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I think, basically just guessing, that it is a node (can't really call it a spine) on the shell of a coiled nautiloid. It is from the Mississippian Chainman Shale where I find a few small coiled nautiloids. This is the thickest piece of shell I have ever found though. Below are a few pics of some of the small nautiloids showing the nodes running along the flank close to the umbilical shoulder. Doing a few calculations on the ratio of diameter/shell thickness of the smaller ones I come up with a diameter of between 450mm and 1050mm (probably closer to the upper figure, shell thickness was greater on the venter where I had to measure some than on the flanks where the nodes are), making it a pretty big shell.