This is an authentic fossilized tooth from a mosasaurus—a giant, aquatic carnivorous lizard that lived until about 66 million years ago.
The item you receive will differ slightly from the one pictured. All teeth are around ¾" long.
Mosasaurus was a type of large, carniverous, aquatic lizard. It was technically not a dinosaur, but a reptile closely related to snakes and monitor lizards. Mosasaurus grew up to 50ft (15m) long and existed for about 4 million years until the mass extinction event that ended the Mesozoic period.