Reptiles

Lizards, dinosaurs, crocodiles, turtles, and snakes – all belong to that ancient and stout class of animals known as the reptiles. This is a diverse group with more than 10,000 different species and a huge representation in the fossil record. Once the dominant land vertebrates on the planet, reptiles still occupy just about every single ecosystem outside of the extreme north and south.

The 8 Reptile Characteristics – Listed

More than anything else, reptile is an evolutionary classification. Every species within this class shares a common ancestor that dates back more than 300 million years. But it also shares a common set of characteristics. At a basic level, all reptiles have four legs, or are descended from creatures with four legs (including snakes, which still apparently carry some of the genes for making legs). They are also vertebrates with a backbone to house the spinal cord. In addition, most reptiles share the following characteristics:

  • Rough scales – Reptile skin is covered in rough horny layers of scales, bony plates, or a combination of the two. These scales are composed of keratin, the same substance in nails, hair, and claws. The skin is actually rather thinner than you might suspect because it lacks the same dermal layer as mammals. But the watertight skin does allow the reptile to live and thrive in drier ecosystems.
  • Regular shedding – Reptiles shed their skin continuously throughout their lifetimes. Shedding tends to be the most frequent during the adolescent phase, because the skin doesn’t actually grow in proportion with the body. The frequency of the shedding tends to decrease once the reptile reaches adulthood. At that point it’s mostly shed to maintain good health.
  • Cold-blooded – Reptiles have a naturally low metabolic rate, which helps to conserve energy but also means they lack any internal means to keep the body temperature consistent. Without fur or feathers for insulation, reptiles do not have the means to stay warm in cold temperatures. And without sweat glands, they also cannot remain cool in hot temperatures. To compensate, they rely on the sunlight or shade as needed to alter their internal temperature. The leatherback sea turtle is the only reptile species with even some elements of warm-blooded physiology.
  • Egg-laying – With the exception of some snakes and lizards, which give birth to live young, all reptiles are oviparous and produce eggs in a nest. The soil temperature at this time determines whether the young are born male or female. Asexual reproduction is very rare, but it has been known to occur in some lizards and snakes.
  • Highly developed lungs – All reptiles rely on their lungs to breathe air. Even species with permeable skin and other adaptations never completely breathe without the use of their lungs.
  • Short digestive tracts – With a few exceptions, reptiles are meat eaters with relatively short digestive tracts. Due to the slower metabolism, they can afford to digest food slowly and consume few meals. In its most extreme form, crocodiles, boas, and other reptiles can survive months on a single meal. Herbivorous reptiles do exist, but they face a problem: because they lack any complex dental system, they must swallow rocks and pebbles to grind up the plant matter in their digestive system, much like some bird species.
  • Chemoreception – Many but not all reptiles have chemically sensitive organs in the nose or the roof of the mouth for identifying prey. This ability supplements or even replaces the sense of smell. Snakes in particular sense chemicals in the air by flicking out their tongues rapidly. This has the effect of transferring odor particles from the tongue to the roof of the mouth.
  • Skull morphology – Several characteristics of the skull distinguish the reptile from other classes of animals. For instance, they have a single bone where the skull attaches to the first vertebrate, a single auditory bone – the stapes – that transmits vibrations from the eardrum to the inner ear, and a very strong jaw. One of the interesting aspects of reptile morphology is that a few of these jaws bones actually correspond to two ear bones in mammals. It is believed that at some point in early mammalian evolution these jaw bones moved to the back of the head and eventually formed the malleus and incus in the mammalian ear to assist with hearing higher frequency sounds.

Exceptions to Oviparous Reproduction

As mentioned previously, the most dominant form of reptile reproduction is oviparous or egg-laying reproduction, but there are a few notable exceptions. Around 20% of all lizards and snakes, including the boas, do produce live young instead of eggs. These viviparous reptiles have a non-mammalian placenta or some other means through which nutrients are transferred from the mother to the offspring and vice versa for the waste. The main advantage of viviparous birth is that it protects the eggs from predators in a hostile environment. But this method of birth has a tradeoff, since it’s taxing on the mother.

