Tapeworm-Like Fossil Suggests Origins of Left and Right

By Elizabeth Howell, Live Science Contributor | June 9, 2014 06:47pm ET

Traces of the tapeworm-like creature Plexus ricei as seen in the fossil record. Individuals were likely 2 to 31 inches (5 to 80 cm) long.

Credit: Droser Lab, UC Riverside

A puzzling fossil find from the Ediacaran period, an era that occurred more than 500 million years ago, has scientists curious about how "bilateral" creatures such as humans evolved.

The Plexus ricei organism resembled a tapeworm or flatworm. Mysteriously, it appears to have had "bilateral", or left-right, symmetry before anything else living 540 million to 575 million years ago.
"Plexus was unlike any other fossil that we know from the Precambrian," said study researcher Mary Droser, a paleobiologist at the University of California at Riverside, in a statement. The Precambrian was the period before abundant animal life appeared on our planet, and represents latter part is called the Ediacaran period.

"It was bilaterally symmetrical at a time when bilaterians — all animals other than corals and sponges — were just appearing on this planet." [In Photos: Spooky Deep-Sea Creatures]

Tube worm

The tubular creature was about 5 to 80 centimeters (2 to 31 inches) in length and 5 to 20 millimeters (0.2 inches to 0.78 inches) wide, and it lived on the seafloor. That wasn't unusual, as all life on Earth lived in the oceans during this time. The earliest evidence of a bilaterally symmetrical organism comes from about 585 million years ago. That slug-like animal, less than a half-inch long, left itsy-bitsy tracks found fossilized in Uruguay.

Here, a reconstruction of the tapeworm-like creature Plexus ricei, which lived more than 500 million years ago.

Credit: Droser Lab, UC Riverside

"Ediacaran fossils are extremely perplexing: They don’t look like any animal that is alive today, and their interrelationships are very poorly understood," Lucas Joel, a former graduate student at UC Riverside who led the research, said in a statement.

The Ediacaran period also lacked bioturbation, Joel said, or churned-up seafloors from marine organisms snuffling for food.

It was only in the Cambrian period — the explosion of life that started around 540 million years ago — that organisms began churning up the seafloor. By contrast, large algae mats carpeted the Ediacaran oceans, a rare occurrence on Earth today.

Tricky identity

Because the seafloor remained undisturbed, organisms that died and drifted to the ocean floor were preserved when sediments accumulated over their bodies, creating a mold.

"What this means is that the fossils we see in the field are not the exact fossils of the original organism, but instead molds and casts of its body," Joel said. That makes it sometimes difficult to tell if a fossil represents an organism like Plexus or simply an empty burrow created by a long-ago creature worming its way through the sand.

Joel and his colleagues discovered that Plexus ricei was not, in fact, a trace fossil (fossil of an organism's path), but rather was an unknown organism. The species gets its name from the Latin word "plexus," which means braided, and from Dennis Rice, a field assistant at the South Australian Museum who excavated many specimens of Plexus.

Droser said researchers need to confirm that Plexus was truly bilateral, but the scientists suspect that this tapeworm-like creature is a distant ancestor of other bilateral organisms — including humans.
The findings were published in March in the Journal of Paleontology.

Images: Bizarre, Primordial Sea Creatures Dominated the Ediacaran Era

By Laura Geggel, Senior Writer | January 12, 2016 11:40am ET

During the Ediacaran period, about 635 million to 541 million years ago, oxygen was sparse, the oceans were murky and marine organisms ate by absorbing nutrients floating around in the water.
It was a very different world, according to fossils left behind by the soft-bodied life-forms that dominated the era.

"The Ediacaran period is the first time in Earth's history where you don't need a microscope to look at the fossils," said Marc Laflamme, an assistant professor of Earth science at the University of Toronto Mississauga. "This is the first time that life is big enough that you can look at it with the naked eye." [See Photos of These Life-Forms Discovered in Newfoundland, Canada]
Here's a look at the wacky organisms that called Earth home at the time:

Bizarre creature

During the Ediacaran — a period named for South Australia's fossil-rich Ediacara Hills — animals weren't the major players.

