The archeocets were the first cetaceans. They appeared in the Early Eocene, only 9 million years after the extinction of the dinosaurs and only 9 million years since mammals were no bigger than mice. The rapid diffusion of placental animals on land, their surprising evolution and the return to sea, in contrast to of the mesozoic marine reptiles, is an incredible event. Early cetaceans are extremely rare in the fossil record, as are for their paleocene ancestors. The history of these primitive whales has therefore still to be written. Zygorhiza had an elongated structure, with only a pair of paddle-shaped arms and with an undulating swim due to its tail and long body. Teeth were well diversified: the front ones were sharp and conic the back ones large and saw-edged. Diet was probably similar to modern spermwhales: deep water fish and cephalopods.