Juramaia

Juramaia 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 1,500 Bells.

Juramaia was a tiny, shrew-sized mammal that lived in China about 160 million years ago. It is one of the earliest known relatives of placental mammals, the group that includes you.