Jawless Fishes

Ostracoderms

Superclass. Earliest major vertebrates on fossil record. Originated in the late Cambrian Period, but first record is from Ordovician and were abundant and diverse, became extinct by late Devonian Period. Important traits: Bony armor (first record of bone in fossils), internal skeleton made of cartilage, numerous gill arches, heterocercal tail, two semicircular canals, lacked true jaws and paired fins(weak swimmers), benthic habitat, small in size, none larger than 15 cm.

Pteraspidomorphi

Class within ostracoderm, has three orders that are all extinct.

Cephalaspidomorphi

Class within ostracoderm, believed to relate to petromyzontiformes (lampreys). Made reconstruction of cranial nerves possibles,

Myxini

Class of hagfish

Myxiniformes

Order of hagfish, date to the mid carboniferous. Known for one semicircular canal, single olfactory capsule, 1 to 16 openings of gill pouches, body naked. More traits include absent dorsal fin, eyes degenerate, barbels present, teeth on reversible plate (tongue), capable of performing rasping action, and fertilized eggs deposited in leathery egg capsule. They feed on flesh by attaching themselves and ripping it off by forming a knot and pushing it down their bodies. Also secrete slime as a defense mechanism.

Petomyzontiformes

Order of lampreys, first appear in the Early Caboniferous. Notable traits include two semicircular canals, single olfactory organ and single innervation, seven openings to gill pouches, naked body, one or two dorsal fins, well developed eyes, no barbels, teeth on tongue and oral disc, fertilized eggs lain in nest, embryos hatch into ammocoetes. Parasitic and nonparastitic.

Petomyzontidae

Family of lamprey. Lives in temperate zones, anadromous and freshwater.