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Teacher's Guide


The family Crocodylidae includes all crocodilian genera, both living and extinct. They are all descendants of a group of reptiles called thecodonts. The thecodonts were also ancestral to pterosaurs (extinct flying reptiles), dinosaurs, and birds. Modern crocodilians include crocodiles, alligators, caimans, and gharials. They are all carnivorous, cold-blooded reptiles that spend a great deal of time in the water, but move about on land as well. Crocodilians live in warm conditions; in fact, most cannot tolerate temperatures that fall below 50-59° F, although there are a few exceptions. Crocodilians live in fresh water, and occasionally the brackish water in estuaries. Most crocodiles cannot handle large quantities of salt water, so only a few venture out to sea for any length of time. No modern crocodile is truly marine. Crocodilians have unique bony armor underneath their scaly skin made up of osteoscutes. Osteoscutes overlap like roof shingles and protect the crocodilian, while still allowing it flexibility. The number of osteoscutes in a row along a crocodilian's back, and their presence or absence on the belly can help identify the type of crocodilian. The crocodilian body plan is hugely successful and only minor changes in it have occurred over the last eighty million years. Adult crocodilians have very few natural predators, but habitat destruction caused by humans has created perhaps the biggest challenge these reptiles have ever faced.


Crocodiles, alligators, and their close relatives such as gharials and caimans, are all called crocodilians. An alligator is a type of crocodilian. Alligators have broad, rounded snouts when compared to other crocodilians, which have a pointy snout. When an alligator shuts its mouth its teeth aren't visible. When a crocodile shuts its mouth, one or more of its lower teeth will still stick out. The largest tooth in the crocodile's upper jaw is the fifth, and the largest tooth in an alligator's upper jaw is the fourth. The Crocodile or Alligator component illustrates these differences for visitors. There are other, more subtle differences between them, but these are the easiest characteristics used to identify crocodiles and alligators. Caimans are closely related to alligators. Gharials (which have an elongated, pointy snout specialized for catching fish), have their own subfamily.

Crocodile and alligator differences


Leidyosuchus is a large prehistoric crocodile found at Wannagan Creek. It's a primitive type that is as closely related to modern crocodiles as it is to modern alligators. It has two protruding lower teeth instead of one. Leidyosuchus was the top predator at Wannagan Creek 60 million years ago; it probably had no enemies (except for other Leidyosuchus!) Skeletons of Leidyosuchus are more common than any other vertebrate animal found at the quarry, and at least 70 individuals have been recovered. Science Museum field crews found Leidyosuchus eggs, hatchlings (about 25 cm), and adults up to 15 feet long. Visit the Top Croc component for more Leidyosuchus facts and adaptations.

Many of the remains of Leidyosuchus are in good shape, as far as fossils go. At Field Camp Mystery visitors discover that some of the skeletons were found belly-up, probably indicating that the bloated corpses of these crocodiles settled at the bottom and were hardly disturbed before they were buried and fossilized.


Wannaganosuchus is a new species of alligator found at Wannagan Creek, and museum curator Bruce Erickson named it. Approximately 40% of one skeleton was found, along with the partial remains of another individual. From this, scientists estimate that the animal was about four feet long. Wannaganosuchus has button shaped teeth in the back of its mouth that helped it to crush mollusks and snails. There are no longer any modern alligators with this type of teeth, although a few other button-toothed alligators are known from fossil remains.


Warm temperatures play a direct role in the global distribution of crocodilians. As continents have shifted over time and climates have changed, so have the environments suitable for crocodiles. For example, Europe was once home to many crocodilians. About 38 million years ago, climate changes in Europe led to a decline in the crocodilian population. By about five million years ago, they were totally gone from Europe. Crocodiles today live in a variety of warm environments including freshwater lakes, swampy or marshy areas, river drainage basins, tropical forests, coastal areas, open grasslands near water, and brackish estuaries. What follows is the approximate distribution of modern crocodilians:


For the most part, yes. Different types of crocodilians tend to defend their territory from others. For example, American alligators and American crocodiles don't share the same territories. Sometimes even crocodilians of the same species don't get along very well, either, but if climatic conditions or scarce resources force them into tight quarters, they seem to put their differences aside out of necessity. The fossil alligator Wannaganosuchus was found relatively far from the remains of the fossil crocodile Leidyosuchus. In life, they probably avoided each other as much as possible. Visitors can watch the Croc Wars video in the diorama area to see modern day intraspecies aggression. The diorama features an alligator encroaching on crocodile territory. Can you spot this encounter?


