A fossil of Dickinsonia, a flat organism that moved around on the sea floor, lacking a mouth and instead absorbing bacteria and algae through its entire bottom surface.Scott Evans/© AMNH
Hundreds of millions of years before the age of dinosaurs, during a time known as the Ediacaran, life existed mostly in the oceans, and the seafloor was covered with soft organisms unlike anything alive today.
With shapes ranging from delicate fern-like fronds to segmented blobs, fossils of Ediacaran organisms represent the first direct evidence of multicellular animal life as well as the origins of animal movement and sexual reproduction.
Now, researchers have uncovered a remarkable site in Canada’s Northwest Territories where they have found more than 100 Ediacaran fossils, including six groups never before seen in North America. These fossils are older and found in much deeper water settings than have been previously described. The work is published today in the journal 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, an assistant curator of invertebrate paleontology at the Museum. “If we want to understand this transition, when life first became large, complex, and unmistakenly animal, this new site has tremendous potential.”
Ediacaran species are linked to a diverse set of animal groups, including mollusks, nematodes, comb jellies, and cnidarians—a group that spans jellyfish and corals. Because most Ediacaran organisms were soft-bodied, their fossils are rare. Although these fossils have been identified on every continent except Antarctica, our understanding of Ediacaran life primarily comes from just a handful of sites, providing limited insight into this roughly 40-million-year interval in Earth’s history.
Ediacaran life is sorted into three groups, representing different times in the geologic record. These “assemblages” are called Avalon (575-559 million years ago), White Sea (559-550 million years ago), and Nama (550-538 million years ago).
Until this study, fossils from the White Sea assemblage had mostly been found in Russia and Australia. Now, researchers have uncovered clear evidence of this assemblage in North America, in the remote, ancient rocks of Canada’s Mackenzie Mountains.
In addition, the researchers determined that some of these North American fossils are estimated to be about 567 million years old, 5-10 million years older than previously documented White Sea specimens and overlapping with the time of the older Avalon assemblage.
“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. The fossils were found on the traditional lands of the Sahtú Dene and Métis, who provided the research team with guidance and permission to access the site.
Among the finds made for the first time in North America are:
- Dickinsonia, a flat organism resembling a round mat, with a divided circular body that moved around on the sea floor, lacking a mouth, and instead absorbing bacteria and algae through its entire bottom surface;
- an immobile tubular organism called Funisia that lived in clusters and offers the oldest evidence of sexual reproduction in the fossil record, likely with coordinated release of sperm and eggs into the water column like coral;
- Kimberella, an organism with a muscular foot that fed by scraping the sea floor, widely interpreted as an early relative of mollusks and now potentially the oldest fossil bilaterian—the group of animals with distinct front, back, top, and bottom with symmetric left and right sides;
- and Eoandromeda, a possible comb jelly with eight spiral arms
The scientists also discovered that these organisms lived in deeper-water environments than previously recognized for the White Sea assemblage. This finding supports a growing hypothesis that early animals may have originated in offshore, deep-marine settings before expanding into shallower waters over time, the opposite of what is typical of animal evolution after this time.
“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,” Evans said. “This stability may have provided key opportunities to support early animal life.”