Me again,

I'm a bit confused with the names of the Eocene horses here. What I believe to be Eohippus has been called by various books, websites, and other sources Hyracotherium, Propalaeotherium, or even Protorohippus. As you can see, everybody seems to be arguing about this. Is there a correct name for this dilemma of a horse?

Post's pictures

eohippushyracotheriumpropalaeotheriumprotorohippus.jpg, 12.75 kb, 250 x 184

My colleagues on the site who know more about fossil mammals may be able to give you more specific information on this one. But basically...

The name Hyracotherium was established in 1841 by the anatomist Richard Owen, for European fossils that proved to belong to an Eocene horse (Owen, incidentally, coined the name Dinosauria in a lecture given in the same year). Later in the 19th century, Eocene horses began turning up in America, and the palaeontologists of the time went to work with their usual enthusiasm for naming fossils. Some of the American Eocene horses were described as species of Hyracotherium, others as new genera including Eohippus.

Around the turn of the 20th century, prevailing opinion began to shift towards the idea that Eohippus was similar enough to Hyracotherium that the two should be classified together in a single genus. In these situations the older name is used, so Eohippus was demoted to a "junior synonym" of Hyracotherium. I believe Protorohippus, another American genus, suffered the same fate. Propalaeotherium is European, but may also have been synonymised with Hyracotherium at some point; I really don't know.

More recently, the pendulum seems to have swung back towards the interpretation that Eohippus and Protorohippus are distinct from Hyracotherium after all, which would imply that these genus names are actually valid. However, I don't know how much consensus currently exists on this issue, and in any case it generally takes a little while for scientific consensus on obscure points of taxonomy to filter through to popular books and the internet. I expect you'll continue to come across conflicting naming schemes for a while yet.