Only three reptile species, including the yellow-bellied three-toed skink of Australia, actually combine both eggs and live birthing methods (the rarity suggests that evolution probably does not favor this in-between stage). The skink’s offspring begins life encased in an egg, the same as any other reptile. But as the embryo develops, the egg begins to thin out until all that’s left upon its birth is a small membrane. The main problem with this method is that the thin egg shells don’t contain enough calcium to nourish the offspring. The mothers appear to compensate for this by secreting calcium from the uterus so it can be absorbed by the developing embryo. The evidence suggests that the skink can choose to lay eggs a few weeks early if it seems like there’s less danger to the offspring. In harsher climates, the mother will keep the offspring insider her body for longer to protect them.

Asexual reproduction is also quite rare, though more common than the egg-live birth method. Around 50 species of lizards and one species of snake partake in this method of reproduction. The evidence suggests that these reptiles may adopt an asexual lifestyle out of necessity – because they are genetically isolated from other groups. The problem with asexual reproduction is the lack of genetic variability: the offspring inherit the same susceptibility to diseases as the parent. But asexual reptiles appear to maintain genetic variation by beginning the reproductive process with twice the normal number of chromosomes.

The Four Orders of Reptiles

The modern class of reptiles is generally divided into four different orders, each with their own distinct characteristics and morphologies.

Testudines – As the only order classified within the subclass Anapsida, Testudines is comprised of all known turtle species. Its main distinguishing characteristic is the hard cartilage-based shell that extends from the ribs and acts as a protective shield. The terms turtle, tortoise, and terrapin are based on regional dialects and do not represent any specific taxonomical or biological differences.

Squamata – The youngest order of reptiles also happens to be the most common. It is comprised of most reptile species, including all known lizards, snakes, geckos, and skinks. Venom is a common feature of at least some species in this order as a means to counter the defenses of their prey. Because the venom evolved from different pre-existing proteins throughout the body, the venoms are very diverse in form and function.

Crocodilia – This ancient order, which includes all modern alligators, crocodiles, caimans, and gharials, is one of the largest carnivorous predators on the planet. Featuring a flattened snout, tough skin, a long tail, and rows of large teeth, the Crocodilia species spend a great deal of their lives in or near water. This order is the closest living relative to the bird class. That’s because the Crocodilia ancestors were closely related to the dinosaurs.

Rhynchocephalia – This order dates all the way back to the Triassic Period some 200 to 250 million years ago. Once incredibly diverse, it has been reduced to only a single living genus, the lizard-like tuatara of New Zealand, which includes only two species. Despite the lizard-like appearance (they are a sister group of Squamata), this order actually shares some basic morphological traits in common with crocodiles and dinosaurs.

The Evolutionary History of the Reptile

When four-legged vertebrates emerged on land some 400 million years ago, semi-aquatic amphibious animals were the first to evolve. This semi-aquatic lifestyle was reflected in its morphology and behavior. But several advances in lungs, bone structure, and egg composition enabled the earliest reptiles to move away from the water and populate the rest of the untapped ecological niches around the world.

The first obvious signs of reptiles appeared in the fossil record some 310 to 320 million years ago. At the time, much of their territory was covered in swamps. The anapsids, characterized by the lack of a hole in the back of the skull, were some of the earliest reptiles to evolve. Diapsids, which have two holes near the back of their skull, include modern reptiles and indeed almost all reptiles from the last 250 million years. Only the turtle is currently placed within the anapsid group (and even then it’s not really considered to be a “true” anapsid, because the solid back skull probably evolved later).

Over millions of years, the reptile class continued to undergo a lot of evolution and change. The archosauromorphs, which encompass dinosaurs, crocodiles, and birds, first appeared in the fossil record around the Late Triassic Period just over 200 million years ago. They flourished and became particularly prominent in the Jurassic Period. Proto-turtles first appeared around the same time and have maintained the same body shape ever since. The Squamata probably evolved in the Middle Jurassic, some 160 to 170 million years ago, but true snakes didn’t evolve until around 100 million years ago.

Part of this evolution was driven by massive sudden changes in the ecosystem, including the mass extinction event 250 million years ago that killed about 90% of all species on the planet and, of course, the asteroid impact and volcanic activity that drove all non-avian dinosaurs extinct about 65 million years ago. Since then reptiles have evolved a smaller size and begun to share the ecosystem with birds and mammals (which actually split off from early reptiles some 300 million years ago).

Types of Reptiles

A Aldabra Giant Tortoise
Aldabra Giant Tortoise

One got to be 255 years old!

A Alligator
Alligator

They have two sets of eyelids!

A Bearded Dragon
Bearded Dragon

Can grow to up 24 inches long!