Crocodilian fossils date back to the Triassic Period, over 200 million years ago, just as the dinosaurs were appearing. The earliest crocodilians would not have looked very familiar to us. Not all of them were amphibious, and some probably moved quickly over land with their long, slender legs. Very primitive crocodiles resembled their ancestors, the thecodonts, which also gave rise to pterosaurs, dinosaurs, and birds. Crocodiles have many features in common with birds and dinosaurs. In fact, when trying to reconstruct dinosaur behavior, paleontologists look to crocodiles and birds. What is true for these living relatives may have been true for dinosaurs as well. The earliest crocodilians that resemble those of today are about eighty million years old (from the Cretaceous Period). It is also about this time in the fossil record that paleontologists see a clear distinction between crocodiles and alligators.

Cretaceous period crocodile


No. So-called "living fossils" are never identical to the relatives that lived millions of years ago. They just look very similar. Crocodilians have maintained the same basic body plan over millions of years because it works so well. But they have undergone changes. Over time, crocodilians evolved a bony palate, their internal nostrils were pushed further back, and their spool-shaped vertebrae went from being biconcave to having a concave front and a convex back--a change that makes the vertebral column more flexible and strong. These subtle changes made crocodilians formidable underwater predators.

Vertebrae of fossil and modern crocodiles

Furthermore, some very strange crocodilians have appeared in the fossil record that challenge the notion that these creatures are unchanged. For example, the strange pug-nosed herbivorous crocodile recently discovered in Madagascar is visibly very different from more familiar crocodilians. There are other oddities, too, like the pristichampsines, which were crocodilians with hoof-like toes. Visitors can read about other so-called "living fossils" in the exhibit, such as the pike in the lake diorama, at the Living Fossils, Primitive Pike, and Dangerous Waters components. The pike isn't really unchanged, either. There is really no such thing as a living fossil because life is always changing.


The biggest crocodile ever discovered was Deinosuchus, which lived near the end of the Cretaceous Period (just before the dinosaurs went extinct, about 70 million years ago). Deinosuchus has also been called Phobosuchus. The lower jaw of this crocodile was six feet long. Conservative estimates for the length of the whole animal put it at 36 feet, and some scientists have claimed Deinosuchus may have been fifty feet long. By comparison, the largest modern crocodiles get to be about twenty feet long.


Marine crocodiles known as the metriorynchids lived during the Jurassic Period, about 150 million years ago. These creatures were so highly adapted for life at sea that they lost many of their crocodilian traits, such as their bony armor. Their limbs looked more like paddles, and they evolved a dorsal fin on their backs.

Modern crocodiles cannot tolerate extremely salty water. Although they have salt glands to help expel salt from the body, they can't survive at sea for long. The American crocodile and the Indopacific crocodile are the most efficient at tolerating salt. American crocodiles live in estuarine environments, and Indopacific crocodiles have been sighted far out at sea. Nevertheless, they are not truly marine animals.



Visitors to the Diorama area can examine the Flippable Croc to learn more about crocodilian biology. This large image of a crocodile has "flips" on various body parts. Visitors discover amazing facts about the crocodile's anatomy under each flip.

Do crocodiles have a gizzard like a dinosaur?
Not quite, although they do swallow stones. Crocodiles do not chew their food. They swallow whole pieces at a time, bones and all. The stones help break up the big pieces. Crocodiles have two stomachs. Stones help to break up the food in the first stomach, and then the meal is passed to the highly acidic second stomach for further digestion. Crocodile stomachs are more acidic than those of any other vertebrate.

Bony palate:
The bony palate in a crocodile's mouth separates its nasal passage from its mouth. A valve at the top of the crocodile's throat keeps water out of the airway, allowing it to hide underwater as long as its nostrils stay above the surface. This is an excellent strategy for sneaking up on prey. With only its nostrils protruding from the water, a crocodile stays well hidden. A crocodile can also hold its breath under water for up to two hours.

