Oldest fossil of a bilaterally symmetrical animal

Ikaria

This is how the ancestor of Bilateria might have looked. (Image: Sohail Wasif / University of California Riverside)

What did the ancestor of all bilateria look like – the bilaterally symmetrical animals from earthworms to dinosaurs and humans? Researchers in Australia may have found an answer to this question. In a rock formation 555 million years old, they discovered the fossils of a living being that was only the size of a grain of rice and which already had decisive features of bilateria. It had a front and rear end, a continuous bowel and symmetrical right and left sides. This creature, baptized Ikaria wariootia, was already moving forward and left dozens of small vaults in the sediment. It could therefore be the oldest known representative of Bilateria.

Whether earthworm, insect, turtle or human – most of today’s animals are bilaterally symmetrical. Your basic blueprint has a front and rear end and usually also a symmetrical right and left side. Individual organs can move out of the middle of the body in the course of embryonic development and disturb this symmetry, as in humans, for example, the heart and liver. Nevertheless, the two-sided basic construction plan forms the common denominator of all tissue animals. This structure enabled the first of these eumetazoa to move in a directed manner. Another feature is an opening in the mouth, which is connected to an outlet opening via a continuous digestive organ.

Searching for traces in the Ediacarium

These similarities suggest that the last common ancestor of all bilateria must have lived before the large groups of the animal kingdom in the Cambrian region developed around 500 million years ago. In the era of the Ediacarium before the Cambrian, the first larger animal forms developed – but these were mostly not similar to Bilateria or the basic building plans of the animal kingdom known today. Instead, the Ediacara fauna resembled stalked leaves, others were rounded or spiral structures, mostly without mouth, intestine or other recognizable organs. Most of these animal forms also became extinct at the end of the Ediacarium without leaving a successor. Biologists therefore often see this first animal world as a dead end of evolution. This raises the question of where the first variants of bilaterally symmetrical animals suddenly came from at the beginning of the Cambrian.

An indication of this can be found in the vaults discovered in many deposits from the early Ediacarium. These are tiny, little more than a millimeter wide tracks that were originally under a thin layer of sand. The shape of these corridors and v-shaped imprints within the tracks indicate that a small animal was moving here using muscle contractions. Thus, the creator of these traces may have been an early representative of Bilateria. The layers in which these traces called helminthoidichnites are preserved have been dated to an age of up to 550 or even 560 million years. But a fossil of the animal that left these traces has been missing – until now.

A fossil as small as a grain of rice

The decisive discovery was made by Scott Evans of the University of California at Riverside and his colleagues in a 555 million year old rock formation in southern Australia. Numerous helminthoidichnites have also been found in this formation. Evans and his team have now captured some of these corridors with a laser scanner and thus obtained high-resolution three-dimensional images of the structures. “As soon as we had the 3D scans, we realized that we had made an important discovery,” says Evans. Because at the end of some aisles, the scans revealed tiny, elongated structures that were roughly the size of a grain of rice, two to seven millimeters in length and one to two and a half millimeters in diameter. Closer analyzes showed that these structures were slightly flattened, symmetrical on both sides and also had a slightly thicker front end and a thinner rear end. The morphology suggests that the body could also have been segmented – this would facilitate the muscle structure for peristaltic movement, ”the researchers report.

From the characteristics of these microfossils and their position at the end of the helminthoidichnites ducts, Evans and his colleagues conclude that this is most likely the animal that once created the ducts. They gave this new genus and type the name Ikaria wariootia. “We suggest that Ikaria is the helminthoidichnites’ trace maker and potentially the oldest unambiguous representative of Bilateria – at least in South Australia,” the researchers say. Not all the body features of this primeval animal have been preserved, but the researchers suspect that Ikaria already had a mouth, anus and a continuous digestive tract. “Ikaria was probably able to sense organic material in the well-ventilated sediments and also potentially toxic conditions, which also indicates rudimentary sensory abilities,” said Evans and his team. That is exactly what evolutionary biologists had predicted for the last common ancestor of Bilateria. “Despite the simple morphology recognizable in the fossil, this organism was already quite complex compared to the Ediacara species that existed at the same time.” If the classification of the researchers is confirmed, a large part of today’s animal diversity could be due to Ikaria and similar mini-worms.

Source: Scott Evans (University of California, Riverside) et al., Proceedings of the National Academy of Sciences, doi: 10.1073 / pnas.2001045117

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