"There are some animals there, but they're not dominating the system like we have in the modern world," Laflamme told Live Science. "Animals churn up sediment, they burrow, they dig" — without them, the seafloor was likely incredibly firm and covered with thick bacterial mats, he said.
However, researchers have found a few exceptions. For instance, this seafloor-dwelling tapewormlike animal, named Plexus ricei, dates to the Ediacaran period. The tubular critter measured about 5 to 80 centimeters (2 to 31 inches) long and 5 to 20 millimeters (0.2 inches to 0.8 inches) wide.
"Plexus was unlike any other fossil that we know from the Precambrian," study researcher Mary Droser, a paleobiologist at the University of California, Riverside, said in a statement. (Image credit: Credit: Droser Lab | UC Riverside)

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Different roots 

A plethora of bacteria probably covered the seafloor like a leathery carpet, Laflamme said. Likewise, many of the organisms that lived during that time didn't have root systems like today's plantsdo but instead had bottoms that were either disc- or ball-shaped (depending on whether they were inflated — it's hard to tell by looking at the fossils), Laflamme said.

The two seaweedlike fossils, pictured here, are small — about 2 inches (5 cm) long. It's unclear if they are plants or animals, as the bulbous structure at the bottom of their stalk could serve as an anchor for a plant. Or, it could be a proboscis, a tubelike feeding structure, of a wormlike creature, Live Science reported. (Image credit: Zhe Chen)

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Rangeomorphs

Other Ediacaran organisms, called rangeomorphs, had these disc-shaped root systems.
"It makes sense if they're living on a firm, rigid substrate," Laflamme said. "They don't dig roots into [the ground]; they just lie flat on it. Or, they burrow within, and this microbial mat that grew around it and held it in place."

Rangeomorphs were bizarre, fractal creatures with self-similar patterns found from the smallest to the largest scale of the organisms, Live Science reported in 2014. (Image credit: Jennifer Hoyal Cuthill | University of Cambridge)

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Next generation

Some rangeomorphs displayed complex procreation. For instance, the soft-bodied rangeomorph known as Fractofusus (shown here) likely reproduced much like a strawberry plant does today, according to a study published Aug. 3, 2015, in the journal Nature. (Image credit: C. G. Kenchington)

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Baby rangeomorph

Researchers have even found juvenile rangeomorphs, including this 0.7-inch-long (17 millimeters) one shown here; the juvenile specimen was found at Mistaken Point Ecological Reserve in Newfoundland, Canada. Notice the detail on its branches. (Image credit: OU | Jack Matthews)

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Suspension feeder

Tribrachidium, a bizarre sea creature that lived some 550 million years ago, is unlike any modern organism. It has three-fold symmetry — a feature that no living thing today has, Live Science reported in November. New research suggests that the creature fed on particles suspended in the water. (Image Credit: M. Laflamme)

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Muscle maniac

Haootia quadriformis, the first known life-form to have muscles, lived during the late Ediacaran, about 560 million years ago. The creature's body looks like a circular disc, which it likely used to anchor itself to the seafloor.
The disc is connected by a short stalk to a sheetlike body made of fibrous bundles — believed to be muscles — arranged in a four-fold symmetry, Live Science reported in 2014. (Image credit: Credit: Martin Brasier)

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Newfoundland fossils

The Ediacaran had low levels of oxygen — likely one to two orders of magnitude lower than oxygen levels on today's Earth, Laflamme said. However, as marine animals evolved into filter feeders, they started eating the bacteria and nutrients in the murky water. The resulting clear water would have allowed sunlight to deliver energy to photosynthesizing bacteria, which, in turn, would have made more oxygen.
Once oxygen levels increased, animals thrived, and larger predators entered the environment. These animals tunneled and tore at the ocean floor, bringing an end to the rangeomorphs and other organisms, Laflamme said. ( Image Credit: OU | Jack Matthews)

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Oldest animal skeleton

Pictured here is Coronacollina acula, an organism with a round center and four needlelike "spicules" that point outward. C. acula lived between 550 million and 560 million years, and is thought to be the oldest animal with a skeleton on record. It likely survived by filtering food from the water, experts told Live Science. (Image Credit: James Gehling)