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Ancestor of all modern animals found in ancient Australian mud

Ancestor of all modern animals found in ancient Australian mud
An artist's impression of the world's earliest bilaterian, a tiny worm-like creature that lived 555 million years ago. Photo by Sohail Wasif/UCR

March 24 (UPI) -- A tiny worm-like species called Ikaria wariootia sits at the top of the family tree that contains most modern animals, including humans.

The creature is the world's earliest bilaterian, which means it had a front and back and symetrical sides, as well as an entrance and exit connected by a gut. Sponges and algal mats, some of Earth's first multicellular organisms, boasted variable shapes.

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Members of the extinct genus Dickinsonia, a group of lily pad-shaped creatures, were part of the explosion of the earliest multicellular creatures, but they are without most of the features of -- and only distantly related to -- modern animals.

The emergence of bilateral symmetry was key to the evolution of animal life, allowing animals to move more purposefully. Symmetry also paved the way for anatomical organization and complexity.

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Genetic models suggest the ancestor of all bilaterians would have been small and simple, boasting only the most basic sensory organs. But scientists have struggled to find remains of such a creature.

For the last 15 years, scientists have, however, been studying burrows made in 555 million-year-old Ediacaran Period deposits in Southern Australia. Until recently, they couldn't be certain what the animal that made the burrows looked like.

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After winning grant funding from NASA, a team of researchers in the United States and Australia were able make out imprints along the rims of the burrows using a three-dimensional laser scanner.

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The high-definition images showed the burrows were made by a small worm-like animal with a cylindrical body, distinct head and tail. The animals were no bigger than a grain of rice. V-shaped ridges found on the burrows suggest the species moved by contracted its ring-pattern muscles, known as peristaltic locomotion.

"We thought these animals should have existed during this interval, but always understood they would be difficult to recognize," Scott Evans, a recent doctoral graduate from the University of California, Riverside, said in a news release. "Once we had the 3D scans, we knew that we had made an important discovery."

Scientists described the new species this week in the journal PNAS.

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Researchers borrowed and augmented words from the Adnyamathanha language, spoken by the region's aboriginal peoples, as homage to the original caretakers of the land where the new species was discovered. Ikara, the inspiration for the genus name, means "meeting place" in Adnyamathanha, while the species name is a reference to the Warioota Creek, which flows through the region.

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The world's earliest bilaterian burrowed in well-oxygenated sand on the ocean floor, and fossil evidence suggests the worm-like creatures ate buried organic matter using its mouth, anus and gut.

"Burrows of Ikaria occur lower than anything else. It's the oldest fossil we get with this type of complexity," Droser said. "Dickinsonia and other big things were probably evolutionary dead ends. We knew that we also had lots of little things and thought these might have been the early bilaterians that we were looking for."

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