FIRST RECORD OF A MOSASAUR FROM IOWA

Circa 2015, Tyler McDonald discovered a fossil in a load of rock that had been quarried in Southwestern Osceola County in Northwest Iowa. In 2021 through a series of social media pages, he was connected with Dr. Kurt Spearing at Morningside University. This fossil turned out to be a set of 3 caudal vertebrae from a Plioplatecarpine Mosasaur. The Fossil is likely either a species of Tylosaurus or Platecarpus as these genera are found in similar Cretaceous deposits in nearby states of Kansas, Nebraska and the Dakotas. While fish and marine invertebrate fossils have been found in northwest Iowa, this appears to be the first record of a Mosasaurus from the state.

The Mosasauridae were a family of extinct marine reptiles that eventually had a global distribution in the later Mesozoic, and lived between 99 and 65 million years ago. They are well known from the inland sea that existed in the Mesozoic of central North America, with many species being found in the great plains, primarily the genera Clidastes, Tylosaurus and Platecarpus.

The vertebrae were identified through comparison with other mosasaur fossils and it conformed favorably with plioplatecarpine caudal vertebrae, which are distinctive in having sockets where the chevrons would attach instead of the chevrons being fused to the vertebrae. From only 3 fused caudal vertebrae it is not possible to identify the exact genus or species, but in looking at the 3 genera that are well known from the time neither Tylosaurus nor Platecarpus had fused chevrons.

Since the load of rock was from a quarry the specific rock unit it came from is yet to be determined, however The bedrock lithology at the locality where the fossil was quarried is either the Dakota and Windrow formations or the Fort Benton Group according to the geological maps that have been consulted.