Oldest ancestor of almost all animals found in Australian fossils


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An artist's impression of Ikaria wariootia, and its burrow
Sohail Wasif/UCR

Researchers have discovered the fossilized remains of the oldest known ancestor of almost every animal in existence 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.

Even if you don’t recognize the term, you know the concept of a bilaterian. It’s an animal whose body has two symmetrical sides, with a mouth at one end and an anus at the other. This basic structure has proven so successful that it’s been conserved across hundreds of millions of years of evolution and countless incarnations, from ducks to dogs to dinosaurs. In fact, only a fraction of animals, like sponges and jellyfish, aren’t bilaterian.

And now, researchers have found the oldest known ancestor of this widespread group, dating back 555 million years. Ikaria looked a little like a slug, measuring between 2 and 7 mm (0.08 and 0.29 in) long and 1 and 2.5 mm (0.04 and 0.1 in) wide. It was very clearly symmetrical, and evidence points to it having a mouth, a gut and an anus – all features of bilaterians.

This timeframe neatly lines up with what evolutionary biologists have long believed about the beginning of bilaterians. Ikaria’s age places it in the Ediacaran Period, a time when life on Earth was really starting to take off.
"This is what evolutionary biologists predicted"
more at link......

the paper:

Discovery of the oldest bilaterian from the Ediacaran of South Australia

The transition from simple, microscopic forms to the abundance of complex animal life that exists today is recorded within soft-bodied fossils of the Ediacara Biota (571 to 539 Ma). Perhaps most critically is the first appearance of bilaterians—animals with two openings and a through-gut—during this interval. Current understanding of the fossil record limits definitive evidence for Ediacaran bilaterians to trace fossils and enigmatic body fossils. Here, we describe the fossil Ikaria wariootia, one of the oldest bilaterians identified from South Australia. This organism is consistent with predictions based on modern animal phylogenetics that the last ancestor of all bilaterians was simple and small and represents a rare link between the Ediacaran and the subsequent record of animal life.


Analysis of modern animals and Ediacaran trace fossils predicts that the oldest bilaterians were simple and small. Such organisms would be difficult to recognize in the fossil record, but should have been part of the Ediacara Biota, the earliest preserved macroscopic, complex animal communities. Here, we describe Ikaria wariootia gen. et sp. nov. from the Ediacara Member, South Australia, a small, simple organism with anterior/posterior differentiation. We find that the size and morphology of Ikaria match predictions for the progenitor of the trace fossil Helminthoidichnites—indicative of mobility and sediment displacement. In the Ediacara Member, Helminthoidichnites occurs stratigraphically below classic Ediacara body fossils. Together, these suggest that Ikaria represents one of the oldest total group bilaterians identified from South Australia, with little deviation from the characters predicted for their last common ancestor. Further, these trace fossils persist into the Phanerozoic, providing a critical link between Ediacaran and Cambrian animals.

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