Any jawless craniate animal. Living representatives are the lampreys and hagfishes. Morphological and molecular evidence now strongly indicates that hagfishes (class Myxini) and lampreys (class Petromyzontida) are sister groups of the same vertebrate clade, Cyclostomata, with hagfishes deemed to have secondarily lost certain characteristic vertebrate features. Fossil agnathans, covered in an armour of bony plates, are the oldest known fossil vertebrates. They have been dated from the Silurian and Devonian periods, 440–345 million years ago. Fossil chordates resembling hagfishes have been found dating from much earlier times, notably Myllokunmingia of the Cambrian Chengjiang fossils.