Question: this change from fins on the lobe finned fish

title: Evolution of fish
text: Pikaia External video Pikaia gracilens animation The Burgess Shale Pikaia , along with Myllokunmingia and Haikouichthys ercaicunensis immediately below , are all candidates in the fossil record for the titles of `` first vertebrate '' and `` first fish '' . Pikaia is a genus that appeared about 530 Ma during the Cambrian explosion of multicellular life . Pikaia gracilens ( pictured ) is a transitional fossil between invertebrates and vertebrates , and may be the earliest known chordate . In this sense it may have been the original ancestor of fishes . It was a primitive creature with no evidence of eyes , without a well defined head , and less than 2 inches ( 5 centimetres ) long . Pikaia was a sideways - flattened , leaf - shaped animal that swam by throwing its body into a series of S - shaped , zig - zag curves , similar to movement of snakes . Fish inherited the same swimming movement , but they generally have stiffer backbones . It had a pair of large head tentacles and a series of short appendages , which may be linked to gill slits , on either side of its head . Pikaia shows the essential prerequisites for vertebrates . The flattened body is divided into pairs of segmented muscle blocks , seen as faint vertical lines . The muscles lie on either side of a flexible structure resembling a rod that runs from the tip of the head to the tip of the tail .