The Narwhal, or Monodon monoceros, belongs to the Cetacean order. Traditionally, the theory of cetacean evolution was that whales were related to the Mesonychids, an extinct order of carnivorous ungulates (hoofed animals), which resembled wolves with hooves and were related to the Artiodactyls (even-toed hoofed mammals). Scientists believed this because these animals had unusual triangular teeth, resembling those of whales, which compose the Cetacean order.
On the contrary, more recent data suggest that whales, including narwhals, are more closely related to the Artiodactyls, and in particular, the hippopotamus. However, the earliest ancestors of hippos do not appear in fossil records until millions of years after Pakicetus, the first known whale ancestor. Due to this, scientists are unsure how closely related hippos and narwhals truly are.
The discovery of skeletons of the Pakicetus family shows that whales did not derive directly from Mesonychids, but are Artiodactyls. Newly located skeletons prove that the Cetacean order are Artiodactyls that began to take to the water soon after Artiodactyls split from Mesonychids.