The Mazon Creek area was once a river delta and it is true there are a lot of Fern fossils but some areas yield more marine life than plant life. It was just a matter of if you were up river more or closer to the shallow sea the river emptied into. Jellyfish actually are a "common fossil" from this area. There are also insects, crustations,worms, and sometimes fish fossils found there.
I don't live very far from the famous pit 11, and have collected at that site and some other areas of Mazon Creek. It's getting hard to find things there as it has been picked over so very much. I have for sale in my shop shrimp, ferns, wood, jellies and even a partial Tully monster. I am sorting a batch of Mazon creek concreations form an old collection. Many interesting things there. I don't have anything posted to my web site right now as I am having some trouble with it but it you're interested in something let me know, we can work something out. By the way I was very impressed with your collection.
I don't have it in front of me right now(it's at another location) but it is about 2 1/2 inches in diameter. Most found at this location are smaller than this one but I do have several in my store (I have a rock/jewelry store) that are larger. One or 2 that might be 3 to 4 inches long. They're not expensive, under $10. I don't remember the exact price on them. Thanks for the inquiry!!