groups was to lighten the skull and give room for the jaw muscles to move, allowing for a more powerful bite.

The Diapsids and later Anapsids are classed as the 'True Reptiles' also known as the 'Sauropsida'

Permian Period

With the close of the Carboniferous, Reptiles became the dominant Tetrapod Fauna. The Synapsids evolved the first Terrestrial Megafauna (giant animals) in the form of 'Pelycosaurs' such as 'Edaphosaurus' and the Carnivorous 'Dimetrodon'.

Early in the Premian Period, the Diapsid Reptiles split into two main lineages, the Archosaurs (forefathers of crocodiles and dinosaurs) and the Lepidosaurs (predecessors of modern snakes, lizards, and tuataras). Both groups remained Lizard-like and relatively small and inconspicuous during the Permian.

In the mid-Permian period the climate turned dryer, resulting in a change of fauna. The Primitive Pelycosaurs were replaced by the more advanced 'Therapsids' (earliest mammals). The Anapsid Reptiles, whose massive skull roofs had no post-orbital holes, continued and flourished throughout the Permian. The 'Pareiasaurs' reached giant proportions in the late Permian, eventually disappearing at the close of the period.

Mesozoic Period

The end of the Permian Period witnessed the 'Greatest Mass Extinction' known as the 'Permian–Triassic Extinction Event'. Most of the earlier Anapsid/Synapsid Megafauna became extinct. They were replaced by the 'Archosauromorph Diapsids'. The Archosaurs had elongated hind legs and an erect pose and the early forms looked somewhat like 'Long-Legged Crocodiles'. The Archosaurs became the dominant group during the Triassic period, developing into the well-known Dinosaurs and Pterosaurs, as well as Crocodiles and Phytosaurs. Some of the Dinosaurs developed into the 'Largest Land Animals' ever to have lived.

Many varieties of Reptiles evolved during this Period. The Lepidosauromorph Diapsids may have been ancestral to the Sea Reptiles. These Reptiles developed into the Sauropterygians in the early Triassic and the Ichthyosaurs during the Middle Triassic. The Mosasaurs also evolved in the Mesozoic era, emerging during the Cretaceous period. The Therapsids came under increasing pressure from the Dinosaurs in the early Mesozoic and developed into increasingly smaller and more nocturnal forms, the first Mammals being the only survivors of the line by the late Jurassic.

Cretaceous - Tertiary Extinction Event

The close of the Cretaceous Period witnessed mass extinction of Mesozoic era Reptilian Megafauna which is known as the 'Cretaceous–tertiary Extinction Event'. Of the large Marine Reptiles, only the Sea Turtles were left, and amongst Dinosaurs, only the small feathered Theropods survived in the form of Birds.

Reptiles Scientific Classification

Kingdom - Animalia.
Phylum - Chordata.
Subphylum - Vertebrata.
(unranked) - Amniota.
Class - Reptilia.

Characteristics of Reptiles

Cold Blooded - The body temperature of Reptiles vary with the surrounding atmosphere. Like in mammals, the body temperature of Reptiles is not regulated by internal mechanisms. So they have to maintain the body temperature by basking in the sun. This is the reason behind the presence of many Reptiles in arid regions, like deserts. However, Reptiles can be seen in different habitats and if the temperature levels are not favourable, some of the Reptiles prefer hibernation too.

Scaly Skin - Almost all Reptiles have Dry Skin with Scales or Scutes. The skin has a few Cutaneous Glands and high levels of Keratin, which prevents water loss through the skin. The Scales and Scutes, are formed from the Epidermis and are also made of Keratin, to protect the body.

Spinal Column - All Reptiles have Spinal Columns and a strong Skeletal System with a rib cage. A Pelvic Region with a minimum of Two Spinal Bones is also seen in most Reptiles.

Tetrapod Characteristic - Reptiles are Tetrapods. They have two sets of paired limbs. Most of them have Five Clawed Toes on each limb. These muscular appendages are angled downward to facilitate faster locomotion. In some Reptiles, like Snakes, Worm Lizards, etc., the legs are absent, though they are considered to have evolved from some Tetrapod ancestors.

Keen Sense Organs - Reptiles have keen sense organs, which help them to find food and escape from predators. Eyes are one of the most important sense organ and in most Reptiles, these are located at the front of the head to facilitate binocular vision. Most Reptiles are Diurnal Animals. The vision is typically adapted to daylight conditions, with colour vision and more advanced visual depth perception than in amphibians and most mammals. Most of the different types of Lizards can move each eye independently, some of them have a protective cover above the eyes. Most of them don't have external ears and the eardrum is located near the eyes.

Respiratory System - All Reptiles breathe using lungs. Reptiles do not pass through an embryonic stage with gills. They start breathing with well-developed lungs, right from birth. Most of them have Two Lungs, except some Snakes, which possess only a Single Lung. Even cutaneous respiration is very negligible. Aquatic Turtles have developed more permeable skin, and some Species have modified their Cloaca to increase the area for gas exchange. Even with these adaptations, breathing is never fully accomplished without lungs.

