Primitive fossil members of the phylum Chordata with a calcite skeleton of echinoderm type. They occur in marine rocks of Cambrian to Pennsylvanian age (530–300 million years old) and, because of their skeletons, have traditionally been placed in the phylum Echinodermata. They are shown to be chordates, however, by many chordate anatomical features. Their calcite skeletons merely confirm an old view—that echinoderms and chordates are closely related.