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tim---mit t1_ism9vsl wrote

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.

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sighthoundman t1_isog9np wrote

This is the correct answer. Before Linnaeus, there were lots of different classification systems. (Genus and species goes back at least as far as Aristotle, but morphological comparisons really weren't a thing until the 1700s. So you could characterize humans as "featherless bipeds" or as "rational animals", but there wasn't a systematic method of characterizing all living things. And it's pretty obvious that limiting yourself to just genus and species is of extremely limited utility. Especially if you have a plucked chicken.) Linnaeus proposed a method of grouping similar to similar. The method was essentially "look at everything, and if it's mostly the same, they're closely related, and if it's almost entirely different, they're not related at all".

Even-toed and odd-toed just happened to be something that indicated all the other stuff really, really well. (Among ungulates.)

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