As fossils go, these stromatolites are pretty old. In fact, as far as anything goes, they are pretty old. What you are looking at is an amazing cross-section of rocks formed by layers of calcareous (limey) secretions from cyanobacteria and from trapped sediments - from all the way back in the Proterozoic some 3.5 billion years ago. Really early life, long before creatures with teeth and bones evolved. Stromatolites were responsible for the first atmospheric oxygen; the relationship between the Earth, the seas, the air and life has a faascinating history.
If you get the chance to go to the Sedgwick Museum, in Cambridge, UK, you and this specimen can have a staring contest. It is worth doing!
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