Mystery Stone Any Ideas?

[jeramme]

Hi all, I found this piece near the river. I thought it might be a fossil, or it was suggested that it is just mud that cracked and turned to stone. It looks like sandstone, but seems harder and denser. Any ideas? Thanks for your time and consideration.

 

[PaleIO]

If I had to guess I would say a piece of a septarian concretion

 

[PaleIO]

Sorry I see this is in another thread. Got a little confused for min.