Clearing the Wastebasket

Vulpavus ovatus, a "miacid"

Apart from whales, dolphins, and their kin, all large carnivorous placental mammals alive today belong to a group called, appropriately enough, the carnivorans. The carnivorans include several families, and they can be divided into two smaller groups, which are broadly described as either "dog-like" or "cat-like". The dog-like carnivores include, besides the dogs themselves, such families as the bears, weasels, and seals, among others. The cat-like carnivores are a slightly smaller group, including cats, hyenas, and a host of animals that look more or less like mongooses.

But, if we go back in time, we find a number of large, carnivorous mammals that, for various reasons, don't really fit. Some of them, such as Hyaenodon, are different enough that we can say that they're definitely not carnivorans in the modern sense. Others, such as sabre-tooth cats and dire wolves, clearly are, because they're so similar to animals we have around right now. And then there's others that are kind of in-between. So where do we draw the line?

One way is to look at what's called the "crown group". The idea here is that you take every living representative of the group you're interested in (in this case, carnivorans as a whole) and trace it back to its last putative ancestor. Anything descended from that animal, including all the extinct ones, belongs to the crown group, and anything that isn't, doesn't.