We usually think of rays as creatures of the ocean. However, these flattened cartilaginous relatives of sharks also inhabit freshwater, and include the striking freshwater stingrays of South America and the giant freshwater stingrays of southeast Asia. During the Late Cretaceous Epoch, freshwater rays also prowled the waterways of North America. The curious little object shown here is a tooth from one of these Late Cretaceous freshwater rays, called Myledaphus bipartitus. The flat, robust crown of the tooth is towards the top of the picture, and the root towards the bottom. When alive, Myledaphus would have had many such teeth in its jaws, for crushing through its meals of freshwater shellfish.
Myledaphus is closely related to the living guitarfish, such as the shovelnose guitarfish Rhinobatos productus (below) that can be found in saltwater off the coast of California:
The Western Science Center’s Myledaphus tooth was collected from the Hell Creek Formation of Montana, the same rock layer that has produced fossils of such dinosaurian giants as Tyrannosaurus, Edmontosaurus, and Triceratops, and an array of other animals that existed 66 million years ago, right before the mass extinction that destroyed all the dinosaurs other than modern birds. Whole skeletons of Myledaphus have been found elsewhere in North America, such as this one featured by the Royal Tyrrell Museum in Alberta, Canada (scroll down for image): https://royaltyrrellmuseum.wordpress.com/2016/11/08/everyone-can-do-science/
by Andrew McDonald