Lab 3: Crocodylia: Matt Knestrick & Megan Thornhill

Introduction

In this lab, we explored members of the Order Crocodylia. Although typically known as just crocodiles or alligators, this Order is very ancient, very diverse, and one of the most recognizable reptile species on the planet. We examined features of the skulls and skeletons, examined morphological differences between the Crocodylian families, and finally classified a group of preserved specimen.

Reptile Families
There are three families in the Crocodylian Order: Gavialidea, Alligatoridea, and crocodyloidea. They can be distinguished by their habitats and anatomy. The Gavialidea are found in Asia and Indonesia. They have notedly long, narrow snouts, with teeth outside of the jaw. This family

is the most aquatic of the Crocodylia, and feed mostly on fish. Alligatoridea are the alligators and caimens. They are located in Eastern North America, Central and South America, and parts of Asia. They have moderately long, particularly broad jaws that set them apart. This family include the notorious “man-eating” crocodiles. Crocodyloidea are the crocodiles and dwarf crocodiles. They are found throughout the southern hemisphere, in Africa, Indonesia, and South America. They have longer, narrower jaws than alligators, but shorter, thicker jaws than Gavialidea. The skulls of these crocodiles illustrate the differences between them.

Skeleton & Skulls

There are variations between the Crocodylian families present in their skulls and skeletons. For the most part, though, all the families exhibit the same skull bones in the cranium and jaw. As

illustrated in the photos, the different families have distinct skull size and shape. However, the bones are all the same. Also of note are the two finestrae (holes located behind the eyes). There is a supratemporal finestra, located above the eyes, and a subtemporal finestra, located below the eyes.

Reptiles may demonstrate morphological variation in the skeleton. However, most possess some sort of skeleton expected in Tetrapoda, at least with regard to general regions of the skeleton. The vertebrae of an Alligator skeleton are separated into four distinct groups. The first of these groups are the cervical vertebrae, which contain the axis-atlas complex with the first and second vertebrae. Second are the trunk vertebrae, which is separated even further into the thoracic vertebrae and the lumbar

vertebrae. The thoracic vertebrae are the rib bearing vertebrae while the lumbar vertebrae are trunk vertebrae without limbs and ribs, but contain broad transverse processes. The third division of the vertebral column is the sacral vertebrae, which have broad transverse processes the articulate with the ilia. The final separation of the vertebrae is the caudal vertebrae, which lack ribs but have conspicuous hemal arches.

Other important regions of the Alligator skeleton are the Pelvic Girdle and Hind Limbs. The pelvic girdle is made of the Ilium, Ischium, and Pubis, which contact each other at the Acetabulum. From this point the Femur is in contact with the Ilium, Ischium, and Pubis. Then the Tibia and Fibula are connected at the joint at the opposite end of the Femur. The Tibia is associated with the organism’s first digit and the Fibula is associated with the outer digit. Then the ankle bones or Tarsals, particularly the fibulare are between the Tibia and Fibula and the Metatarsals. Finally are the Phalanges of which the ungula phalanx is the one that bears the claws of the crocodile.

Also, there is the Pectoral Girdle and Forelimbs. The Scapula and Coracoid of the organism contact one another at the head of the humerus bone of the upper forelimb. The Interclavicle and Sternum are in contact with one another at the place where they are also contacting the Coracoid. The Humerus is in contact with the Radius and Ulna at the end opposite that which has contact with the Sacpula and Coracoid. The Radius is associated with the crocodile’s first digit and the Ulna with the outer digit. The Carpals, or wrist bones, are assimilated in a way similar to those associated with the pelvic girdle, except for the lack of Fibulare. Following the Carpals are the presence of the Metacarpals and following the Metacarpals are the Phalanges.

Classification

The final part of the lab involved classifying a collection of Crocodylia specimen as

gavialidea, alligatoridea, or crocodyloidea. The scale ridge along the front of the eyes, as well as the scale formation on its back helped us (after a great deal of time) to determine the specimen as caimens. They had the scaley ridge in front of their eyes, and had the scale pattern on their backs. We were also able to determine the sex of the individual specimen by examining the cloaca. The male caimens had a small “penis” within the anterior wall of the cloaca, while the females did not

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