BIOMINERALIZATION BY ATTACHMENT OF NANOPARTICLES FROM THE EDIACARAN THROUGH ALL OF THE PHANEROZOIC (Invited Presentation)

Crystalline biominerals cost energy but provide the organism making them with scaffolding, shielding, locomotion, mastication, gravity and magnetic field sensing, etc. Diverse marine organisms form via the same two amorphous calcium carbonate (ACC) precursor phases, which then crystallize into either calcite or aragonite, respectively, in echinoderms^1-3 or mollusk shell nacre⁴ and coral skeletons⁵, as revealed by difficult, lengthy, synchrotron spectromicroscopy experiments.

Much simpler SEM experiments show that when the biominerals are formed via attachment of ACC nanoparticles, they also appear nanoparticulate after cryofracturing, with nanoparticle size in the 50-400 nm range. The latter is thus a proxy for the former. Tunicate spicule crystals, which do not form via ACC⁶, do not exhibit nanoparticulate cryofracture-figure.

Having validated this proxy on well-known, modern biominerals, we can now use nanoparticulate texture as a proxy for crystallization by particle attachment (CPA)⁷ in many more modern and fossil samples throughout the Phanerozoic.

We find that CPA is convergent: it has evolved independently in diverse phyla, starting with the oldest know animal fossil, the Ediacaran Cloudina (550 Ma). The nanoparticulate proxy confirms CPA in Cambrian (500 Ma) shelly fossils, as well as nacre from Ordovician (450 Ma), Cretaceous (100 Ma), Miocene (13 Ma) and modern (0 Ma) mollusks.

Surprisingly, the nanoparticulate proxy is not altered by phosphatization as demonstrated by the nacre results, which were collected from both phosphatized fossils and fossils that are still aragonite. This suggests that re-crystallization of aragonite or calcite to apatite does not occur by dissolution and (re)precipitation, but is an ion-exchange mechanism.

1. Y Politi 2008 DOI: 10.1073/pnas.0806604105
2. CE Killian 2009 DOI: 10.1021/ja907063z
3. YUT Gong 2012 DOI: 10.1073/pnas.1118085109
4. RT DeVol 2015 DOI: 10.1021/jacs.5b07931
5. T Mass 2017 DOI: 10.1073/pnas.1707890114
6. B Pokroy 2015. DOI: 10.1021/acs.chemmater.5b01542
7. JJ De Yoreo 2015 10.1126/science.aaa6760