Exploring Our Fluid Earth

What is a Reptile?

Reptiles are a group of vertebrate animals with dry scaly skin and hard bony skeletons. Examples of reptiles include turtles, lizards, snakes, and crocodilians (Fig. 5.21). Biologists currently consider birds as a type of reptile. Reptiles are a diverse group of animals that includes almost 10,000 species not including birds. They are adapted to live in a wide variety of environments. Most reptile species live in terrestrial environments, but many live in freshwater or marine environments. For 186 million years—from approximately 252 to 66 million years ago—reptiles were the dominant land animals on Earth. Dinosaurs and other extinct reptile groups thrived during the Mesozoic era, also known as the age of reptiles.

 

Fig. 5.21. (A) Florida box turtle (Terrapene carolina bauri)

Image courtesy of Jonathan Zander, Wikimedia Commons

Fig. 5.21. (B) Flat-headed rock agama (Agama mwanzae) lizard, Serengeti National Park, Tanzania

Image courtesy of Christian Mehlführer, Wikimedia Commons

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Fig. 5.21. (C) San Bernardino ring-necked snake (Diadophis punctatus modestus)

Image courtesy of Mark Herr, Wikimedia Commons

Fig. 5.21. (D) American alligator (Alligator mississippiensis) juveniles, Miami-Dade County, Florida

Image courtesy of Ianaré Sévi, Wikimedia Commons

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Evidence of Common Ancestry and Diversity

The last common ancestor of all mammals and reptiles (including birds) first evolved from amphibians approximately 350 to 300 million years ago (Fig. 5.22). Thick scaly skin was one of several adaptations that allowed reptiles to survive on land and replace amphibians as the dominant terrestrial vertebrates on Earth.

 

Fig. 5.22. (A) Phylogenetic tree of all vertebrate animals

Image by David Lin, adapted from image by Petter Bøckman, Wikimedia Commons

Fig. 5.22. (B) Phylogenetic tree illustrating the evolutionary relationships among different reptile groups (shaded in blue)

Image by David Lin, adapted from image by Jacek FH, Wikimedia Commons

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Modern reptiles are represented by five general groups: turtles, lizards, snakes, crocodilians, and birds. Although birds are now widely regarded as a type of reptile, they are discussed separately in Birds.

 

Turtles

Turtles (order Testudines) are four-legged reptiles with hard bony shells covering their torsos. The shell is made of two pieces (Fig. 5.23). The carapace is the upper, or dorsal, side of the shell. The plastron is the flatter lower, or ventral, side of the shell. Turtle species vary greatly in their diets and habitats. They eat plants, invertebrates, and fish. Many turtle species have adapted aquatic lifestyles in freshwater, brackish, and marine habitats (Fig. 5.23 B, C and D). Other species, including several tortoises, can survive in harsh desert climes (Fig. 5.23 A). The term tortoise is used to describe some non-swimming terrestrial turtles.

 

Fig. 5.23. (A) A Galápagos Pinta Island tortoise (Chelonoidis nigra abingdoni) named Lonesome George was the last member of his species when he died in 2012.

Image courtesy of Arturo de Frias Marques, Wikimedia Commons

Fig. 5.23. (B) Yellow-bellied slider (Trachemys scripta scripta)

Image courtesy of Joe Ravi, Wikimedia Commons

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Fig. 5.23. (C) Alligator snapping turtle (Macroclemys temminckii)

Image courtesy of Melissa Wilkins, Flickr

Fig. 5.23. (D) Diamondback terrapin (Malaclemys terrapin)

Image courtesy of Ryan Hagerty, U.S. Fish and Wildlife Service (USFWS)

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Marine or sea turtles form one single group (superfamily Chelonioidea; Fig. 5.24). The leatherback sea turtle (Dermochelys coriacea) is the world’s largest extant turtle species weighing up to 700 kilograms (kg) and spanning 2.2 meters (m) in total length (Fig. 5.24 D). Sea turtles occur in all ocean basins with the exception of the cold polar regions. They generally feed on jellyfish, sponges, crustaceans, seagrasses, and macroalgae.

 

Fig. 5.24. (A) Green sea turtle (Chelonia mydas), Keauhou, Island of Hawai‘i

Image courtesy of Brocken Inaglory, Wikimedia Commons

Fig. 5.24. (B) Hawksbill sea turtle (Eritmochelys imbricata)

Image courtesy of Caroline S. Rogers, National Oceanic and Atmospheric Administration (NOAA)

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Fig. 5.24. (C) Kemp’s ridley sea turtle (Lepidochelys kempii), Bon Secour National Wildlife Refuge, Alabama

Image courtesy of Jereme Phillips, U.S. Fish and Wildlife Service (USFWS)

Fig. 5.24. (D) Leatherback sea turtle (Dermochelys coriacea), Trinidad

Image courtesy of Alastair Rae, Flickr

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Snakes

Snakes (suborder Serpentes) are legless reptiles with slender, elongated bodies (Fig. 5.25). All snakes are strictly carnivorous. They eat other reptiles, birds, small mammals, fish, and insects. Most snakes are terrestrial, living in habitats ranging from deserts to rainforests. However, some species have adapted to living in the water. The large and infamous anaconda snakes (Eunectes spp.) live primarily in freshwater swamps and slow-moving rivers. They are also known as “water boas.”

