The dawn of life remains mysterious, as do the antecedents of Animalia — the multicellular set. Sinuous tunnels passing through rigid layers of sediment 2.1 billion years ago in today’s Gabon are taken by some to show that multicellular animals existed and were motile that long ago, but that’s well short of proof. Finding the dead body right in its tracks is a whole other story, though. Now, a Chinese-American collaboration of scientists reports the fossilized death march of an Ediacaran-era animal, complete with the animal itself, from around 550 million years ago in what is today China.
This was no humdrum worm, as the creature moving on Earth 2.1 billion years ago is postulated to have been. This was a complex animal with legs, a member of the bilateria — meaning every bilaterally symmetric animal that isn’t a jellyfish or sponge, and that sports a mouth and anus. You are a bilaterian.
This bilaterian living back in the Ediacaran Period, around 635 million to 539 million years ago, had a segmented body with repetitive segments and looked like an elongated wormy trilobite.
The fossil animal lying dead in its spoor was reported Wednesday by scientists from the Nanjing Institute of Geology and Paleontology, of the Chinese Academy of Sciences, and Virginia Tech in Nature.
Life is believed to have begun to evolve over 4 billion years ago as simple single-celled creatures, like bacteria or archaea. The earliest bilaterians were wormlike, and when they arose is debatable — it could have been more than 2 billion years ago. Maybe motility arose then too. But most likely, the animals 2 billion years ago weren’t as complex as this one.
This creature, dubbed Yilingia spiciformis, was discovered in the Dengying Formation in the Yangtze Gorges area — a hot spot for the Ediacarans. And it was big: Based on 35 specimens, the longest was 27 centimeters (almost a foot) in length. That specimen was incomplete, hence the whole animal was longer than 27 centimeters, Zhe Chen of the Chinese Academy of Sciences told Haaretz.
“We named the fossils after their locality,” he explained. “The fossil was discovered from Yiling district of Yichang, Hubei province, in China. Yilinigia is derived from the Yiling district.”
In one sense, its discovery was no surprise: “Molecular clock” estimates predicted that segmented, frisky bilaterians evolved in the Ediacaran Period, though most of the Ediacarans are believed to have been sessile: living attached to a rock or spot on the seafloor.
The surprise about this fossil is mainly that the animal was found directly connected with its trail. Ergo, around 550 million years ago, something was definitely moving and it looked something like a wheat sheaf, and a lot like an elongated trilobite. And it stopped and died and was mineralized in its tracks.
One happy result of this extraordinary discovery is that paleontologists will be able to interpret other similar trace fossils preserved in the same geological unit, which, naturally, do not have the animals that made them lying right there.
What Yilingia spiciformis actually is, we may never know. The surmise is that it might be a proto-arthropod — bilaterally symmetrical animals with exoskeletons (think crabs), typically segmented bodies (think centipedes) and joined limbs (think spiders). Trilobites were early arthropods and some were small and cute, some were small and spiky, and some were over 2 feet long. Might it have been the earliest segmented bilaterian?
“Several other (and slightly older) Ediacaran fossils have been thought to be segmented bilaterian, but there’s still a lot of debate about whether they’re animals or not. They are potential candidates for segmented bilaterian. If more evidence is found in future studies that they are animals, they should be the earliest segmented bilaterian,” Zhe Chen explains.
The Ediacarans were extraordinary mainly for looking like nothing that exists anymore. It is not known whether any of the life-forms existing today are their descendants, or whether the lot went extinct and today’s life came from someone else. Some looked like leaves with their stems stuck in the ground; others were tubelike; and others are reconstructed by imaginative artists like Disney-esque monsters. In June, a separate study concluded that a certain type of sessile Ediacaran called Ernietta developed gregarious eating. These amiable Ediacarans are thought to have looked tubular.
Truth be told, more than half a billion years later, it’s hard to say what any of the Ediacarans actually looked like in real life. They were soft-bodied, leaving behind no more than impressions in sediment. What looks like a veiny pancake today could have been spherical, who knows? But we do know one thing: this Ediacaran moved.