567-million-year-old fossils rewrite the dawn of animal life

Researchers have discovered an unusually rich fossil site in a remote region of Canada's Northwest Territories, revealing new details about some of the earliest complex animals on Earth. The fossils belong to the Ediacaran biota, a collection of mostly soft-bodied organisms that lived on ancient seafloors more than 500 million years ago.

Some specimens appear to be about 567 million years old. Their age suggests that animal movement and sexual reproduction began 5-10 million years earlier than previous evidence indicated. The study, led by scientists at the American Museum of Natural History and Dartmouth, was published in Science Advances.

"For 3 billion years, life on Earth was dominated by microbes. Then, all the sudden, we get these strange-looking marine animals big enough to see and capable of behaviors we would find familiar today," said the study's lead author Scott Evans, assistant curator of invertebrate paleontology at the American Museum of Natural History. "If we want to understand this transition, when life first became large, complex and unmistakably animal, this new site has tremendous potential."

A Rare Window Into the First Animals

Ediacaran organisms came in many unfamiliar forms, including flat discs, leafy shapes, and ribbed ovals. Their fossils provide the earliest direct record of multicellular animal life.

Some Ediacaran species have been connected to animal groups that still exist, including mollusks, nematodes, comb jellies, and cnidarians (a group that spans jellyfish and corals). Other species resemble nothing alive today. Even so, they include the oldest known animals capable of moving to find food or reproducing sexually.

Most of these organisms lived before animals commonly developed shells, bones, or other hard body parts. Their soft bodies rarely fossilized, meaning that only exceptional environmental conditions could preserve them.

Ediacaran fossils have been found on every continent except Antarctica, but sites with more than 10 species are extremely uncommon. As a result, scientists have had only limited evidence from this crucial period, which lasted roughly 40 million years.

A White Sea Community Found in North America

Researchers divide Ediacaran organisms into three major groups, known as assemblages, based on where they fall in the geologic record. These are the Avalon assemblage (575-559 million years ago), the White Sea assemblage (559-550 million years ago), and the Nama assemblage (550-538 million years ago).

Before this discovery, White Sea fossils had been documented in Europe, Asia, and Australia, but not in North America. The new fossils were found in ancient rock layers in Canada's Mackenzie Mountains.

The site is located on the traditional lands of the Sahtú Dene and Métis. Members of those communities gave the researchers guidance and permission to enter and study the area.

The work builds on previous geological research in the region, but the scale and diversity of the discovery mark a major advance. Scientists identified more than 100 fossils, including six groups that had never previously been recorded in North America.

Even more surprising was their age. Some specimens date to approximately 567 million years ago, making them 5-10 million years older than other known White Sea fossils. That places them within the same broad time period as the older Avalon assemblage.

The fossil-bearing layers are also covered by hundreds of feet of rock that may contain additional specimens, suggesting that much more remains to be discovered.

"Not only is this new site highly diverse, but also it is from a part of the rock succession where we have previously lacked fossil remains," said study co-author Justin Strauss, an associate professor of Earth and Planetary Sciences from Dartmouth, who has been exploring this area for about 15 years. "This is really exciting. Given our understanding of the regional geology in northwestern Canada, there is great potential here to revisit our understanding of Ediacaran Earth history."

Strange Creatures From an Ancient Seafloor

Several of the fossils represent organisms never before found in North America.

Dickinsonia was a flat animal that traveled across the seafloor. It had no mouth and apparently obtained nutrients by absorbing bacteria and algae through its entire lower surface. Evans compared its divided circular body to a "bathmat" or "pancake."

Funisia was a stationary, tube-shaped organism that lived in clusters of similarly sized individuals. It provides the oldest known fossil evidence of sexual reproduction. Like modern corals, it may have reproduced through a coordinated release of sperm and eggs into the surrounding water.

Kimberella moved with the help of a muscular foot and scraped food from the seafloor. Scientists commonly interpret it as an early relative of mollusks. The newly discovered specimens may also make it the oldest known fossil bilaterian, the group of animals with distinct front, back, top, and bottom with symmetric left and right sides that makes up more than 99% of all known animals.

The site also preserved Eoandromeda, a possible comb jelly with eight arms arranged in a spiral.

Did Animal Innovation Begin in Deep Water?

The fossils indicate that these organisms lived in deeper water than scientists had previously associated with the White Sea assemblage.

This supports a developing idea that early animals may have first appeared in offshore, deep marine environments before gradually spreading into shallower coastal waters. That pattern is the reverse of what became more common in later animal evolution.

"These results suggest a pattern where evolutionary innovation begins in deeper environments and later spreads toward the coast," Evans said. "We think of the deep ocean as a dark, inhospitable place, but it is also relatively stable, with few fluctuations in things like temperature and oxygen essential to most animal life. This stability may have provided key opportunities to support early animal life."

The stable conditions of the deep ocean may therefore have offered a favorable setting for the earliest stages of animal diversification.

Preserving Canada's Fossil Record

The fossils will eventually become part of the permanent collection at the Prince of Wales Northern Heritage Centre in Yellowknife, Northwest Territories.

Other coauthors on the study include Erik Sperling, Stanford University; and Kimberly Lau, The Pennsylvania State University.

This work was supported by a NASA Exobiology grant (# 80NSSC25K7024); a U.S. National Science Foundation (NSF) grant (# EAR-20 2143164); and NSF Frontier Research in Earth Science grants (# EAR-2021324 EAR-2021176).

Reporting from ScienceDaily contributed to this story.