Oldest Bilaterian Fossil Found in Australia
Ikaria wariootia, a wormlike creature that lived more than 555 million years ago (Ediacaran period) in what is now Australia, is the earliest bilaterian, according to new research led by University of California, Riverside researcher Scott Evans.
Bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. They have a head and a tail as well as a back and a belly.
The development of bilateral symmetry was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organize their bodies.
A multitude of animals, from worms to insects to dinosaurs to humans, are organized around this same basic bilaterian body plan.
Evolutionary biologists studying the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs.
Preserving and identifying the fossilized remains of such an animal was thought to be difficult, if not impossible.
For 15 years, paleontologists agreed that Helminthoidichnites, fossilized burrows found in Ediacaran-period deposits in Nilpena, South Australia, were made by bilaterians.
But there was no sign of the creature that made Helminthoidichnites-types burrows, leaving scientists with nothing but speculation.
Dr. Evans and colleagues noticed miniscule, oval impressions near some of these burrows.
They used a 3D laser scanner that revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature.
Dubbed Ikaria wariootia, the animal ranged between 2-7 m long and about 1-2.5 mm wide, with the largest the size and shape of a grain of rice — just the right size to have made the burrows.
“We thought these animals should have existed during this interval, but always understood they would be difficult to recognize. Once we had the 3D scans, we knew that we had made an important discovery,” Dr. Evans said.
“Burrows of Ikaria wariootia occur lower than anything else. It’s the oldest fossil we get with this type of complexity,” said University of California, Riverside’s Professor Mary Droser.
“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.”
In spite of its relatively simple shape, Ikaria wariootia was complex compared to other fossils from this period.
It burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, indicating rudimentary sensory abilities.
The depth and curvature of Ikaria wariootia represent clearly distinct front and rear ends, supporting the directed movement found in the burrows.
The burrows also preserve crosswise, V-shaped ridges, suggesting Ikaria wariootia moved by contracting muscles across its body like a worm, known as peristaltic locomotion.
Evidence of sediment displacement in the burrows and signs the organism fed on buried organic matter reveal Ikaria wariootia probably had a mouth, anus, and gut.
“This is what evolutionary biologists predicted. It’s really exciting that what we have found lines up so neatly with their prediction,” Professor Droser said.
The study was published in the Proceedings of the National Academy of Sciences.
Scott D. Evans et al. Discovery of the oldest bilaterian from the Ediacaran of South Australia. PNAS, published online March 23, 2020; doi: 10.1073/pnas.2001045117