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PurplePeggysus t1_islp0iv wrote

It is because we determine things as more closely related if they share a more recent common ancestor. So orcas and moose shared a common ancestor more recently than orca and horse or moose and horse. DNA supports Perissodactyla and Artiodactyla (or Cetartiodactyla) as two separate groups. Within mammals, convergent evolution (traits that look similar but do not share a single origin), is pretty common. This makes determining relationships from morphology alone much more challenging! If you are interested in cetacean evolution and how they came about from the artiodacyls you could look into transitional fossils for this group. It's actually really cool (to me anyway) to see the mix of traits as the cetaceans adapted back into life in the water!

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AppleFart t1_ism7klo wrote

I can expand on this and answer specifically the question in the title. Organisms are classified based on how closely related they are to one another, not based on a single morphological trait, but a morphological trait can be a convenient way to distinguish between two groups. Even when phylogenies are built from morphological data (not common in the age of molecular biology) they use dozens of traits, which can be as simple as number of toes, but more often are measurements of different body parts or ratios of those measurements. It might be just as correct to say that Perissodactyla and Artiodactyla are distinguishable by the ratio of femur length to pelvis width (just for example, not a real measurement they used), but number of toes is much more convenient.

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