I found this fragment of a fish fin during my last trip to the North Sulphur River. The ribbed structure looked so much like a piece of tile that I almost walked past it at first! I've been looking at some of the photos in the Dallas Paleo Society's book on the NSR since I've been back, and the closest match I can find are the pectoral fins of Protosphyraena tenius. The only other thing I can think of is maybe part of the tail fin of a Xiphactinus audax? If anyone has any suggestions, they'd be much appreciated!