Lancelets are somewhere in between, with a notochord that remains throughout life. There is good evidence that Cathaymyrus and Pikaia had notochords. But the matter is more debatable for the strange fossil Yunnanozoon , also from Chengjiang. Some researchers think that Yunnanozoon represents a whole group of extinct chordates. Dr Shu and colleagues disagree, placing it among distant relatives of chordates called hemichordates.
Chordates have other defining features.
In one, the throat is modified to make a kind of filter, in which particles of food entering the mouth are strained from the water, the 'exhaust' leaving through the gills. The larvae of lampreys also have this organ, but it becomes transformed, in adults, into the thyroid gland. The existence of goitres and cretinism, conditions related to iodine deficiency, can be blamed on our filter-feeding heritage. Cathaymyrus preserves faint traces of gill slits, but these have not yet been seen in Pikaia . Neither have gills been traced in conodonts, a large, extinct group of vaguely fishy creatures with formidable teeth.