Fossils reveal the earliest animal with a head was also the oldest known ‘righty’

By Mindy Weisberger, CNN

(CNN) — If you happen to be right-handed, you may be able to trace the origins of that trait to a wormlike animal that lived about 550 million years ago and had a tendency to bend to the right.

Spriggina floundersi appeared in oceans during the Ediacaran Period (635 million to 542 million years ago) at the dawn of the first forms of animal life.

The small creature’s flat, segmented body, known only from fossils found in what’s now South Australia, was a stretched-out oval. It tapered to a tip at one end and had a large, curved structure at the other, making it the earliest known animal with a head.

Paleontologists described the first Spriggina fossils in 1958. Since then, scientists have debated whether the animal could move on its own. To answer that question, researchers recently examined more than 100 fossils in the most comprehensive analysis of Spriggina since its discovery.

The scientists concluded that not only did Spriggina wriggle across the seafloor, but the abundance of fossil specimens curving left meant that these early animals favored their right sides, a behavioral preference seen in modern animals that are right-handed.

No animal quite like Spriggina exists today, but it set the evolutionary stage for directional preference, a trait possessed not only by most humans but also by other primates, mice, frogs and insects.

“Fossils of early animals, to most people — even to me — they look weird,” said Scott Evans, lead author of the study detailing the findings that published Thursday in the journal Scientific Reports. But if you push past that weirdness, “what we see is that a lot of the fundamental characters that we associate with animals today, things like the ability to move and even having this behavioral handedness, are present in these earliest animal communities.”

A ‘really surprising’ discovery

In some of the fossils, the bodies were straight, while others were curved. The fossils were mirrored imprints of the animals’ soft bodies. Most bent left in the rock, indicating that their bodies curved right in life.

Researchers examined body curves and compared them across fossils. S. floundersi measured no more than 4 inches (10 centimeters) long, though most were just 0.8 to 1.2 inches (2 to 3 centimeters) in length.

The team studied the rocks around the fossils for telltale signs of currents and storms to learn whether the curved bodies meant that the animals were pushed by moving water or could move on their own.

“The really surprising thing was that they had this ‘handedness,’” said Evans, an assistant curator of invertebrate paleontology at the American Museum of Natural History in New York City and an assistant professor at the museum’s Richard Gilder Graduate School.

“About twice as many of these things are bent to the left as are bent to the right.”

He thought that the abundance of left-bending fossils was odd but was unsure what it meant.

“Then I looked into how researchers identify handedness” in animals alive today, he explained. Evans found the same 2-1 ratio of individuals favoring their right sides.

“That suggests that this is a significant behavioral preference in Spriggina,” he said. “I never thought that for an impression of a half- billion-year-old organism, we’d be able to say it preferred to turn one way versus the other.”

Identifying right-handedness in this way is statistically significant and suggests that Spriggina already had a nervous system connected to muscles, allowing it to curve in a preferential direction, said Diego García-Bellido, a senior paleontology researcher at the South Australian Museum and an associate professor of paleontology at Adelaide University. He was not involved in the new research.

“I am very cautious when interpreting the fossil record, and I believe Evans and coauthors have been as well,” said García-Bellido, a specialist in animals from the Ediacaran and Cambrian periods, in an email. “They have clearly considered and stated all alternative hypotheses and they offer clear, valid arguments for their interpretations.”

Behind the bend

Some extinct animals leave fossilized trails behind them, proving that they could creep, slither or crawl. Spriggina was not so obliging, so scientists had to dig deeper for their proof.

First, the research team examined Spriggina anatomy to check its range of motion. Curves in various fossils showed that Spriggina could bend in both directions, and deeply enough to form a U shape.

Evans’ next question was: “Is that bending because of something in the environment — a wave or a storm that bent the specimen — or is that because the specimen can, in fact, bend its body?”

The researchers excavated layers of rock covering tens of square meters and containing hundreds of different fossil specimens. If the Spriggina specimens were all facing in the same direction or all bent in the same way, the researchers reasoned, that could suggest an external force was acting on all the animals equally.

Instead, the team found specimens oriented in different directions with different magnitudes of body bending, a sight that told them “this is a thing that could move around in any way it wants, and we capture it at various stages of that movement,” Evans said.

“We even have Spriggina that are found on some of those beds where other fossils show impact from currents,” he added, “and they too are not bent in ways consistent with that current or consistent with each other.”

Another possibility was that after the animals died and dried out, their bodies curved. Again, variation between curvature in specimens from the same bed hinted that desiccation was not the cause of their curving.

The fact that Spriggina individuals fossilized close together while bending in different directions — “sometimes even with more than one bend” — and that some were preserved with sediment between their bodies and the sea bottom below “is a compelling reason to indicate that these organisms were not fixed to the seafloor and could actually move, even if we don’t see the telltale of their feeding ‘footprints,’” García-Bellido said.

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