Why does Old Russian have epenthesis of /u/ only before the syllabic sonorant "l"? (before the syllabic sonorants "r,m,n" the epenthesis is /i/)? I thought earlier that only Proto-Germanic zero-grade have this epenthesis. The examples: вълкъ /wolf/ пълнъ /full/
Epenthesis with /u/ is also seen in Lithuanian (near labiovelars) with all syllabic resonants, and in Albanian (with unclear conditioning) for both l & r, whilst Sanskrit uses a /u:/ (or /u/) in its reflexes of long syllabic l & r (*l̥H & *r̥H and *l̥l &r̥r respectively) near labials. Additionally, Slovene vȏłk & pȏłn and Kashubian wôłk & pôłny also show rounded reflexes.
Lithuanian apparently depending on the existence of labiovelars would require a very early rounding of the epenthetic vowel (before the merger of labiovelars with plain velars, so well before Proto-Balto-Slavic), but this seems inconsistent with the Slavic outcomes which depend on other factors and so require a later (post Proto-Slavic split).
It's possible there was dialectal variation within early Balto-Slavic with Pre-Lithuanian undergoing this early split, and East Slavic (and also Slovene & Kashubian amongst others) rounding the epenthetic ǐ (or ь depending on transcription) in slightly different environments at a later stage, likely due to the cross-linguistically common trend for coda l to velarise to ł and cause rounding (or vocalise completely, losing any lateral character cf Polish <ł> /w/).
For what it's worth, wiktionary gives Proto-Slavic (and Proto-Balto-Slavic) reconstructions of both words (*vь̑lkъ, *pь̀lnъ) with ǐ, citing Vasmer, Max (1964–1973), in Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), translated from German and supplemented by Oleg Trubachyov, Moscow: Progress. I do note however that despite wolf having a labiovelar, the Lithuanian does not show a rounded vowel.
There has never been any sound
u before the syllabic sonorant
l in the Slavic words for “wolf” and “full”.
Proto-Balto-Slavic (PBS) *wilkás (“wolf”) and *pilnas (“full”) begat Proto-Slavic (PS) *vĭ̯l̥kŭ and *pĭ̯l̥nŭ respectively:
*wilkas> Early PS
*vĭlkŏs> Late PS
*pilnas> Early PS
*pĭlnŏs> Late PS
The change was conditioned by the law of increasing sonority within a syllable a.k.a. the law of the open syllables which appeared in Proto-Slavic. According to this law, in every syllable the sonority could only rise, the voiceless fricatives being of the least sonority and the vowels of the greatest sonority, which made every syllable open, no closed syllables were allowed anymore. Consequently,
l couldn't directly precede
k within the same syllable, since
l is more sonorous than
k. To fix the situation,
l became syllabic
l̥, that is, the most sonorant sound of the syllable, and
i became non-syllabic reduced (ultra-short) glide
ĭ̯ ('ь̯'), that is, less sonorant than the following syllabic liquid: Early PS
*vĭl-kŏs > Late PS
*vĭ̯l̥-kŭ. Later this
ĭ̯ stopped to be pronounced at all and only the syllabic liquid remained, that is the stage we see in the Old Church Slavonic
[pl̥nŭ]. This syllabic liquid
*ĭ̯l̥ didn't have a special letter in Cyrillic and was written with digraphs
ъл, the choice of the type and position of the reduced vowel letter in the digraph was irrelevant since the vowel letter was silent anyway:
Later, when the reduced vowels (“the yers”) disappeared and the law of the open syllables ceased functioning, a new sound change called pleophony (also known as “polnoglasie” or “full vocalization”) occurred in the East Slavic (ES) languages. During this stage the change
[ol] occurred which was reflected in writing as using only
As you can see, there was no [u] of any kind in these words, the letter
ъ (ŭ) being just an orthographic convention there. However, while describing the development of the PBS liquid diphthongs with high vowels (*ir, *il, *ur, *ul) Wikipedia says that
there is a divergence of opinion, with some scholars assuming that the high-vowel liquid diphthongs evolved into syllabic sonorants early in the Common Slavic period (even before the metathesis of the mid-vowel liquid diphthongs), while others assume that the change to syllabic sonorants was one of the last changes in the Common Slavic period and did not occur at all in many languages (e.g. East Slavic).
As for me, it is quite problematic to agree that “the change to syllabic sonorants did not occur at all in East Slavic”, since if we agree, we will have to acknowledge the change of Early PS
*ĭ to Early East Slavic
*ŭ which is very unlikely in general and practically impossible in PS dialects since they held a consistent distinction between
*ŭ. That is why the two Old East Slavic words you list never had any
u-like sound in their first syllables, the development was like this:
*il> Early PS
*ĭl> Late PS
*ĭ̯l̥> Early ES