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Physical Characteristics (Internal & External):

1. Eyes (vertical pupils) and nostrils that are positioned on top of the head

2. Powerful jaws and muscular tails

3. Cold-blooded

4. Have a palatal valve which is a flap that prevents water getting into their mouths when submerged

5. Do not have sex chromosomes

Behavioral Characteristics:

1. Carnivores, sometimes cannibals

2. Adults provide extensive parental care to young

3. Oviparous (lay eggs)

4. Feed during both day and night

5. Usually live in smaller groups

Specific Habitat Requirements:

1. Aquatic Habitat

1. Crocodilians are semi-aquatic predators and have adapted to life on both land and water

2. Favor freshwater and calmer waters

2. Tropic (Warm) Climate

1. Crocodilians are cold-blooded and therefore cannot withstand temperatures colder than those in temperate climates

3. Proximity to Land

1. Crocodilians need land for shelter as well as breeding. "Gator holes" are usually dug up in land for shelter and protection. Land is also needed for breeding as crocodilians build their nests and lay their eggs outside of water

4. Sufficient Amount of...

1. Prey - feeding

2. Mud - shelter

3. Vegetation - breeding

Source1 & Source2

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1. American Alligator

2. Australian Freshwater Crocodile

3. Indian Gharial

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Habitat: Southeastern United States

Classification:
Common Name | American Alligator
Kingdom | Animalia
Phylum | Chordata
Class | Reptilia
Order | Crocodilia
Family | Alligatoridae
Genus | Alligator
Species | A. mississippiensis

Unique Characteristic: Broad Snout

Other Characteristics:

1. Appearance: juveniles are pale yellow with black stripes and spots along their back while the body color of adults usually darken into tan or gray

2. Functional: thermoregulate, lack functional salt-glands, jaw consists of around 68 teeth, and the edge of the upper jaw overlaps teeth in the lower jaw

3. Structural: broad snout, bony nasal bridge is present, males reach ~ 4-4.5 meters long while females reach ~ 3 meters long, ventral osteoderms (bony plates) are present, and are known to fall into two general forms: those which are long and thin, and those which are short and stock

4. Behavior: opportunistic feeders, their feeding time is governed by water temperature, eat a large range of organisms from fish , to small mammals, to other smaller crocodiles, hunt/feed both during the day and night, can "high walk" on land, usually live in small groups or in solitary, build nests, and give extensive to young. Have also been know to mainly avoid human interaction

Source1 & Source2

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Habitat: Northern Australia

Classification:
Common Name | Australian Freshwater Crocodile
Kingdom | Animalia
Phylum | Chordata
Class | Reptilia
Order | Crocodilia
Family | Crocodylidae
Genus | Crocodylus
Species | C. johnsoni

Unique Characteristic: Small Size (rarely exceed 2.5 - 3 meters long) and unique snout

Other Characteristics:

1. Appearance: the body colour is light brown with darker bands on the body and tail which tend to be broken up near the neck. Some individuals possess distinct bands or speckling on the snout

2. Functional: thermoregulate and naturally lack functioning salt-glands however, endangered habitats have lead to an increase of freshwater crocodiles moving into more saltine environments, therefore slowly making the glands more functional as required

3. Structural: unusually narrow and rampering snout, rarely exceed 2.5 - 3 meters, and have relatively large body scales.

4. Behavior: piscivorous (fish-based diet), favor the sit-and-wait hunting method, rarely feeds during the dry season, breeding starts in the early months of the dry season, live in small groups or in solitary, build nests (often in groups), dig/live in "gator holes", provide protection for young for a variable period.

Source

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Habitat: India and Nepal

Classification:
Common Name | Indian Gharial
Kingdom | Animalia
Phylum | Chordata
Class | Reptilia
Order | Crocodilia
Family | Gavialidae
Genus | Gavialis
Species | G. gangeticus

Unique Characteristic: "Ghara" and elongated, narrow snout

Other Characteristics:

1. Appearance: large size (one of the largest species with individuals reaching 5-6 meters in length), elongated narrow snout, males have a ghara, elongated jaw lined with many interlocking razor-sharp teeth, laterally flattened well-developed tail, and extensively webbed rear feet

2. Functional: thermoregulate, lack a functioning salt gland, and males have a ghara that can used for many functions. It works as a vocal resonator which can be used to produce buzzing noises during social behavior, a visual stimulus for females during courtship, and an aid to producing bubbles during courtship

3. Structural: elongated jaw as an adaption for their diet, poorly leg musculature, have extensively webbed rear feet, and has a laterally flattened well-developed tail

4. Behavior: exclusively fish eaters at maturity, hunt as opportunists, hunt during both day and night, live in small groups, "belly slide" due to poor leg musculature, live mainly in water to thermoregulate, mate in a two month period between November, December, and into January, build nests, females do not assist new borns (most likely because their jaws are unsuitable for carrying hatchlings), and protect young around the nesting area for some time after hatching.

