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Crocodilia

Crocodilia

Overview
Crocodilia is an order of large reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s that appeared about 84 million years ago in the late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period (Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

 stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

ia. Members of the crocodilian total group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

, the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Crurotarsi
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...

, appeared about 220 million years ago in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 Period and exhibited a wide diversity of forms during the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 Era.

Although the term 'crocodiles' is sometimes used to refer to alligators, caiman, and gharials
Gavialidae
Gavialidae is a family of reptiles within the order Crocodilia. Gavialidae consists of only one surviving species, the gharial , which is native to India. Many extinct species are also known...

, or even to their distant prehistoric relatives the marine crocodiles
Thalattosuchia
Thalattosuchia is the name given to a clade of marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution. They are sometimes colloquially referred to as marine crocodiles or sea crocodiles, though they are not actually members of Crocodilia.The term...

, a less ambiguous vernacular term for this group is 'crocodilians'.

The group is often spelled 'Crocodylia' for consistency with the genus Crocodylus
Crocodylus
Crocodylus is one of three genera from the Crocodylinae subfamily extending from the Crocodylidae family.Established species include three extinct species:* † Crocodylus acer - probably represents a distinct genus....

(Laurenti
Josephus Nicolaus Laurenti
Josephus Nicolaus Laurenti was an Austrian naturalist of Italian origin.Laurenti is considered the auctor of the class Reptilia through his authorship of on the poisonous function of reptiles and amphibians...

, 1768).
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Encyclopedia
Crocodilia is an order of large reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s that appeared about 84 million years ago in the late Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 Period (Campanian
Campanian
The Campanian is, in the ICS' geologic timescale, the fifth of six ages of the Late Cretaceous epoch . The Campanian spans the time from 83.5 ± 0.7 Ma to 70.6 ± 0.6 Ma ...

 stage). They are the closest living relatives of birds, as the two groups are the only known survivors of the Archosaur
Archosaur
Archosaurs are a group of diapsid amniotes whose living representatives consist of modern birds and crocodilians. This group also includes all extinct non-avian dinosaurs, many extinct crocodilian relatives, and pterosaurs. Archosauria, the archosaur clade, is a crown group that includes the most...

ia. Members of the crocodilian total group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

, the clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 Crurotarsi
Crurotarsi
The Crurotarsi are a group of archosauriformes, represented today by the crocodiles,...

, appeared about 220 million years ago in the Triassic
Triassic
The Triassic is a geologic period and system that extends from about 250 to 200 Mya . As the first period of the Mesozoic Era, the Triassic follows the Permian and is followed by the Jurassic. Both the start and end of the Triassic are marked by major extinction events...

 Period and exhibited a wide diversity of forms during the Mesozoic
Mesozoic
The Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...

 Era.

Although the term 'crocodiles' is sometimes used to refer to alligators, caiman, and gharials
Gavialidae
Gavialidae is a family of reptiles within the order Crocodilia. Gavialidae consists of only one surviving species, the gharial , which is native to India. Many extinct species are also known...

, or even to their distant prehistoric relatives the marine crocodiles
Thalattosuchia
Thalattosuchia is the name given to a clade of marine crocodylomorphs from the Early Jurassic to the Early Cretaceous that had a cosmopolitan distribution. They are sometimes colloquially referred to as marine crocodiles or sea crocodiles, though they are not actually members of Crocodilia.The term...

, a less ambiguous vernacular term for this group is 'crocodilians'.

Spelling


The group is often spelled 'Crocodylia' for consistency with the genus Crocodylus
Crocodylus
Crocodylus is one of three genera from the Crocodylinae subfamily extending from the Crocodylidae family.Established species include three extinct species:* † Crocodylus acer - probably represents a distinct genus....

(Laurenti
Josephus Nicolaus Laurenti
Josephus Nicolaus Laurenti was an Austrian naturalist of Italian origin.Laurenti is considered the auctor of the class Reptilia through his authorship of on the poisonous function of reptiles and amphibians...

, 1768). However, Richard Owen
Richard Owen
Sir Richard Owen, FRS KCB was an English biologist, comparative anatomist and palaeontologist.Owen is probably best remembered today for coining the word Dinosauria and for his outspoken opposition to Charles Darwin's theory of evolution by natural selection...

 used the -i- spelling when he published the name in 1842, so it is generally preferred in the scientific literature. The -i- spelling is also a more accurate Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

ization of the Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...

 κροκόδειλος (crocodeilos, literally "pebble-worm", referring to the texture and shape of the animal). However, the y-spelling is increasingly common, especially in the context of phylogeny-based names in which Crocodylia is limited to the crown group.

