The Shark Tooth is a stand-alone fossil that can be donated to the museum in Animal Crossing: Wild World, Animal Crossing: City Folk, and Animal Crossing: New Leaf.
Donating to the museum
In Wild World
"It is believed that sharks existed long before even dinosaurs appeared... However, a shark's skeleton is made from cartilage and is softer than bone. As a result, very few have survived as fossils, eh wot? Fortunately, we CAN research them when we find a shark tooth. Ah! Oh, so very sorry! I... I cannot help it, really. And that's the tooth! Hoo!"
In City Folk
"Capital! Unheard of! What a first-rate fossil! Sharks were actually around a long time before dinosaurs appeared on the scene, eh wot? They developed into the perfect predator, and since then they haven't changed a smidgen!"
In New Leaf
"The fossilized shark tooth has a distinctive serrated edge, like a steak knife. Sharks have existed since before the dinosaurs, but their appearance seems to have hardly changed at all. Their age actually puts them at a time even before plants had fully propagated over every continent."
In Real Life
Sharks can have up to 50000 teeth in their lifetime. The tooth itself can be used to identify the kind of shark it came from but using the image alone that's not possible.
As a furniture item