Is This a Tortoise Fossil?

[BoiseNoise]

My sister found this in a creek in St. Clair County, Missouri. It looks very much like a turtle shell, but we don't know enough about fossils to determine whether it is an actual turtle fossil or a concretion. I've included photos of the rock when it was covered with algae from the creek (since that seems to show the details better) as well as after the algae was cleaned off. Hopefully my sister's hand will be a good enough reference point for size.

Edited May 19 by BoiseNoise
Added info

[Fin Lover]

Welcome to the forum!  I think the cracks and algae are just giving the appearance of a turtle shell.  I'm not seeing the right texture on the broken end.

[Fossildude19]

Welcome to the Forum.

 

Looks like a chert nodule, to me.

[Coco]

I can’t help you, but I can say that a hand is not a good scale to give the size of a fossil. Indeed, there is a whole range of hand sizes, between women, men, children etc... And sometimes according to the trades... Nothing will ever be worth a meter in cm or inches because there are many members (like me) who are not Americains and are from around the world.

 

Coco

Edited May 20 by Coco

[val horn]

As others have said not a turtle as it lacks bone texture and suture lines.  It does have concentric lines and smooth curves.  I wonder at a bivalve external mold?  I am not familiar with the area.