Source

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One way to see how different organisms are related is though taxonomy. A lot of information about relatedness of different species can be shown through taxonomy as it shows connects and similarities between the species. In this comparison – between the american alligator, australian freshwater crocodile, and indian gharial – taxonomy allows us to see that the species are indeed, very closely related. This conclusion is shown through the similarities that the species have in terms of what kingdom, phylum, class, and order they are in. Being classified similarly shows that the species are related and how they share many traits, ancestors, and characteristics with each other. This is how taxonomy alone can be used to gather information about the relatedness of different species.

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Crocodilians have been around for a very very long time. Evidence found of their existence has not only shown us that they have been on earth for a long time, but also that they have been slowly evolving and changing into the modern crocodilians they are now. This evidence can be found in many different areas including

1. Embryology

2. The Fossil Record

3. Anatomy and Physiology

4. Chromosomal Analysis

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The embryos of crocodilia are very similar physically however, have small differentiating characteristics that develop later on in embryonic development depending on the species.

All crocodilians have the same outwards appearance of 4 short legs, a powerful tail, two eyes, and similar skin color and texture which can be shown through the embryonic development of all species. The embryos of all species start to develop eyes at about 3 weeks, limbs and distinct snout shape at 5 weeks, and then the embryos are fully developed and ready to be laid as eggs after 8 weeks.

In terms of differences, crocodilians tend to have different shapes of teeth and snouts or mouths which can be seen in the embryos at around 5 weeks (as stated above). For example, the American Alligator has a wider mouth while the indian gharial has a longer and skinnier mouth. The teeth each species have are different as well to accommodate the different types of mouths however, despite their differences the embryos still develop the same way and at around the same rate.

Source

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Indian Gharial Embryo ~ 8 weeks

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Many crocodilian fossils have been found over time and can date back to as far as the dinosaur age. These fossils can be used as key evidence for evolution since they serve as physical proof of a time before. By comparing fossils to the bones and structures of modern day crocodilians, scientists and researchers are able to draw conclusions about how the organisms changed and evolved over time

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Around 70 Million Years Old - Late Cretaceous Period

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Around 70 Million Years Old - Late Cretaceous Period

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Around 70 Million Years Old - Late Cretaceous Period

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Large Leidysuchus Crocodylian Claw from Hell Creek Formation

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Large Leidysuchus Crocodylian Claw from Hell Creek Formation

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Around 70 Million Years Old - Late Cretaceous Period

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110 Million Years Old - Cretaceous Period

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Around 1.8 Million Years Ago - Pleistocene Era

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These fossils show evidence of evolution through their shape, size, appearance, and more. By comparing these fossils to crocodilians living today, there are many changes and differences between the two. These changes can range in size and significance however, all changes serve as important evidence for evolution. They can help scientists and researchers determine why, when, and how crocodilia species have been changing over time. For example, the fossils of Sarcosuchus Imperator show the organism having a very large head, and therefore body as well, which is very different when compared to the size of modern crocodilia today. This comparison acts as evidence for evolution because it physically shows the differences from times before when crocodilians were much larger and different

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Chromosomal analysis has also shown evolution in crocodilia in terms of reproduction. Species of crocodilia do not contain X and Y chromosomes and instead have evolved to allow the temperature of egg incubation to determine the sex of their offspring. This genetic and biochemical evidence supports evolution because it shows how crocodilians have become more efficient in reproducing in terms of producing a certain sex of offspring.

Crocodilians also have a similar number range of chromosomes since there is not a large range of species found throughout the world.

Source

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Picture

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Picture

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Similarities:

1. American Alligator (A. mississippiensis) vs Siamese crocodile (Crocodylus siamensis):

1. The size of each pair decrease in size as they go on

2. The first pair of chromosomes are significantly larger than the rest

3. Both lack sex chromosomes, as all crocodilians do

Differences:

1. American Alligator (A. mississippiensis) vs Siamese crocodile (Crocodylus siamensis):

1. Number of chromosomes

2. Crocodilians vs. Humans:

1. Humans have 23 pairs of chromosomes while crocodilians have around 15-17 depending on the species

2. Humans have sex chromosomes while crocodilians do not

3. The chromosomes of humans seem to be longer in size while the chromosomes of crocodilians seem to mainly be short and small in size

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Anatomy:
Within the crocodilia taxonomy group, every species' anatomy is fairly similar. Almost all crocodilia share the same body structure ㅡ short legs, sharp teeth, long powerful tails, etc. The biggest differences in their anatomy is their skulls which significantly influences their physical appearance. The differences in the head are usually correlated to the habitat/location of the species. These differences show evolution through adaptations of crocodilian when they are put in different locations and habitats. Although crocodilia skulls are the most prevenlent evidence of evolution other changes, both large or minute, within certain bones of crocodilia also serve as evidence. Recorded changes in the size or function of a bone overtime can found in fossils and used as evidence for evolution.

