Agnatha (Lampreys, Hagfishes, Ostracoderms)

Agnathans are an ensemble of jawless (nongnathostome) vertebrates, some of which are more closely related to the gnathostomes than others. Living agnathans fall into two groups, hagfishes (Hyperotreta) and lampreys (Hyperoartia), united into a higher group, the cyclostomes (Cyclostomi). Key Concepts • Phylogenomics now demonstrates that cyclostomes are a monophyletic group contrary to what morphological features suggested. • The rise of bone and dentine occurred after the divergence of the cyclostomes. • The first mineralised skeleton was in the form of small ‘odontodes’ made up by acellular bone and dentine in the dermis, which later fuse into larger plates. • All jawless vertebrates that produce bone and dentine are stem jawed vertebrate, or gnathostomes, and informally referred to as ‘ostracoderms’. • Nonmineralised soft tissues can sometimes be preserved in fossils under particular environmental conditions, and provide information about the early history of cyclostome and jawless fishes in general.