A Box Turtle
Box Turtle

This reptile has an S-shaped neck allowing it to pull its entire head into its shell.

A Burmese Python
Burmese Python

These snakes can swallow their prey as whole.

A Caiman
Caiman

Can grow to up 6 meters long!

A Caiman Lizard
Caiman Lizard

Caiman lizards are among the largest lizards.

A Crocodile
Crocodile

Have changed little in 200 million years!

A Desert Tortoise
Desert Tortoise

Lives in burrows underground!

A Dwarf Crocodile
Dwarf Crocodile

Digs burrows in river banks to rest!

A Frilled Lizard
Frilled Lizard

Mainly lives in the trees!

A Galapagos Tortoise
Galapagos Tortoise

The biggest species of tortoise in the world!

A Gecko
Gecko

There are thought to be over 2,000 species!

A Gharial
Gharial

Found in the murky waters of Northern India!

A Gila Monster
Gila Monster

It's tail acts as a fat storage facility!

A Glass Lizard
Glass Lizard

Can grow up to 4ft long!

A Green Anole
Green Anole

It communicates with head movements, color and dewlap

A Horned Lizard
Horned Lizard

The horned lizards are able to squirt blood from their eyes.

A Iguana
Iguana

Uses visual signals to communicate!

A Indian Star Tortoise
Indian Star Tortoise

Popular in the exotic pet trade!

A King Cobra
King Cobra

They are the longest poisonous snake in the world.

A Komodo Dragon
Komodo Dragon

Only found on five Indonesian islands

A Leaf-Tailed Gecko
Leaf-Tailed Gecko

Only found on Madagascar!

A Leopard Tortoise
Leopard Tortoise

The most widely distributed tortoise in Africa!

A Lizard
Lizard

There are around 5,000 different species!

A Marine Iguana
Marine Iguana

Adult marine iguanas vary in size depending on the size of the island where they live.

A Mexican Alligator Lizard
Mexican Alligator Lizard

Mexican alligator lizards shed their skin like snakes.

A Monitor Lizard
Monitor Lizard

Some species are thought to carry a weak venom!

A Nile Crocodile
Nile Crocodile

When a female Nile crocodile’s hatchlings are in danger, she may hide them in a special pouch inside her throat.

A Painted Turtle
Painted Turtle

Male painted turtles have longer nails.

A Radiated Tortoise
Radiated Tortoise

The most protected tortoise in the world!

A Rattlesnake
Rattlesnake

It's venom digests it's prey before it even swallows it!

A River Turtle
River Turtle

Inhabits freshwater habitats around the world!

A Sand Lizard
Sand Lizard

Males turn green in spring!

A Sea Turtle
Sea Turtle

Always return to the same beach to lay eggs!

A Sharp-Tailed Snake
Sharp-Tailed Snake

This snake uses its sharp tail to steady itself when capturing prey.

A Skink Lizard
Skink Lizard

Some skinks lay eggs in some habitats while giving birth to skinklets in other habitats.

A Slow Worm
Slow Worm

Found widely throughout British gardens!

A Snake
Snake

There are around 3,000 known species worldwide

A Sulcata Tortoise
Sulcata Tortoise

Some cultures in Africa believe the sulcata tortoise is an intermediary between the people and their ancestors and gods.

A Thorny Devil
Thorny Devil

Found only on mainland Australia!

A Tortoise
Tortoise

Can live until they are more than 150 years old!

A Virgin Islands Dwarf Gecko
Virgin Islands Dwarf Gecko

The Virgin Islands dwarf gecko is among the smallest reptiles in the world

A Water Dragon
Water Dragon

Spends most of it's time in the trees!

A Wood Turtle
Wood Turtle

Temperature determines the sex of turtle eggs

List of Reptiles

Reptiles FAQs (Frequently Asked Questions) 

What are reptiles?

The reptiles are a class of cold-blooded animals characterized by rough skin and egg-laying. It is only one of three vertebrate classes, along with mammals and birds, that have an amnion, or an inner sac, during the embryonic stage of development.

What are the characteristics of reptiles?

The reptile has rough scales or bony plates covering the skin which are shed on a regular basis. These cold-blooded creatures cannot maintain a consistent internal body temperature and rely completely on the external environment to warm up or cool down.

What are the types of reptiles?

The three major orders of reptiles are the turtles, the lizards and snakes, and the crocodiles and all of its variants. A fourth order, Rhynchocephalia, is composed of only two living species though many more extinct ones.