Originating from an ancestor covered entirely by uniform rhomboid scales with numerous, highly segmented fin rays, the Triassic basal actinopterygian fish Saurichthys radiated into species diagnosed by different degrees of loss in scales, rays, and dermal bones. Such changes are analogous to those reported in mutants of different extant species, such as sticklebacks, zebrafish, and medaka. With this background, we infer a loss of function or a regulatory change of a signaling pathway as a key mechanism behind the morphological diversity of Saurichthys. Either the fibroblast growth factor pathway was affected, assuming that gene duplication had occurred, as for example in the closely related acipenserids or in teleosts, or the ectodysplasin pathway was involved, assuming that its pleiotropic effects led to viable morphological diversification. In contrast to a gradualistic model of evolution, a change in a major developmental gene serves as a viable explanation for the essential differences among the species of Saurichthys in the paleoecological context in which they originated. J. Exp. Zool. (Mol. Dev. Evol.) 314B:519–526, 2010. © 2010 Wiley-Liss, Inc.