Reptiles

Characteristics of Amniotes

The distinguishing characteristic of amniotes, a shelled egg with an amniotic membrane, allowed them to venture onto land.

Learning Objectives

Discuss the evolution of amniotes

Key Takeaways

Key Points

  • The amniotes include reptiles, birds, and mammals; shared characteristics between this group include a shelled egg protected by amniotic membranes, waterproof skin, and rib ventilation of the lungs.
  • In amniotes, the shell of the egg provides protection for the developing embryo and allows water retention while still being permeable to gas exchange.
  • Amniotic eggs contain albumin, which provides the embryo with water and protein, and an egg yolk that supplies the embryo with energy.
  • The chorion, amnion, and allantois are key membranes found only in amniotic eggs.
  • The chorion facilitates gas exchange between the embryo and the egg’s external environment.
  • The amnion protects the embryo from mechanical shock and supports hydration, while the allantois stores nitrogenous wastes and facilitates respiration.

Key Terms

  • amnion: the innermost membrane of the fetal membranes of amniotes; the sac in which the embryo is suspended; protects the embryo from shock and carries out hydration
  • chorion: allows exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment
  • allantois: membrane in an egg that stores nitrogenous wastes produced by the embryo and facilitates respiration
  • monotreme: a mammal that lays eggs and has a single urogenital and digestive orifice; only the echidnas and platypuses

Characteristics of Amniotes

The amniotes, reptiles, birds, and mammals, are distinguished from amphibians by their terrestrially-adapted egg, which is protected by amniotic membranes. The evolution of amniotic membranes meant that the embryos of amniotes were now provided with their own aquatic environment, which led to less dependence on water for development, allowing the amniotes to branch out into drier environments. This was a significant development that distinguished them from amphibians, which were restricted to moist environments due their shell-less eggs. Although the shells of various amniotic species vary significantly, they all allow retention of water. The shells of bird eggs are composed of calcium carbonate and are hard, but fragile. The shells of reptile eggs are leathery and require a moist environment. Most mammals do not lay eggs (except for monotremes). Instead, the embryo grows within the mother’s body; however, even with this internal gestation, amniotic membranes are still present.

The amniotic egg is the key characteristic of amniotes. In amniotes that lay eggs, the shell of the egg provides protection for the developing embryo while being permeable enough to allow for the exchange of carbon dioxide and oxygen. The albumin, or egg white, provides the embryo with water and protein, whereas the fattier egg yolk is the energy supply for the embryo, as is the case with the eggs of many other animals, such as amphibians. However, the eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois. Extra-embryonic membranes are those present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac. The chorion facilitates exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration. In mammals, membranes that are homologous to the extra-embryonic membranes in eggs are present in the placenta. Additional derived characteristics of amniotes include waterproof skin, due to the presence of lipids, and costal (rib) ventilation of the lungs.

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Amniotic eggs: The key features of an amniotic egg are the chorion, amnion, and allantois.

Evolution of Amniotes

Modern amniotes, which includes mammals, reptiles, and birds, evolved from an amphibian ancestor approximately 340 million years ago.

Learning Objectives

Outline the evolution of amniotes

Key Takeaways

Key Points

  • Synapsids include all mammals and therapsids, mammal-like reptiles, from which mammals evolved.
  • Sauropsids, which are divided into the anapsids and diapsids, include reptiles and birds.
  • The diapsids are divided into lepidosaurs (modern lizards, snakes, and tuataras) and archosaurs (modern crocodiles and alligators, pterosaurs, and dinosaurs).
  • Skull structure and number of temporal fenestrae are the key differences between the synapsids, anapsids, and diapsids; anapsids have no temporal fenestrae, synapsids have one, and diapsids have two.
  • Turtle classification is still unclear, but based on molecular evidence, they are sometimes classified under diapsids.
  • Although birds are considered distinct from reptiles, they evolved from a group of dinosaurs, so considering them separately from reptiles is not phylogenetically accurate.

Key Terms

  • synapsid: animals that have one opening low in the skull roof behind each eye; includes all living and extinct mammals and therapsids
  • anapsid: amniote whose skull does not have openings near the temples; includes extinct organisms
  • diapsid: any of very many reptiles and birds that have a pair of openings in the skull behind each eye
  • temporal fenestrae: post-orbital openings in the skull of some amniotes that allow muscles to expand and lengthen

Evolution of Amniotes

The first amniotes evolved from their amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye. Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles (Testudines), which have an anapsid-like skull with one opening. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.

