The earliest existing codification of the prayerbook is the Siddur (order) drawn up by Amram Gaon of Sura about 850.
Half a century later the famous Gaon Seadiah, also of Sura, issued his Siddur, in which the rubrical matter is in Arabic. Besides the Siddur, or order for Sabbaths and general use, there is the Mahzor (cycle) for festivals and fasts.
The office of Gaon lasted for something over 400 years, beginning about A.D.
They begin with Mar Rab Sheshna (7th century) and continue to Hai Gaon, who died in 1038, and are full of historical and literary interest.
Besides the Responsa, but closely related to them, we have the lesser Halakhoth of Yehudai Gaon of Sura (8th century) and the great Halakhoth of Simeon Qayyara of Sura (not Gaon) in the 9th century.