Even the earliest ichthyosaurs had flippers, or fin-shaped limbs.
There are many ways to call them, but it has become customary among
paleontologists over the past few decades to call the front pair forefins,
and the back pair hindfins.
Ichthyosaurs probably used their forefins to maneuver during swimming, as
in living fishes and cetaceans (dolphins and whales). Some paleontologists
have suspected that certain ichthyosaurs used their limbs for propulsion,
but it seems that their shoulders and hips were not very robust, unlike in
vertebrates that use their limbs for propulsion.
The limb skeletons went through a drastic modification as ichthyosaurs
evolved. You can clearly see it by plotting the shape of the forefin
skeleton along the family tree of ichthyosaurs, as in the above figure.
You can make the following observations about the evolution of forefin
skeletons in ichthyosaurs: (1) The lower arm bones became shorter and
shorter along the family tree, although there are exceptions; (2) The
finger bones also became shorter and shorter, and eventually became
disk-shaped; (3) The number of finger bone increased early in the
evolution; (4) The thumb disappeared at one point, and then additional
digits (light blue) appeared on both sides of the remaining digits.