Three Chambered Heart - Almost all Reptiles have 'Three-Chambered Hearts', except Crocodiles, which have 'Four-Chambered Hearts' (2 atria, 2 ventricles), like mammals and birds. The Three Chambers in Reptiles consist of Two 'Atria' to receive blood and One Partially Divided 'Ventricle' for pumping blood.

Lack of Secondary Palate - Most Reptiles lack a Secondary Palate, meaning that they must hold their breath while swallowing. Crocodilians have evolved a 'Bony Secondary Palate' that allows them to continue breathing while remaining submerged (and protect their brains against damage by struggling prey). Skinks (family Scincidae) also have evolved a 'Bony Secondary Palate', to varying degrees. Snakes took a different approach and extended their Trachea instead. Their Tracheal Extension sticks out like a fleshy straw and allows these animals to swallow large prey without suffering from asphyxiation.

Slow Digestive System - Most Reptiles are Carnivorous and have rather simple and comparatively short guts, meat being fairly simple to break down and digest. Digestion is slower of their inability to divide and masticate their food. Even the Herbivorous Reptiles slow digestive system because they lack complex teeth of Mammals. Many Speciess swallow rocks and pebbles (so called gastroliths) to aid in digestion. The rocks are washed around in the stomach, helping to grind up plant matter. Sea Turtles, Crocodiles, and Marine Iguanas also use Gastroliths as Ballast, helping them to dive.

Less Energy Requirement - Reptiles being Poikilotherms, i.e. having varying body temperature regulated by the environment, have energy requirement which is about a fifth to a tenth of that of a Mammal of the same size. Large Reptiles like Crocodiles and the large constrictors can live from a single large meal for months, digesting it slowly.

Small Brain - Reptiles are generally considered less intelligent than Mammals and Birds. The size of their brain relative to their body is much less than that of Mammals, the encephalization quotient being about one tenth of that of mammals. Crocodiles have relatively larger brains and show a fairly complex social structure. Larger Lizards like the monitors are known to exhibit complex behaviour, including cooperation.

Amniotic Eggs - Reptiles lay Amniotic Eggs (covered with a special membrane), that can be laid on land and not in water. These amniotic eggs can survive without water, as they have protective shells and membranes, that allow oxygen and other gases to enter. Most of the Reptiles lay eggs, but some of them give birth to young ones, by hatching the eggs inside the body of the mother.

Weight - The 'Largest Extant Reptile' weighs over 1,000 kg and the 'Smallest Reptile' weighs between 20-50 grams.

Size - The 'Largest Extant Reptile' reaches approximately 6 metres in length and the 'Smallest Extant Reptile' grows to only 1.6 cm (0.6 in).

Geographical Range and Habitat

Reptiles inhabit every Continent with the exception of Antarctica. Their Habitats include Deserts, Forests, Mountains etc.

Diet - Most Reptiles eat a variety of food but most are Carnivorous. But there are some Reptiles which are Omnivorous. A small number of Reptile Species eat only plants like some Species of Turtles, Chuckwalla and Desert Iguana. Some Reptiles eat only certain type of food such as the Striped Crayfish Snake, which eats only Crayfish. Their Diet includes a wide variety of Mammals, Birds, Insects, other Reptiles etc.

Reptiles Reproduction

Almost all the Reptiles reproduce sexually, though some are capable of Asexual Reproduction. All reproductive activity occurs through the Cloaca, the single exit/entrance at the base of the tail where waste is also eliminated. Most Reptiles have Copulatory Organs, which are usually retracted or inverted and stored inside the body. In Turtles and Crocodilians, the Male has a single median penis, while Snakes and Lizards, possess a pair of 'Hemipenes'. Tuataras, however, lack copulatory organs, and so the Male and Female simply press their Cloacas together as the Male excretes sperm. Asexual reproduction has been identified in Squamates in six families of Lizards and one Snake. In some Species of Squamates, a population of Females is able to produce a 'Unisexual Diploid Clone' (organism having two sets of chromosomes) of the mother. This form of Asexual Reproduction is called Parthenogenesis. In Parthenogenesis an unfertilized egg develops into a new individual. It also occurs in several Species of Gecko and is particularly widespread in the Teiids (new world lizards especially Aspidocelis) and Lacertids (old world terrestrial lizard, Lacerta). In captivity, Komodo dragons (Varanidae) have reproduced by Parthenogenesis. Some Species among Chameleons, Agamids, Xantusiids and Typhlopids are also suspected of Parthenogenesis.

Most Reptiles lay amniotic eggs covered with leathery or calcareous shells. An amnion, chorion, and allantois are present during embryonic life. There are no larval stages of development. Viviparity and Ovoviviparity have evolved only in Squamates, and many Species, including all boas and most vipers, utilize this mode of reproduction. The degree of Viviparity varies: some Species simply retain the eggs until just before hatching, others provide maternal nourishment to supplement the yolk, and yet others lack any yolk and provide all nutrients via a structure similar to the mammalian placenta.Some Reptiles exhibit temperature-dependent sex determination (TDSD), in which the incubation temperature determines whether a particular egg hatches as Male or Female. TDSD is most common in Turtles and Crocodiles, but also occurs in Lizards and Tuataras.