Description


The smallest species of crocodilian is the Cuvier's Dwarf Caiman
Cuvier's Dwarf Caiman
The Cuvier's dwarf caiman or Musky caiman, Paleosuchus palpebrosus, is a relatively small crocodilian reptile from northern and central South America. It is found in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Surinam and Venezuela...

 and the largest is the Saltwater Crocodile
Saltwater Crocodile
The saltwater crocodile, also known as estuarine or Indo-Pacific crocodile, is the largest of all living reptiles...

. The basic crocodilian body plan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...

 is a very successful one; modern species closely resemble their Cretaceous
Cretaceous
The Cretaceous , derived from the Latin "creta" , usually abbreviated K for its German translation Kreide , is a geologic period and system from circa to million years ago. In the geologic timescale, the Cretaceous follows the Jurassic period and is followed by the Paleogene period of the...

 ancestors of 84 million years ago. Mammals, too, have adapted to this body plan at least once in history. One ancestral whale family, the Ambulocetidae
Ambulocetidae
Ambulocetidae is a family of early cetaceans from Pakistan that still were able to walk on land. The genus Ambulocetus, after which the family is named, is by far the most complete and well-known ambulocetid genus due to the discovery by Thewissen et al. of a partially complete specimen of...

, were aquatic predators living in rivers and lakes, and they filled an ecological niche similar to the crocodilians. Much earlier, before the age of reptiles, Amphibians like Prionosuchus
Prionosuchus
Prionosuchus is a genus of extremely large temnospondyl amphibians from the Late Permian which was found in an area of what is now Brazil.-Description:...

 and Platyoposaurus
Platyoposaurus
Platyposaurus was a Temnospondyli amphibian from the late Permain period about 255-250 Mya .One of the discoveries includes a skull 28 cm long with a purported body length of 250 cm representing a carnivorous adult specimen....

 were the first animals to adopt this body plan and niche.

Locomotion


Crocodilians have a flexible semi-erect (semi-sprawled) posture. They can walk in low, sprawled "belly walk", or hold their legs more directly underneath them to perform the "high walk". Most other reptiles can only walk in a sprawled position, and chameleon
Chameleon
Chameleons are a distinctive and highly specialized clade of lizards. They are distinguished by their parrot-like zygodactylous feet, their separately mobile and stereoscopic eyes, their very long, highly modified, and rapidly extrudable tongues, their swaying gait, the possession by many of a...

s are the only modern reptiles with a more erect posture than crocodilians. The semi-erect posture makes it possible for some species to gallop on land if necessary. An Australian species can reach a speed of over 16 km/h while galloping on an irregular forest floor. Crocodilian ancestors, fast-moving terrestrial predators like the rauisuchia
Rauisuchia
Rauisuchia is a group of predatory and mostly large Triassic archosaurs. As a clade, Rauisuchia includes these Triassic forms and all crocodylomorphs, which are descendants of Triassic rauisuchians. The group in its traditional sense is paraphyletic, because it does not include crocodylomorph...

ns, actually had a fully erect posture, indicating that the sprawling and semi-erect posture of crocodilians evolved after they adapted to life as semi-aquatic ambush predators. Their ankle bones, or tarsi
Tarsus (skeleton)
In tetrapods, the tarsus is a cluster of articulating bones in each foot situated between the lower end of tibia and fibula of the lower leg and the metatarsus. In the foot the tarsus articulates with the bones of the metatarsus, which in turn articulate with the bones of the individual toes...

 are highly modified. Modern crocodilian locomotion is not a primitive trait, but a specialization for their semi-aquatic lifestyle.

Teeth and jaws


All crocodilians have thecodont dentition (teeth set in bony sockets) but unlike mammals, they replace their teeth throughout life (though not in 'extreme' old-age). Juvenile crocodilians replace teeth with larger ones at a rate as high as 1 new tooth per socket every month. After reaching adult size in a few years, however, tooth replacement rates can slow to two years and even longer. Very old members of some species have been seen in an almost "edentulous" (toothless) state, after teeth have been broken and replacement slowed or ceased. The result of this is that a single crocodile can go through at least 3,000 teeth in its lifetime. Each tooth is hollow, and the new one is growing inside the old. In this way, a new tooth is ready once the old is lost.

