Rethinking Evolution: Extinct Creatures Unlike Any Living Today

Life in the Ediacaran Sea

 

“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 paleobiologist Jennifer Hoyal Cuthill at the University of Essex referring to ocean dwellers, rangeomorphs, 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 snowflake.

“It’s probably too early to conclude exactly which geochemical changes in the Ediacaran oceans were responsible for the shift to large body sizes,” Hoyal Cuthill notes,, “but there are strong contenders, especially increased oxygen, which animals need for respiration.”

Scientists believe that they may have been some of the earliest animals to live on Earth.

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 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?”

This fractal-like body form lived on well into the Cambrian period, surviving the onset of the Cambrian explosion, but eventually going extinct.”

In an email to The Daily Galaxy, Hoyal Cuthill  wrote: “Rangeomorphs are unusual in that almost the whole body appears to have had a fractal-like branched structure, something like an animate gill or digestive system (and likely a combination of the two). The production of sheets or tubes of tissue arguably requires animal-level innovations in the organization of cells to form tissues, building on billions of years of prior evolution. However, fractal structures can operate at multiple scales, from small to large, because even when large they still contain the small. This means that small rangeomorphs could have been ideally positioned to pounce on increases in ocean oxygen or other nutrients and very rapidly achieve large body size, potentially using plasticity in growth responses without the necessity for extensive evolutionary change from their fractal starting point. Our new research suggests that this fractal-like body form lived on well into the Cambrian period, surviving the onset of the Cambrian explosion, but eventually going extinct.”

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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.

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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.

The Ediacaran organisms represent the first major explosion of complex life on Earth”

“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 said. 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.”

Jennifer Hoyal Cuthill, Science and Nature

Image credits: With thanks to Ryan Somma – Life in the Ediacaran Sea

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