Physiology:
It is well know that crocodilians have short legs that allow them to live on both land and in water. Their limbs are used for land transportation, movement through water, digging holes and more to accommodate their lives however, the functionality and structure of the their forelimbs are not especially unique. The forelimb is an example of a homologous structure that exists within the animal kingdom. The structure resides in a great range of organisms including frogs, birds, rabbits, lizards and more however, the way the different organisms use the structure varies. The common set of bones that create the structure (the radius, ulna, and humerus) look and function differently based on the species. For example the bones in the forelimb of the bird may be elongated to help serve the function of wings and flying whereas the forelimbs of alligators are shorter to prevent water resistance when swimming. Other species may use the structure for jumping, walking, hanging, gliding, etc. The original use of the structure was for transportation as prehistoric fish transistioned from water to land however the structure has evolved throughout time to accommodate the many different species who use today.

Source
A. Forelimb of a modern crocodilian
B. Geosaurus (Upper Jurassic)

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Head/Snout Shape

This picture shows the differences and common relationships shared between crocodilians. The picture connects all the different species to a common ancestor and then separates the species as evolution occurs and their traits change. As the species continue to break down more and more, relationships between the species are shown by common ancestors that carried the same traits. Eventually by the end of the graph, the different species of crocodilians have their own specialized features, in this case head appearence, that other species may not have. This graph also shows that these  changes in traits may be due to the crocodilian’s environment or location in the world. Relationships of common location can be found within the graph and can help link certain species to a common ancestor. The tree successfully shows the relationships between crocodilian species and why they came to be in terms of common ancestors and locations around the world.

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The range of different crocodilian species has been a result of gradual speciation. Over the long period of time that crocodilians have been on earth, they have changed and evolved over time. Most of the changes were gradual and lead to speciation as individuals migrated and inhabited new environments. New environments forced individuals to adapt to their new surroundings and resources causing them to inherit certain traits and skills. Over long periods of time, the populations conformed to fit their different environments, specializing them from one another – from diet, to way of movement, to behavior, etc.

One current example of this is related to the Australian Freshwater Crocodile. As their freshwater habitats become more endangered, many are finding home further out in more saltine locations. This has forced the new inhabitants to change and adapt in order to survive in the saltier waters. Over long periods of time, if more individuals follow in pursuit, there may be speciation between the original population living in freshwater and the other species inhabiting salt water. Traits including functionality of salt-glands, diet, appearance, and behavior are just some ideas of how they may speciate from one another.

Source

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As stated briefly earlier, immigration and emigration have served as a major role for evolution in crocodilians. Crocodilians' habits of needing space and living in small groups have significantly impacted their migration habits and have lead to the constant migration of populations. It is harder for individuals or small groups to find permanent places to live due to competition and resources, therefore causing many to migrate and move constantly. While this helps the gene pool and gene flow of the species, it also forces evolution to occur slower. The constant change of individuals causes stabilizing natural selection to occur because both the extremes are unbeneficial which then reduces the amount of variation. However, when populations migrate and move into new environment, evolution occurs faster as populations change to conform into their new habitat.

Source

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Crocodilians have gone through many instances of divergent evolution as they have migrated and inhabited many places around the world over time. This movement throughout time has caused different populations to take on new traits as individuals conform to new environments and conditions. Over time these new traits, lifestyles, and other aspects of the new individuals spread to more individuals and cause them to create a new population or group. Over more time as the two groups grow, the differences between them become more prominent and create a clear distinction between the two which then leads to the formation of a new species. This process is very long and occurs very slowly which explain why there are not too many different species of crocodilians.

Source

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Adaptive radiation is very prevalent within the crocodilia order and is shown through the different species' physical characteristics. All alligators have a similar shape however certain features may look different from species to species because of environmental adaptations. For example, the American Alligator and the Indian Gharial look pretty similar in terms of their bodies however, their heads look very different. This most likely has to do with their locations and the adaptation they had to make to live there. The Indian Gharial has a long slender head which helps it swim and hunt fish in its habitat of fast-moving rivers. The American Alligator does not need a similiarily shaped head as the Gharial since it lives in swamps and other slow moving bodies of water. It instead has a very broad head.

Source

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Crocodilian populations often experience stabilizing natural selection. This is due to the advantages of being average. Although smaller alligators do not need as much food to survive and are small enough to hide from predators they are often at risk of being hunted. Despite being able to hide, they are not fast enough to escape larger predators and are often eaten, even by other crocodilians. Larger crocodiles are also at a disadvantage because they need a large food source to survive (which is not always available), have trouble finding enough space for themelsves to live and are often sought out and hunted by humans. These factors have caused populations to favor the average individuals, as opposed to those of extremes, which has lead to stabilized natural selection.

Source

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All evidence of evolution points towards common descent for crocodilians. From the relatedness of taxonomy and similarities in characteristics, embryology, fossils, anatomy, physiology, and chromosomal data it can be concluded that all crocodilians came from a common ancestor. Modern species are already very similar to each other and can also be considered similar to ancient species when compared. The functions and characteristics of crocodilians from around the world today and in the past, all the way up to prehistoric times, all show similarities. Chromosomes and embryos as well show even more similarities which all lead to the conclusion that crocodilians are indeed similar and are most likely from a common descent to explain why they're so similar.