They are known from pre-Islamic Syria, where the form was used in the fourth century CE in the Baptistery of Mar Ya'qub (St. Jacob) at Nisibin and Qasr Ibn Wardan (564 CE). Some argue that it was used earlier in the Sassanian Taq-Kasra in Iraq. It was adopted immediately by the Islamic caliphates, with a form of it appearing in the Mosque of Amr ibn al-As in Cairo, the Great Mosque of Damascus (706-715 CE), Qasr al-Hayr al-Gharbi (727 CE), it appears in friezes in the ruins of and it is extremely prominent throughout the Umayyad palace at Amman Citadel in Jordan. Horseshoe and semicircular arches are the predominant type of arch used in the Umayyad desert castles in Jordan, Syria, and Lebanon. However, it was in Spain and North Africa that horseshoe arches developed their characteristic form. Prior to the Muslim invasion of Spain, the Visigoths used them sporadically, although it is a matter of debate whether the arches in the extant churches are pre-Moorish or rebuilt after the Islamic conquest of Iberia. Some tombstones from that period have been found in the north of Spain with horseshoe arches in them, with speculation about a pre-Roman local Celtic tradition. Also, the arch of the Church of Santa Eulalia de Boveda - part of a previous Roman temple—in Lugo, points in that direction.