500-Million-Year-Old Leaf-Like Creatures Turn Out To Be A Strange Class Of Ancient Animals

Ediacaran organisms.
Ryan Somma / Wikimedia Commons / Cropped and Resized (CC BY-SA 2.0)

More than seven decades ago, scientists stumbled upon the curious fossils of ancient plant-looking creatures dating back more than half a billion years ago, to the Ediacaran Period.

Unearthed from the Ediacara Hills in South Australia in 1946, these strange fossils became known as Ediacaran organisms and later turned up all over the world, with 200 different varieties being discovered to date, reports Science Magazine.

Ever since their discovery, Ediacaran organisms have been shrouded in mystery because no one really knew exactly what they were. Due to their bizarre appearance — a leaf-like structure similar to many life forms, including protozoans, algae, fungi, and lichens — scientists had a hard time placing these creatures in the tree of life.

But a new study published yesterday in the journal Palaeontology claims to have discovered their true identity and documents that Ediacaran organisms were actually a very strange class of animals with their own separate branch on the evolutionary tree.

Conducted by Jennifer Hoyal Cuthill of the Tokyo Institute of Technology in Japan and Jian Han from Northwest University in Xi’an, China, the research focuses on a particular species known as Stromatoveris psygmoglena and provides evidence that this creature is animal in origin and also an Ediacaran organism.

Stromatoveris psygmoglena is an odd little fellow. Aside from being an animal that looks like a plant, the timeline of its existence has proven very confusing. This creature dates back to 518 million years ago, long after most of the other Ediacaran organisms had already died out.

The ancient life forms of the Ediacaran Period, which stretched from 635 million to 541 million years ago, were gradually replaced by more evolved and better-adapted creatures and are thought to have already gone extinct by the time the Cambrian Explosion rolled around.

As reported by the Inquisitr, this major life-sparking event took place right at the end of the Ediacaran Period and ushered in the dawn of a new chapter, eventually giving rise to most of the major animal groups we see today.

Yet one group of Ediacaran organisms seems to have made it through the Cambrian Explosion, instead of falling into oblivion with the rest of them. And the proof of that lies in the Stromatoveris psygmoglena fossils discovered in the Yunnan province of southwestern China, argues the new study.

Ediacaran organisms.
Ediacaran fossil. Alicejmichel / Wikimedia Commons/Resized (CC BY-SA 4.0)

Most Ediacaran organisms were marine creatures, the Inquisitr previously reported, and so was the 10-centimeter-tall (3.9 inches) Stromatoveris psygmoglena. Its close resemblance to the plant-like Ediacarans became obvious once Hoyal Cuthill and Han started examining fossils of this ancient creature.

After looking at 206 impeccably preserved Stromatoveris psygmoglena fossils, the two researchers established that the species is clearly an Ediacaran organism, displaying the same frond (leaf-like) shape and branching structure.

“Exceptionally preserved soft‐tissue anatomy shows that Stromatoveris was a soft‐bodied, radially symmetric animal with multiple, sub‐branched petaloids and a differentiated holdfast,” Hoyal Cuthill and Han wrote in their paper.

Ediacaran organism Stromatoveris psygmoglena
Reconstruction of ‘Stromatoveris psygmoglena.’ Apokryltaros / Wikimedia Commons (CC BY-SA 3.0)

The species’ identity as an animal and its place on the evolutionary tree were uncovered with the help with computer analysis, which showed that Stromatoveris psygmoglena fits in right “between sponges and complex animals with a digestive cavity,” such as worms, mollusks, and vertebrates, notes Science Magazine.

Therefore, Stromatoveris psygmoglena was designated to a new clade of animals, along with the rest of the Ediacaran organisms. Dubbed Petalonamae, this is now a separate branch in the evolutionary tree.

“This branch, the Petalonamae, could well be its own phylum, and it apparently lacks any living descendants,” said Hoyal Cuthill, who is also affiliated with the University of Cambridge in the U.K.

The trouble with this find is that it now creates a new puzzle, states Science Magazine. If Ediacaran organisms were indeed animals and not primitive aquatic plants, then these creatures were more evolved than previously anticipated. Since they apparently survived the Cambrian Explosion and were still alive 20 million years later in the form of Stromatoveris psygmoglena, then what triggered their demise?

“It’s not quite so neat anymore,” says Hoyal Cuthill. “As to what led to their eventual extinction I think it’s very hard to say.”