[PatrickHenry]

Were all the horse niches filled 30,000 years ago?

Probably. 30K years isn't a long time -- as these things go -- and their environment hasn't changed all that much since then, so yes, I suppose they were filling all the "horse-friendly" niches 30K years ago.

[dubyagee]

After horses acquired the digestive, dental and axial body structures for life in the open, came an explosion in distal limb length (and the development of large body mass in a few lineages).(equinestudies.org)

What is a scientist's definition of a "few" lineages?