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Crocodilia

Crocodilia    (“small lizard”) arose with a distinct lineage by the middle Triassic; extant species include alligators, crocodiles, and caimans. Crocodilians ( [link] ) 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, and spend most of their time in water. Some species are able to move on land due to their semi-erect posture.

The photo shows a crocodile sitting in the mud.
Crocodilians, such as this Siamese crocodile ( Crocodylus siamensis ), provide parental care for their offspring. (credit: Keshav Mukund Kandhadai)

Sphenodontia

Sphenodontia    (“wedge tooth”) arose in the Mesozoic era and includes only one living genus, Tuatara , comprising two species that are found in New Zealand ( [link] ). 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.

This photo shows a green lizard with short spines on its back.
This tuatara from New Zealand may resemble a lizard but belongs to a distinct lineage, the Sphenodontidae family. (credit: Sid Mosdell)

Squamata

Squamata    (“scaly”) arose in the late Permian, and extant species include lizards and snakes. Both are found on all continents except Antarctica. Lizards and snakes are most closely related to tuataras, both groups having evolved from a lepidosaurian ancestor. Squamata is the largest extant clade of reptiles ( [link] ). 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.

The photo shows a green lizard with its tail curled like a snail shell. The lizard has two horns and matches the leaves of the plant on which it sits.
This Jackson’s chameleon ( Trioceros jacksonii ) blends in with its surroundings.

Snakes are thought to have descended from either burrowing lizards or aquatic lizards over 100 million years ago ( [link] ). Snakes comprise about 3,000 species and are found on every continent except Antarctica. They range in size from 10 centimeter-long thread snakes to 10 meter-long pythons and anacondas. All snakes are carnivorous and eat small animals, birds, eggs, fish, and insects. The snake body form is so specialized that, in its general morphology, a “snake is a snake.” Their specializations all point to snakes having evolved to feed on relatively large prey (even though some current species have reversed this trend). 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.

The photo shows a snake with orange and black bands and white stripes.
The garter snake belongs to the genus Thamnophis , the most widely distributed reptile genus in North America. (credit: Steve Jurvetson)

Testudines

Turtles are members of the clade Testudines    (“having a shell”) ( [link] ). 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.

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.

The photo shows a very large tortoise.
The African spurred tortoise ( Geochelone sulcata ) lives at the southern edge of the Sahara Desert. It is the third largest tortoise in the world. (credit: Jim Bowen)

Section summary

The amniotes are distinguished from amphibians by the presence of a terrestrially adapted egg protected by amniotic membranes. The amniotes include reptiles, birds, and mammals. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids (mammals) and sauropsids. Sauropsids can be further divided into anapsids (turtles) and diapsids (birds and reptiles). Reptiles are tetrapods either having four limbs or descending from such. Limbless reptiles (snakes) are classified as tetrapods, as they are descended from four-limbed organisms. One of the key adaptations that permitted reptiles to live on land was the development of scaly skin containing the protein keratin, which prevented water loss from the skin. Reptilia includes four living clades: Crocodilia (crocodiles and alligators), Sphenodontia (tuataras), Squamata (lizards and snakes), and Testudines (turtles).

Art connections

[link] Which of the following statements about the parts of an egg are false?

  1. The allantois stores nitrogenous waste and facilitates respiration.
  2. The chorion facilitates gas exchange.
  3. The yolk provides food for the growing embryo.
  4. The amniotic cavity is filled with albumen.

[link] D

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[link] Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?

[link] The ancestor of modern Testudines may at one time have had a second opening in the skull, but over time this might have been lost.

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Questions & Answers

how are proteins digested?
Trox Reply
how are proteins digested
Kaka
guys what is locomotion?
Misheal Reply
what is anatomy
Mohamed Reply
no idea
Benard
Anatomy is the branch of science that deals with the study of internal and external structures of an organism
Bigenis
Anatomy is the branch of science that deals with the study of the internal structure of an organism
Nana
what is locomotion?
Misheal
no idea
John
Locomotion may simply mean the movement of an organism from one point to another without permanent displacement of the organidm it'self
Bigenis
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MissMeriiit
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Misheal
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Dickens
locomotion as an art student is the ability to move from on place to another
John
muscles that are concerned with locomotion
John
Anatomy deals with the study of internal structures of an organism
Osei
The ability of cells or organisms to move and propel itself from place to place. supplement. locomotion in biology pertains to the various movements of organisms (single-celled or multicellular organisms) to propel themselves from one place to another.
garnhial
Anatomy is the branch of biology concerned with structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structure organization of living things - human anatomy is one of the essential basic sciences that are applied in medicine
garnhial
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What's the function of epiglottis
Ugo Reply
What Is The Other Name For Intestinal Juice?
Justin Reply
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rick Reply
what are the types of cell
Bernard Reply
prokaryotic and eukaryotic
Yazi
prokaryotic cell and eukaryotic cell
Grace
what is the protein found in the blood?
Tobias Reply
globin
Joelia
Globin
globulins
EZRA
globulins
SASMITA
globulins
Grace
what is parasitic movement
Emmanuel Reply
Parasitic movement is a problem for all of us. So is its companion, parasitic tension. Parasitic movement is the excess contraction of muscles that you don't actually need to complete an action.
freya
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Nyandera
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Bigenis
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Ocen
absorption may simply mean utilization of food in the body
Bigenis
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Thiza Reply
eukaryotic cells which posses a true nucleus that is the DNA is enclosed and covered by a nuclear membrane
Grace
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hi
Lagos
23 haploid and 23diploid
Patson
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Angela
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Aremu Reply
why did human being need sex?
Aremu
because he/she have feelings
Chripine
reproduction...to make more
Yazi
due to active harmon
Manish
One important of sex is to reproduce
Emma
to ensure the countinuty of life
Yusuf
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Edith
for sexual satisfaction and birth
Grace
what is momentum
Asiya Reply
The strength or force that allows something to continue or grow stronger or faster as time pass
Emma
What is Centripetal Force?
Justin
centrepital force is the inward force required to keep a body moving with constant speed in a circular path
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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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