Current accounts of this explosion in animal diversity rely heavily on records from fossilised shells and other hard parts, since these structures are the most likely to survive as fossils.
However, since most marine animals are ‘soft-bodied’ this represents only a small fraction of the total diversity.
Rare sites of exceptional fossilisation, like the world-famous Burgess Shale, have revolutionised palaeontologists understanding of ‘soft-bodied’ Cambrian life. Because of the special conditions of fossilisation at these localities, organisms that did not produce hard mineralized shells or skeletons are also preserved. Such sites offer a rare glimpse into the true diversity of these ancient seas, which were filled with a dazzling array of soft and squishy predatory worms and arthropods (the group containing modern crustaceans and insects).
One of the oldest of these truly exceptional fossil bonanzas is the Sirius Passet site in the far north of Greenland. Unfortunately, during their long history, the rocks at Sirius Passet have been heated up and baked to high temperatures as the northern margin of Greenland smashed into various tectonic plates and buried these rocks deep beneath the surface.
All this heating has boiled away the delicate organic remains that once formed the fossils of soft bodied animals at Sirius Passet, leaving only faint impressions of their remains.
Not far to the south of Sirius Passet, the rocks have escaped the worst effects of this heating. By applying a low-manipulation acid extraction procedure to dissolve some of these less intensively cooked mudrocks, a wealth of previously unknown microscopic animal fossils was revealed, preserved in spectacular detail.
Most of the fossils are less than a millimetre long and need to be studied under the microscope. Fossils at the nearby Sirius Passet site typically preserve much larger animals, so the new finds fill an important gap in our knowledge of the small-scale animals that probably made up the majority of these ecosystems. Among the discoveries are the tiny spines and teeth of priapulid worms – small hook shaped sensory and gripping structures that allowed these worms to efficiently burrow through the sediments and capture prey. Other finds include the resistant outer cuticles and defensive spines of various arthropods, and perhaps most surprisingly, microscopic fragments of the oldest known pterobranch hemichordates – a group of tube-dwelling filter feeders that are distant relatives of the vertebrates. This group became very diverse after the Cambrian Period and are among some of the most commonly found fossils in rocks from younger deposits, but were entirely unknown from the early Cambrian. This new source of fossils will also help aid our understanding of the famously difficult to interpret fossils at the nearby Sirius Passet site, where the flattened animal fossils are usually completely articulated, but missing crucial microscopic details. The sheer abundance of these miniature animal fossils means that we have only begun to scratch the surface of this fossil resource, but it is already clear that this discovery will help to reshape our view of the non-shelly animals that crawled and swam among the early Cambrian seas more than half a billion years ago.
Link to the article
Slater, B. J., Willman, S., Budd, G. E., Peel, J. S. 2018. Widespread preservation of small carbonaceous fossils (SCFs) in the early Cambrian of North Greenland. Geology https://doi.org/10.1130/G39788.1