The missing skull parts are the most fragile parts which are least likely to be preserved.
(The missing skull parts; are; the most fragile parts which are least likely to be preserved) (the most fragile parts; are; least likely to be preserved) (the most fragile parts; to be preserved; )
The missing skull parts the most fragile parts the most fragile parts
are are to be preserved
the most fragile parts which are least likely to be preserved
least likely to be preserved