Fossil bonanza fills important knowledge gap

A team of researchers have uncovered a rich diversity of microscopic animal fossils from over half a billion years ago in rocks from the northern tip of Greenland.

Current accounts of the ‘Cambrian explosion’ in animal diversity rely heavily on records from fossilised shells and other hard parts, as 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, like the world-famous Burgess Shale, have specific conditions which allow the fossilisation of organisms that did not produce hard mineralised shells or skeletons. Now a team from Uppsala University in Sweden have discovered that one of the oldest of these exceptional locations is just south of the Sirius Passet site in the far north of Greenland.

“The sheer abundance of these miniature animal fossils means that we have only begun to scratch the surface of this overlooked 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,” said Sebastian Willman, a researcher at the Department of Earth Sciences, Uppsala University.

During their history, the rocks at Sirius Passet have been heated up to high temperatures as the northern margin of Greenland smashed into various tectonic plates and buried these rocks deep beneath the surface. 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. However the team discovered that not far to the south of Sirius Passet, the rocks have escaped the worst effects of this heating.

Most of the fossils were less than a millimetre long and had 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 were the tiny spines and teeth of priapulid worms – small hook shaped structures that allowed these worms to efficiently burrow through the sediments and capture prey.

Other finds included the tough outer cuticles and defensive spines of various arthropods, and perhaps most surprisingly, microscopic fragments of the oldest known pterobranch hemichordates – an obscure group of tube-dwelling filter feeders that are distant relatives of the vertebrates.

The work is published in Geology.