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                                   General Description

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                                   General Description

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The Order crocodilia includes about 20 different species including the American Alligator, Spectacled Caiman, Chinese Alligator, Nile Crocodile, American Crocodile, Mugger, and the Indian Gharial. They are all cold blooded, a palatal valve to prevent water from going to the back of their throat, and eyes on top of their head. All of them have slow digestive systems and set up territory during the winter around female nurseries. Also, all crocodilia subspecies hatchling's gender is determined based on the temperature during incubation. Meaning that the eggs will be male or female based on the temperature of the eggs. They all have unique courtship dances for mating, but generally consist of the showing off of the organism's size and power. Crocodilia, mainly live in waterways, and even if they are saltwater, they will spend some of their life in rivers and warmer areas especially in the tropics. The order crocodilia includes a variety of reptilian animals that dominate waterways, and have powerful jaws that prove them to be ferocious predators around the world. Crocodile is divided into subgroups of alligators, gharials, and crocodiles. The size of these animals varies on the species, but some of the largest include the Nile Crocodile exceeding 18 feet long.

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The taxonomy shown below shows the three subspecies of Crocodilia are similar in the kingdom, phylum, class, and order, but as they concise, they differentiate.

| Exemplary Example 1 | Exemplary Example 2 | Exemplary Example 3
Common Name | Indian Gharil | Nile Crocodile | American Alligator
Kingdom | Animilia | Animilia | Animilia
Phylum | Chordata | Chordata | Chordata
Class | Reptilia | Reptilia | Reptilia
Order | Crocodilia | Crocodilia | Crocodilia
Family | Gavialidae | Crocodylidae.  | Alligatoridae
Genus | Gavialis   | Crocodylus.  | Alligator
Species | gangeticus | niloticus   | acutus

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Location found & habitat specific information: Subsaharan Africa
Physical characteristics (internal & external): Contains 68 teeth, and our short, splayed legs, a flat, powerful tail, a scaly hide and a strong jaw. Adult males can reach up to 20 feet in length and weigh well over 1,000 pounds.
Behavioral characteristics: The Nile crocodile drags large prey underwater, using a twirling method, called a “death roll,” to tear off large chunks of flesh.

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Location found & habitat specific information: Southeastern U.S
Physical characteristics (internal & external): It has a long armored body with thick scales or  bony plates
Behavioral characteristics: During very cold and very hot weather, the American alligator may dig a den or gator hole in the mud.

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Location found & habitat specific information: India, Nepal, Pakistan.
Physical characteristics (internal & external): Thin flat mouth, 13-19ft, small head, sharp teeth with about 58 on top and 52 on the bottom.
Behavioral characteristics: Oviparous breeding pattern, weak feet, and move mainly in water. The males are very territorial during mating season.

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The tree below shows the genetic connections among the subspecies within the order crocodilia. The order splits into Alligator, and another species which split into the Gharial and Crocodile species. the tree shows recent evolution, where the Crocodile and Gharial  spectated , and split off to fill their niches more recently than the Alligator, which is older than the other two species.

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Deinosuchus rugosus
Age of fossil (give time period as well) : Cretaceous, 70 million years old

Phytosaurus tooth
Age of fossil (give time period as well): Triassic, 55 million years old

Pcrocodilius Clavirostris
Age of fossil (give time period as well): Paleocene, 66 million years old

Crocodile scute
Age of fossil (give time period as well): Paleocene, 67 million years old

Uberabasuchus Terrificus
Age of fossil (give time period as well): Cretaceous, 73 million years oldEosuchus
Age of fossil (give time period as well): Paleocene, 59 million years old

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All of the organisms have a powerful tail that allows them to swim rapidly through the water, and all of them have sharp teeth and claws that make the predators even more ferocious. The bone structure of them show dense thick bones, needing powerful muscles to move. The crocodilia order has evolves to be as cunning and destructive as possible. The size of these organisms has lowered do to a decrease in mammal sizes after the last extiction. These large reptiles sizes adjusted on how much prey they can consume to survive. These organisms evolved to contain powerful tail muscles to help them survive in the tropical areas where they need to sun because they are cold blooded. They must receive sun to warm up, and function, so they typically are seen in tropical estuaries. The teeth size and head size varied by evolution to suit the type of prey that different crocodilia feed on. For example, nile crocodiles eat large water buffalo, and need a large head with powerful jaws to take down large prey. While the small Chinese Alligator has a small head to feed on small fish in Chinese rivers.

