Dickinsonia is a group of extinct organisms that lived during the late Ediacaran period in what is now Australia, China, Russia, and Ukraine. It had a round body that was roughly symmetrical on both sides, with many body parts running along its length. Dickinsonia could be as small as a few millimeters or as large as over a meter long. It likely lived in shallow waters and fed on layers of tiny organisms that covered the seafloor at that time.

As part of the Ediacaran biota, scientists have long debated Dickinsonia’s relationship to other life forms. Early theories suggested it might be a jellyfish, a land-dwelling lichen, a placozoan, or a giant protist. Today, the most widely accepted idea is that it was an animal that lived on the seafloor, possibly an early ancestor of animals with symmetrical body parts. However, this view is still debated. Dickinsonia resembles other Ediacaran organisms with segmented bodies, such as Vendia, Yorgia, and Spriggina. It has been classified as part of the group Proarticulata or the group Dickinsoniomorpha. Scientists are still unsure whether its body parts were perfectly symmetrical along a central line or shifted in a pattern called glide reflection, or if both possibilities are true.

Description

Dickinsonia fossils are found only as imprints or casts in sandstone layers. These fossils vary in size, with lengths ranging from a few millimeters to about 1.4 meters (4 feet 7 inches) and thickness from less than a millimeter to several millimeters. They are nearly mirror-image on both sides, have body segments, and are shaped like rounded or oval outlines, with one end slightly wider (egg-shaped). The rib-like segments point toward the wide and narrow ends, and the size of the segments increases toward the wide end. A ridge or groove divides the body in half, except for one unpaired segment at one end, called the "anterior most unit," which may have been the front of the organism. Scientists debate whether the segments are shifted or mirrored (isomers) or perfectly mirrored across the midline (true bilateral symmetry), as some shifted patterns may result from how the fossils were preserved. Dickinsonia may have had both types of symmetry at different times, with one side showing shifts and the other showing true symmetry.

The body of Dickinsonia is thought to have been sack-like, with an outer layer made of a strong but non-mineral material. Some Russian fossils show branched internal structures. Some scientists suggest the underside may have had tiny hair-like structures called cilia and folded pockets. Dickinsonia is believed to have lacked a mouth, anus, or digestive tract.

Dickinsonia likely grew by adding new pairs of segments or isomers at the end opposite the unpaired "anterior most unit." It probably grew without a set maximum size, though the rate of adding new segments may have slowed as it matured. Deformed fossils from Russia suggest Dickinsonia individuals could regenerate after damage.

Ecology

Dickinsonia is thought to have been an animal that moved around in the ocean and lived on the seafloor. It ate tiny microbes growing on the ocean floor using parts on the bottom of its body. Fossil marks that look like Dickinsonia, possibly showing where it fed, are sometimes found in groups, suggesting it moved in chains. These marks are named Epibaion. A study in 2022 found that Dickinsonia used mucus to stick to the ocean floor. This might have helped it live in very shallow water.

Taphonomy

Dickinsonia fossils are found as negative impressions on the bottom of sandstone layers. These impressions show the top sides of benthic organisms that were buried under sand. The impressions formed when the sand hardened before the organism’s body fully broke down. The exact process of hardening is not fully understood, but it may have happened through conditions that created pyrite "death masks" on the decaying body or through the hardening of sand by carbonate. The body imprints are often flattened, twisted, and sometimes extend into the rock above. These changes seem to show the organisms tried to move away from falling sand.

Rarely, Dickinsonia fossils are preserved as casts in large sandstone layers, where they are found with other fossils like Pteridinium and Rangea. Large areas with hundreds of Dickinsonia fossils (along with many other species) are found in place within Nilpena Ediacara National Park. Park rangers offer guided tours there during the cooler months. These fossils formed when organisms were removed from the seafloor, carried by sand flows, and deposited. In these cases, Dickinsonia fossils are stretched or torn. One such fossil was first named as a separate species, Chondroplon bilobatum, but was later identified as Dickinsonia.

Taxonomy

Dickinsonia was first found in 1946 in the Ediacara Member of the Rawnsley Quartzite in the Flinders Ranges of South Australia. Reg Sprigg described Dickinsonia the next year and named it after Ben Dickinson, who was then the Director of Mines for South Australia and head of the government department that employed Sprigg. Additional Dickinsonia fossils have also been discovered in the Mogilev Formation in the Dniester River Basin of Ukraine, the White Sea in Russia, and the Dengying Formation in the Yangtze Gorges area of South China. (approximately 551–543 million years ago).

Reg Sprigg initially believed Dickinsonia was a jellyfish-like organism from the early Cambrian period. He thought the fossil imprint showed the flattened bell of the organism, and the grooves radiating from the center might have been a canal system or rigid structure. In 1949, further study suggested Dickinsonia’s bilateral symmetry could indicate higher complexity, but scientists did not assign it to a specific taxonomic group. In 1955, Harrington and Moore classified Dickinsonia into class Dipleurozoa, order Dickinsoniida, and family Dickinsoniidae, all part of the now-discontinued group Coelenterata.

After the discovery of Charnia in 1958, which was clearly a Precambrian organism, scientists accepted that Proterozoic life existed. This discovery confirmed Dickinsonia and other South Australian fossils were Precambrian in age. The segmentation of Spriggina, found in the same area, led scientists to classify both Spriggina and Dickinsonia as annelids. This classification remained the leading theory for several decades, though some scientists had doubts.

In 1985, studies showed Dickinsonia and related organisms had glide symmetry instead of bilateral symmetry. This led to the creation of a new phylum, Proarticulata, to include Ediacaran organisms with glide reflection symmetry, such as Spriggina, Vendia, and others. Their relationships to other life forms remain unclear, and many hypotheses have been proposed. Adolf Seilacher suggested most Ediacaran organisms were closely related, forming a group called "Vendobionta." However, recent scientists argue this group is likely polyphyletic. Some researchers avoid using Proarticulata and instead use the clade Dickinsoniomorpha. In 2013, Gregory Retallack proposed Dickinsonia and other Ediacaran lifeforms were lichens, citing similar preservation methods. This idea has been widely rejected, as evidence supports a marine environment. Other theories suggest Dickinsonia could be giant protists, placozoans, or cnidarians.

Although Dickinsonia’s relationships to other organisms remain debated, most scientists believe it was an animal with stem-bilaterian characteristics. In 2018, Russian Dickinsonia fossils were found to contain cholesterol, a substance only produced by animals, supporting an animal classification. However, some scientists question whether the cholesterol is definitively linked to Dickinsonia fossils. The predictable growth patterns, clear left and right sides, and a posterior-anterior axis suggest Dickinsonia was a bilaterian. However, modern bilaterians have a mouth and anus connected by a gut, which has not been found in Dickinsonia. This rules out Dickinsonia being a crown-bilaterian but supports the possibility it was a stem-bilaterian.

Since 1947, 11 species of Dickinsonia have been described. Three are currently considered valid, two are still being studied, and six have been deemed invalid.