Crocodilians have a secondary bony palate that enables them to breathe when partially submerged, even if the mouth is full of water. Their internal nostrils open in the back of their throat, where a special part of the tongue called the "palatal valve" closes off their respiratory system when they are underwater. This way they can open their mouths underwater without choking. Most reptiles lack a secondary palate
Secondary palate
The secondary palate is an anatomical structure that divides the nasal cavity from the oral cavity in many vertebrates.In human embryology, it refers to that portion of the hard palate that is formed by the growth of the two palatine shelves medially and their mutual fusion in the midline...

, but some skink
Skink
Skinks are lizards belonging to the family Scincidae. Together with several other lizard families, including Lacertidae , they comprise the superfamily or infraorder Scincomorpha...

s (family Scincidae) have evolved a bony secondary palate too, to varying degrees.

Crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s and gharial
Gharial
The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...

s have modified salivary glands on their tongue (salt gland
Salt gland
The salt gland is an organ for excreting excess salts. It is found in elasmobranchs, seabirds, and some reptiles. In sharks, salt glands are found in the rectum, but in birds and reptiles, they are found in or on the skull, in the area of the eyes, nostrils or mouth. In crocodiles, the salt is...

s), which are used for excreting excess salt ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...

s from their body. Alligator
Alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

s and caimans have them too, but here they are non-functioning. This indicates that at some point the common origin of the Crocodylia were adapted to saline/marine environments. This also explains their wide distribution across the continents (i.e. marine dispersal). Species like the saltwater crocodile
Saltwater Crocodile
The saltwater crocodile, also known as estuarine or Indo-Pacific crocodile, is the largest of all living reptiles...

 (C. porosus) can survive protracted periods of time in the sea, and can hunt prey within this environment.

Crocodilians are often seen lying with their mouths open, a behavior called gaping. One of its functions is probably to cool them down, but since they also do this at night and when it is raining, it is possible that gaping has a social function as well.

Internal organs


Crocodilians lack a vomeronasal organ
Vomeronasal organ
The vomeronasal organ , or Jacobson's organ, is an auxiliary olfactory sense organ that is found in many animals. It was discovered by Frederik Ruysch and later by Ludwig Jacobson in 1813....

 (except in the embryonic stage) and a urinary bladder.
Like mammals and birds and unlike other reptiles, crocodiles have a four-chambered heart.
While the four-chambered heart is traditionally characteristic of endotherms, it is thought that the ectothermic crocodilia have a four-chambered heart because of an endothermic ancestry, originating in the archosaurs or in an earlier predecessor.

When crocodilian ancestors transitioned back to aquatic ectothermy, it was advantageous for them to have a heart more akin to the normally three- or five-chambered heart found in most ectotherms. In order for their four-chambered heart to function more like the ectothermic heart, they adopted a mechanism for shunting blood in an alternative pathway through the heart. The right ventricle
Right ventricle
The right ventricle is one of four chambers in the human heart. It receives deoxygenated blood from the right atrium via the tricuspid valve, and pumps it into the pulmonary artery via the pulmonary valve and pulmonary trunk....

 has two arteries
Artery
Arteries are blood vessels that carry blood away from the heart. This blood is normally oxygenated, exceptions made for the pulmonary and umbilical arteries....

 leaving it; a pulmonary artery, which goes to the lungs, and the left aortic arch, which goes to the body, or systemic circulation. There is also a hole, the foramen of Panizza
Foramen of Panizza
The Foramen of Panizza is a hole with that connects the left and right aorta as they leave the heart of all animals of the order Crocodilia.Crocodilians have a completely separated ventricle with deoxygenated blood from the body, or systemic circulation, in the right ventricle and oxygenated blood...

, between the left and right aortic arches. Because the left aortic arch goes directly to the gut, the shunting of oxygen depleted blood which is high in CO2 may serve to aid in creating stomach acid to assist in digesting bones from its prey. Their blood has been shown to have strong antibacterial properties.

Crocodilians have lungs with alveoli
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...

. They have a unique muscle called the diaphragmaticus that attaches to the liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 and viscera
Viscus
In anatomy, a viscus is an internal organ, and viscera is the plural form. The viscera, when removed from a butchered animal, are known collectively as offal...

 and acts as a piston to assist in breathing. The diaphragmaticus is not homologous
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...

 to the diaphragm
Thoracic diaphragm
In the anatomy of mammals, the thoracic diaphragm, or simply the diaphragm , is a sheet of internal skeletal muscle that extends across the bottom of the rib cage. The diaphragm separates the thoracic cavity from the abdominal cavity and performs an important function in respiration...

 of mammals and the proto-diaphragm of tegu lizards. Like other amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...

s, crocodilian breathing uses muscles between the ribs to both increase and decrease thoracic volume. In addition, expiration
Exhalation
Exhalation is the movement of air out of the bronchial tubes, through the airways, to the external environment during breathing....

 is accomplished by contracting muscles to move the liver towards the head to rotate the pubic bones to decrease abdominal volume. Inspiration
Inhalation
Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

 involves contraction of the diaphragmaticus muscle to push organs to the back of the body and other muscles to make space for these organs. In crocodilians, expiration is mostly passive (involves little muscle contraction) during rest while inspiration always involves muscle contraction. Because many of these ventilatory muscles are used for maneuverability in water, and because the muscles were originally used for locomotion, it is possible that these muscles became ventilatory muscles after they evolved to move air around in the lungs for maneuverability.

