Morphology and phylogeny. Pufferfish lack ribs and pelvis and have few vertebrae, as revealed by Alizarin red staining of the skeleton of the pufferfish Takifugu rubripes (a) and the zebrafish Danio rerio (b). (c) A phylogenetic tree for vertebrates (see Nelson 1994), times according to Hedges (2002), Hedges and Kumar (2002), and Santini and Tyler (1999). (fv) Few vertebrae; (mv) many vertebrae;(np) no pelvic appendage; (nr) no ribs; (p) pelvic apparatus; (r) ribs.