As the universal nature of Roman law was eroded and replaced by Islamic law in part of the Iberian Peninsula, Shariah law allowed most ethnic groups in the medieval Islamic world to be judged by their own judges, under their own law: Mozarabs had their own tribunals and authorities. Some of them even held high offices in the Islamic administration under some rulers. A prominent example being that of Rabi ibn Zayd, a palace official, who, sometime between 961 and 976, wrote the famous "Calendar of Córdoba" for Abd ar-Rahman III, undertook various diplomatic missions in Germania and Byzantium, and was rewarded with the bishopric of "Elvira" (present-day Granada). Furthermore, in 1064 Emir Al-Muqtadir of Zaragoza sent Paternus, the Mozarabic bishop of Tortosa, as an envoy to king Ferdinand I of León in Santiago de Compostela, while the Christian Abu Umar ibn Gundisalvus, a Saqaliba (a Slav), served the same taifa ruler as the "Wazir" (Vizier, or the equivalent to prime minister).