Complex sentence: The widths of "Cooksonia" fossils span an order of magnitude; while the smaller ones could not possibly be self-sufficient, the larger axes could; this trend may represent the evolution of an independent sporophyte generation.
Split into simpler sentences: The widths of "Cooksonia" fossils span an order of magnitude. While the smaller ones could not possibly be self-sufficient, the larger axes could--a trend that may represent the evolution of an independent sporophyte generation.