freyar said:+4 might be good here, or maybe even +6. The original AC is 2 (or 18 in 3e); what's the usual natural armor bonus for animals?
It varies quite a bit, from +0 for bats to +12 for dire hippopotami.
Looking at similar templates...
Earth element creature improves by +3.
Mineral warrior improves by +3.
Revived fossil is based on size:
Tiny, Diminutive, Fine +6