Crocodylians are known to swallow stones, gastroliths ("stomach-stones"), which act as a ballast in addition to aiding post-digestion processing of their prey. The crocodiylian stomach is divided into two chambers; the first one is described as being powerful and muscular, like a bird gizzard. This is where the gastroliths are found. The other stomach has the most acidic digestive system of any animal, and it can digest mostly everything from their prey; bones, feathers, and horns.

Crocodile sex determination is temperature-dependent
Temperature-dependent sex determination
Temperature-dependent sex determination is type of environmental sex determination in which the temperatures experienced during embryonic development determine the sex of the offspring. It is most prevalent and common among amniote vertebrates that are classified under the reptile class, but is...

; i.e., the sex of developing crocodilians is determined by the incubation temperature of the eggs. This means crocodilians do not have genetic
Genetics
Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

 sex determination
Sex-determination system
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual...

, but instead have a form of environmental sex determination which is based upon the temperature embryos are subjected to early in their development.

Sensory organs



Like all non-avian reptiles, crocodilians have a relatively small brain, but it is more advanced than in other reptiles. Among other things they have true cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...

es.

As in many other aquatic or amphibious tetrapods, the eyes, ears, and nostrils are all located on the same plane. They see well during the day and may even have color vision, plus the eyes have a vertical, cat-like pupil which also gives them excellent night vision. The iris is silvery and a light reflecting layer of tapetum behind the retina greatly increases their ability to see in weak light and also makes their eyes appear to glow in the dark when a light is pointed at them. A third transparent eyelid, the nictitating membrane
Nictitating membrane
The nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten it while maintaining visibility. Some reptiles, birds, and sharks have a full nictitating membrane; in many mammals, there is a small...

, protects their eyes underwater. However, they cannot focus under water, meaning other senses are more important when submerged.

While birds and most reptiles have a ring of bones around each eye which supports the eyeball (the sclerotic ring
Sclerotic ring
Sclerotic rings are rings of bone found in the eyes of several groups of vertebrate animals, except for mammals and crocodilians. They can be made up of single bones or small bones together. They are believed to have a role in supporting the eye, especially in animals whose eyes are not spherical,...

), the crocodiles lack these bones, just like mammals and snakes. The eardrums are located behind the eyes and are covered by a movable flap of skin. This flap closes, along with the nostrils and eyes, when they dive, preventing water from entering their external head openings. The middle ear cavity has a complex of bony air-filled passages and a branching eustachian tube
Eustachian tube
The Eustachian tube is a tube that links the nasopharynx to the middle ear. It is a part of the middle ear. In adult humans the Eustachian tube is approximately 35 mm long. It is named after the sixteenth-century anatomist Bartolomeo Eustachi...

. There is also a small muscle (which is also seen in gecko
Gecko
Geckos are lizards belonging to the infraorder Gekkota, found in warm climates throughout the world. They range from 1.6 cm to 60 cm....

s) next to or upon the stapes, the stapedius, which probably functions in the same way as the mammalian stapedius muscle does, damping strong vibrations.

The upper and lower jaws are covered with sensory pits, visible as small, black speckles on the skin, the crocodilian version of the lateral organ seen in fish and many amphibians, though arising from a completely different origin. These pigmented nodules encase bundles of nerve fibers that respond to the slightest disturbance in surface water, detecting vibrations and small pressure changes in water, making it possible for them to detect prey, danger, and intruders even in total darkness. These sense organs are known as DPRs (Dermal Pressure Receptors). While alligator
Alligator
An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

s and caimans only have them on their jaws, crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

s have similar organs on almost every scale on their body. The function of the DPRs on the jaws are clear, but it is still not quite clear what the organs on the rest of the body in crocodiles actually do. They are probably doing the same as the organs on their jaws, but it seems as if they can do more than that, like assisting in chemical reception or even salinity detection.

