Complex sentence: The first of the mega-fauna emerged with the Brontotherien already in the Middle and Upper Eocene, the "Megacerops" known from North America reached a shoulder level of and could have weighed just over.
Split into simpler sentences: The first of the mega-fauna emerged with the brontotherien already in the Middle and Upper Eocene. The "Megacerops" known from North America reached a shoulder height of and could have weighed just over.