The most common and most likely answer to this crossword clue is the 9 letter word AMMONITES
I've seen this crossword clue in Independent
A
M
M
O
N
I
T
E
S
More clues leading to the result AMMONITES
Buddy explains!

The straight part
We know that "Ancient sea-creatures" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ancient sea-creatures = ammonites.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does AMMONITES mean?
AMMONITES noun- Any of an extinct group of cephalopods of the subclass Ammonoidea; a fossil shell of such an animal.