Why - if you are a young earth creationist ideologue - the NAUTILUS species found today have managed to evade extinction (while the trilobites went extinct 250 million years ago and the ammonites 65 million years ago).
After all, YECs inform us that marine creatures such as these must have been decimated (though presumably not rendered fully extinct) during Noah's Flood - when according to Genesis 7:11 "all the springs of the great deep burst forth" which they say refers to massive undersea volcanism issuing boiling water and noxious gases into the oceans - and then gradually and inexplicably gone fully extinct sometime after that Flood ie within the last 4,500 years they would insist (as they have to do for land creatures since the Bible never alludes to extinction at all and the ark was meant to preserve land animals from total destruction, forever).
But the fortunate nautilus has managed to cling on despite the ravages of the Flood and any subsequent unknown 'recent' oceanic or wider-scale disasters that apparently did for the trilobites and the ammonites.