Everything about Crocodilia totally explained
Crocodilia is an order of large
reptiles that appeared about 84 million years ago in the late
Cretaceous Period (
Campanian stage). They are the closest living relatives of
birds, as the two groups are the only known survivors of the
Archosauria. Members of the crocodilian
stem group, the
clade Crurotarsi, appeared about 220 million years ago in the
Triassic Period and exhibited a wide diversity of forms during the
Mesozoic Era.
The correct vernacular term for this group is "
crocodilians", not "crocodiles," although the latter term is sometimes used incorrectly to refer to
alligators and
caiman, or even their distant prehistoric relatives, "
marine crocodiles".
Spelling
The group is often spelled 'Crocodylia' for consistency with the genus
Crocodylus (
Laurenti, 1768). However,
Richard Owen used the -i- spelling when he published the name in 1842, so it's generally preferred in the scientific literature . The -i- spelling is also a more accurate
Latinization of the
Greek κροκόδειλος (
krokodeilos, literally "pebble-worm", referring to the shape and texture of the animal).
Description
The basic crocodilian
body plan is a very successful one; modern species closely resemble their
Cretaceous 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, were aquatic predators living in rivers and lakes, and they filled an ecological niche similar to the crocodilians.
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
chameleons 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. Because the left aortic arch goes directly to the gut, the shunting of oxygen depleted blood which is high in CO
2 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.
They have alveoli in their lungs and a unique muscular attachment to the liver and viscera that acts as a piston to breathing, separating the thoracic and abdominal cavities (similar to the
diaphragm of mammals). Although tegu lizards have a primitive proto-diaphragm, separating the pulmonary cavity from the visceral cavity and allowing greater lung inflation, this has a different evolutionary history.
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.
The sex of developing crocodilians is determined by the incubation temperature of the eggs. This means crocodilians don't have
genetic sex determination, 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 reptiles, crocodilians have a relatively small brain, but it's more advanced than in other reptiles. Among other things it has a true cerebral cortex.
As in many other aquatic or amphibian 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 glow in the dark. A third transparent eyelid, the
nictitating membrane, protects their eyes underwater. However, they can't focus under water, meaning other senses are more important when submerged under water.
While birds and most reptiles have a ring of bones around each eye which supports the eyeball (the
sclerotic ring), 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. There is also a small muscle (which is also seen in
gecko) next to or upon the stapes, the stapedius, which probably functions in the same way as the mammalian stapedius muscle does, dampening strong vibrations.
The upper and lower jaws are covered with sensory pits, seen as small, black speckles on the skin, the crocodile version of the lateral organ we see in fish and many amphibians. But they've 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
alligators and
caimans only have them on their jaws,
crocodiles have similar organs on almost every scale on their body. The function of the DPRs on the jaws are clear, but it's 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 like they can do more than that, like assisting in chemical reception or even salinity detection.
Skin and skeleton
The skin is covered with non-overlapping scales composed of the protein
keratin (the same protein that forms hooves, skin, 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
osteoderms or scutes, 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's 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.
Evolution
Eusuchia, a modern
clade which includes the
crown group Crocodilia, first appeared in the Lower Cretaceous of Europe.
Isisfordia duncani lived approximately 95 to 98 million years ago, during the
Cenomanian epoch of the Upper Cretaceous.
Isisfordia is the second oldest known eusuchian, and the earliest crocodylomorph 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); hooved, terrestrial carnivores (
Pristichampsus), and 'hatchet'-shaped skulled forms (
Baru).
Classification
Superorder Crocodylomorpha
In popular culture
Several science fiction movies have giant crocodilians as their stars, such as Lake Placid, DinoCroc, and the crocodile series.Further Information
Get more info on 'Crocodilia'.
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