Calcite orientations and composition ranges within teeth across Echinoidea

Sea urchin’s teeth from four families of order Echinoida and from orders Temnopleuroida, Arbacioida and Cidaroida were studied with synchrotron X-ray diffraction. The high and very high Mg calcite phases of the teeth, i.e. the first and second stage mineral constituents, respectively, have the same crystallographic orientations. The co-orientation of first and second stage mineral, which the authors attribute to epitaxy, extends across the phylogenic width of the extant regular sea urchins and demonstrates that this is a primitive character of this group. The range of compositions Δ<i>x</i> for the two phases of Ca_1- <i>_x</i>Mg<i>_x</i>CO₃ is about 0.20 or greater and is consistent with a common biomineralization process.