Martin Huld seems to be of the opinion that West Baltic (= Old Prussian and
related languages) needs to be seen as an independent branch of Balto-Slavic.
'Macro-Baltic' is his term. He says:
--start quote--
In light of the equally great division between Old Prussian (with the old
thematic genitive singular) on the one hand and Latvian and Lithuanian (where,
as in Slavic, the thematic ablative has replaced it), the unanswered question is
whether West Baltic is a separate grouping.
--end quote--
--"Meillet's Northwest Revisited" in The Indo-Europeanization of
Northern Europe, JIES Monograph #17, 1966, p. 114.
West Baltic survived just long enough to get itself scantily documented in
Lutheran religious books, before being absorbed into Germanic, or East
Baltic.
I don't have much to say here, only just to say this is interesting. There
is some sort of distinct, ancient relationship between Germanic and Baltic,
perhaps one that begins with the two forming a distinct unified dialect even in
PIE times, together with the Northwestern IE block (Germanic, Balto-Slavic,
Italic, Celtic, Albanian, perhaps Messepic and Thracian).
One idle thought I'm having is the idea of a convergent
pre-Germanic/pre-Baltic dialect located somewhere on the coast between Jutland
and the Vistula, one that contributed elements to what became Germanic and what
became Baltic, or to say it another way, acted as the bridge between the
two emerging branches of IE. This would be around 2000 BCE, perhaps later.
My reading does indicate that there were distinct cultural differences
between West and East Balts, with those in the west cremating their dead vs. the
east's inhumation practices.
Mark.