bone?

Looking at this specimen found in the Mississippi in MN it appears to have mineralized and turned to rock. Heavy for the size, also appears it’s hollowed from what I could see through one slit in the middle. Also wondering what the black staining is?

Need a little bit of help, So came across this sort of oblong thick piece at a park in Southeast Pennsylvania. Was hesitant to pick it up wasn't sure if it was bone or something else can anyone identify it, especially most importantly if it's bone or not And if it is bone has it been modified...