Complex sentence: Whittington, believing that "Opabinia" had no legs, thought that it crawled on its lobes and that it could also have swum slowly by flapping the lobes, especially if it timed the movements to create "Mexican waves".
Split into simpler sentences: Whittington, believing that "Opabinia" had no legs, thought that it crawled on its lobes. He thought it could also have swum slowly by flapping its lobes, especially if it timed the movements to create "Mexican waves".