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All organisms of the crocodilia family have around 30 to 42 chromosomes, but crocodiles are the exception with 56 to 60 chromosomes. Crocodiles have more genetic information and chromosomal pairs. This explains why  crocodiles are a split off of alligators that speciated. Crocodiles have more DNA, because they are newer species within the crocodilia order. The small differences within the order show that the species, American Alligator’s DNA was very low at some point and many of the varying characteristics within the species were lost, causing all American Alligators today to be almost identical. The order of the karyotypes listed below go from caimans, humans, and alligators. The DNA contained by both species of the crocodilia family have fewer DNA than humans, because crocodilia subspecies are prehistoric and contain the same amount of DNA since they have existed. They have evolved less than humans, and the newest species of the crocodilia order is the crocodile, which contains the most chromosomes compared to all of the crocodilia subspecies. The DNA of the Crocodilia order is very different, and contains fewer DNA and advanced modifications compared to humans. This is a Caiman karyotype.

This is a human karyotype.
This is an American Alligator karyotype.

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Protein analysis:
Protein analysis has been used in the divergence among different species in the crocodilia order. A large body of traditional (morphological and paleontological), as well as recent non-traditional (cytogenetic and molecular) evidence found by research suggest that birds are the closest living sister group to the Crocodilia. However, relationships within the order are much harder to solve, because of
convergence/parallelism and general morphological conservatism. Recent studies have shown that with traditional interpretations of affinities between the alligators and caimans, the true and false gharials are more closely related to each other than to other crocodilians, and that the true crocodiles are all very close relatives that may have diverged recently. "Preliminary analyses from an ongoing study of restriction endonuclease analysis of crocodilian mitochondrial and ribosomal DNAs acknowledge both of these observations, suggesting that the molecular approach will be very valuable for resolving crocodilian phylogeny" (Dr. Densmore). The order Crocodilia came into existence during the Middle Triassic. DNA analysis suggest that the primitive members of the order are known only from a few specimens, apparently of two groups, the earliest of which is found in South America. Information about the Crocodilia during the early and middle Jurassic is practically non-existent, but this interval of time marked the rise of a number of independent groups, highly varied but with many common characters, forming the suborder Mesosuchia (possibly an artificial group). In the late Cretaceous a further expansion of the Crocodilia took place, that of the Eusuchia. This expansion continued into the warm early part of the Tertiary and became greatly reduced with the cooling which began during the later part of this period. "The present distribution is a relict of the early Tertiary radiation. Crocodiles are sensitive to climatic fluctuation and their distribution correlates well with the climatic changes of the Tertiary as represented by fossil floras. The endemic groups of crocodiles present in South America and Africa indicate that these continents were isolated for long periods of time" (Dr. Sill). The evidence shows that all species within the Crocodilia order have common DNA showing many common characteristics, but they differentiate slightly in adapted traits that are the result of evolution.

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The image below shows an alligator at the top with a crocodile on the bottom. The two embryos show differences and similarities caused by evolution from embryonic development. The embryos along with other crocodilia species show a common tail-like, and snout structure, along with small and weak legs that will cause them to rely on their tail for swimming. However, due to evolution, the head structure adapts to the food that they eat and environment that they live in. Alligators have a broader snout and crocodiles show a v-like snout used for slimming the crocodile down to a thin creature that hunts in rapidly moving waters. The Alligators hunt in stagnant waters leading to a bulkier shape. The species show slight embryotic differences based on what they eat and their environmental differences, but they both show signs of powerful tails yet to fully form to thrash through the water and hunt down prey.

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The anatomy of all species within crocodile differentiate mainly in the snout, where the shape of the species snouts are unique based on their food habits and environment. the head structure adapts to the food that they eat and environment that they live in. Alligators have a broader snout and crocodiles show a v-like snout used for slimming the crocodile down to a thin creature that hunts in rapidly moving waters. The Alligators hunt in stagnant waters leading to a bulkier shape. The species show slight embryotic differences based on what they eat and their environmental differences. Crocodile order subspecies are very recognizable, with a shared set of claws, scales, powerful tails, sharp teeth, but they speciate and differentiate based on their unique environments around the world.

