A record of the Cambrian explosion
In 1984, geologist A. Higgins, working with the Geological Survey of Greenland, made a discovery in the remote, icy landscape of north Greenland. He found an outcrop of the Buen Formation—dark, silty mudstones packed with exceptionally preserved fossils from the Cambrian Period, approximately 518 million years ago. This site, named Sirius Passet after the local Sirius sledge patrol, is a Lagerstätte, a sedimentary deposit that preserves soft tissues in remarkable detail. Since its discovery, expeditions have collected around 10,000 fossil specimens.
The fossils are found in mudstones that were deposited in a deep-water, low-oxygen environment at the bottom of the sea. During the Cambrian, this part of Greenland was located on the northern margin of the paleocontinent Laurentia, near the equator. The conditions were perfect for capturing a snapshot of life during the "Cambrian Explosion," a period when animal life rapidly diversified. Sirius Passet is about 10 to 15 million years older than the more famous Burgess Shale in Canada, showing an even earlier stage of animal evolution.
The bizarre fauna of an ancient sea
The Sirius Passet ecosystem was dominated by arthropods and predators. Many of the animals are entirely alien compared to modern life, representing early animal body plans.
One of the largest inhabitants was Pambdelurion whittingtoni, a formidable predator that could grow up to 55 centimeters long. It had 11 pairs of swimming flaps and 11 pairs of stout, lobopodous legs. Its head featured a pair of large, spiny frontal appendages for sensing or capturing prey. Another major predator was Kerygmachela kierkegaardi. This swimming lobopodian had 11 pairs of lateral flaps for propulsion and large, spined frontal limbs to impale soft-bodied prey. Its species name honors the Danish philosopher Søren Kierkegaard.
Among the most informative fossils is Halkieria evangelista, an armored, slug-like creature between 1.5 and 8 centimeters long. Its back was covered in a "chain mail" of tiny mineralized plates, known as sclerites, and it had a prominent shell at each end of its body. Complete, articulated specimens from Sirius Passet were essential for understanding how these widespread but often fragmented fossils fit together. Halkieria shows the early evolution of mollusks and brachiopods.