(See the horse tree for a lovely ASCII depiction.) Small preface: All equids (members of the family Equidae) are perissodactyls -- members of the order of hoofed animals that bear their weight on the central 3rd toe.

(Other perissodactyls are tapirs and rhinos, and possibly hyraxes.) The most modern equids (descendents of Parahippus) are called "equines".

With this in mind, I'll take you through a tour of the major genera of the horse family, Equidae.

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[Note: the particular species that probably gave rise to the rest of the equids, H.

vassacciense, may be renamed, perhaps to "Protorohippus".] Though in retrospect we may consider Hyracotherium to be "primitive", it was a very successful animal in its time, and seems to have found a nice stable niche for itself.

The sequence of horse fossils that Marsh described (and that T. Huxley popularized) was a striking example of evolution taking place in a single lineage.

Here, one could see the fossil species "Eohippus" transformed into an almost totally different-looking (and very familiar) descendent, Equus, through a series of clear intermediates.

As new fossils were discovered, though, it became clear that the old model of horse evolution was a serious oversimplification.

The ancestors of the modern horse Overall, the horse family demonstrates the diversity of evolutionary mechanisms, and it would be misleading -- and would be a real pity -- to reduce it to an oversimplified straight-line diagram.Strictly speaking, only the very modern genus Equus contains "horses", but I will call all equids "horses" rather indiscriminately.Most horse species, including all the ancestors of Equus, arose in North America.All the names on the tree are genus names, so recall that each genus encompasses a cluster of closely related species.The is a brief description of the tree for those who are visually impaired. There are many branches alive during most times until two million years ago when only the various species of Equus remain.I apologize in advance for the length; I didn't want to cut it down any more than this, because horse evolution has been oversimplified too many times already.