Biostratigraphy of Mosasauridae (Squamata, Varanoidea) from the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of Western Kansas

The remains of the mosasaur genera Clidastes, Platecarpus, Tylosaurus, and Ectenosaurus are found in the Smoky Hill Chalk Member, Niobrara Formation (upper cretaceous) of western Kansas. Some taxa are found only in limited horizons, whereas others appear to occur at all levels. Ectenosaurus clidastoides is a very rare mosasaur, and its occurrence has been documented only from near the middle of the Smoky Hill Member. Platecarpus seems to be the most common genus, although Tylosaurus is at least as cornmon in the lower horizons. Clidastes seems to be nearly absent in the lower horizons, and becomes relatively more common in the chalk's uppermost levels. Clidastes liodontus is known to occur lower in the chalk than Clidastes propython. Clidastes propython occurs in the chalk's uppermost levels. Platecarpus tympaniticus occurs at all levels in the chalk, while Platecarpus planifrons seems to be limited to the lower horizons. Tylosaurus consists of three taxa: Tylosaurus nepaeolicus and an undescribed tylosaur(s) which occur in the lower chalk and Tylosaurus proriger which is found in the upper chalk. Shifts in generic abundance through the stratigraphic column may be due to changing environmental conditions. The relatively abrupt morphologic changes in some mosasaurs at various levels in the Smoky Hill Member is thought to be the result of evolution. If this interpretation is valid, the varying number of species per genus might be evidence of differing evolutionary rates in closely related genera.