Animal Biology :D

Had a great time in my PASS class for animal biology today.
They made us did some riddles on phylum/sub phylum/super class/class and all, based on our choice.
The riddles were to be read out loud to the others to guess.

My riddle
I'm first in my super class,
I could be your knight in shining armour,
Watch me sink once i stop,
Chew chew chew i have to when it comes to meals.

Hehe was happy when they said my riddle was good :D
I'm progressing in class and enjoying my courses and also the new friends i made

BTW answer to the riddle is:
Placodermi (armoured fish)- sadly they are extinct already.