Andrew Jarette wrote:
> (However, r-stems in *-e:r and *-o:r go back to original *-ers and
> *-ors, and these do not have -h. in Sanskrit, simply rather -a:.
It > may be that only original syllabic *r. before final *s gave rise
to > -h. in Sanskrit, and not diphthongal *-ers and *-ors, which
early lost > the *-s and later lost the *-r in Sanskrit.)
The *-ers and *-ors endings had already changed into *-e:r and *-o:r,
respectively, before (most of) IE broke apart. So, the Sanskrit forms
in _-a:_ stem directly from *-e:r and *-o:r. The /s/ had already been
lost with, presumably, compensatory lengthening.
- Rob