From: G&P
Message: 63392
Date: 2009-02-23
>As
for -tt- > -ss- (typically ppp's):
In Greek the change /-tt-/ > /-ss-/ is dialectal, and affects all words, not just ppp’s. E.g. tettar- / tessar-/ pittar- = 4; thalatta / thalassa = sea; and so on. Since the last word is probably non-IE, it shows that the change is an active phonological change within Greek, independent of what has happened in other IE language groups, and not due to something inherited.
It has typically been explained by an intrusive fricative /-tt-/ > /-t(tH)t-/ > /-tst-/ and so on. But I doubt if there is actual evidence for that.
Peter