Complex sentence: The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm), which selectively affected the shallow seas where most organisms lived.
Split into simpler sentences: The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm). This selectively affected the shallow seas where most organisms lived.