Hindlimb Development in a Dolphin
The first sign of limbs in mammalian embryos is the development of a small bump on the side of the animal, called a limb bud.
Forelimb buds develop near the middle of the embryo, and hindlimb buds develop near the root of the tail.
Modern dolphins, newborns as well as adults, lack any external signs of hindlimbs.
However, in dolphin embryos, a small hindlimb bud does develop and grow (circled in the embryos above), but then (in a later embryonic stage, on the right) the hindlimb bud reduces again.
The presence of the initiation of hindlimb development suggests that dolphins had terrestrial ancestors with four limbs,
and several of these ancestors have been found as fossils (http://www.neoucom.edu/DEPTS/ANAT/Ambulocet.html)
Modern adult dolphins still have a small internal remnant of the bony pelvis.
Some of the muscles to the genitals attach at this short piece of bone which is located ventrally at the root of the tail in the pelvis.