The ungulates (hoofed mammals) often share a superficial resemblance to one another because they are all loosely related. Before the genetic era taxonomists attempted to classify them based on morphology alone. There was a fair amount of guesswork involved in deciding which morphological features were taxanomically relevant.

At some point it was realised that the ungulates fell into two broad groups. Those who's axis of their limb (ie the distribution of weight down the leg) passed through the middle digit, and those who's passed between two digits. The former condition was described as mesaxonic and the latter paraxonic. This formed the basis of the division of the ungulates into the"odd toed" Perissodactyla and the "even toed" Artiodactyla.

There were other morphological features that seemed to follow that same division, lending support to it being a taxonomically relevant grouping. These were things like the presence of horns, the number of dorsolumbar vertebrae, and the anatomy of the skull and femur.

It seems that the morphologists got it right in this case. The division of the odd and even toed ungulates has held up after being examined genetically.

The Cetaceans (including orcas) were moved into Artiodactyla based on genetic work, which showed them to be most closely related to Hippos. This relationship was not realised based on morphology.