Skin and skeleton


The skin is covered with scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...

 composed of the protein keratin
Keratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...

 (the same protein that forms hooves, horns, feathers, hair, claws, and nails in other tetrapods), which are shed individually. On the head the skin is actually fused to the bones of the skull. There are small plates of bone, called osteoderm
Osteoderm
Osteoderms are bony deposits forming scales, plates or other structures in the dermal layers of the skin. Osteoderms are found in many groups of extant and extinct reptiles, including lizards, various groups of dinosaurs , crocodilians, phytosaurs, aetosaurs, placodonts, and hupehsuchians...

s or scute
Scute
A scute or scutum is a bony external plate or scale, as on the shell of a turtle, the skin of crocodilians, the feet of some birds or the anterior portion of the mesonotum in insects.-Properties:...

s, under the scales. Like the scales comprising the shell of a turtle, or the cross-section of a tree trunk, crocodile osteoderms have annual growth rings, and by counting them it is possible to tell their age. Osteoderms are found especially on the back, and in some species also on the belly. The rows of scutes cover the crocodile's body from head to tail, forming a tough protective armor. Beneath the scales and osteoderms is another layer of armor, both strong and flexible and built of rows of bony overlapping shingles called osteoscutes, which are embedded in the animal's back tissue. The blood-rich bumpy scales seen on their backs act as solar panels.

Their spool-shaped vertebrae in their ancestors went from being biconcave to having a concave front and a convex back in the modern forms. This made the vertebral column more flexible and strong.

They possess ribs of dermal origin restricted to the sides of the ventral body wall. The collar bone (clavicle) is absent.

Differences between alligators and crocodiles


While alligators (and caimans) are often confused with crocodiles, they belong to two quite separate taxonomic families.

The most obvious external differences are visible in the head—alligators and caimans have wider and shorter heads, and a more U-shaped than V-shaped snout. The alligator's upper jaw is wider than its lower jaw, and the teeth in the lower jaw fit into small depressions in the upper jaw. The upper and lower jaws of the crocodiles are the same width, and teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed. When the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. For hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define a species. However, in captivity, alligators and caimans may show jaw deformities which result in lower teeth protruding.

Alligators lack the jagged fringe which appears on the hind legs and feet of the crocodile and have the toes of the hind feet webbed, not more than halfway to the tips. Alligators strongly prefer freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...

, while crocodiles can better tolerate seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 due to specialized glands for filtering out salt. However, both can survive in either.

Both species of alligator also tend to be darker in color than crocodiles—often nearly black—but color is very dependent on water quality. Algae-laden waters produce greener skin, while tannic acid
Tannic acid
Tannic acid is a specific commercial form of tannin, a type of polyphenol. Its weak acidity is due to the numerous phenol groups in the structure...

 from overhanging trees can produce often darker skin.

When cleaning alligator pools, some zookeepers can tread on alligators without eliciting a response, though crocodiles almost invariably react aggressively and are for the most part more aggressive in their natural habitat.

Evolution and classification


Eusuchia
Eusuchia
Eusuchia is a clade of crocodylomorphs that first appears in the Early Cretaceous with Hylaeochampsa. All living crocodilian species are eusuchians, as are many extinct forms.-Description:...

, a modern clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 which includes the crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...

 Crocodilia, first appeared in the Lower Cretaceous of Europe. Isisfordia
Isisfordia
Isisfordia is an extinct genus of crocodyliform closely related to crocodilians that lived during the Middle Cretaceous . Its fossils were discovered in the Winton Formation in Isisford, Queensland, Australia in the mid 1990s...

 duncani
lived approximately 95 to 98 million years ago, during the Cenomanian
Cenomanian
The Cenomanian is, in the ICS' geological timescale the oldest or earliest age of the Late Cretaceous epoch or the lowest stage of the Upper Cretaceous series. An age is a unit of geochronology: it is a unit of time; the stage is a unit in the stratigraphic column deposited during the corresponding...

 epoch of the Upper Cretaceous. Isisfordia is the second oldest known eusuchian, and the earliest crocodylomorph
Crocodylomorpha
The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...

 yet found in Australia. Eusuchians underwent a mass radiation during the Late Cretaceous and the Paleogene, in which they evolved into numerous forms, such as semi-aquatic dinosaur-eating species (Deinosuchus
Deinosuchus
Deinosuchus is an extinct genus related to the alligator that lived 73 to 80 Ma , during the late Cretaceous period. The name translates as "terrible crocodile" and is derived from the Greek deinos , "terrible", and soukhos , "crocodile"...

