Reptiles, along with amphibians and fish, are some of the oldest vertebrates on Earth. They are common in many areas worldwide, and it is likely that you frequently come into contact with them in the warmer months, when lizards can be found basking on brick walls and snakes stretch themselves across walking trails!
But, have you ever wondered what exactly reptiles are? If your answer is yes, then you came to the right place!
- We will first talk about the characteristics of reptiles.
- Then, we will look at the difference between reptiles and amphibians.
- After, we will look at types and examples of reptiles.
- Lastly, we will learn about underground reptiles.
Reptile Characteristics
Let's start by looking at the definition of a reptile and understanding reptile characteristics.
Broadly speaking, a reptile is any animal of the class Reptilia, though the classical definition of a reptile has changed in recent years.
The first reptiles evolved around 320 million years ago, during the Carboniferous Period. Historically, reptiles were believed to consist of four taxonomic orders. However, we now know there are five orders of reptiles:
Squamata (lizards and snakes)
Testudines (turtles and tortoises)
Crocodylia (all crocodilians)
Rhynchocephalia (tuataras)
Aves (birds)
Physical characteristics of reptiles
Let's dive into the characteristics of reptiles. Reptiles vary greatly in size and shape.
They can range in size from as small as half an inch, as in the case of the tiny nano-chameleon (Brookesia nana), to as large as the saltwater crocodile (Crocodylus porosus).
In general, reptiles are externally covered by a skin consisting of scales, which can range from microscopic in size to the large scales of crocodilians, called scutes.
Birds are not covered in scales, but they do have them on their legs and feet.
Like most other vertebrates, all reptiles are tetrapods, which are animals with four limbs. Now, this might have got you thinking: what about snakes? Well, snakes are descended from four-limbed animals and still retain vestigial limbs.
Most reptiles are ectothermic (cold-blooded) rather than endothermic (warm-blooded). The only reptiles that are endothermic are birds. Ectothermic animals rely on environmental sources, such as the sun, to maintain body temperature, while endothermic animals maintain their own body temperature through internal physiological processes.
The process of maintaining body temperature is known as thermoregulation. When you see crocodilians basking in the sun along river banks, for example, they are thermoregulating.
Reptile Reproduction
While the vast majority of reptiles reproduce sexually, a small number are capable of asexual reproduction, such as the New Mexico whiptail lizard (Aspidoscelis neomexicana).
In addition, although most reptiles lay eggs (oviparous), some species give birth to live offspring. These species can be further separated into viviparous and ovoviviparous.
Viviparous species include most mammals, including humans.
Viviparous reptiles include around 20% of snake species! These species give birth to live young without the need to lay eggs at all.
Ovoviviparous reptiles emerge as live young from eggs laid inside their mother's body.
Now, while ovoviviparous reptiles enter the world as smaller versions of adults, they do hatch from the internally laid eggs.
Examples of ovoviviparous reptiles would be all members of the Agkistrodon genus of snakes, which include the copperheads and cottonmouths of North America.
Reptiles and Amphibians
Now that you know a bit about the characteristics of reptiles, let's talk about amphibians and what makes them different from reptiles.
Amphibians are ectothermic, tetrapod vertebrates consisting of all animals in the Lissamphibia group, represented by Anura (frogs), Caudata (salamanders), and Gymnophiona (caecilians).
While there may be some superficial similarities, amphibians are very different from reptiles. For starters, amphibians have gills, which allow them to breathe underwater, as opposed to lungs, which reptiles have.
Amphibians also do not have scales and must maintain constant moisture by living in aquatic environments. What about aquatic reptiles like crocodilians and turtles? While they spend much of their time in aquatic environments, they still have lungs and cannot breathe underwater. They merely hold their breath for extended periods of time. Crocodilians, for example, are capable of remaining submerged underwater for two hours! This allows them to wait and sneak up on prey species undetected.
Types and Examples of Reptiles
Now, let's go over each order of reptiles in greater detail here.
Squamata
First up is the reptile order squamata.
The order squamata consists of all snakes and lizards. It is the biggest of all reptile orders, containing nearly 11,000 species.
The largest members of Squamata include the reticulated python (Malayopython reticulatus), which can reach over 25 feet (ca. 8 m) in length, and the Komodo dragon (Varanus komodoensis), at over 10 feet (ca. 3 m).
Fig 1: The Komodo dragon is one of the world's largest squamate
According to research, the earliest known squamates evolved during the Triassic Period (some 240 million years ago) under the genus Megachirella. This small squamate (6 inches in length) closely resembled modern lizards.
Snakes, on the other hand, are believed to have first diverged from lizards during the Jurassic Period around 130 million years ago. So, lizards pre-date snakes by over 100 million years. Some squamates also evolved to be venomous in order to aid in predation. While most venomous reptiles are snakes, a few lizards are also venomous, including the Gila monster (Heloderma suspectum) of the southwestern United States.
Fun fact: The world's largest venomous snake is the king cobra (Ophiophagus hannah) of Asia, which can reach over 19 feet (ca. 6 m) in length!
Testudines
The second order of reptiles is called testudines.
The order testudines consist of all turtles, tortoises and, terrapins, which are reptiles that have shells that aid in protection from predators.
There are currently around 356 living species of turtle, widely distributed on every continent except Antarctica and found in many climates, from tropical to temperate.
Living turtles can be categorized into two groups:
Pleurodira, or "side-necked" turtles, bend their heads to the side when retreating into their shells. Pleurodira are found in the Southern Hemisphere, in Australia, Africa, and South America.
Cryptodira, or "hidden-necked" turtles, move their heads straight back into their shells. Cryptodira are found worldwide. All sea turtles and tortoises are members of Cryptodira.
