 Following the description of H. bickelli , four additional species of sixgill stingray were described on the basis of morphological differences . However , their validity was brought into question after comparative studies revealed that traits such as snout shape , body proportions , and tooth number vary greatly with age and among individuals . Taxonomists therefore concluded tentatively that there is only a single species of sixgill stingray , though genetic analysis is needed to determine whether this is truly the case . Phylogenetic studies using morphological and genetic data have generally concurred that the sixgill stingray is the most basal member of the stingray lineage . An extinct relative , H. <unk> , lived during the Middle Eocene ( 49 – 37 million years ago ) . 
