Crallsnaper

The crallsnaper has undergone neoteny and replaced its ancestor. The tilebacks of the Arctic should have gone extinct long ago with no place to haul out and little ability to live for long times in the ocean as adults. The neoteny is only partial, while lungs have been lost in favor of gills the armour has been retained. The limbs are highly reduced, serving only as articulation points for the claws which are backwards curving and allow the crallsnaper to hall itself forward. In times of danger they can bury themselves in the sand.They don't select mates any more, simply releasing gametes when the time is right.