Intervocalic s > r (rhotacism [1]) is a well known phenomenon in early Latin. Any Latin grammar from the last 150 years should mention it. The comparative meth

Intervocalic s > r (rhotacism [1]) is a well known phenomenon in early Latin. Any Latin grammar from the last 150 years should mention it. The comparative method proves the original phoneme was /s/ in many cases: generis - compare Greek gene(s)os, Sanskrit janasas jus, juris - compare justitia where the /s/ was not intervocalic robur (archaic robos), roboris - compare robustus The 3rd declension has many s-stems like these though there are some original r-stems also like jecur (< PIE jekwor, compare Greek hepar). 1. https://en.wikipedia.org/wiki/Rhotacism https://en.wikipedia.org/wiki/Rhotacism