A study of the fossils of the Cambrian period to trace the birth of the first animals

Anomalocaris Canadensis fossil from the Cambrian (Image courtesy A. Daley)
Anomalocaris Canadensis fossil from the Cambrian (Image courtesy A. Daley)

An article published in the journal “Proceedings of the National Academy of Sciences” describes a research on ancient animal fossils connected to the Cambrian explosion. A team of researchers from the British University of Oxford and the Swiss University of Lausanne carried out the most complete examination ever made of fossils of early euarthropods. The conclusions are that the origins of these animals date back to no earlier than 550 million years ago and their diversification took about 40 million years.

Euarthropods (Euarthropoda) form a huge phylum that includes more than 80% of all animal species because it includes insects, arachnids, organisms such as centipedes, crustaceans and various groups now extinct such as trilobites. It’s a group so vast that many relationships are not clear but their traces date back over 500 million years ago and help to understand when the first animals started evolving.

In the field of paleontology it’s normal to do research based on limited amounts of fossils and that’s even more true when they try to study such ancient periods. For this reason, there are scientists who believe that animals started emerging 650 million years ago and then evolved gradually while according to others there was a rapid evolution that began about 540 million years ago.

A team led by Professor Allison Daley of Oxford examined early fossils of euarthropods to try to understand if the absence of fossils older than 540 million years ago was due to a lack of conservation problem. From that time, in the space of a few tens of millions of years, there was the evolution of the phylum existing today but what was the starting point for the evolution of animals?

The examination of fossils from deposits such as Burgess Shale, where conservation is very good, dating back to the Cambrian period and to the previous one, the Ediacaran, shows that in both periods there are abundant amounts of fossils. However, even in the richest deposits the fossils of euarthropods are present in the strata relative to the Cambrian period but not in the layers related to the Ediacaran, where there’s an abundance of algae but not of animals.

The scientists led by Professor Allison Daley examined a wide range of fossils to understand the situation concluding that the absence of euarthropods older than 541 million years was not due to a conservation problem. According to them the only possible explanation is that that was the moment when this huge phylum emerged.

According to the scientists there was a period of about 40 million years after the origin of the euarthropods in which there was a diversification with the evolution of the various groups that partly exist today. In essence, the conclusion is a middle ground between the hypotheses that have been discussed by paleontologists for a long time.

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