Mongolic languages
In her PhD thesis, "Long Vowels in Mongolic Loanwords in Tuvan", Baiarma Khabtagaeva states that the history of long vowels is ambiguous. While the long vowels may originate from Mongolic languages, they could also be of Tuvan origin. In most Mongolic languages, the quality of the long vowel changes depending on the quality of the second vowel in the conjunction. The only exception to this rule is if the conjunction is labial. The ancient Tuvan languages, in contrast, depended upon the first vowel rather than the second to determine the long vowels.