The discovery of one of the planet’s strangest organisms in the history of evolution could help explain how life on Earth, which once consisted only of microscopic organisms, changed so that huge organisms like dinosaurs and blue whales could ultimately evolve.
So-called Ediacaran organisms have puzzled biologists for decades. To the untrained eye they look like fossilized plants, in tube or frond shapes up to two meters long. These strange life forms dominated Earth’s seas half a billion years ago, and scientists have long struggled to figure out whether they’re algae, fungi, or even an entirely different kingdom of life that failed to survive. Now, two paleontologists think they have finally established the identity of the mysterious creatures: They were animals, some of which could move around, but they were unlike any living on Earth today.
Scientists first discovered the Ediacaran organisms in 1946 in South Australia’s Ediacara Hills. To date, reports Colin Barras in Science, researchers have identified about 200 different types in ancient rocks across the world. Almost all appear to have died out by 541 million years ago, just before fossils of familiar animals like sponges and the ancestors of crabs and lobsters appeared in an event dubbed the Cambrian explosion. One reason these creatures have proved so tricky to place in the tree of life is that some of them had an anatomy unique in nature. Their bodies were made up of branched fronds with a strange fractal architecture, in which the frond subunits resembled small versions of the whole frond.
One of the strangest of the the Ediacaran organisms, rangeomorphs, reported in the journal Nature Ecology and Evolution in July of 2017, were some of the earliest large organisms on Earth, existing during a time when most other forms of life were microscopic in size. Some rangeomorphs were only a few centimeters in height, while others were up to two meters tall.
These organisms were ocean dwellers that lived during the Ediacaran period, between 635 and 541 million years ago. Their soft bodies were made up of branches, each with many smaller side branches, forming a geometric shape known as a fractal, which can be seen today in things like lungs, ferns and snowflakes.
Since rangeomorphs don’t resemble any modern organism, it’s difficult to understand how they fed, grew or reproduced, let alone how they might link with any modern group. However, although they look somewhat like plants, scientists believe that they may have been some of the earliest animals to live on Earth.
“What we wanted to know is why these large organisms appeared at this particular point in Earth’s history,” said Dr Jennifer Hoyal Cuthill of Cambridge’s Department of Earth Sciences and Tokyo Tech’s Earth-Life Science Institute, the paper’s first author. “They show up in the fossil record with a bang, at very large size. We wondered, was this simply a coincidence or a direct result of changes in ocean chemistry?”
“During the Ediacaran, there seem to have been major changes in the Earth’s oceans, which may have triggered growth, so that life on Earth suddenly starts getting much bigger,” said Hoyal Cuthill. “It’s probably too early to conclude exactly which geochemical changes in the Ediacaran oceans were responsible for the shift to large body sizes, but there are strong contenders, especially increased oxygen, which animals need for respiration.”
Their analysis shows the earliest evidence for nutrient-dependent growth in the fossil record. All organisms need nutrients to survive and grow, but nutrients can also dictate body size and shape. This is known as ‘ecophenotypic plasticity.’ Hoyal Cuthill and her co-author Professor Simon Conway Morris suggest that rangeomorphs not only show a strong degree of ecophenotypic plasticity, but that this provided a crucial advantage in a dramatically changing world. For example, rangeomorphs could rapidly “shape-shift”, growing into a long, tapered shape if the seawater above them happened to have elevated levels of oxygen.
Fast forward to today, Cuthill and Jian Han at Northwest University in Xi’an, China, have now found key evidence that the Ediacaran organisms were animals. They analyzed more than 200 fossils of a 518-million-year-old marine species named Stromatoveris psygmoglena.
Paleontologists had previously concluded that the 10-centimeter-tall species was some sort of animal—in part, says Hoyal Cuthill, because it was found alongside other known animals, and all of the fossils are preserved in a similar way. Hoyal Cuthill and Han argue S. psygmoglena was also an Ediacaran organism, a rare “survivor” that somehow clung on through the Cambrian explosion.
The Stromatoveris fossils, which were all unearthed in Yunnan province in southwestern China, are beautifully preserved, Hoyal Cuthill says. As she examined specimen after specimen she became increasingly excited. “I began thinking: My goodness, I’ve seen these features before.” Like some of the strange Ediacaran organisms, Stromatoveris was made up of several radially repeated, branched fronds with a fractal internal architecture.
To find out what sort of animals Stromatoveris and the other Ediacaran organisms were, Hoyal Cuthill and Han ran a computer analysis that uses anatomical features to reconstruct evolutionary relationships. They found that Stromatoveris and the other Ediacaran organisms don’t belong to any living animal group or “phylum.” Instead, they cluster on their own branch in the animal evolutionary tree, between the sponges and complex animals with a digestive cavity like worms, mollusks, and vertebrates, the team reports today in Palaeontology. “This branch, the Petalonamae, could well be its own phylum, and it apparently lacks any living descendants,” Hoyal Cuthill says.
“It looks very likely [the Ediacaran organisms] are animals,” says Simon Conway Morris, a paleontologist at the University of Cambridge, who worked with Han on the first description of Stromatoveris in 2006, but who was not involved in the current study. At that point there were just a handful of known Stromatoveris fossils. The researchers argued that they were similar to some Ediacaran organisms, although others later questioned that link. Conway Morris says the new study “extends the story very nicely” by exploring the Ediacaran nature of Stromatoveris in more detail.
Geobiologist Simon Darroch at Vanderbilt University in Nashville is also comfortable with the idea that the Ediacaran organisms were animals and that a few survived into the Cambrian. But on a first look he is not convinced that Stromatoveris was one such survivor; he thinks the evidence that it had the fractal architecture of an Ediacaran organism isn’t strong—yet he’s open to persuasion.
If the new conclusion settles one mystery, though, it introduces another. The Ediacaran organisms represent the first major explosion of complex life on Earth, and they thrived for 30 million years. Their demise has been linked to the appearance of animals in the Cambrian Explosion, Hoyal Cuthill says. But that simple explanation doesn’t work as well if Ediacaran organisms were animals themselves, and some were still alive tens of millions of years later. “It’s not quite so neat anymore,” she says. “As to what led to their eventual extinction I think it’s very hard to say.”
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