Modern Mg calcite echinoderm skeletons are thought to lose Mg so rapidly that no pre-Pleistocene ossicles retain their original Mg concentrations; however, Carboniferous echinoderm skeletons from the Imo Formation of Arkansas and the Holder Formation of New Mexico are reported herein that are still composed of Mg calcite (7.8 to 14.5 mol% MgCO3). Their Sr concentration is also high (620 to 3710 ppm). The pore system in these echinoderm skeletons is filled with ferroan calcite or siderite that has not grown in optical continuity with the skeleton. The discovery of Carboniferous Mg calcite confirms an interpretation already made from their diagenetic replacements. Marine Mg calcite has been associated with proposed icehouse conditions, and although the Carboniferous echinoderm skeletons are compatible with this as they formed during Gondwana glaciation, they are too few to be of significance. Also they are organic secretions and so can override secular mineral trends based on inorganic precipitates. The Carboniferous echinoderm skeletons, produced in tropical waters, have a concentration of Mg similar to values for modern echinoderms. The simplest explanation for this similarity is that both precipitated by similar processes from oceans of similar composition. This does not support predictions, based on mass-balance modeling, that the Mg/Ca ratio of the Carboniferous ocean was half that of today's. The Sr content of Carboniferous echinoderms is less than that for modern echinoderms, leading to a prediction of 6 ppm Sr concentration for the Carboniferous ocean. Further examples of Mg calcite are required to substantiate these interpretations.