Crocodiles have very thick scales on their skin, but they also have protection under their skin. Osteoscutes are bony pieces of armor underneath the crocodile's skin. The surface of an osteoscute is pitted, similar to a golf ball. Osteoscutes overlap each other like roof tiles. Different types of crocodilians vary in the number and placement of osteoscutes, but osteoscutes most typically line the crocodilian's back. Some crocodilians have them on their bellies. Big, broad-nosed crocodilians often have fewer osteoscutes than ones with slender noses do. Fossil osteoscutes are found throughout Wannagan Creek matrix and are easy to recognize. Visitors can touch cast osteoscutes at the Scutes and Cannibals rail in the diorama area of the exhibit.

Crocodile hearts have four chambers like a mammal's (reptiles usually have three). New research into crocodile hearts shows that a valve in the heart operates like a cog to reroute blood in the crocodile's body. When a crocodile is administered adrenaline, the valve opens to allow blood flow to the lungs. When the crocodile is in a relaxed state, the valve remains closed so that "used" blood recycles back through the body instead of entering the lungs. Scientists used to think this adaptation allowed crocodiles to stay underwater longer during dives. New research suggests that may not be true. Scientists still do not fully understand how the crocodile's heart works.

Tooth replacement:
Crocodiles (as well as sharks and dinosaurs) are continuously replacing teeth. A single crocodile might go through more than 2,000 teeth in its lifetime, although there are typically only about eighty teeth in its mouth at one time. An individual tooth has a life span of about two years. Teeth at the front of the mouth, not surprisingly, get replaced more often than teeth at the back of the mouth.


Nesting and rearing young:

Most crocodilians lay a nest of eggs once a year. Nests can have anywhere from 20 to 80 eggs in them, depending on the species. Some of these eggs never hatch, and many hatchlings may never make it to adulthood. Lots of predators seek out crocodile eggs, as well as unguarded hatchlings. The Safety in Numbers media interactive, in the diorama area, shows that although adult crocodiles may be at the top of the food chain, they start out near the bottom. Visitors can see how many eggs a crocodile lays, and how many eggs or hatchlings disappear over time.

Mother crocodilians defend their nests. Some crocodilians build hole nests; others build mound nests made from vegetation and leaf litter. These techniques keep the eggs warm. Severe temperature fluctuations or flooding can destroy a whole nest of eggs. Temperatures below 81° or above 93° F are usually deadly. Interestingly, the temperature at which many (and perhaps all) crocodilian eggs are incubated determines the sex of the hatchlings. In crocodiles, high temperatures and low temperatures usually result in females, and intermediate temperatures produce males. Among alligators, high temperatures produce males and low temperatures produce females. Some crocodilians make their nests next to termite mounds, which generate heat. Termites aren't the only creatures that may share space with a crocodilian nest. Turtles, and at least a few types of snakes and lizards, will also lay their eggs next to or with crocodile nests (less work, added warmth).

Incubation time can be as little as 35 days in small species of crocodilians, or as many as 100 days in larger species. Babies start vocalizing before they hatch. Eventually, their calls alert the mother that it is time to assist in hatching. She may gently roll the eggs against the roof of her mouth to help the hatchlings break free. She also carries the hatchlings in her mouth to safety in the water. Young crocodilians stick close to their mother, often basking on her head. She submerges into the water with them if there is danger.

Crocodilians are the most vocal reptiles. The dense vegetation that often surrounds them makes it difficult for them to see each other. When visual recognition is not an option, vocalization is a more efficient means of communication. Noises include roars, bellows, and hisses. Crocodilians also communicate with each other through body movements, such as aggressive "head slaps" (the crocodile slaps its head on the water) or by lifting up the head in submission. Visitors can learn more about the non-vocal communication between crocodilians at the Silent Speech component in the diorama area.

As already mentioned, baby crocodilians start making noises before they have even hatched! The noises they make help the mother determine when they are ready to hatch, so that she may help them. Hatchlings make all sorts of squealing noises and call out for help when they are distressed. Hatchling distress calls often cause all nearby adults--not just the mother--to come to their aid. Visitors can hear recordings of crocodile vocalizations at the Big Talkers media interactive and then discover what behavior each noise is associated with.