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Tempral fenestrae: The image illustrates the differences in the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.

The diapsids diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period. The lepidosaurs include modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct pterosaurs (“winged lizard”) and dinosaurs (“terrible lizard”). Clade Dinosauria includes birds, which evolved from a branch of dinosaurs.

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Evolution of amniotes: This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.

In the past, the most common division of amniotes has been into the classes Mammalia, Reptilia, and Aves. Birds are descended, however, from dinosaurs, so this classical scheme results in groups that are not true clades. Birds are considered as a group distinct from reptiles with the understanding that this does not completely reflect phylogenetic history and relationships.

Characteristics of Reptiles

Reptiles are ectothermic tetrapods that lay shelled eggs on land and possess scaly skin and lungs.

Learning Objectives

Summarize the key adaptations of reptiles

Key Takeaways

Key Points

  • All reptiles, including aquatic ones, lay their eggs on land.
  • Reptiles reproduce sexually through internal fertilization; some species are ovoviviparous (lay eggs) and others are viviparous (live birth).
  • Because of the development of impermeable, scaly skin, reptiles were able to move onto land since their skin could not be used for respiration in water.
  • Reptiles are ectotherms: they depend on their surrounding environment to control their body temperature; this leads to advantages, such as not being dependent on metabolic energy from food for body heat.
  • Reptiles are also poikilotherms: animals whose body temperatures vary rather than remain stable.
  • Some reptiles go into brumation: a long period during cold weather that consists of no eating and a decreased metabolism.

Key Terms

  • viviparous: being born alive, as are most mammals, some reptiles, and a few fish (as opposed to being laid as an egg)
  • ovoviviparous: a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother’s body until they are ready to hatch
  • ectotherm: a cold-blooded animal that regulates its body temperature by exchanging heat with its surroundings

Characteristics of Reptiles

Reptiles are tetrapods. Limbless reptiles (snakes and other squamates) have vestigial limbs and, as with caecilians, are classified as tetrapods because they are descended from four-limbed ancestors. Reptiles lay on land eggs enclosed in shells. Even aquatic reptiles return to the land to lay eggs. They usually reproduce sexually with internal fertilization. Some species are ovoviviparous, with the eggs remaining in the mother’s body until they are ready to hatch. Other species are viviparous, with the offspring born alive.

One of the key adaptations that permitted reptiles to live on land was the development of their scaly skin which contains the protein keratin and waxy lipids, reducing water loss from the skin. Due to this occlusive skin, reptiles cannot use their skin for respiration, as do amphibians; all breathe with lungs.

Reptiles are ectotherms: animals whose main source of body heat comes from the environment. This is in contrast to endotherms, which use heat produced by metabolism to regulate body temperature. In addition to being ectothermic, reptiles are categorized as poikilotherms: animals whose body temperatures vary rather than remain stable. Reptiles have behavioral adaptations to help regulate body temperature, such as basking in sunny places to warm up and finding shady spots or going underground to cool down. The advantage of ectothermy is that metabolic energy from food is not required to heat the body; therefore, reptiles can survive on about 10 percent of the calories required by a similarly-sized endotherm. In cold weather, some reptiles, such as the garter snake, brumate. Brumation is similar to hibernation in that the animal becomes less active and can go for long periods without eating, but differs from hibernation in that brumating reptiles are not asleep or living off fat reserves. Rather, their metabolism is slowed in response to cold temperatures; the animal becomes very sluggish.

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Ectotherms: Reptiles, such as these sunbathing Florida redbelly turtles, are ectotherms: they rely on their environment for body heat.

Evolution of Reptiles

Dinosaurs and pterosaurs diverged from early amniotes and dominated the Mesozoic Era.

Learning Objectives

Outline the evolution of reptiles

Key Takeaways

Key Points

  • Diapsids diverged into archosaurs and lepidosaurs, but these groups did not dominate the ecosystem until the Triassic following the Permian extinction.
  • Archosaurs diverged into the dinosaurs and the pterosaurs about 250 million years ago.
  • Pterosaurs had the ability to fly because of their wings and hollow bones, a trait convergent to modern birds, but were not ancestral to birds.
  • Dinosaurs were quadrupeds or bipeds, carnivorous or herbivorous, and laid eggs.
  • It is unknown whether dinosaurs were endothermic or ectothermic, but since birds are endothermic, the dinosaur ancestors of birds were probably endothermic.
  • Dinosaurs dominated the Mesozoic Era until the Cretaceous -Tertiary extinction wiped out most of these large-bodied animals.

