Parthian rule brought little change in the administration and institutions of Mesopotamia as it had existed under the Seleucids, except for a general weakening of central authority under the feudal Parthians. The Parthians instituted a new era, beginning in 247 bce, but it paralleled rather than replaced the Seleucid era of reckoning, and the Parthian vanished at the end of the dynasty. As far as can be determined, Hellenism was never proscribed under the Parthians, although it grew weaker toward the end of Parthian rule. From archaeological surveys around Susa, located in the kingdom of Elymais in modern Khūzestān, and from the Diyālā plain northeast of Ctesiphon, it seems that the population of the land increased greatly under the Parthians, as did trade and commerce. The coinage of the later Parthian rulers became more and more debased, probably as a result of the many internecine wars and the lack of control by the central authority. Local rulers also issued their own coinages in Persis, Elymais, Mesene, and elsewhere.
Changes took place in the demography of Mesopotamia under the Parthians, and perhaps the most striking development among the population was the increase of Arab infiltration from the desert, which resulted in Arab dynasties in the oasis settlements of Palmyra and Hatra. Similarly, an influx of Armenian settlers in the north changed the composition of the local population. After the fall of the Temple of Jerusalem to the Romans in 70, many Jews fled to Mesopotamia, where they joined their coreligionists; Nehardea, north of Ctesiphon, became a centre of Jewish population. Naturally also many migrants from the east came to Mesopotamia in the wake of the Parthian occupation. With many merchants from east and west passing through or remaining in Mesopotamia, the population became more diverse than it had previously been.
During the Parthian occupation the ancient religion and cults of Mesopotamia came to an end and were replaced by mixed Hellenic and Oriental mystery religions and Iranian cults. Local Semitic cults of Bel, Allat, and other deities flourished alongside temples dedicated to Greek gods such as Apollo. The sun deity Shamash was worshiped at Hatra and elsewhere, but the henotheism of the ancient Middle East was giving way to acceptance of universalist religions, if the prevalent view cannot yet be called one of monotheism. In Mesopotamia, in particular, the influence of Jewish monotheism, with the beginning of rabbinic schools and the organization of the community under a leader, the exilarch (resh galuta in Aramaic), must have had a significant influence on the local population. Toward the end of the reign of Artabanus III, the royal family of Adiabene converted to Judaism. In the first two centuries of the Common era, Christianity and various baptismal sects also began to expand into Mesopotamia. So far no Mithraeums (underground temples for the worship of the god Mithra), such as existed in the Roman Empire, have been found in Mesopotamia, except at Dura-Europus, where Roman troops were stationed. Many local cults and shrines, such as that of the Sabians and their moon deity at Harran, however, continued to exist until the Islamic conquest. Parthian Zoroastrianism reinforced local Zoroastrian communities in Mesopotamia left from the time of the Achaemenians, and one of the gnostic baptismal religions, Mandaeanism, which is still in existence, had its beginning at this time. Although Christian missionaries were active in Mesopotamia in the Parthian period, no centres, such as the one established later at Nisibis, have been reported, and it may be supposed that their activity at first was mainly confined to Jewish communities.
Archaeological evidence indicates that the Parthians had a more marked influence on art and architecture. Local schools of art flourished, and at first Greek ideals predominated, but in the last two centuries of Parthian rule a “Parthian style” is evident in the art recovered from Mesopotamia and other regions. Whereas Achaemenian and Sāsānian art are royal or imperial and monumental, Parthian art, like Seleucid art, can be characterized as “popular.” Parthian works of art reflect the many currents of culture among the populace, and one may say that it is expressionist and stylized, in contrast with Greek and Roman naturalistic or realistic art. The characteristics of Parthian art in Mesopotamia are total frontality (i.e., the representation of figures in full face) in portraits, along with an otherworldly quality. In Middle Eastern art from previous periods, figures were almost always shown in profile. Another new feature of Parthian art is the frequent portrayal of the “flying gallop” in sculpture and painting, not unexpected in view of the importance of cavalry and mounted archers in the Parthian armies. Likewise, Parthian costume, with baggy trousers, became the mode over much of the Middle East and is portrayed in painting and sculpture. In architecture the use of ayvans (arches in porticoes) and domed vaults is attributed to the Parthian period; they may have originated in Mesopotamia. Parthian art influenced that of the Nabataeans in Roman territory, as it did others throughout the Middle East.
Parthian was an Iranian language written in the Aramaic alphabet. It had an enormous number of words and even phrases that were borrowed from Aramaic, and scribal training was necessary to learn these. Syriac, being a Semitic language with emphasis on consonants, evolved several alphabets based on the Aramaic alphabet. The Aramaic alphabet was better suited to Syriac than to Parthian phonology. Parthian was therefore difficult to read and was mainly used by scribes or priests for official or religious writings.
The largest lacuna is in literature from the Parthian period. The largely oral literature of the Parthians, famous for their minstrels and poetry, does not seem to have found many echoes in Mesopotamia, where the settled society contrasted with the heroic, chivalric, and feudal society of the Iranian nomads that continued to dominate Parthian mores even after they had settled in Mesopotamia. Nonetheless, the end of the Parthian period saw the beginning of Syriac literature, which is Christian Aramaic, and some of early Syriac literature, such as the “Song of the Pearl,” contains Parthian elements. In the realm of language, rather than literature, the writing of Aramaic changes to Parthian in the 2nd century ce, as can be seen from a bilingual (Greek and Parthian) inscription on a bronze statue from Seleucia dated 150–151 ce. It tells how Vologeses III defeated the king of Mesene and took over the entire country. After this period one no longer speaks of Aramaic, but of Parthian and Syriac written in a new cursive alphabet. The Sāsānian period
The Sāsānian period marks the end of the ancient and the beginning of the medieval era in the history of the Middle East. Universalist religions such as Christianity, Manichaeism, and even Zoroastrianism and Judaism absorbed local religions and cults at the beginning of the 3rd century. Both the Sāsānian and the Roman empires ended by adopting an official state religion, Zoroastrianism for the former and Christianity for the latter. In Mesopotamia, however, older cults such as that of the Mandaeans, the moon cult of Harran, and others continued alongside the great religions. The new rulers were not as tolerant as the Seleucids and Parthians had been, and persecutions occurred under Sāsānian rule.