So, last week, scientists of the Mississippi Geological Survey were mapping the physiographic regions near the Bluff Lake outside of Starkville, Oktibbeha County, Mississippi.
In an outcrop of the Maastrichtian chalky limestone of the Prairie Bluff Formation, geologist James Starnes spotted this:
I thinks it's a postpygal, although don't quote me on that - I've never found any mosasaur fossils and am no pro at IDing them
It measured over 18 centimeters at its widest point
[Mosasaurus hoffmanni paleoart by Aberrantologist]
It was found beneath a bridge on the studied Bluff Lake quadrangle.
Turned over to the Mississippi Museum of Natural Science, the vertebra was identified as that of the one and only Hoffmann's mosasaur (Mosasaurus hoffmanni), and represents the remains of the largest known Mississipian lizard. The given size estimates are at least 30 feet (9.1 meters) and about 20000 pounds (9072 kg).
(Left to right: nautiloid, gastropod, and horn coral fossils found in the region. Credit: Lexyorow
[Bluff Lake, at the Sam D. Hamilton Noxubee National Wildlife Refuge]
Prairie Bluff was once a rich shallow ocean habitat, teeming with ammonites, squids, marine reptiles, and fish - all sorts of great prey sources for mosasaurs.
The nearby Flatwoods region is dominated by clays from the sort of environment just after the extinction of all mosasaurs.
A Mississippi Department of Environmental Quality post announced this in a Fossil Friday post: https://www.mdeq.ms.gov/geology/fossil-friday/
Some news articles covering the story.
Smithsonian magazine: https://www.smithsonianmag.com/smart-news/geologists-stumble-upon-remains-of-giant-sea-monster-in-mississippi-likely-the-largest-mosasaur-ever-identified-in-the-state-180986523/
Popular Science: https://www.popsci.com/science/mississippi-mosasaur-fossil/
Popular Mechanics: https://www.popularmechanics.com/science/archaeology/a64591551/mosasaur-mississippi/