Dissected and stained larvae of the Slender Sunfish (Ranzania laevis). This research demonstrated that sunfish — including of course Mola mola — have lost their caudal fin and replaced it with a lookalike structure made out of the dorsal and anal fins.
Johnson, G. & Britz, R. (2005) Leis’ Conundrum: Homology of the Clavus of the Ocean Sunfishes. 2. Ontogeny of the Median Fins and Axial Skeleton of Ranzania laevis (Teleostei, Tetraodontiformes, Molidae). Journal of Morphology 266 11–21.