In the United States, the term "tortoise" most often refers to terrestrial turtles, such as the Galapagos giant tortoise (Chelonoidis niger), but the term can be used to describe some aquatic turtles elsewhere, such as in Australia, where tortoise is used to differentiate freshwater turtles from marine turtles. The largest living turtle species is the leatherback turtle (Dermochelys coriacea), which can weigh over 1000 lbs.
Crocodylia
The third reptile order is known as crocodylia.
The order Crocodylia consists of all crocodiles, alligators, caiman, and gharials, which are collectively known as crocodilians.
Due to advances in genetics and other forms of research, the number of living crocodilian species continues to increase, with the recent splitting of species in West Africa and Oceania resulting in 27 species currently recognized.
Crocodilians range greatly in size, from Cuvier's dwarf caiman (Paleosuchus palpebrosus), which rarely exceeds 5 feet (1.52 m), to the saltwater crocodile, which can exceed 20 feet (6.1 m) in length on rare occasions. They also vary widely in appearance, including the slender-snouted Indian gharial (Gavialis gangeticus) and the heavily armored smooth-fronted caiman (P. trigonatus).
Crocodilians are widely distributed throughout tropical regions worldwide, though their distribution is limited by cooler temperatures and habitat requirements.
There are three extant crocodilian families:
Crocodylidae (true crocodiles)
Alligatoridae (alligators and caiman)
Gavialidae (gharials and Tomistoma).
While they may look similar, crocodiles and alligators last shared a common ancestor around 200 million years ago! That means that crocodiles and alligators have been on separate evolutionary paths since the early Jurassic Period! For comparison, humans and chimpanzees (Pan troglodytes) last shared a common ancestor between 6 and 7 million years ago.
Fig 2: The saltwater crocodile is the largest living reptile.
You may have heard the question, "What is the difference between a crocodile and an alligator?" This question can be answered by looking at the distribution of ISOs (integumentary sensory organs) in their bodies (Fig. 3).
ISOs are small sensory organs that detect changes in the water (such as the waves created by an animal entering the water) to help locate potential prey. On alligators and caiman, the ISOs are restricted entirely to the head, while true crocodiles and gharials have ISOs distributed throughout their entire body. However, it is wise not to get close enough to a wild crocodilian to be able to discern ISO distribution!
Fig. 3: The integumentary sensory organs are the tiny dots you can see on the jaw of this crocodile.
Rhynchocephalia
The only living member of the Rhynchocephalia order is the tuatara (Sphenodon punctatus), which closely resembles some modern lizards, such as dragons and iguanas. Sadly, all other members of Rhynchocephlaia died out around 60 million years ago.
The tuatara is endemic to New Zealand, and while once widespread throughout the main islands, it is now restricted to 32 small islands in the north, as well as one reintroduced population on the main North Island at Zealandia Wildlife Sanctuary.
Tuataras generally grow to around 1.5-2 feet in length, and while visually similar to and closely related to lizards, they have some important differences. Unlike lizards, tuataras have no ears on their external surface (but they can hear), and they are nocturnal! Tuataras also have an arrangement of teeth that is unique among reptiles- they have a double row of teeth on the top jaw, but only a single row on the bottom jaw.
Aves
Birds are warm-blooded reptiles with feathers. You may have heard before that birds are related to dinosaurs.
What you may not know is that we now know that birds actually ARE dinosaurs!
That's why nowadays when scientists refer to extinct dinosaurs, they call them "non-avian" dinosaurs.
Birds are archosaurs, which means they are members of the Archosauria clade, which also includes crocodilians and the extinct, non-avian dinosaurs. Yes, you read that right; the closest living relatives of birds are crocodilians!
This may seem odd until you look closer at bird and crocodilian behavior. Like birds, crocodilians have a "homing instinct" that allows them to travel long distances to return to locations. This has actually become quite an issue when relocating "problematic" crocodiles since even when they have moved hundreds of miles away, they tend to swim right back to where they were originally captured.
The earliest modern birds emerged during the Cretaceous Period, around 100 million years ago, and were the only known dinosaurs to survive the Cretaceous–Paleogene extinction event 66 million years ago.
Underground Reptiles
To finish off, let's talk about underground reptiles. Now, you may think, "How can a reptile live underground? Don't they need the sun to thermoregulate?" The answer is no!
Temperatures underground tend to be much more stable than at the surface. Evolutionary adaptations to living in an underground environment have resulted in these reptiles more closely resembling worms and other underground animals than they do other reptiles.
Take mole lizards of the Bipedidae family, for example. They have front legs with digging claws, similar to a mole, but no back legs. They are also covered in ring-shaped scales and are pink in color, leading to a mole-like appearance.
Other examples of underground reptiles include blind snakes and worm lizards, neither of which have legs and closely resemble worms. Many of these underground species are poorly studied due to their cryptic nature and the difficulty of observing them in their natural habitat.
Reptiles - Key takeaways
- There are five orders of reptile: Squamata, Testudines, Crocodilia, Rhynchocephalia, and Aves.
- Reptiles can be differentiated from amphibians in a number of ways, including by their lack of gills, presence of lungs, and presence of scales.
- Squamata includes all snakes and lizards, and Testudines include all turtles. Crocodylia includes all crocodiles, alligators, caiman, and gharials. The only living member of Rhynchocephalia is the tuatara.
- Birds, along with crocodilians and extinct non-avian dinosaurs, are members of the Archosauria clade.
- Some reptiles, such as mole lizards and blind snakes, live underground and do not require the sun to thermoregulate.
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