); hooved, terrestrial carnivores (Pristichampsus
Pristichampsus
Pristichampsus is an extinct genus of pristichampsid crocodylian that grew to approximately three metres in length....

), and 'hatchet'-shaped skulled forms (Baru
Baru
Baru was an extinct genus of Australian mekosuchine crocodilian. It was semi-aquatic, around 4 m in length. Being semi-aquatic its habitat was around fresh pools of water in wet forests, ambushing their prey, much like modern species...

).

Crocodilian Snouts


Although crocodilians were thought of as living fossils for many years, review of their fossil record in the last decade has shown that their snout morphology has been continuously changing throughout evolutionary history. Furthermore, snout and skull morphology seem to be functional adaptations to the selection pressures that crocodilians face from the diet in their environment. For instance, long-snouted crocodilians, such as Gavialis gangeticus, that live in habitats where small, rapid, agile fish are the main food source need to be able to move, open and close their snouts rapidly through the water. Thus, long snouts have evolved in such habitats in order to benefit from hydrodynamic efficiency since a long out-lever allows crocodilians to quickly open and close their jaws while in water. On the other hand, the need for a strong mandible is crucial for short-snouted crocodilians, such as Alligator mississippiensis, which are found in habitats where large terrestrial vertebrates, amphibians, and fish are the food sources. Thus, these crocodilians have evolved to increase the mechanical strength of their mandibles by developing a larger mass of M. ptergoideus posterior muscles. Therefore, natural selection seems to lead to the evolution of an effective crocodilian snout by creating a mechanical compromise between bite force and hydrodynamic efficiency.

Extant taxonomy

  • Family Gavialidae
    Gavialidae
    Gavialidae is a family of reptiles within the order Crocodilia. Gavialidae consists of only one surviving species, the gharial , which is native to India. Many extinct species are also known...

    • Genus Gavialis
      • Gharial
        Gharial
        The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...

         (Gavialis gangeticus)
  • Family Alligatoridae
    Alligatoridae
    Alligatoridae is a family of crocodylians that includes alligators and caimans.- True alligators :Alligators proper occur in the fluvial deposits of the age of the Upper Chalk in Europe, where they did not die out until the Pliocene age. The true alligators are now restricted to two species, A...

    • Genus Alligator
      Alligator
      An alligator is a crocodilian in the genus Alligator of the family Alligatoridae. There are two extant alligator species: the American alligator and the Chinese alligator ....

      • American Alligator
        American Alligator
        The American alligator , sometimes referred to colloquially as a gator, is a reptile endemic only to the Southeastern United States. It is one of the two living species of alligator, in the genus Alligator, within the family Alligatoridae...

         (Alligator mississippiensis)
      • Chinese Alligator
        Chinese Alligator
        The Chinese alligator or Alligator Alligator sinensis) is one of two known living species of Alligator, a genus in the family Alligatoridae. The Chinese alligator is native only to China...

         (Alligator sinensis)
    • Genus Paleosuchus
      Paleosuchus
      Paleosuchus is a South American genus of reptile in the Alligatoridae family. They are the smallest members of the order Crocodilia in the Americas.The genus contains two species:* Cuvier's Dwarf Caiman ....

      • Cuvier's Dwarf Caiman
        Cuvier's Dwarf Caiman
        The Cuvier's dwarf caiman or Musky caiman, Paleosuchus palpebrosus, is a relatively small crocodilian reptile from northern and central South America. It is found in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana, Paraguay, Peru, Surinam and Venezuela...

         (Paleosuchus palpebrosus)
      • Smooth-fronted Caiman
        Smooth-fronted Caiman
        The smooth-fronted caiman , also known as Schneider's smooth-fronted caiman, is a crocodilian reptile from South America...

         (Paleosuchus trigonatus)
    • Genus Caiman
      Caiman
      Caimans are alligatorid crocodylians within the subfamily Caimaninae. The group is one of two subfamilies of the family Alligatoridae, the other being alligators. Caimans inhabit Central and South America. They are relatively small crocodilians, with most species reaching lengths of only a few...

      • Yacare Caiman
        Yacare Caiman
        The Yacare caiman is a species of caiman found in central South America, including northeastern Argentina, Uruguay eastern Bolivia, central/south-west Brazil, and the rivers of Paraguay...

         (Caiman yacare)
      • Spectacled Caiman
        Spectacled Caiman
        The spectacled caiman , also known as the white caiman or common caiman, is a crocodilian reptile found in much of Central and South America. It lives in a range of lowland wetland and riverine habitat types and can tolerate salt water as well as fresh; due in part to this adaptability it is the...

