Fossil traces from more than half a billion years ago offer clues to the evolution of animals

3D reconstruction of fossil burrows (Image courtesy Luke Parry - University of Bristol)
3D reconstruction of fossil burrows (Image courtesy Luke Parry – University of Bristol)

An article published in the journal “Nature” describes the study of traces left by organisms that lived over half a billion years ago that could be important to understand how animals evolved. A team of researchers discovered fossil traces of tracks and burrows left by some of the first organisms capable of active movements. Those creatures were very small, with a thickness comparable to that of a hair that could range from 40 to 300 micrometres, but among them there could be the ancestors of most animals.

The evolution of the first animals is a subject of discussion because the various hypotheses are supported by limited data. Many phyla existing today seem to have evolved in a short time from a biological point of view over the geological period known as the Ediacaran-Cambrian transition, between 635 and 541 million years ago. Fossils dating back and that period are limited and particularly those belonging to the so-called Ediacara fauna are the subject of many controversies.

In the Corumbá region, in western Brazil, fossils from that transition have been discovered. There are no fossils of organisms, perhaps because their bodies didn’t fossilize, but their tracks and their burrows. Their analysis indicates that the organisms that left those traces were part of the ancient meiofauna, which today indicates aquatic organisms that are microscopic but larger microbes, and were bilaterians, that is, they a had bilateral symmetry like most of today’s animals.

The examination of the fossils was made using modern technologies and in particular an X-ray microtomography technique. It allows to examine objects and create a virtual 3D model without the need to destroy them. This allowed to create a virtual map of the burrows, which have a diameter between 50 and 600 nanometers left by those ancient tiny organisms.

According to the researchers, the animals that created those burrows were similar to some modern nematodes, those commonly known as roundworms. Luke Parry, the article’s lead author, explained that the fossils of bilaterian animals appeared in the Cambrian period but the fossil traces studied in this research are more ancient.

The fossil traces provide limited information on those ancient organisms but indicate that they already had the ability to move actively and influence their environment. This is already a remarkable step forward compared to the simple organisms that existed earlier so this discovery gives new clues as to what happened in a crucial period in the evolution of life on Earth.

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