Interesting, I had a fossil found in an Eastern Iowa river that looked just like a mushroom. I posted pics in a fossil web site, and someone there told me that it looked like a river worn rogue horned coral. In prehistoric time everything around here was a shallow sea. Another time here in a Iowa river I found a ball shaped stone...almost like a ball of mud about the size of an egg. The weird thing was, it was like twice as heavy as river rock. That went into a friends aquarium. Wishing now that I would have split it in half.
Those do not look like artifacts. Artifacts - whether flint, granite, or slate, will show some sign of human workmanship. Either chipping or grinding will be apparent on close inspection. There are a lot of very interesting stones - like yours - that leave you wondering how they got that way. Possibly fossils or parts of fossils, but that is not my specialty.