Crocodilia is an order of large reptiles that appeared about 220
million years ago. They are the closest living relatives of birds.
Like mammals and unlike most other reptiles, crocodiles have a
four-chambered heart (although, monitor lizards have a four-chambered
heart, as well); however, unlike mammals, oxygenated and deoxygenated
blood can be mixed when the foramen of Panazzi is open, which bridges
both ventricles in the heart. This opening is typically only open
during diving, in order to shunt blood away from the lungs. Their
blood has shown to have strong antibacterial powers.
All crocodilians have "thecodont" dentitions (teeth set
in bony sockets) but unlike mammals, they replace these teeth throughout
life. 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 the "edentulous" (toothless) state,
after teeth have been broken and replacement has apparently 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.
From the top: Head of an American alligator (Alligator mississippiensis),
a Nile crocodile (Crocodylus niloticus), and an Indian gharial (Gavialis
gangeticus).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 flap of skin 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, but some skinks
(family Scincidae) have evolved a bony secondary palate too, to
The tongue is attached to the floor of its mouth, making it hard
to move at all. Like all reptiles they have a relatively small brain,
but it is more advanced than in other reptiles. Among other things
it has a true cerebral cortex.
Crocodiles are often seen laying with their mouths open, called
gaping. One of its functions is probably to cool them down, but
since they are also doing this at night and/or when it is raining,
it is possible that gaping has a social function too.
The crocodile basic body plan has turned out to be very successful,
as it appears to be a good solution which works very well. This
is the reason why modern forms look very much the same way today
as they did when the dinosaurs were still around. Even mammals have
more or less adapted their body plan at at least one point in history.
One primitive ancestor of the whales, Ambulocetidae, was an aquatic
predator living in rivers and lakes. Ambulocetids seems to have
filled an ecological niche similar to the modern crocodiles.
They have a semi-erect, semi-sprawled posture, holding their legs
more directly underneath them than most other reptiles (the chameleons
are probably the only reptiles with a more erect posture than Crocodilia).
This makes it possible for some species to even gallop on land if
necessary; an Australian species can reach a speed of over 16 km/h
while galloping on an irregular forest floor. But their ancestors
actually had a fully erect posture; their sprawling and semi-erect
posture are secondary and evolved after they adapted to a life in
water. Especially the ankle bones (tarsals) are highly modified.
In other words, their locomotion is not primitive, instead it turns
out it is very specialized and quite unique. It seems their distant
ancestors most likely were fast-moving terrestrial predators, like
Junggarsuchus sloani. An extinct and very early terrestrial crocodile,
pristichampsus rollinatii, even had hoof-like toes. The front feet
have five toes and the hind feet have four webbed toes. The three
inner toes on the front feet and the three inner toes on the hind
feet have claws.
As in many other aquatic or amphibian tetrapods, the eyes, ears,
and nostrils are all located on the same plane. They see well at
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, a light reflecting layer of tapetum behind the
retina greatly increases their ability to see in weak light, this
also makes their eyes glow in the dark. A third transperant eyelid,
the nictitating membrane, protects their eyes underwater. But they
can't focus under water, which means other senses are more important
when they are under the surface. While birds and most reptiles have
a ring of bones around each eye which supports the eyeball, a ring
called 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
What sex the juvenile will be is determined by the incubation temperature.
The skin is covered with non-overlapping scales composed of the
protein keratin (the same protein that forms hoofs, 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. Just like a tree, 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 overlapping 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 acts as solar panels.
Crocodiles and gharials have modified salivary glands on their
tongue, salt glands, used for excreting excess salt from their body.
Alligators and caimans have them too, but here they are non functioning.
This tells us that at some point the common origin of the Crocodilia
were adapted to saline water and marine environments. This also
explains their wide distribution across the continents. Species
like the saltwater crocodiles (C. porosus) can still survive long
periods in the sea. True crocodiles are probably the most original
forms, while alligators and caimans have evolved from the crocodiles.
They are known to swallow stones, gastroliths ("stomach-stones"),
which act as a ballast as well as help to crush up the bones of
their prey. The crocodile 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
Crocodiles are an old group of animals, but they have evolved much
since their body plan first formed many million years ago. After
dinosaurs became extinct, some crocodiles became more terrestrial,
filling niches earlier occupied by meat-eating dinosaurs. Some extinct
forms were probably herbivorous (Simosuchus clarki and Chimaerasuchus
paradoxus). Primitive and long-extinct species such as Hesperosuchus
and Gracilisuchus were facultatively bipedal. Others were much more
adapted to a life in water than any of the species living today.
The marine crocodile Metriorhynchus had modified its legs into flippers,
and Dakosaurus andiniensis had a skull that was adapted to eat large
sea reptiles. If extinct forms are included, the crocodiles are
a very diverse and adaptive group of reptiles.
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