Minuscule fossil finds shed light on the aftermath of the Cambrian Explosion

What happened to the bizarre animals that typified Cambrian oceans, 540 to 480 million years ago? These aberrant organisms look very different from anything that we know of today, and were thought to have died out soon after their initial diversification. New findings revealed that some of them persisted for another 40 million years, albeit in a miniature form.

A previously unexplained gap in the fossil record of animals

About 540 million years ago, the Earth was profoundly altered by the sudden diversification of animals that transformed it into the dynamic, lively planet that we know today. This pivotal event, known as the Cambrian Explosion, is most famously understood from sites of exceptional preservation such as the Burgess Shale, where fossils are preserved as thin carbon films. These deposits provide snapshots of ancient life that help palaeontologists track the evolution of animal groups in deep time. However, Burgess Shale-type preservation disappears soon after the beginning of the Ordovician period 485 million years ago, leaving a crucial gap in our knowledge of how Ordovician animals evolved from Cambrian ones.

A micro-Burgess Shale

A new Ordovician site has now been identified in Castle Bank (Wales, UK) that, in many respects, matches the type of organic preservation seen in the Burgess Shale; albeit with a conspicuous difference: the size of recovered fossils rarely exceeds a few millimetres. Researchers at Uppsala University have submerged rock samples from Castle Bank into an acid solution in order to recover these delicate organic remains and analyse their fine details using powerful microscopes. This laboratory technique involves hand-picking individual microfossils from organic residues using a small pipette, a long and laborious process that often discourage initial investigations. The outcome, however, was worth the struggle: a range of exquisitely preserved animal tissues was recovered, confirming the occurrence of Burgess Shale-type preservation well within the Ordovician.

The missing link between Cambrian and Ordovician biotas

Amongst the large diversity of animal remains that were recovered were numerous modern groups such as marine worms and various crustaceans; but also typical members of Cambrian biotas, including diverse sponges and enigmatic arthropod predators known as opabiniids. The strikingly small size of these fossils compared to their Cambrian counterparts is suggestive of an evolutionary process of miniaturization, whereby diminutive organisms are selected against larger forms under specific conditions. Many animal groups identified at Castle Bank have small-sized representatives today, which suggests that the adoption of minute dimensions among these Ordovician animals was a significant and widespread event in their evolution.

The untapped microfossil record of animals

If miniaturized animals became widespread in post-Cambrian oceans, gentle acid-maceration of additional rock samples from Ordovician strata is likely to provide many more insights into the origin of diminutive biotas emerging at this time. This acid-based laboratory technique can be applied to a wide range of fine-grained sedimentary rock samples, and is known to yield microfossils even when their larger counterparts (if any) cannot be detected with the naked eye. Traditionally used in Cambrian palaeontology, the method shows great potential to fill in the gaps of the wider fossil record, in turn enhancing our understanding of animal evolution from its early beginnings to the present.