 

Fig. 5.25. (A) Green anaconda (Eunectes murinus), Orinoco River basin, Venezuela

Image courtesy of Fernando Flores, Wikimedia Commons

Fig. 5.25. (B) Southern copperhead (Agkistrodon contortrix contortrix), Liberty County, Florida

Image courtesy of Tim Ross, Wikimedia Commons

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Fig. 5.25. (C) Yellow-bellied sea snake (Pelamis platura), Costa Rica

Image courtesy of Aloaiza, Wikimedia Commons

Fig. 5.25. (D) Banded sea krait (Laticauda colubrina) hunting on a coral reef, Zamboanguita, Phillipines

Image courtesy of Elias Levy, Flickr

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All sea snakes (subfamily Hydrophiinae) share common ancestry with several other species of venomous snakes like cobras and mambas.

 

Findings from the field of evolutionary developmental biology (informally known as “evo-devo” biology) have indicated that snakes lost their limbs secondarily over the course of evolution. Secondary loss is the evolutionary reversion of a trait back to its ancestral condition. Legless snakes evolved from lizard-like ancestors with legs, but superficially appear more similar to earlier ancestors like eels or fish. Snakes are most closely related to lizards (order Squamata) rather than the other reptile groups.

 

Lizards

Lizards (suborder Lacertilia) are reptiles mostly with four legs (Fig. 5.26). Some lizard groups have independently evolved limbless body forms and appear superficially similar to snakes. Lizards range in size from the tiny 16-millimeter-long dwarf gecko (Sphaerodactylus ariasae) to the three-meter-long Komodo dragon (Varanus komodoensis). They occur on every continent on the planet except for Antarctica. Unlike other vertebrate groups, lizards have largely been successful in colonizing small, remote islands.

 

Fig. 5.26. (A) Geckos like this gold dust day gecko (Phelsuma laticauda) are the most diverse group of lizards.

Image courtesy of Samantha Levang, Flickr

Fig. 5.26. (B) The Australian thorny devil (Moloch horridus) has sharp spines that defend against predators.

Image courtesy of Kevin D. Hartnell, Wikimedia Commons

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Fig. 5.26. (C) The Nile monitor (Varanus niloticus) is a large swimming lizard found near rivers throughout sub-Saharan Africa.

Image courtesy of Klausf, Wikimedia Commons

Fig. 5.26. (D) The marine iguana (Amblyrhynchus cristatus), found only in the Galapagos Islands, is the world’s only species of marine lizard.

Image courtesy of David Adam Kess, Wikimedia Commons

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Crocodilians

Crocodilians (order Crocodilia) are large aquatic reptiles with thickly scaled skin, long tails, and flattened heads. Examples include crocodiles, alligators, and caimans (Fig. 5.27). All crocodilians are predatory carnivores feeding mostly on fish, molluscs, crustaceans, birds, and mammals. The saltwater crocodile (Crocodylus porosus; Fig. 5.27 B) is the largest living reptile, measuring up to seven meters in length and 1,200 kilograms in weight. There are 23 extant species of crocodilians on Earth, mostly in warm tropical climates.

 

Fig. 5.27. (A) American alligator (Alligator mississippiensis)

Image courtesy of Postdlf, Wikimedia Commons

Fig. 5.27. (B) Saltwater crocodile (Crocodylus porosus)

Image courtesy of fvanrenterghem, Flickr

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Fig. 5.27. (C) Cuvier's dwarf caiman (Paleosuchus palpebrosus)

Image courtesy of Piotr Naskrecki, Oregon State University

Fig. 5.27. (D) Gharial or gavial (Gavialis gangeticus)

Image courtesy of Bo Link, Flickr

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Extinct Reptiles

Many groups of large-bodied reptiles ruled Earth’s terrestrial and aquatic habitats during the Mesozoic era, approximately 252 to 66 million years ago. Figure 5.28 shows some examples of life during the Mesozoic era. The most familiar group of these ancient reptiles is probably the dinosaurs. Dinosaurs are a diverse group of terrestrial reptiles, mostly large in size (Fig. 5.28 B). Schoolchildren around the world know dinosaurs by their complex scientific names like Tyrannosaurus rex, Stegosaurus sp., Triceratops sp., and Velociraptor sp. Most species went extinct at the end of the Mesozoic era around 66 million years ago, but one group of dinosaurs has survived to present day: the birds.

