The lines connecting those animals are important -- both their lengths
and where they branch. At the far left of the tree (it's still called a
tree even though it is sideways), at about 54 million years ago, would
be the common ancestor of whales and hippos. It is not pictured because
no fossils of it have been found. From it, one branch evolved for 54
million years to produce the hippo; the intermediate steps and any
further branches of that lineage are not shown on the diagram. The
other branch evolved for 54 million years, branching again several times
along the way, to produce whales and dolphins.
Note, too, that the fossils shown -- Pakicetus, Kutchicetus, and the
rest -- are not the actual common ancestors at the branches. They are
merely not far removed from the common ancestors, so they give a fairly
good idea of what the common ancestor looked like. At least, usually
they do. Indohyus is about 5 million years evolved from its common
ancestor with Pakicetus, which is about the same amount of time between
Pakicetus and Kutchicetus, so the degree of difference between those two
is what we might expect between Indohyus and the actual ancestor it had
with Pakicetus.
You say you see a big jump between Pakicetus and Ambulocetus. But the
changes you see are merely to the size of body parts. The legs are
shorter, but they are still there, and still jointed in pretty much the
same way. And the feet are broader, and the ear flaps shrunk to
practically nothing, and that's about it. Consider that you yourself
have probably seen people less than four feet and more than seven feet
tall just within your own population, and such changes don't seem all
that drastic. At least they don't to me.
As for the difference between Indohyus and Mysticetes, remember that
Indoyus spent 5 million years evolving in a different direction. Also,
the picture used for Mysticetes (and Odonotcetes) are of modern animals
(probably for convenience of the artist), so the difference you are
seeing in the pictures is actually 30 million years greater. The short
answer to your question is that the time was closer to 15 million years,
and yes, a lot can happen in 15 million years. Even if were just 10
million years, that amount of change would still be plausible.
--
Mark Isaak eciton (at) curioustaxonomy (dot) net
"The evil that is in the world always comes of ignorance, and good
intentions may do as much harm as malevolence, if they lack
understanding." - Albert Camus, _The Plague_