Crocodilians are opportunistic. Fish, birds, and mammals, both small and large, are all a good meal if they happen to be in or near the water. Young crocodilians eat mostly fish, insects, crabs, and snails. As they get older, they incorporate larger animals in their diet, but will still consume small meals if they don't have to expend much energy to get them. Large crocodiles will kill big animals like wildebeest and cattle if they pass through the crocodile's waters or step up for a drink. Although crocodiles can move quickly on land, they rarely chase prey for any distance. The crocodile relies on surprise attacks, and has an extreme advantage in the water. Even humans, stuck in the water with a hungry crocodile, don't stand much of a chance. Like other reptiles, crocodiles expend as little energy as possible. This allows some crocodiles to go a long time without feeding. Most will eat once a week, but a very large crocodile might make it two years without a meal! Leidyosuchus probably ate a lot of fish and turtles (and perhaps an occasional champsosaur, Wannaganosuchus, or even one of its own kind!). We have direct evidence of its turtle dinners. Visitors to the diorama area can touch a cast of the One That Got Away, which is a turtle shell found fossilized with crocodile bite marks in it. Visitors can also test the jaw strength of a crocodile at the Jaw Power component. All the Better to Eat You With provides visitors with photographs of crocodilian feeding behavior.



Are crocodilians endangered?
Several are. The good news is that many crocodilians that were once endangered aren't anymore, but illegal poaching and destruction of habitat are still damaging several species of crocodilians more quickly than they can recover. Black caimans, Broad-snouted caimans, American crocodiles, Orinoco crocodiles, Morelet's crocodiles, Philippine crocodiles, Gharials, and False Gharials all suffer from over-hunting. While some of these species enjoy protection on parkland or reserves, the Orinoco crocodile, the Philippine crocodile, and Morelet's crocodile have almost no protection. The Chinese alligator, the Cuban crocodile, and the Gharial have experienced major habitat loss. Habitat loss has made it difficult for the Cuban crocodile to compete with the Common caiman, which is taking over much of its former territory. Gharials, in addition to hunting and habitat loss, are frequently drowned in fishing nets. The Siamese crocodile may be entirely extinct in the wild, as the only known ones are bred captively in Thailand. As of 1998, the most critically endangered crocodilians were the Philippine crocodile, the Siamese crocodile, the Chinese alligator, and the Orinoco crocodile. To date, there are probably less than 150 Chinese alligators in the wild due to habitat loss. Still endangered but not "critically endangered" are the Black caiman, the Cuban crocodile, the Gharial, and probably the Slender-snouted crocodile (although data are poor on this last one). Many other crocodilians are listed as vulnerable, but not yet endangered.

Stopping the poaching of these species will not entirely solve the problem. It is also crucial that people maintain varieties of habitats for them. Wetland preservation has been an important part of conservation in the United States. Laws concerning the import and export of crocodilian skins have also helped. For a while, laws only affected the exporting countries. Importing or consuming countries were not held accountable. Now the laws affect many of these countries as well. Many steps are being taken to ensure the skins that make it to the market were legally taken.

What are they used for commercially?
Skins are the most profitable products derived from crocodilians, but their meat is sold for food as well. Alligator and crocodile hunting was at an all time high around the time of the Civil War, and the demand became so great that skins were being sought in Central America before the close of the nineteenth century. At the same time, colonists in Africa were paying hefty sums for crocodile skins. Some of the first legislation to protect crocodilians was passed in Florida in 1944 (which protected very small alligators, as well as alligators during their breeding season). More legislation was passed in the 1960s and 1970s protecting crocodilians in several other countries as well.

Ranching and farming is one way to collect crocodilian skins without damage to any wild populations. Most farms were started in the U.S. during the 1960s. All alligators on an alligator farm are born on site. Alligator ranches occasionally collect wild alligators, babies, and eggs to add to their population, when and where it is legal. Although this has helped natural populations of crocodilians dramatically, in many areas of the world crocodilians are still illegally poached. About 2 million caimans, crocodiles, and alligators are killed each year for the skin trade. Only half of these killings are legal (according to data published in 1989).

Do they attack people?
Crocodile attacks are rare. However, this does not mean that one should tempt fate by swimming in infested waters. If a hungry crocodile is large enough to take on a human, it probably will. Smaller crocodiles and alligators don't present much of a danger. But if a crocodile is big enough to capture a water buffalo, a human isn't much of a challenge. Children and the elderly are the most vulnerable. Some of the most dangerous individual crocodilians may be those in areas heavily populated by people. It seems that as the animals lose their fear of people, they are more likely to attack. Indopacific and Nile crocodiles are the most dangerous and are responsible for the largest amount of human deaths (American alligators and American crocodiles have attacked humans, but very rarely). Most attacks seem to be motivated by hunger, and perhaps defense of territory as well.

Other animals at Wannagan Creek >

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