What environmental shift led to the contraction of forests supporting Hyracodontines like Paraceratherium?

Global climate cooling and drying

The success of the Hyracodontines, exemplified by Paraceratherium, was intrinsically linked to the specific environment of the Oligocene: vast, relatively continuous forest landscapes across Eurasia. When the global climate began to cool and dry toward the latter part of the Oligocene epoch, these essential forest environments contracted significantly. This habitat loss directly impacted the specialized niche filled by these enormous herbivores, which required substantial amounts of foliage to sustain their massive metabolic needs. Consequently, the ecological pressures that supported such gigantism vanished, leading to the eventual decline and extinction of the Hyracodontine branch of the rhino family.