Ancestor of all animals identified in Australian fossils

James Marshall
March 25, 2020

Artist's rendering of Ikaria wariootia, the ancestor of most animals today. The creature, named Ikaria wariootia, is a wormlike animal about the size of a grain of rice, and it appears to be the earliest example of the bilaterian body shape that's common to the overwhelming majority of animals ever since. According to geologists writing in Proceedings of the National Academy of Sciences, the development of bilateral symmetry was a crucial evolutionary step.

Ikaria wariootia impressions in stone.

The earliest multicellular organisms , such as sponges and algal mats, had variable shapes.

While bilaterians run rampant today (insects, humans and most other animals among them), the identity of that progenitor organism has long eluded discovery.

Evolutionary biologists considering the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been small and straightforward, with rudimentary sensory organs.

Preserving and identifying the fossilized remains of such an animal was thought to be hard, if not impossible.

The burrows were clearly made by wriggling creatures with distinct front and back sides, but to get a more detailed picture of those ancient burrowers the researchers analyzed the fossils with a 3D laser scanner.

Dr Scott Evans of the University of California, Riverside, and Professor Mary Droser noticed miniscule, oval impressions near some of the burrows. They found that the tiny animals not only had a clear head and tail, but also had a bilaterally symmetrical body and faintly grooved musculature, similar to a worm. Managing to reconstruct the dimensions of the creature, they found that it would have been between 2-7mm long and 1-2.5mm wide, with the largest growing up to the size of a grain of rice.

"We thought these animals should have existed during this interval, but always understood they would be hard to recognize".

Writing in the Proceedings of the National Academy of Sciences, Scott and colleagues in the United States and Australia report how they made their discovery in sandstone at sites including fossil-rich Nilpena.

As for actually finding a fossil of Ikaria wariootia itself, chances are incredibly slim. The worm-ish organism has been named Ikaria wariootia, but what it lacks in a flashy name it makes up for by being, well, nearly entirely unremarkable. The name was taken from an indigenous Adnyamathanha language, and means "meeting place". It's the Adnyamathanha name for a grouping of mountains known in English as Wilpena Pound. And the name of the species is a variation on a waterway in the area, called Warioota Creek. "Dickinsonia and other big things were probably evolutionary dead ends". The newly discovered Ikaria wariootia is thought to be the earliest bilaterian.

In spite of its relatively simple shape, Ikaria wariootia was complex compared to other fossils from this period. The depth and curvature of Ikaria represent distinct front and rear ends, supporting the directed movement found in the burrows. Given that the burrows track through sand that was oxygenated, rather than toxic spots, suggest the creature had basic senses.

She explained that evidence of sediment displacement in the burrows indicates the organism fed on buried organic matter and probably had a mouth, anus, and gut.

Its discovery underscores the humble evolutionary beginnings of this large segment of the animal kingdom, humankind included, the researchers said. "It's really exciting that what we have found lines up so neatly with their prediction", Professor Droser said.

Other reports by Click Lancashire

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