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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.

(In fact, even at this length, this post is still only a summary! Marsh published a description of newly discovered horse fossils from North America.

) People who are in a hurry may just want to read the intro and summary and look at the tree. At the time, very few transitional fossils were known, apart from Archeopteryx.

Do not be misled into thinking that Equus was the target of evolution!

Bear in mind that there are other branches of the horse tree that I will mention only in passing.

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.

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.

I wanted people to see some of the detail and complexity of the fossil record of a fairly well known vertebrate group.

)----------------------------------------- 25My \ | / \ | / | 35My | Miohippus Mesohippus | | 40My Mesohippus | | | 45My Paleotherium | | Epihippus | | Propalaeotherium | Haplohippus | | | 50My Pachynolophus | Orohippus | | | | | | ------------------------------ \ | / \ | / 55My Hyracotherium The first equid was Hyracotherium, a small forest animal of the early Eocene.