Carbon. Calcium. Phosphorus. These three ingredients are used to build the bones of every living animal. And every extinct animal. Indeed, the first bones were also built using carbon, calcium and phosphorus, but the bones of animals are not the only hard tissues made using these key ingredients. Some living algae also build phosphatic skeletons. And so did some extinct algae, including three mid-Neoproterozoic (717–812 Ma) species from Canada.
Phoebe Cohen and colleagues (including J. William Schopf) recently presented the “…earliest compelling evidence of biologically controlled eukaryotic biomineralization known in the fossil record…” Eukaryotes are organisms whose cells contain complex structures enclosed within membranes, like you, me and trees, and unlike bacteria (which are prokaryotes). The evidence for biomineralization of the algal scales includes traces of carbon, calcium and phosphorus detected using energy-dispersive x-ray fluorescence spectroscopy. These three elements are remnants of ancient phosphatic tissues. The chemistry of phosphatic tissues can be mimicked by minerals that form during burial, however, but Cohen and colleagues state that “…none of the scales… are deformed or show any evidence of either phosphate replacement or secondary apatite overgrowths…” Another possibility for the mineralised structures is replacement of calcium carbonate with calcium phosphate over time, but “…the possibility that these fossils were originally composed of calcium carbonate can be discounted due to the absence of (1) carbonate pseudomorphs, (2) any carbonate signal in the Raman spectra, and (3) cast and mold structures characteristic of phosphate replacement…” So, the earliest origin of hard tissues in the eukaryotic Domain can now be traced back to the mid-Neoproterozoic thanks to spectroscopy.
Cohen PA, Schopf, JW, Butterfield NJ, Kudryavtsev AB, Macdonald FA, 2011. Phosphate biomineralization in mid-Neoproterozoic protists. Geology 39, 539-542.
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