What is the lineage of a jawless vertebrate?

Lamprey (agnathans) is the lineage of a jawless vertebrate.

Explanation:

Lampreys are jawless fishes (or agnathans), closely related to other living vertebrates, the jawed vertebrates (or gnathostomes). They, along with hagfish, are the only known surviving lineage of once diverse groups of jawless fishes. Living cyclostomes are modern yet they have some anatomic elements that appear to be retained from primitive members of their own groups, and possibly of primitive ancestral vertebrates. Lampreys are readily obtainable, and comparisons between lampreys and vertebrates are useful for the identification of developmental traits that are putatively derived from ancestral vertebrates. Of the two cyclostome groups, lampreys are the more experimentally tractable developmental models, and work has been done on a variety of species.