Complex sentence: The earliest lions known in the Americas south of Alaska are from the Sangamonian Stage-the last interglacial-following which, the American lion spread widely from Alberta to Maryland, reaching as far south as Chiapas, Mexico.
Split into simpler sentences: The earliest lions known in the Americas south of Alaska are from the Sangamonian Stage (the last interglacial). After that, the American lion spread widely from Alberta to Maryland, and as far south as Chiapas, Mexico.