Jawless fish

Jawless fish are vertebrates that lack hinged jaws. Living forms include lampreys and hagfish; extinct forms include many armored lineages.

Vertebrate: An animal with a vertebral column or its evolutionary precursor, the notochord. Jawless fish are early-diverging vertebrates, though their backbones vary in form.
Lamprey feeding: A feeding mode in which many lampreys attach to hosts with a toothed oral funnel and rasp tissue. It demonstrates how a jawless fish can feed parasitically using a specialized mouth.
Lamprey: An eel-shaped jawless fish with a cartilaginous skeleton and a round oral disc. Lampreys are one of the two surviving jawless fish groups and include parasitic species.
Vertebrate evolution: The history of vertebrate diversification, anatomical change, and relationships through geological time. Jawless fossils illuminate vertebrate history before and after the origin of jaws.
Jawed vertebrate: A vertebrate with upper and lower jaws derived from the anterior pharyngeal arches. Jawed vertebrates contrast with jawless forms in feeding anatomy and evolutionary history.
Notochord: A flexible rod of tissue that supports the body in chordate embryos and some adults. It provides axial support in jawless fish, alongside or instead of vertebral elements.
Hagfish slime: A rapidly expanding mucus-and-fiber secretion that hagfish release when threatened. This defense compensates for vulnerability and can clog a predator's gills.
Hagfish: A marine jawless craniate with a skull, reduced vertebral elements, and copious slime glands. Hagfish are the other living jawless group and differ markedly from lampreys.
Ordovician: A geological period lasting from about 485 to 444 million years ago. Some of the earliest evidence for vertebrates comes from Ordovician fossils.
Gnathostome: A member of the vertebrate clade containing all jawed vertebrates. Gnathostomes are the major vertebrate lineage that evolved jaws after jawless forms appeared.

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