More recent research has reclassified these animals into ten different genera and at least three families, of which many are not supposed to have anything to do with the horse series but are similar to e.g. tapirs (family Tapiromorpha).Interesting. I took his word for it when I wrote my first response. That was a mistake. Molén's reference for that claim is this paper by David Froehlich:
Froehlich. 2002. Quo vadis eohippus? The systematics and taxonomy of the early Eocene equids (Perissodactyla). Zool J Linn Soc 134:141-256.
I just happen to have a copy of that paper in my files. Molén is correct that Froehlich reclassifies a number of Eocene 'hyracotheres' into different genera (some of which he proposed in that paper). Molén is not correct when he says that "many are not supposed to have anything to do with the horse series." Here is part of Froehlich's phylogeny (from Figure 1) (click to enlarge):
See those taxa named Sifrhippus, Minippus, Arenahippus, Xenicohippus, Eohippus, Pliolophus, and Protorohippus? Those are taxa into which Froehlich reclassified specimens previously attributed to 'Hyracotherium.' See how they're all part of the clade Froehlich labels Equidae? That means Froehlich considers them still horses. How many hyracotheres ended up in non-horse genera? Froehlich lists them out for us: Hyracotherium leporinum (which he considers a paleothere), Cymbalophus cuniculus (which he considers a tapiromorph), and Systemodon tapirinus (which he also considers tapiromorph). Anybody keeping score? Of the specimens previously referred to 'Hyracotherium,' Froehlich places them in seven horse genera and three non-horse genera. I don't see how anyone could reasonably interpret three out of ten as "many" taxa.
For lots more reasons why Molén is still wrong, see my original post.
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