An extremely rare collection of 160-million-year-old sea spider fossils from southern France, closely related to the living species, unlike older fossils of their kind.

These fossils are very important for understanding the evolution of sea spiders. They show that the diversity of sea spiders that exists today began to form by the Jurassic period.

Lead author Dr Romain Sabroux, from the University of Bristol’s School of Earth Sciences, said: “Sea spiders (Pyctonogonida) are a group of marine animals that have been poorly studied as a whole.

“However, it is very interesting to understand the evolution of arthropods [the group that includes insects, arachnids, crustaceans, centipedes and millipedes] Because they appeared relatively early in the arthropod tree of life. That’s why we are interested in their development.

“Sea spider fossils are very rare, but we do know some of them from different periods. One of the most remarkable fauna, both because of its diversity and its abundance, is the one from La Volte-sur-Rhône which dates to the Jurassic period. is, about 160 million years ago.”

In contrast to older sea spider fossils, the La Volta pycnogonids are morphologically similar (but not identical) to living species, and previous studies have suggested that they may be closely related to living sea spider families. But these hypotheses were restricted by the limitation of their observational instruments. Since it was impossible to access what was hidden in the rock’s fossils, Dr. Sbroux and his team traveled to Paris and set out to investigate this question with cutting-edge methods.

Dr Sbroux explained: “We used two methods to re-examine the fossils’ morphology: X-ray microtomography, to ‘look inside’ the rock, finding hidden morphological features, and 3D imaging of the fossil specimen reconstructing the model; and reflectance change imaging, an imaging technique that relies on different orientations of light around them to enhance the visibility of obscure features on a fossil’s surface.

“With these new insights, we gained new morphological information to compare them with existing species,” Dr. Sbroux explained.

This confirmed that these fossils are close relatives of living pycnogonids. Two of these fossils belong to the two living pycnogonid families: Colosopantopodus boisinensis One was Colocendidae while the other, Paleoendis Elmi There was an Andy day. third species, Palaeopycnogonides gracilisSeems to belong to a family that is now extinct.

“Today, by calculating differences between DNA sequences of samples of species and using DNA evolutionary models, we are able to estimate the evolutionary time that binds these species together,” said Dr. Sbroux.

“This is what we call molecular clock analysis. But like a real clock, it needs to be calibrated. Basically, we need to tell the clock: ‘We know that at that time, that group was already there. ‘ Thanks to our work, we now know that Colocendidae, and Endidae were already ‘there’ by the Jurassic.”

Now, the team can use these minimum ages as a calibration for the molecular clock, and probe the timing of Pycnogonida evolution. This could help them understand, for example, how their diversity was affected by the various biodiversity crises that spanned Earth’s history.

They also plan to investigate other pycnogonid fossils, such as fauna from the Hunsrück Slate in Germany, which dates to the Devonian period, about 400 million years ago.

With the same approach, his goal would be to re-describe these species and understand their affinities with existing species; and finally, substituting all pycnogonid fossils from all periods into the tree of life of the Pycnogonida.

Dr Sabroux said: “These fossils give us information about sea spiders that lived 160 million years ago.

“It’s very exciting when you’ve been working on living pycnogonids for years.

“It’s interesting how these pycnogonids look both very familiar and very exotic. Familiar, because you can certainly recognize some of the families that exist today, and because of small differences like the shape of the legs, the length of the body are exotic, and some other morphological features that you don’t find in modern species.

“Now we look forward to the next fossil discoveries – from the Jurassic and other geological periods – so that we can complete the picture!”

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