By Martyn Herman RIO DE JANEIRO, Aug 19 (Reuters) - Peerless Colombian Mariana Pajon retained her women's BMX title and Connor Fields became the first American to win Olympic gold in cycling's most spectacular discipline on Friday. Pajon, 24, roared on by hundreds of yellow-clad fans in the sun-bathed grandstand at Rio's X-Park, put not a wheel out of place as she pedalled to victory in the final, repeating her feat from London four years ago. Blasting out of the gate she quickly hit the front in the final and flowed smoothly over the humps and bumps to the finish line. More...