Complex sentence: The diversity within the rhino group was much larger in prehistoric times, with sizes ranging from dog-sized to the size of "Paraceratherium", with long-legged, cursorial forms, and squat, semi aquatic forms.
Split into simpler sentences: The diversity within the rhinoceros group was much larger in prehistoric times; sizes ranged from dog-sized to the size of "Paraceratherium". There were long-legged, cursorial forms and squat, semi aquatic forms.