Ancient sea-creatures with shells, say, paladins picked up (9)

'ancient sea-creatures' is the definition.
I can't judge whether this defines the answer.

'with shells say paladins picked up' is the wordplay.
'with' is a charade indicator (letters next to each other).
'shells' becomes 'ammo' (I've seen this before).
'say paladins' becomes 'setin' (I can't justify this - if you can you should give a lot more credence to this answer).
'picked up' shows that the letters should be reversed in order.
'setin' backwards is 'nites'.
'ammo' next to 'nites' is 'AMMONITES'.

Can you help me to learn more?