Lab 3: Crocodylia (Maggie and Kelly)

Although the Crocodyliformes have been around about 220 million years, the extant crocodilians of today have their roots in the Late Cretaceous, or around 65-70 million years ago. Twenty-three extant species have been described, and they all fall into three extant superfamilies.

General Anatomy:

(A) Dorsal view (B) Ventral view of skull (C) Lateral view (D) Jaw

The bodies of  extant crocodilians are adapted to a semiaquatic or aquatic life. Their tails are laterally compressed and make them efficient swimmers. The eyes are positioned on the top of the head, allowing the body to be concealed beneath the water while searching for prey.

Crocodilians have long, robust snouts, which are necessary for their strictly carnivorous diet. The actual shape of the snout can provide information about what type of diet the particular croc species consumes: a more slender snout indicates a fish diet, while a heavier snout would indicate a diet of food items which require more crushing power.

 The skull of a crocodilian displays a good example of the diapsid condition. The subtemporal and supratemporal fenestra can easily be seen when looking at the skull.

Left to Right: Gharial, Caiman, Alligator. Note the differences among the snouts.

Crocodylian "penis"

Crocs have internal fertilization and direct development, which takes place inside a cleidoic egg. The "penis" of a crocodilian extends from the anterior wall of the cloaca. By slightly dissecting into the cloaca, we can determine is a specimen is a male or female.

In addition, crocodilians exhibit parental care in the form of nest building and guarding. Females also help the young to hatch out of their eggs and continue to guard the new hatchlings until they are larger.

Crocodilian Superfamilies:

There are three Superfamilies within Crocodylia: Gavialoidea, Alligatoroidea, and Crocodyloidea.

Gavialoidea: Gavialidae

Gharial. Note the placement of the teeth and the presence of the boss. (1)

Crocs in this family are know as gharials. Gharials are found in deep, fast-flowing rivers in India and  Southeast Asia. Males of the species Gavialis gangeticus have a process known as a "boss" located on the end of the snout. In addition to the boss, another way to identify this species is the location of the teeth when the mouth is closed: all teeth are located outside of the jaw when the jaws are closed. This species is partial to fish. Another species of gharial, Tomistoma schlegelii, has the characteristic long and slender snout; however, this species preys on much more than fish, and their teeth remain inside the mouth when the jaws are closed.

Alligatoroidea: Alligatoridae

Caiman

Species in this family are found in southeastern North America, Central America, South America, and China. When the jaws are closed, all the teeth are inside the mouth. The snout is broader than that of the gharial species. Within this family, there are two subfamilies: Alligatorinae (Alligators) and Caimaninae (Caimans). Species in both subfamilies are freshwater species. The genus Alligator is an opportunistic carnivore, which basically means they will eat anything. The Caimaninae subfamily is divided up into three genera: Caiman, Melanosuchus, and Paleosuchus. These are found in Central and South America.


Crocodyloidea: Crocodylidae

Crocodylus. (2)

This family contains some of the largest crocodylians, with some members reaching up to seven meters in length. Their snouts are broader than gharial snouts, but longer than alligator and caiman snouts. When the jaws are closed, the fourth tooth in the lower jaw is outside of the mouth. Most of the species within Crocodylidae are distributed throughout the New and Old World tropics, but some have ranges that extend into temperate zones in the Southern hemisphere. Unfortunately, many of these species are on the brink of extinction.

Maggie instructs her fellow classmates on parts of the skull.

Crocodilian Phylogeny:

The exact phylogeny of crocodilians is up for debate at the moment. Molecular data suggests that the family Alligatoridae is sister to all other crocodilians; however, morphological data points to Gavialis (from the family Gavilidae) as the sister to all other crocodilians.

Stay tuned for our next adventure in Biology of the Reptilia, when we take over the Cleveland Natural History Museum!

KG & MH

Citations:
(1) http://www.crocodilenepal.org/images/croc1.jpg
(2) http://www.worldofstock.com/slides/NAN6572.jpg