Complex sentence: The Chera trading stations of Naura and Tyndis began as busy fishing villages, but later the presence of large number of pirates posed a major threat to the Roman ships and Roman merchants had to resort to more southerly ports of Muziris and Nelcynda.
Split into simpler sentences: The Chera trading stations of Naura and Tyndis began as busy fishing villages, but later large number of pirates began to operate in the region. These pirates became on the "Pirate coast" a major threat to the Roman shipping and Roman merchants began to head directly for the more southerly ports of Muziris and Nelcynda.