That’s the upper jaw upside down. Surprised they didn’t find the tusks. Probably collected by the natives. Since the site is on a river (thru limestone rock more than soil and sand) the mammoth may have been butchered there.
There is a guy on the E coast of FL who excavates ICW dredge pile islands since they have debris from 10s of feet below prevailing ground. Some of his finds are Mastodon and Mammoth bones and tusks. He has a Mammoth tusk with graffiti carved into it showing a hunting scene. Pre-historic Scrimshaw.
It will be interesting to see what kind of marks are found on this find once it is cleaned up. Scars from wild creature attacks, or signs of human hunting like spear gashes.