Munda in Early NW India

[petegray]

>  is there any convincing scenario for how the
> retroflex series became phonemic in Sanksrit from internal causes alone?

The distribution of the retroflex series is severely limited in the older
language:
(a) after a lost vocalic /r/ (which can often be established from IE
evidence).
(b) rarely before a lost /r/
(c) practically never initially
(d) after retroflex /s/, or as a substitute for it or other stops in certain
situations.  (The aspirate retrofelx t.h occurs almost only here in the
older language)
(e) in loan words

The source appears to be a retroflexion after /r/, assisted by the presence
of retroflex articulations in loan words.  I am sure this is disputable, but
it does not seem on the face of it unlikely.

Peter