Complex sentence: At a later stage, "C. grandis" disappears from the rock record, leaving only "C. lentus", which then also disappeared at the same time "C. supremus" appears in the uppermost layers.
Split into simpler sentences: At a later stage, "C. grandis" disappeared from the rock record, leaving only "lentus". Then "lentus" too disappeared; at the same time "supremus" appeared in the uppermost layers.