Niobrara Chalk Sponges?

[kipper327]

Found these five mysteries in an outcropping of Niobrara chalk in southeast SD. I think they resemble sponges or coral, though I'm well aware that they may not be. The four sand-colored ones are all roughly the diameter of a penny, though the first pic is a little larger. The red-stained one is about the diameter of a nickel. Thanks in advance!

[Rockwood]

Marcasite nodules ?