Vertebrate fossils can occur in large local concentrations. Secondary deposition in bone beds often preserves only the toughest parts of the skeleton e.g. teeth. The Moroccan Kem-Kem beds are the name given to the Cretaceous ages layers of rock exposed close to the Moroccan-Algerian border, locally dug for commercial dinosaur remains. A composite assemblage of many types of teeth and bone are found together in a bone bed that best represents a large river system or possibly mangrove like environment.