Some heirs to Sarvāstivādin monastic lineages, initially clustered around Mathurā and regions to its northwest, claimed primacy among the Sarvāstivādins in calling themselves the Mūlasarvāstivādin, or “Original Sarvāstivādins” (Fumi Yao, “On the Name ‘Mūlasarvāstivādin,’ ” Journal of Indian and Buddhist Studies 55, no. 2 (2007): 246–47). Their vinaya, the Mūlasarvāstivāda Vinaya, which was written and compiled in Sanskrit circa the second through the sixth centuries ᴄᴇ, is the longest of all known vinayas.

Unfortunately, the most accurate description of “Mūlasarvāstvādin” is tautological: the Mūlasarvāstivādins are the upholders of the Mūlasarvāstivāda Vinaya, because the only reliable means we have of distinguishing the “Mūlasarvāstivādins” from the Sarvāstivādins is by their respective vinayas‍—the former contains extensive “settings” and avadāna while the latter does not. (See also the entry on Mūlasarvāstivāda Vinaya). Furthermore, the Mūlasarvāstivādins seem to have shared much of their sūtra and abhidharma texts with the Sarvāstivādins. Although other ways of distinguishing them from other nikāya or “ordination lineages” are recorded in Indic texts‍—which were included in the Vinaya section of the Tengyur (Toh 4138–4140)‍—these are, in fact, extracts from śāstra, and the descriptions they give are not entirely consistent.