to mint coins at Seleucia as late as 228 AD. The Sassanians would not only assume Parthia's legacy as Rome's Persian nemesis, but they would also attempt to restore the boundaries of the Achaemenid Empire by briefly conquering the Levant, Anatolia, and Egypt from the Eastern Roman Empire during the reign of Khosrau II (r. 590–628 AD). However, they would lose these territories to Heraclius—the last Roman emperor before the Arab conquests. Nevertheless, for a period of more than 400 years, they succeeded the Parthian realm as Rome's principal rival. Native and external sources Local and foreign written accounts, as well as non-textual artifacts, have been used to reconstruct Parthian history. Although the Parthian court maintained records, the Parthians had no formal study of history; the earliest universal history of Iran, the Khwaday-Namag, was not compiled until the reign of the last Sasanian ruler Yazdegerd III (r. 632–651 AD). Indigenous sources on Parthian history remain scarce, with fewer of them available than for any other period of Iranian history. Most contemporary written records on Parthia contain Greek as well as Parthian and Aramaic inscriptions. The Parthian language was written in a distinct script derived from the Imperial Aramaic chancellery script of the Achaemenids, and later developed into the Pahlavi writing system. The most valuable indigenous sources for reconstructing an accurate chronology of Arsacid rulers are the metal drachma coins issued by each ruler. These represent a "transition from non-textual to textual remains," according to historian Geo Widengren. Other Parthian sources used for reconstructing chronology include cuneiform astronomical tablets and colophons discovered in Babylonia. Indigenous textual sources also include stone inscriptions, parchment and papyri documents, and pottery ostraca. For example, at the early Parthian capital of Mithradatkert/Nisa in Turkmenistan, large caches of pottery ostraca have been found yielding information on the sale and storage of items like wine. Along with parchment documents found at sites like Dura-Europos, these also provide valuable information on Parthian governmental administration, covering issues such as taxation, military titles, and provincial organization. The Greek and Latin histories, which represent the majority of materials covering Parthian history, are not considered entirely reliable since they were written from the perspective of rivals and wartime enemies. These external sources generally concern major military and political events, and often ignore social and cultural aspects of Parthian history. The Romans usually depicted the Parthians as fierce warriors but also as a culturally