This is a bumpy, Pithocrinus cooperi crinoid calyx from the Alpena Limestone of Michigan. The calyx is where the arms and stem of the crinoid would have attached and is frequently the only part of the crinoid preserved intact as they typically fall apart prior to fossilization.
Crinoids, sometimes commonly referred to as sea lilies are animals not plants. They are echinoderms related to starfish, sea urchins and brittle stars. Many crinoid traits are like other members of their phylum. Such traits include tube feet, radial symmetry, a water vascular system, and appendages in multiples of five (pentameral). They first appeared in the Ordovician (488 million years ago) and some species are still alive today.