Haikouichthys

They grew to the size of a man's thumb, with a segmented, streamlined body and a single dorsal fin that merged with the tail. Along his side, Haikouichthys had from six to nine gills and multiple circular structures on his bottom that could be gonads or slime organs (as seen in modern hagfish). He lived during the Lower Cambrian period in China, some 525 MYA.