These are Megalodon shark teeth. That was a dinosaur shark that lived millions of years ago. These fossolized teeth can be 4 or even 6 inches long. Some are even larger than that. Megalodon sharks lived in the Miocene-Pliocene Epochs (appx 24 million to 2 million years ago). I actually have one of these fossil teeth that's 5 inches in length. I'd like to collect more dinosaur teeth with time. There are many types available. They tend to fossilize well and last the test of time.
You can also see a T. Rex tooth here. Most are just the crown without the root. Here also is a replica of a Tyrannosaurus Rex tooth. This replica wih root is 11 inches long. With that kind off root, you can imaging the torque it could sustain when in use. An actual tooth is shown without the root. Check out the serrations on the edge. T. rex lived during the late Cretaceous period, about 85 million to 65 million years ago.