Two Siddurim from the era of the Geonim are important sources for studying the development of the Siddur; Seder Rav Amrom Gaon and the Siddur of Rav Sa'Adiya Gaon. Seder Rav Amrom Gaon, considered to be the first Siddur, was copied many times during the Middle Ages. Because of it being copied so often, suspicions have arisen as to whether local customs become incorporated within it. The Siddur of Rav Sa'Adiya Gaon, written about one hundred years later, disappeared from circulation during the Middle Ages. A manuscript of it was discovered in the Bodlean Library in the 1800's and thanks to fragments found in the Cairo Geniza, an authoritative copy of it was put together by three academic scholars and published for the first time in 1938.
There are significant differences between the two Siddurim. Some of those differences are attributed to Rav Sa'Adiya Gaon spending the early part of his life in Egypt where he may have learned of alternate Nusacha'Ot. What is important in my mind is what we can learn about the development of the Siddur from Rav Sa'Adiya Gaon's text.
The first example concerns the Bracha of מגביה שפלים, a Bracha that still is part of ברכות השחר in Nusach Roma and which was included in many Ashkenazic Siddurim until the 1800's. Here is how Rabbi Zeligman Baer in his סדר עבודת ישראל presents the Bracha:
Apparently, the community of Metz was one of the last Ashkenazic communities to recite the Bracha. Why was the Bracha slowly eliminated from Ashkenazic Siddurim? Because of the rule that if a Bracha does not appear in the Talmud, it should not be said with שם ומלכות. Masechet Brachot Daf 60b presents the ברכות השחר and the Bracha of מגביה שפלים does not appear there.
The issue that the Siddur of Rav Sa'Adiya Gaon raises is whether there was a version of that Gemara that did include the Bracha. Notice how Rav Sa'Adiya describes the Bracha:
Rav Sa'Adiya appears to be quoting the version of the Gemara that was available to him. This version included the Bracha.
Another contribution that Rav Sa'Adiya Gaon provides concerns the text of ברוך שאמר. A myth developed beginning with the אור זרוע that ברוך שאמר was a Bracha that appeared for the first time by way of a note that fell from heaven which contained a Bracha that consisted of 87 words. The reason I describe this story as a myth is because only one version of ברוך שאמר that appeared in handwritten manuscripts consisted of 87 words. Seder Rav Amrom Gaon provides a version with many more words. Sephardic practice until very recently provided a version that consisted of more than 87 words and included inserts for Shabbat and Yom Tov. Here is an example that provided those present a choice:
But Rav Sa'Adiya Gaon provides the earliest proof that ברוך שאמר is not a prayer that consists of 87 words. He presents two versions; one to be said on weekdays and one to be said on Shabbat. What he reveals in his Shabbat version is that the first part of ברוך שאמר is a Piyut, known as a רשות, a request for permission. First, I will present the weekday version:
Now the Shabbat version:
Truth be told, the myth developed because the Talmud does not mention the Bracha of ברוך שאמר. In order to justify reciting it, the myth was created. What surprises me is that no one saw that ברוך שאמר is one manifestation of ברכת השיר that is mentioned in the Talmud concerning the Seder night. The Talmud explains that ברכת השיר is one of two Brachot: נשמת or יהללוך. Both those Brachot (and ישתבח) end with the same wording מלך מהולל בתשבחות. Coincidentally so does ברוך שאמר. Since the most important part of a Bracha is its concluding words, the Chatima, שאמר ברוך fits the definition of ברכת השיר.