“Agnathans”

By: J.Q

Agnathans

. Agnathans meaning “without jaws” these “jawless” fishes lack a biting apparatus.

Examples: Hagfishes (Myxinoidea) and lamprey (Petromyzontida)- together, these two living groups are known as cyclostomes (meaning “round” and “mouth”).

Extinct group of agnathans-conodonts-ostracoderms-pteraspidomophs

Living Agnathans - arose within Cambrian period

- all living agnathans lack bone and possess a single nostril

MyxinoideaHAGFISHES- deep sea, mud-burrowing, eel-like- scavengers- use teeth like processes on their muscular “tongue” to rasp flesh from prey or reel in worms. -possess slime glands beneath the skin release mucus - mucus, or “slime,” may serve to slip them from the grip of a predator or clog its gills.- hagfishes can knot their bodies to escape capture or give them force to tear off food

Hagfishes

Petromyzontida

LAMPREY.- uses its oval mouth to grasp a stone and hold its

position in a current. Two types: parasitic – marine non-parasitic - freshwater- parasitic lampreys, constitute about half of all

living lampreys, the mouth clings to live prey so that the rough “tongue” can rasp away flesh or clear skin, allowing the fish to open blood vessels below and drink of the fluid within.

- Medial fins are present, but paired fins and limbs areabsent. - Vertebrae are represented by individual blocks ofcartilage that ride atop the lamprey’s prominent notochord. - vestibular organ or ear for balancing

Lamprey

Life cycle of a lamprey

- Difference between lamprey and hagfish

Lamprey Hagfish- Ammocoete larva - No larva stage

- Indirect development -direct development

- Marine and freshwater - Marine only

- Mostly parasitic - scavengers

3 Early living fossils

- Conodonts- Ostracoderms- Pteraspidomorpha

Early living fossil

Conodonts

- slender, soft-bodied animal bearing a complete set of conodont elements in its pharynx. -the trunk exhibited evidence of a series of V-shapedmyomeres, a notochord down the midline, and caudal fin rayson what could be interpreted to be a postanal tail. -the notochord was a streak consistent with the interpretation of a dorsal nerve cord - conodont elements suggested the presence of mineralized dental tissues known from vertebrates, such as cellular bone, calcium phosphate crystals, calcified cartilage, enamel, anddentin.

Conodonts

Ostracoderms

- appeared in the very Late Cambrian - they had complex eye muscles and dentin like

tissues.- a few possessed paired appendages.- They were the first vertebrates to possess an

intricate lateral-line system, - an inner ear with two semicircular canals, and bone, although the bone is located almost exclusively in the outer exoskeleton that encases the body.- The endoskeleton of these later agnathans was not well developed and, when present was usually of cartilage inside the body.

Pteraspidomorpha

- appear in the Ordovician - they are represented at first only by splinters of

primitive bone lacking true bone cells (acellular bone).

- a vestibular apparatus with two semicircular canals and the presence of paired nasal openings seem to characterize most pteraspidomorphs.

- pteraspidomorphs had head shields formed by fusion of several large bony plates

Pteraspidomophs