This week we’re going properly prehistoric! Today’s fossil isn't an animal, or even a plant. It's a rock… made by microbes!
Stromatolites are layered structures formed by cyanobacteria, which are tiny, photosynthesising microbes that have been around for over 3.5 billion years. It's the stuff often referred to as 'blue-green algae' but it's actually a type of photosynthesising bacteria!
So how does this squishy sticky gooey stuff end up making a fossil? Well, the cyanobacteria grow in sticky mats of 'mucus' (actually called mucilage) in shallow water. As fine sediment settles onto the mat, the microbes move upward to stay near the light to photosynthesise. Layer after layer of a very long time, this creates a domed mushroom-like structure — like nature's version of a mille-feuille, but with microbes and mud instead of pastry and cream. Over time, these layers harden into stone by leaching out the calcium carbonate from the sediment forming a sort of 'cement', leaving behind the beautiful banded patterns we see in fossil stromatolites.
I’ve made a sketch this week showing the process in five stages - from the first sticky 'mucus' coverage to the final dome ready to go round again. These fossils might not have teeth or tentacles or tell tales of giants roaming the earth, but they tell an epic story of Earth’s earliest days!
A common mix-up: You may have seen Kambaba "Jasper" sold as fossilised stromatolite. While it looks similar in some cases, it’s not the same at all. Kambaba is actually a type of rhyolite (a volcanic rock), and its swirling green patterns are caused by mineral formations, not fossilised microbes. You can see these formations as orbs with radiating rings growing out giving it this appearance.
You'll often hear the arguement - but it's green like Algae?! - yes, but remember, stromatolites are rocks made from sediments - which aren't green - trapped in place by the bacteria, so the colour only helps to disprove it as a Stromatolite.
Kambaba is still gorgeous - but it's not a Stromatolite!
Fun fact: Without cyanobacteria and stromatolites, we probably wouldn’t be here. They helped create the oxygen-rich atmosphere we all rely on today by saturating the oceans with Oxygen that was then released into the atmosphere allowing life to move onto land - meaning you owe your next deep breath to a squishy greeny blue slime mat from 3 billion years ago!