Key Terms

  • pterosaur: any of several extinct flying reptiles, of the order Pterosauria, including the pterodactyls
  • Cretaceous-Tertiary extinction: mass extinction of three-quarters of plant and animal species on earth, including all non-avian dinosaurs, that occurred over a geologically-short period of time 66 million years ago

Evolution of Reptiles

Reptiles originated approximately 300 million years ago during the Carboniferous period. One of the oldest-known amniotes is Casineria, which had both amphibian and reptilian characteristics. One of the earliest undisputed reptiles was Hylonomus. Soon after the first amniotes appeared, they diverged into three groups (synapsids, anapsids, and diapsids) during the Permian period. The Permian period also saw a second major divergence of diapsid reptiles into archosaurs (predecessors of crocodilians and dinosaurs) and lepidosaurs (predecessors of snakes and lizards). These groups remained inconspicuous until the Triassic period when the archosaurs became the dominant terrestrial group due to the extinction of large-bodied anapsids and synapsids during the Permian-Triassic extinction. About 250 million years ago, archosaurs radiated into the dinosaurs and the pterosaurs.

Although they are sometimes mistakenly called dinosaurs, the pterosaurs were distinct from true dinosaurs. Pterosaurs had a number of adaptations that allowed for flight, including hollow bones (birds also exhibit hollow bones, a case of convergent evolution). Their wings were formed by membranes of skin that attached to the long, fourth finger of each arm and extended along the body to the legs.

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Pterosaurs: Pterosaurs, which existed from the late Triassic to the Cretaceous period (210 to 65.5 million years ago), possessed wings, but are not believed to have been capable of powered flight. Instead, they may have been able to soar after launching from cliffs.

The dinosaurs were a diverse group of terrestrial reptiles with more than 1,000 species identified to date. Paleontologists continue to discover new species of dinosaurs. Some dinosaurs were quadrupeds; others were bipeds. Some were carnivorous, whereas others were herbivorous. Dinosaurs laid eggs; a number of nests containing fossilized eggs have been found. It is not known whether dinosaurs were endotherms or ectotherms. However, given that modern birds are endothermic, the dinosaurs that served as ancestors to birds were probably endothermic as well. Some fossil evidence exists for dinosaurian parental care. Comparative biology supports this hypothesis since the archosaur birds and crocodilians display parental care.

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Quadruped dinosaurs: Edmontonia, an example of an extinct quadruped reptile, was an armored dinosaur that lived in the late Cretaceous period, 145.5 to 65.6 million years ago.

Dinosaurs dominated the Mesozoic Era, which was known as the “Age of Reptiles.” The dominance of dinosaurs lasted until the end of the Cretaceous period, the end of the Mesozoic Era. The Cretaceous-Tertiary extinction resulted in the loss of most of the large-bodied animals of the Mesozoic Era. Birds are the only living descendants of one of the major clades of dinosaurs.

Modern Reptiles

Class Reptilia, amniotes that are neither mammals nor birds, has four living clades: Crocodilia, Sphenodontia, Squamata, and Testudine.

Learning Objectives

Differentiate among the types of modern reptiles

Key Takeaways

Key Points

  • Reptile are amniotes that lay their eggs on land; they have scales or scutes and are ectothermic.
  • Crocodilia includes the alligators, crocodiles, and caimans; they are mostly aquatic species, but some are able to move on land because of their semi-erect posture.
  • Tuataras are classified as the only group under Sphenodontia; they may be lizard-like, but skull and jaw differences set them apart from true lizards.
  • Squamata, the largest group of reptiles, includes the lizards and snakes.
  • Snakes, which lack the eyelids and external ears present in lizards, are believed to have evolved from burrowing or aquatic lizards.
  • Turtles are grouped under the Testudines; species in this group all have bony or cartilaginous shells.

Key Terms

  • scute: a horny, chitinous, or bony external plate or scale, as on the shell of a turtle or the skin of crocodiles
  • plastron: the nearly flat part of the shell structure of a tortoise or other animal, similar in composition to the carapace
  • amniote: a group of vertebrates having an amnion during the development of the embryo; mammals, birds, and reptiles

Modern Reptiles

Class Reptilia includes many diverse species that are classified into four living clades. These are the 25 species of Crocodilia, 2 species of Sphenodontia, approximately 9,200 Squamata species, and the Testudines, with about 325 species. A reptile is any amniote (a tetrapod whose egg has an additional membrane, originally to allow them to lay eggs on land) that is neither a mammal nor a bird. Unlike mammals, birds, and certain extinct reptiles, living reptiles have scales or scutes (rather than fur or feathers) and are cold-blooded. Modern reptiles inhabit every continent with the exception of Antarctica.

