Fine Mississippian Starfish Cast Fossil

This is a fine example of a cast fossil of a Mississippian starfish. Fossils can come about in 3 ways:
1) Replacement Fossils: Fossils resulting from the actual replacement of the organic matter in the cells by minerals (permineralization)
2) Mold Fossils: Impressions left behind after an organism decays, leaving behind an opening in the rock
3) Cast Fossils: Caused by infilling of a mold fossil by new minerals resulting in an exact likeness of the original specimen
Echinoderms typically disarticulate soon after death, so this specimen must have been buried quickly in order to preserve its shape.