As the Apostles and their successors evangelized throughout the world (both East and West, and beyond these boundaries of the Roman Empire), various regional liturgies were used in the earliest times. These liturgies were "cultural expressions" of the faith. This means that the faith itself and the key outline of the Mass and Divine Office were unchanged, but the musical tones (cadence, tonal system) and means of rhetoric (circular or linear; analogical or typological or otherwise) and ceremonies (usually based on local court customs) were employed. This was noticed primarily, then, in the prayers and chants, as well as the 'flow' of the liturgical services.
In the Western part of the Roman Empire (and later extending into the remainder of Europe), all of these liturgies were built, in some way, on the Roman Rite (i.e., the rites used in the Church of Rome) with some influences (in a few scattered instances) from Eastern customs. Over time, in both East and West, for various reasons usually related to the dominance of a capital city or the need for more uniform practices, these various liturgies collapsed into four major rites, centered around the major regions of Christianity: The Antiochian Rite; the Alexandrian Rite; the Roman Rite; the Carthaginian Rite; and the Edessan Rite. (See the chart below.) In turn, these rites (among the canonical Orthodox and Roman Catholic churches) collapsed, again for various reasons usually having to do with desire uniformity, into two: the Roman Rite (which was influenced by the Gallican Rite and which, in turn, influenced the still extant Ambrosian and Mozarabic Rites as well as the rites of a few religious orders); and the Constantinopolitan Rite (which was heavily influenced by the Antiochian Rite, but is not at all identical with it; and which influenced all the other Byzantine Rites.) In Europe, the collapsing into the Roman Rite was chiefly completed by 600 AD (although it underwent a few minor changes in the 9th century) and has remained in tact (excluding the protestant changes in England, etc., and the revolutionary change in Rome in 1970). In Asia and Eastern Europe, the collapsing into the Constantinopolitan/Byzantine Rite was not completed until the 13th century and has remained in tact (although different minor 'uses' appear in Russia and elsewhere).
Given this background, it is perhaps clear that the 'pre-schism' Western Rite is the Roman Rite which was used (most often in Latin) from 600 AD until 1970 AD in the Church of Rome; and has been used in the Orthodox Church since the 1880s. To use 'ancient Western liturgies' offers an historical, liturgical, and theological challenge. The historical challenge is that the documentation for exactly how those Rites were used (the ceremonies and chants, as well as the texts) is spotty at best; certainly it is not in tact fully. Furthermore, the best of these rites have already been folded into the Roman Rite, and so teasing them out may leave lacunae. The liturgical challenge is that 're-creating' a Rite that hasn't been used for at least 1000-1400 years follows the Protestant model that the Rites are not received nor the work of the Holy Spirit, but devised by humans and therefore open to 'liturgical archeology' (the exact same argument that lies behind the Novus Ordo of the Roman Church). The theological challenge is centered around this question: what is objectionable now to the Roman Rite that was never objectionable before the so-called 'Great Schism.' (Remember, the date for the Great Schism is arbitrarily chosen by post-reformation European scholars, mainly Anglican [i.e., Protestant]). Some will answer that question by presuming that the liturgy carries the seed for whatever heresies might be extant in the Church of Rome; a dubious premise, at best.
Given this history with the attendant challenges, when the Churches of Russia, Constantinople, Antioch, Romania, Serbia, and Alexandria issued decrees concerning the use of the liturgical tradition of the Western liturgy in their churches, they collectively approved the continued use of the Roman Rite (with the use of the Benedictine Breviary) as it was practiced prior to the 'Great Schism'--which is no different (except in a few feasts) than what was done in the Church of Rome and the Benedictine monasteries until 1970.