While fishing from the banks of the Missouri River in eastern Montana, you notice some shiny fragments of fossils lying in the grey shaly mud of the banks. The nearby outcrops are known to be composed of Cretaceous strata. Upon closer inspection, you discover that the fragments were from a coiled "critter" that had very ornate "curved" lines across the very shiny pearly or lustrous outer layer. Required:
What is this fossil "critter"?