Calcite orientations and composition ranges within teeth across Echinoidea

Area of Science:

• Biomineralization
• Paleontology
• Materials Science

Background:

• Sea urchin teeth are complex biomineral structures.
• Understanding their formation provides insights into evolutionary processes.

Purpose of the Study:

• To investigate the crystallographic orientation of mineral phases in sea urchin teeth.
• To determine if observed orientations are conserved across different sea urchin orders.
• To elucidate the primitive nature of sea urchin biomineralization.

Main Methods:

• Synchrotron X-ray diffraction was used to analyze sea urchin teeth.
• Teeth from four families of Echinoida and orders Temnopleuroida, Arbacioida, and Cidaroida were examined.

Main Results:

• High and very high Mg-calcite phases in sea urchin teeth exhibit identical crystallographic orientations.
• This co-orientation, attributed to epitaxy, is consistent across the phylogenetic breadth of extant regular sea urchins.
• The compositional range (Δx) for the two phases (Ca1-xMgxCO3) is approximately 0.20 or greater.

Conclusions:

• The co-orientation of mineral phases is a primitive character in regular sea urchins.
• This finding suggests a common and ancient biomineralization process.
• Epitaxy plays a crucial role in the structural organization of sea urchin teeth.