 

Fig. 5.28. (A) Fossil leaf of Ginkgo biloba, a tree species that has survived to the present day

Image courtesy of Anders Sandberg, Flickr

Fig. 5.28. (B) Replica fossil skeleton of Tyrannosaurus rex, a dinosaur

Image courtesy of ScottRobertAnselmo, Wikimedia Commons

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Fig. 5.28. (C) Replica fossil skeletons of Geosternbergia sp., a flying pterosaur related to dinosaurs

Image courtesy of Kenn Chaplin, Flickr

Fig. 5.28. (D) Fossil skeleton of Eurhinosaurus sp., a 6.4 m long swimming ichthyosaur related to dinosaurs

Image courtesy of Kevmin, Wikimedia Commons

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Ancient reptiles also ruled Earth’s skies during the Mesozoic era. Pterosaurs evolved the ability to fly independent from other flying animals (e.g., insects, birds, and, later, bats). The Mesozoic “age of reptiles” also saw several groups of large predatory marine reptiles adapted for swimming with paddle-like appendages. Ichthyosaurs resemble large modern-day dolphins (Figs. 5.28 D and 5.29 B). Plesiosaurs had four large flippers, broad flat bodies and long necks (Fig. 5.29 C). Other examples of extinct marine reptiles include mosasaurs, nothosaurs, thalattosaurs, and placodonts (Fig. 5.29). While dinosaurs dominated Earth’s terrestrial environment in the Mesozoic era, these diverse marine reptile groups ruled the world ocean.

 

Fig. 5.29. (A) Dracorex hogwartsia dinosaur skeleton

Image courtesy of The Children's Museum of Indianapolis

Fig. 5.29. (B) Artist concept drawing of Platypterygius kiprjianovi ichthyosaurs

Image courtesy of Dmitry Bogdanov, Wikimedia Commons

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Fig. 5.29. (C) Plesiosaurs (Plesiosaurus sp.) skeleton

Image courtesy of Kim Alaniz, Wikimedia Commons

Fig. 5.29. (D) Plesioplatecarpus planifrons mosasaur skeleton

Image courtesy of MCDinosaurhunter, Wikimedia Commons

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Marine Reptiles

There are approximately 70 extant reptile species adapted to the marine environment.

 

All marine reptiles need to breathe air and have evolved adaptations to survive underwater for long periods of time. The leatherback sea turtle (Dermochelys coriacea) can hold its breath underwater for more than two hours. Marine reptiles also need to regulate their body temperature, often with behavior such as basking in the sun, either on shore or near the water surface.

 

There are seven extant species of marine turtles: the green sea turtle (Chelonia mydas), loggerhead (Caretta caretta), Kemp’s ridley (Lepidochelys kempii), olive ridley (Lepidochelys olivacea), hawksbill (Eretmochelys imbricata), flatback (Natator depressus), and leatherback (Dermochelys coriacea). The diamondback terrapin (Malaclemys terrapin) thrives in brackish waters (Fig. 5.23 D), but it is not considered marine like the seven sea turtle species listed above. Sea turtles occur in all ocean basins with the exception of the cold polar regions. The leatherback sea turtle (Fig. 5.24 D) occurs primarily in the open ocean far away from shore, but all sea turtles must return to land to lay eggs. They generally feed on jellyfish, sponges, crustaceans, seagrasses, and macroalgae.

 

There are 62 species of sea snake closely related to cobras in the subfamily Hydrophiinae (Figs. 5.25 C and D). All sea snakes are highly venomous but rarely attack humans unless provoked.

 

The marine iguana (Amblyrhynchus cristatus) is the only extant marine lizard (Fig. 5.26 D). This species feeds primarily on submerged macroalgae and is found only in the Galapagos Islands.

 

Several species of crocodilians live in brackish waters but none are considered truly marine. Both the aptly named saltwater crocodile (Crocodylus porosus; Fig. 5.25 B) and the American crocodile (Crocodylus acutus) inhabit beaches, estuaries, salt marshes, mangrove swamps, and coastal lagoons but rarely venture out to sea.

 

Structure and Function

All reptiles have respiratory systems with lungs, hard bony skeletons (Fig. 5.29 D) and thick, dry skin covered in scales (Fig. 5.30). Thick scaly skin offered reptiles an evolutionary advantage over amphibians for surviving on land in dry conditions. Reptile scales are made of a fibrous protein called keratin. Keratin is also found in horns, claws, turtle shells, bird feathers, and mammal hair.