         (Caiman crocodilus)
      • Broad-snouted Caiman
        Broad-snouted Caiman
        The broad-snouted caiman is a crocodilian reptile found in eastern and central South America, including southeastern Brazil, northern Argentina, Uruguay, Paraguay and Bolivia. It is found mostly in freshwater marshes, swamps, and mangroves. Usually, in still or very slow moving waters...

         (Caiman latirostris)
    • Genus Melanosuchus
      • Black Caiman
        Black Caiman
        The black caiman is a crocodilian. It is a carnivorous reptile that lives along slow-moving rivers and lakes, in the seasonally flooded savannas of the Amazon basin, and in other freshwater habitats in South America. Once common, it was hunted to near extinction primarily for its commercially...

         (Melanosuchus niger)
  • Family Crocodylidae
    Crocodile
    A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

    • Genus Crocodylus
      Crocodylus
      Crocodylus is one of three genera from the Crocodylinae subfamily extending from the Crocodylidae family.Established species include three extinct species:* † Crocodylus acer - probably represents a distinct genus....

      • American crocodile
        American Crocodile
        The American crocodile is a species of crocodilian found in the Neotropics. It is the most widespread of the four extant species of crocodiles from the Americas. Populations occur from the Atlantic and Pacific coasts of southern Mexico to South America as far as Peru and Venezuela. It also lives...

         (Crocodylus acutus)
      • Slender-snouted crocodile
        Slender-snouted Crocodile
        The African Slender-snouted Crocodile is a species of crocodile. Recent studies in DNA and morphology suggest that it may belong in its own genus, Mecistops, but at present most continue to use Crocodylus for this species.African Slender-snouted Crocodile are native to freshwater habitats in...

         (Crocodylus cataphractus)
      • Orinoco crocodile
        Orinoco Crocodile
        The Orinoco crocodile, Crocodylus intermedius, is a critically endangered crocodile found in freshwater in northern South America, in particular the Orinoco River. Typical measurements for adults range from 3 to 4.8 m . Typical weight for mature females is 200 kg and 380 kg for males...

         (Crocodylus intermedius)
      • Freshwater crocodile
        Freshwater Crocodile
        The freshwater crocodile , also known as the Australian freshwater crocodile, Johnston's crocodile or colloquially as freshie, is a species of reptile endemic to the northern regions of Australia...

         (Crocodylus johnsoni)
      • Philippine crocodile
        Philippine crocodile
        The Philippine crocodile is a crocodile found in the Philippines. It is also known as the Mindoro crocodile and the Philippine freshwater crocodile. In the Philippines, it is strictly prohibited to kill a crocodile, but its status is critically endangered from exploitation and unsustainable...

         (Crocodylus mindorensis)
      • Morelet's crocodile
        Morelet's Crocodile
        Morelet's crocodile, also known as the Mexican crocodile, is a modest sized crocodilian found only in fresh waters of the Atlantic regions of Mexico, Belize and Guatemala. It usually grows to about in length. It is an endangered species.-History:...

         (Crocodylus moreletii)
      • Nile crocodile
        Nile crocodile
        The Nile crocodile or Common crocodile is an African crocodile which is common in Somalia, Ethiopia, Uganda, Kenya, Egypt, Tanzania, Zambia, Zimbabwe, Gabon, South Africa, Malawi, Sudan, Botswana, and Cameroon...

         (Crocodylus niloticus)
      • New Guinea crocodile
        New Guinea Crocodile
        The New Guinea crocodile is a small species of crocodile found on the island of New Guinea.-Description:Crocodylus novaeguineae grows to a length of up to for males and for females, although most specimens are rather smaller. The body is grey-brown in colour, with dark brown to black markings on...

         (Crocodylus novaeguineae)
      • Mugger crocodile
        Mugger Crocodile
        The mugger crocodile , also called the Indian, Indus, Persian, or marsh crocodile, is found throughout the Indian subcontinent and the surrounding countries...

         (Crocodylus palustris)
      • Saltwater crocodile
        Saltwater Crocodile
        The saltwater crocodile, also known as estuarine or Indo-Pacific crocodile, is the largest of all living reptiles...

         (Crocodylus porosus)
      • Cuban crocodile
        Cuban crocodile
        The Cuban crocodile is a small species of crocodile found only in Cuba's Zapata Swamp and the Isle of Youth, and highly endangered, though it formerly ranged elsewhere in the Caribbean...

