August 8, 2022

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Re-analysis of the fossil found it to be from the oldest branch of vertebrates

Re-analysis of the fossil found it to be from the oldest branch of vertebrates

Zoom / Yes, these are the gills on this Cambrian animal.

Since we are a member of the group, it is easy to see vertebrates as the pinnacle of evolution, a group capable of producing bats, birds, and giant whales in addition to ourselves. But when vertebrates first evolved, it wasn’t a sure thing. They branched out from a group that lived in mud and didn’t need to distinguish top from bottom or left from right, and ended up losing an organized nerve cord. Our closest non-vertebrate relatives have reestablished the nerve cord (on the wrong side of the body, naturally) but they can’t be bothered with compliments like the skeleton.

It wasn’t clear exactly how vertebrates got away with this, and the potential lack of skeletons in our direct ancestors helped ensure there weren’t many fossils to help clear things up.

But in Thursday’s issue of Science, researchers reassessed some of the mysterious Cambrian fossils, solving several arguments about what exactly the features were. Yunanozoan It was. The answers include the cartilaginous structures that support the nostrils and a possible ancestor of what became the mandible. In the process, they show that Yunanozoan It is probably the closest branch of the vertebrate tree.

Yunnanoowhatans?

You can find out what a file is Yunanozoan It appears from the picture above. The soft tissues down their sides were segmented into segments, a feature of our closest living non-vertebrate relatives (amphioxus or lancelet) and is present in vertebrate embryos, but is generally lost as they develop into adults. Near the animal’s head – it has a pronounced head and mouth – there are also a group of arched structures that closely resemble the gill arches found near the head of modern fish.

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If this interpretation is correct, it means Yunanozoan It is very similar to amphioxus, but has a feature that is found only in modern vertebrates. This means that it preserves features crucial to understanding the origin of vertebrates.

But the “if” that starts from the previous paragraph is big. Many people in this field disagreed with this interpretation and put it Yunanozoan in another place. Or rather several other places, depending on who exactly was arguing. Some put them in the same group as the amniotes. Others have propped them away from vertebrates and placed them in a group of mud dwellers who do not have the two body axes found in vertebrates. However, others have suggested that they were ancestors to a huge group of organisms that includes things like sea urchins.

A small team from China has now attempted to settle these arguments. It does this in part by photographing more than 100 new fossils of the species. But the great part is that they used some of the most sophisticated imaging techniques available. This included 3D X-ray imaging, electron microscopy, a technique that bombards microscopic areas of the sample with electrons and then uses the emitted light to identify which elements are present.

Ignore the nomenclature - the level of detail in the new imaging helps us understand the structure of features resembling gill arches.
Zoom / Ignore the nomenclature – the level of detail in the new imaging helps us understand the structure of features resembling gill arches.

Tian et al.

I show one of the images from the paper below to give an idea of ​​the details these imaging techniques provide.

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