Complex sentence: During the Nara period the the base of the stirrup which supported the riders sole was elongated past the toe cup this half tongued style of stirrup ("hanshita abumi") remained in use until the late Heian period when a new stirrup was developed.
Split into simpler sentences: During the Nara period, the the base of the stirrup which supported the rider's sole was elongated past the toe cup. This half-tongued style of stirrup ("hanshita abumi") remained in use until the late Heian period when a new stirrup was developed.