We start with Sinonyx, a wolf-sized mesonychid (a primitive ungulate from the order Condylarthra, which gave rise
to artiodactyls, perissodactyls, proboscideans, and so on) from the late Paleocene, about 60 million years ago. The
characters that link Sinonyx to the whales, thus indicating that they are relatives, include an elongated muzzle, an
enlarged jugular foramen, and a short basicranium (Zhou and others 1995). The tooth count was the primitive
mammalian number (44); the teeth were differentiated as are the heterodont teeth of today's mammals. The molars
were very narrow shearing teeth, especially in the lower jaw, but possessed multiple cusps. The elongation of the
muzzle is often associated with hunting fish — all fish-hunting whales, as well as dolphins, have elongated muzzles.
These features were atypical of mesonychids, indicating that Sinonyx was already developing the adaptations that
later became the basis of the whales' specialized way of life.