Rugose Coral Fossil!

The fossil in the image appears to be a cross-section of a rugose coral, also known as a horn coral. Rugose corals are an extinct group of solitary or colonial corals that lived from the Ordovician to the Permian periods (about 485 to 252 million years ago). The distinctive features in this fossil include the radiating, rib-like structures (septa) that extend from the center outward, which are typical of rugose corals. These septa are the internal partitions that supported the coral polyp, and their arrangement in a radial pattern is a key identifier.
The circular shape and the concentric growth lines around the edge suggest this is a transverse section, likely cut or naturally broken to show the internal structure. The white and brownish coloration is due to mineral replacement over millions of years, often with calcite or silica, which is common in coral fossils. The texture and structure are different from the tabulate corals you showed earlier, as rugose corals tend to have more pronounced septa and a less honeycomb-like appearance.
This particular specimen might belong to a genus like Heliophyllum or Zaphrentis, which are common rugose corals with well-defined septa.