3.7" Cretaceous Rudist (Durania) - Kansas

A rudist is a ring-shaped marine heterodont bivalve that arose during the Late Jurassic and died out near the end of the Cretaceous. Durania maxima rudists had heavy cone-shaped shells, often with a large circular collar, and are typically found in the lower third of the Smoky Hill Chalk.

The Smoky Hill Chalk Member of the Niobrara Chalk formation is a Cretaceous conservation Lagerstätte, or fossil-rich geological formation, known primarily for its exceptionally well-preserved marine reptiles. It outcrops in parts of northwest Kansas--its most famous localities for fossils--and in southeastern Nebraska. Large, well-known fossils excavated from the Smoky Hill Chalk include marine reptiles such as plesiosaurs, large bony fish such as Xiphactinus, mosasaurs, pterosaurs, and turtles.