Especially in the period when many groups of organisms first evolved, researchers find puzzling fossils which cannot be assigned to any known group. Problematic microscopic organisms are often assigned to blue-green algae because of their superficial similarity to microscopic sheaths made of calcium carbonate produced by these algae. Blue-green algae are one of the oldest organisms on Earth and play a fundamental role in most marine and land ecosystems. In spite of their importance, little is known about their evolution, because their fossils are almost featureless tubes or bubbles of carbonate. As a result, until now it has been very difficult to identify whether even the most abundant fossils belonged to blue-green algae or an entirely different group of organisms.

We used electron backscatter diffraction (EBSD), a method derived from materials sciences, to analyse crystal structures if carbonate skeletons of problematic fossils. This is a cross section through a microscopic problematicum (white outline). Each crystal in the skeleton is coloured according to its crystallographic orientation.

We based our study on the observation that biologically formed carbonate structures are more ordered than those formed chemically. What is more, different organisms produce their skeletons in different ways and a trace of it may be left in the way crystals are arranged. EBSD allowed us to measure not only the directions of crystal growth, but their slight misorientations between adjacent crystals.

We found that mysterious fossils with typical blue-green morphologies had their skeletons, or sheaths, with poorly ordered crystals and a wide range of misorientations.

Undebatable organisms such a trilobites had shells with more ordered crystal structures and limited misorientations. This is in agreement with what we know about biomineralization: multicellular organisms can control biomineralization precisely. Blue-green algae can mediate crystal growth in their sheaths, but cannot control the orientations of individual crystals. Using our approach, it may be possible to finally resolve the biological affinity of many mysterious fossils in Earth's history.