Gnathostomes Shark dissection

Introduction One of the most significant changes to the early vertebrate was the development of jaws in primitive fishes. These prey grasping and biting devices derived from the anterior pharyngeal aches. Early jaws could grasp, bite, or crush prey, allowing these fishes to capture and process larger food. This adaptation opened the way for advances in predatory life.

Overall, this radiation of gnathostomes proceeded along two lines; Early gnathostomes possessed two sets of paired fins (pectoral and pelvic) that were supported on girdles and controlled by specialized musculature. Paired fins allow organisms to maneuver and prowl within the aquatic environment by resisting roll. Overall, this radiation of gnathostomes proceeded along two lines; One producing the Chondrichthyes, And another that lest to the Teleostomi

Chondrichthyes The modern chondrichthyans consist of two groups, the sharks and rays (elasmobranchs) and the ratfishes (holocephalans). Members of these two groups possess skeletons predominately made of cartilage infused with calcium. Since bone was present in early agnathans this lack of bone is probably a secondary adaptation.

Like most fishes, chondrichthyes are more dense that the water in which they live. In bottom dwellers this is not an issue, but open water swimmers must compensate for this in some way. Large livers filled with oil, fixed pectoral fins to act as hydrofoils, and heterocercal tails all help supply lift. In the earliest species, the notochord is the primary axial skeletal structure. Modern chondrichthyans have a vertebral column, made of cartilage, that replaces the notochord as the functional support for the body.

Elasmobranchii The majority of Devonian sharks lived in fresh water (as do a few modern species). But, most modern sharks live in marine environments. In general, sharks are characterized by having a mouth full of pointed, serrated teeth. The functional teeth are lost in a regular pattern and replaced by teeth waiting in the successive rows. Another key feature in modern sharks is the electric organ, or Ampuli of Lorenzini, that is located on the snout.

All rays belong to the family Batoidea. Most modern rays are bottom-dwelling, specialists with a fossil record from the Jurassic. The pectoral fins of all rays are enlarged and fused to the head to give the dorsoventrally flattened body an overall disc shape. The tail is reduced, and flapping of the pectoral fins is the primary method of locomotion. Unlike sharks, ray teeth are designed for crushing mollusks and crustaceans. Many rays spend time resting on the bottom of their aquatic habitat, in order to successfully exchange gases a spiracle is located on the top of the body, usually behind the eyes.

Lab Objectives Today we are examining the external anatomy and musculature of the dogfish shark. Begin with identification of all external features. Removal of the skin can be done by making an incision along the dorsal surface and peeling back the skin as you carefully separate it from the fish. You do not have to remove all of the skin, just remove skin from one side, and do not go below the upper abdomen or it will tear into the body cavity.

Identify External Muscles Placoid Scales Pectoral, Pelvic, Dorsal and Caudal Fins Ampullae of Lorenzini Cloaca External gill slits and Spiracle Claspers (Male Only) Muscles Myomeres and Myosepta (Epaxial and Hypaxial Divisions) Horizontal Septum Pectoral and Pelvic abductor and adductor muscles Dorsal and ventral superficial constrictors Cucullaris Adductor mandibulae Intermandibulae Coracomandibularis and Coracohyoids