This is the tooth of a clam-crushing shark called Ptychodus. Ptychodus is a common shark in rocks of Late Cretaceous age (96-66 million years old) in North America. This particular fossil was found in east-central New Mexico near Tucumcari in rocks called the Graneros Formation. To date, no one has found a skeleton of Ptychodus, only isolated teeth.
These fossils show that the jaws of Ptychodus were filled with closely packed, knobby teeth that looked like a cobblestone pavement. These Cretaceous sharks used teeth like this to crush hard-bodied prey such as mollusks.