Hyracotherium is the "fashionable name for eohippus . Blme the Europeans even though they inherited horses and we di all the work.
Im not sure of the climate peaks but during the "Grand Coupure" (I think thats how itwas spelt) , a time to the late oligocene, all the areas of the world bgan cooling . The shallow sea in the mid US bcame a great grasslan and grasslands all over were associated with a mass extinction (the " bigass breakage" which sounds better in french), and all the paleo forms of animals died out and their neer forms began rising.
Horses were tied pretty much to the evironment because as the grasslands appeared , theMesotherial toes (3 hooflets) became vestigial , as did the limb patterns of the ancient paleocetaceans. From the end of the Oligocene the estuarine areas along the sothern
Indo -African areas where whale evolution began, became deeper water areas with very few remaining estuaries that originally gave rise to the packycetus and ambliocetus. The beginning of the Miocene saw the appearances of many modern form species, and a big hiatus in species of earlier "asian" types .
During these times two bolides were discovered in the stratigraphy within the last 25 years.
The first, the Chesapeake "Invader" was discovered by dr Wiley Poag of USGS from data he couldnt understand from UGS ground water monitoring systems. Wells drilled within the areas of the Potomac and Rappahannock and James Rivers showed weird water levels, as if all these areas were part of a single aquifer zone. Also the chemistry of the water was indicative of some extra aquifer metallic ions.later drilling showed the typical layers of shocked quartz and really smashed up deep zones of paleocene and Cretaceous underlying rocks.
Then another one was found in the Siberian rocks of the Creatceous. This was called the Popagai bolide. It was apparently a final smack that marched the climate to a cooler, lower marine water stand . So the "little extinction event" was an opportunity for newer forms of species to develop and become "overadapted" to a new environmental system