Hello, I need your help.
I am writing my master thesis on phonological developments of English
and Danish in historical comparison. My question concerns the voiced
sibilant /z/ which was derived from */s/ by Verner's Law. In English
this sound became /r/ in all positions quite early and thus
disappeared from the phoneme system for some time. Around the 6th-7th
century it came back, however, when all voiceless fricatives became
voiced between other voiced segments. Thus, /s/ > /z/ again.
In Old Nordic there was a similar development, but the literature
says that it affected /f/ and the interdental /th/ only.
Vernerian /z/ had become 'palatal r', i.e. /z~R/ at that stage in
word-final positions and had been assimilated or lost in other
positions. Now my question: why the hell was there no new /z/
from /s/ like in English?? Or was there, but it was not reflected in
orthography (voice was normally not marked in runic writing)?? And if
there were new instances of /z/ (Brondum-Nielsen, Gammeldansk
Grammatik argues that occasional <z> spellings in later manuscripts
could reflect voiced /s/), what happened to these?
I hope any of you know the solution to my problem. Thanks in advance.

yours, Lily