Around 10 million years after the dinosaurs (~56 MYA), the first traceable ancestor of the modern-day dolphin showed up.
Mesonychids, as they were called, somewhat resembled lions, even though they did not evolve into them. Mesonychids modern
descendants consist of Cetaceans (whales, dolphins, porpoises, and monodons), and Ungulates (horses, llamas, etc.)
next 10 or 15 million years, the first cetaceans evolved. There is some dispute to the order, but the generally accepted
order based on body shape is Pakicetus, Ambulocetus, Rodhocetus, Protocetus, and finally Basilosaurus. These, along with
a few other side-genera, are known as Archaocetes. However, it is generally accepted that Basilosaurids did not have anything
to do with modern day cetaceans.
Around 35 MYA, Protocetoids split into two suborders, Mysticetes, or baleen whales, and
Odontocetes, or toothed whales. Cetaceans were now completely marine, and evolution through natural selection has not carried
them much farther.
Around 10-15 MYA, the first dolphins as we know them today evolved. Originally, there were 5 families
of Odontocetes, but 2 have since become extinct. This is illustrated in greater detail on the Cladogram, which can be downloaded
as a Word document from this site.