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The Crocodilia order has evolved for many millions of years without many drastic changes.Crocodilian subspecies have mainly evolved gradually. Many of these changes lead to speciation and divided the Crocodilian order into species. Speciation occurred through changes in the environment and surroundings. For example, the American Alligator spectated in the America's where the stagnant water, and everlasting swamps caused them to look dark to camouflage into the mud, and they formed broader snouts to eat large fish whole. However, Nile Crocodiles show a v-like snout used for slimming the crocodile down to a thin creature that hunts in rapidly moving waters.The environment and changes in food sources have led to speciation within the crocodilia order. A more current example of gradual speciation, is seen in the Australian Freshwater Crocodile, which have turned to living in salt water habitats, because the fresh water habitats are endangered. Gradual speciation mainly exists within the crocodilia order to help species adapt to rapidly changing enviornments.

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Species within the Crocodilia order are scattered around the world and because of their diversity, and different environments, they evolve through divergent evolution. The species within the crocodilian order are throughout the world and therefor they must conform to new conditions and environments. The reason why the Nile Crocodile and Australian Freshwater Crocodile are different, is because they live on two different continents this division does not cause the species to evolve together, but it causes them to evolve further apart from one another. This is a long process in which the different environments will push species to change and to evolve to the most fit organisms to survive. This is why there are only around twenty different species within the crocodilia order even though they have lived on Earth for millions of years

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Adaptive radiation is one of the most prominent signs of evolution within the crocodilian order. All of the species within the crocodilian order share common physical charecteristics such as the same body structure, with sharp teeth and a powerful tail. However, because species of the crocodilian order are diffused throughout the world, the different environmental pressures will cause evolution within the crocodilian order. This is the reason why the small Chineese Alligator that lives in small rivers, feeding on small fish looks different than the massive Nile Crocodile that feeds on Water Buffalo. Environmental pressures cause animals to adapt and become more suited to three and live in their environment. This is the reason why the most fit organisms survive for their environment, and why the Nile Crocodile cannot live in China without adaptations contained by the Chinese Alligator.

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Geography Isolation is a main reason for evolution within the crocodilian order. All species within the crocodilian order spend atlas sometime of their life in the Ocean or Sea, but that does not mean that they connect with other species within the order. For example, geographic Isolation caused evolution between the Indian Gharials and Nile Crocodiles. Thick deserts in the Middle East separate these two species and prevent interaction. Even though they must spend a part of their life in the ocean, they can not physically swim across the Indian Ocean or walk across the Middle East to Interact with one another. This causes the Indian Gharials to live and stay in India and the Nile Crocodile to thrive in the Nile River. Another barrier is the Himalyan Mountains which separates the Indian Gharials from the Chinese Alligator. The two species may seem relatively close by a few miles, the treacherous Himilayan mountains separate the two species and prevent interaction. Physical features on Earth are some of the reason for evolution within the Crocodilia order.

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Decent with modification can be explained within the Crocodilian order through all of the evidence mentioned previously. DNA analysis, taxonomy, physical and behavioral characteristics, fossils, anatomy and physiology, and chromosomal analysis all prove that the species within the Crocodilian order have a common ancestor that they originate from. All of the evidence shows the passing of traits and characteristics within the DNA from Crocodilian ancestors to common species of the order crocodilia. The facts and evidence explains why the modern species of the order crocodilia and the ancestors are simualar in characteristics, and traits..

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Most of the species within the crocodilia order must be in the middle of the variant scale, where they need to be in the relative middle of the Bell Curve in order to be fit and survive. An example of this is in the Nile Crocodile where small crocodiles will be eaten by the larger ones and die, while being the biggest has it's advantages, but is unsustainable in such harsh and competitive environments. The large Crocodiles can not survive if they need much more food in order to survive, when food is limited and sauce. Species within the Crocodilian order need to be in the middle of the variant scale and extreme phenotypes are either unattainable or too weak to survive the competitive crocodilian world.

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http://www.nhptv.org/natureworks/americanalligator.htm
https://www.nature.org/newsfeatures/specialfeatures/animals/reptiles/nile-crocodile.xml
http://www.theanimalfiles.com/reptiles/crocodiles_alligators/gharial.html
http://www.fossilguy.com/sites/potomac/liv_col.htm
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4742319/

https://www.ncbi.nlm.nih.gov/pubmed/7720416

http://carnivoraforum.com/topic/9333543/1/

https://link.springer.com/chapter/10.1007%2F978-1-4615-6971-8_8#page-1
https://www.researchgate.net/publication/272548912_The_Zoogeography_of_the_Crocodilia
https://www.researchgate.net/publication/249287127_Molecular_Systematics_of_the_Order_Crocodilia
http://crocodilian.com/cnhc/csl.html
http://ordercrocodilia.weebly.com/characteristics.html