Crocodilia

Crocodilia (“small lizard”) arose with a distinct lineage by the middle Triassic; extant species include alligators, crocodiles, and caimans. Crocodilians are large, solidly built lizard-like reptiles with long flattened snouts and laterally-compressed tails, and eyes, ears, and nostrils at the top of the head. Their skin is thick and covered in non-overlapping scales. They have conical, peg-like teeth and a powerful bite. As with birds, they have a four-chambered heart and a unidirectional system of air flow around the lungs; however, in contrast to birds, they are ectotherms, as are all other reptiles. Crocodilians live throughout the tropics and subtropics of Africa, South America, Southern Florida, Asia, and Australia. They are found in freshwater, saltwater, and brackish habitats, such as rivers and lakes; they spend most of their time in water. Some species are able to move on land due to their semi-erect posture.

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Crocodilians: Crocodilians, such as this Siamese crocodile (Crocodylus siamensis), have large flattened snouts and thick skin.

Sphenodontia

Sphenodontia (“wedge tooth”) arose in the Mesozoic era and includes only one living genus, Tuatara, which comprises two species that are found in New Zealand. Tuataras measure up to 80 centimeters and weigh about 1 kilogram. Although quite lizard-like in gross appearance, several unique features of the skull and jaws clearly define them and distinguish the group from the squamates. Their dentition, in which two rows of teeth in the upper jaw overlap one row on the lower jaw, is unique among living species. They are also unusual in having a pronounced photoreceptive eye, dubbed the “third eye”, whose current function is a subject of ongoing research, but is thought to be involved in setting circadian and seasonal cycles.

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Tuatara: Tuataras may resemble a lizard but belongs to a distinct lineage, the Sphenodontidae family.

Squamata

Squamata (“scaly”) arose in the late Permian; extant species include lizards and snakes. They are most closely-related to tuataras; both groups evolved from a lepidosaurian ancestor. Squamata is the largest extant clade of reptiles. Most lizards differ from snakes by having four limbs, although these have been variously lost or significantly reduced in at least 60 lineages. Snakes lack eyelids and external ears, which are present in lizards. Lizard species range in size from chameleons and geckos, which are a few centimeters in length, to the Komodo dragon, which is about 3 meters in length. Most lizards are carnivorous, but some large species, such as iguanas, are herbivores.

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Squamata: The Squamata, which includes lizards and snakes, are the largest group of reptiles. The garter snake belongs to the genus Thamnophis, the most widely-distributed reptile genus in North America.

Snakes are thought to have descended from either burrowing lizards or aquatic lizards over 100 million years ago. Snakes comprise about 3,000 species and range in size from 10 centimeter-long thread snakes to 10 meter-long pythons and anacondas. All snakes are carnivorous, eating small animals, birds, eggs, fish, and insects. Although variations exist, most snakes have a skull that is very flexible, involving eight rotational joints. They also differ from other squamates by having mandibles (lower jaws) without either bony or ligamentous attachment anteriorly. Having this connection via skin and muscle allows for great expansion of the gape and independent motion of the two sides; both advantages in swallowing big items.

Testudines

Turtles are members of the clade Testudines (“having a shell”). Turtles are characterized by a bony or cartilaginous shell. The shell consists of the ventral surface called the plastron and the dorsal surface called the carapace, which develops from the ribs. The plastron is made of scutes or plates; the scutes can be used to differentiate species of turtles. The two clades of turtles are most easily recognized by how they retract their necks. The dominant group, which includes all North American species, retracts its neck in a vertical S-curve. Turtles in the less speciose clade retract the neck with a horizontal curve.

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Testudines: Testudines include all shelled reptiles, such as the African spurred tortoise (Geochelone sulcata) that lives at the southern edge of the Sahara Desert. It is the third-largest tortoise in the world.

Turtles arose approximately 200 million years ago, predating crocodiles, lizards, and snakes. Similar to other reptiles, turtles are ectotherms. They lay eggs on land, although many species live in or near water. None exhibit parental care. Turtles range in size from the speckled padloper tortoise at 8 centimeters (3.1 inches) to the leatherback sea turtle at 200 centimeters (over 6 feet). The term “turtle” is sometimes used to describe only those species of Testudines that live in the sea, with the terms “tortoise” and “terrapin” used to refer to species that live on land and in fresh water, respectively.