The end of some extinct creatures shown in useful reference books (8)
I believe the answer is:
thesauri
'reference books' is the definition.
(thesaurus is a kind of reference book)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for thesauri that I've seen before include "Word founts" , "Word books" , "reference books" , "Reference works providing synonyms" , "References" .)