         (Crocodylus rhombifer)
      • Siamese crocodile
        Siamese Crocodile
        Siamese crocodile is a freshwater crocodile native to Indonesia , Brunei, East Malaysia, Laos, Cambodia, Burma, Thailand, and Vietnam...

         (Crocodylus siamensis)
    • Genus Osteolaemus
      • Dwarf crocodile
        Dwarf Crocodile
        The dwarf crocodile is an African species of crocodile. It is also the smallest extant crocodile species in the world. Recent sampling has identified three genetically distinct populations...

         (Osteolaemus tetraspis)
    • Genus Tomistoma
      • False gharial
        False gharial
        The false gharial , also known as the Malayan gharial, false gavial, or Tomistoma is a freshwater crocodile of the Crocodylidae family with a very thin and elongated snout...

         (Tomistoma schlegelii)

Taxonomy

  • Superorder Crocodylomorpha
    Crocodylomorpha
    The Crocodylomorpha are an important group of archosaurs that include the crocodilians and their extinct relatives.During Mesozoic and early Tertiary times the Crocodylomorpha were far more diverse than they are now. Triassic forms were small, lightly built, active terrestrial animals. These were...

    • Order Crocodilia
      • Superfamily Gavialoidea
        • Family Gavialidae
          Gavialidae
          Gavialidae is a family of reptiles within the order Crocodilia. Gavialidae consists of only one surviving species, the gharial , which is native to India. Many extinct species are also known...

          : gharials & false gharials
      • Superfamily Alligatoroidea
        Alligatoroidea
        Alligatoroidea is a superfamily of crocodilians that evolved in the Late Cretaceous period. Cladistically, it is defined as Alligator mississippiensis and all crocodylians more closely related to A. mississippiensis than to either Crocodylus niloticus or Gavialis gangeticus ....

        • Family Alligatoridae
          Alligatoridae
          Alligatoridae is a family of crocodylians that includes alligators and caimans.- True alligators :Alligators proper occur in the fluvial deposits of the age of the Upper Chalk in Europe, where they did not die out until the Pliocene age. The true alligators are now restricted to two species, A...

          • Subfamily Alligatorinae
            Alligatorinae
            Alligatorinae are one of two subfamilies of the family Alligatoridae.-Taxonomy:* Subfamily Alligatorinae** Genus Chrysochampsa ** Genus Hassiacosuchus ** Genus Navajosuchus ** Genus Ceratosuchus...

            : alligators
          • Subfamily Caimaninae: caimans
        • Family †Diplocynodontidae
      • Superfamily Crocodyloidea
        • Family Crocodylidae
          Crocodile
          A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

          • Subfamily Crocodylinae
            Crocodile
            A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...

            : crocodiles
          • Subfamily †Mekosuchinae
            Mekosuchinae
            Mekosuchinae was a subfamily of crocodiles from Australia and the South Pacific that have now become extinct. They first appear in the fossil record in the Eocene in Australia, and survived until the Pleistocene in Australia and until the arrival of humans in the Pacific islands of Fiji, New...


Phylogeny


Cladogram
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 after Brochu (1997).

Fossil record


Neonate sized crocodilians fossils have been documented in the scientific literature
Scientific literature
Scientific literature comprises scientific publications that report original empirical and theoretical work in the natural and social sciences, and within a scientific field is often abbreviated as the literature. Academic publishing is the process of placing the results of one's research into the...

.

In popular culture

  • Several science fiction movies have giant crocodilians as their stars, such as Alligator
    Alligator (film)
    Alligator is a 1980 monster movie, directed by Lewis Teague with a screenplay by John Sayles. It stars Robert Forster, Robin Riker, and Michael V. Gazzo...

    , Lake Placid
    Lake Placid (film)
    Lake Placid is a 1999 American monster movie with elements of comedy, horror, bromance film, and the buddy genre. It was written and produced by David E. Kelley, directed by Steve Miner, and stars Bill Pullman, Bridget Fonda, Brendan Gleeson, Kevin Bacon, Phillip Seymour Hoffman, Oliver Platt, and...

    , Primeval, DinoCroc
    DinoCroc
    Dinocroc is a 2004 horror film, starring Charles Napier and Joanna Pacuła, produced by Roger Corman, and directed by Kevin O'Neill. It had a limited theatrical release in early 2004 before premiering on the Syfy Channel in April of that year...

    , and Rogue
    Rogue (film)
    War is a 2007 American action thriller film, directed by Phillip G. Atwell who makes his film debut, with fight choreography by Corey Yuen. The film was released in North America on August 24, 2007 and stars action film actors Jet Li and Jason Statham, making their second collaboration after the...

    .

External links