Conway Morris classified the Burgess Shale fossil Pikaia as a chordate because it had a rudimentary notochord, the rod of cartilage that evolved into the backbone of vertebrates.[63] Doubts have been raised about this, because most of the important features are not quite like those of chordates: it has repeated blocks of muscle along its sides but they are not chevron-shaped; there is no clear evidence of anything like gills; and its throat appears to be in the upper part of its body rather than the lower.[64] It also has "tentacles" on the front of its head, unlike living chordates.[65] At best it may be a stem group chordate, in other words an evolutionary "aunt" of living chordates.[64] Metaspriggina, also found in the Burgess Shale but even rarer, may be a chordate, if the repeated chevron-shaped structures along its sides represent muscle blocks.[63] While Pikaia was celebrated in the mid-1970s as the earliest known chordate,[66] three jawless fish have since been found among the Chengjiang fossils, which are about 17 million years older than the Burgess Shale.[63]