Agnathans

Abstract

Living jawless vertebrates were formerly included in the cyclostomes, a group comprised of lampreys (petromyzontids), hagfishes (myxinoids), and various groups of extinct jawless fishes. Cyclostomes, however, are no longer considered a natural group. Researchers have also grappled with whether lampreys and hagfishes are representatives of a monophyletic group of vertebrates, having shared a common ancestor, or whether they represent two separate lines of jawless craniates. A further issue is the relationship between extant agnathans on the one hand and jawed (gnathostome) vertebrates on the other: are lampreys more closely related to jawed vertebrates than are hagfish? Although debate continues, the weight of evidence supports the view that lampreys and hagfishes have quite different and separate evolutionary histories, and that only lampreys share a close evolutionary relationship with the jawed vertebrates (Fig. 4.1, also see Fig. 1.1). Indeed, while lampreys and hagfishes are both craniates, hagfish lack an axial vertebral skeleton and so are not vertebrates (see Fig. 3.1).¹