Complex sentence: During the Late Precambrian, western Montana was covered by a warm, shallow sea where local bacteria formed stromatolites and bottom-dwelling marine life left tracks on the sediment that would later fossilize.
Split into simpler sentences: During the Late Precambrian, western Montana was covered by a warm, shallow sea. This sea was home to stromatolites and to bottom-dwelling marine life whose tracks on the benthic sediment would later fossilize.