 

 

Like amphibians, all reptiles are tetrapods. While most tetrapods have four limbs, snakes and legless lizards are examples of tetrapods without limbs. These species lost their limbs secondarily, meaning their recent evolutionary ancestors had limbs. Snakes and legless lizards evolved slender body forms as these were advantageous for survival in certain habitats and conditions.

 

Other reptiles have evolved specialized limbs. Sea turtles, as well as ancient reptiles like ichthyosaurs and plesiosaurs, evolved paddle-shaped limbs that enable them to move through water efficiently. Extinct pterosaurs (Fig. 5.28 C) and modern birds evolved wing-shaped forelimbs that allow for flight.

Adaptations

Reptiles evolved several adaptations that allowed them to become the dominant terrestrial vertebrates on Earth for almost 250 million years, until the extinction of the dinosaurs and the rise of mammals. Probably the most important of these adaptations was the evolution of the amniotic egg. Unlike the eggs of amphibians and fish, the eggs laid by reptiles were adapted for a wide variety of conditions on dry land. These amniotic eggs contain an embryo surrounded by a protective amniotic sac and shell. Figure 5.32 shows the evolution of the amniotic egg allowed reptiles to survive in a wide variety of dry terrestrial habitats, in contrast to the amphibians. The amnion, or amniotic sac, is a watertight fluid-filled tissue that nourishes the developing embryo and protects it from drying out.

 

Fig. 5.32.  (A) Diagram of an amniotic egg

Image courtesy of CNX OpenStax

Fig. 5.32. (B) Desert tortoise (Gopherus agassizii) hatchling emerging from its egg shell

Image courtesy of K. Kristina Drake, U.S. Geological Survey (USGS)

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Another important adaptation that allowed the reptiles to thrive in terrestrial environments is the thick, scaly skin that all reptiles have (Fig. 5.30). This allowed reptiles to survive not only in terrestrial environments, but in the most arid conditions.

 

Energy Acquisition

Living organisms require energy to survive. All snakes and crocodilians are predators that acquire their energy from other animals. Examples of their prey include smaller reptiles, birds, insects, fish, and small mammals. Some snakes, such as rattlesnakes, vipers, and all sea snakes, inject their prey with deadly venom when biting. Crocodilians are generally ambush predators—they hide in shallow water before surprising their prey with powerful bites.

 

Lizards and turtles display a wide diversity of diets. Most lizard species feed on small invertebrates such as insects and spiders. Iguanas, including the marina iguana, eat vegetation. Marine iguanas use their long powerful tails to dive to the seafloor where they graze on macroalgae, or seaweed. Sea turtles generally eat jellyfish, sponges, crustaceans, seagrasses, and macroalgae.

 

Growth, Development, and Reproduction

Almost all reptile species produce offspring through sexual reproduction. Male reptiles fertilize female eggs internally. Most reptiles give birth to their young through eggs laid externally. These amniotic eggs typically have a soft leathery shell or a hard protective shell. All sea turtles must return to land to dig nests in sandy beaches. Most species return to the same coastlines where they were born.

 

 

Fig. 5.33. Female crocodilians are among the few reptiles to exhibit parental care behavior.

Image courtesy of Florida Fish and Wildlife Commission (FWC) Fish and Wildlife Research Institute

Unlike amphibians, reptiles do not have larval stages of development. Recall how newly hatched frogs appear in the form of a tadpole larval stage with a long swimming tail and no limbs. Tadpoles eventually undergo metamorphosis to change into adult frogs. In contrast, a newly hatched sea turtle will simply grow and mature into adulthood without changing dramatically in form.

 

Parental care behavior is rare among reptiles, with the exception of birds and crocodilians. Female crocodilians not only guard their nests from potential egg-eating predators, they also assist their offspring (Fig. 5.33).

 

Behavior

Reptiles display a wide range of behaviors in response to other organisms and to their environment. One of the most important things reptiles achieve through their behavior is the regulation of their internal body temperatures.

 

Thermoregulation is the ability to control the body temperature inside a living organism. Maintaining an appropriate body temperature is important for metabolism, locomotion, and many cellular processes inside living organisms. Recall that all living amphibians and reptiles are ectothermic or “cold-blooded” animals. Ectothermic animals are unable to generate and maintain body heat internally. They must absorb heat from their surrounding environment. Reptiles can often be seen increasing their body temperatures by basking in the sun at dawn or resting on warm rocks at dusk. They might decrease their body temperatures by hiding underground, in water, or in shade. Some reptiles are even capable of regulating body temperatures by changing their skin coloration. Both amphibians and reptiles use behavior to regulate body temperature. In contrast, endothermic animals are capable of maintaining body temperatures through internal chemical reactions from the digestion of food. Examples of endotherms—also called “warm-blooded” organisms—include mammals and birds. All extant amphibians, lizards, turtles, snakes, and crocodilians are ectotherms.