Just got home from a sorrowful trip to Texas. I stopped at the Denison Dam on Lake Texoma for a relaxing hunt. I did find some oysters (brought one home for my grandson), and found a nice exterior mold of an ammonite. However, there was one piece I saw and photographed that I'm not sure what it was. I'd have brought it home, but I noticed two cottonmouths too close for comfort.
The object was approximately 6"×10" (15×24 cm). Although it was in jumbled rip-rap, I believe it from the Duck Creek Formation.
I hope the photo is clear enough.