Ammonite

Ammonite is a standalone fossil: one piece completes it. Dig up fossils at the cracks on the ground, have Blathers assess them, then donate it to the museum or sell spares for 1,100 Bells.

Ammonites were sea creatures related to squid and octopuses, living inside coiled, chambered shells. They thrived for over 300 million years before vanishing at the same time as the dinosaurs.