Nervous system preserved in fossilized chelicerate ancestor

(ORDO NEWS) — Mollisonia symmetrica was a primitive Cambrian arthropod, the ancestor of modern spiders, mites, and scorpions. Paleontologists drew attention to the 508-million-year-old fossilized remains of this creature and studied its well-preserved nervous system in detail.

Recently, paleontology pleases with sensational news from the most distant past. Some look truly fantastic: for example, the study of the chemical composition of Dickinsonia from the Ediacaran period, or a detailed study of the eyes of Mesozoic crabs . The new article describes the well-preserved remains of the nervous system of Mollisonia symmetrica , an arthropod from the Cambrian period.

Approximately 540 million years ago on Earth, the long Proterozoic era was replaced by the Paleozoic. Its first period, the Cambrian, marked a sharp turn in the evolution of the biosphere. It was then that the famous Cambrian explosion occurred , when many new groups of animals arose almost simultaneously (by geological standards).

Among them were the first primitive arthropods – for example, representatives of the genus Mollisonia . Mollisonia is considered the ancestor of a large group of chelicerae arthropods, which includes modern spiders, mites, scorpions and horseshoe crabs.

During life, mollisonia had several pairs of limbs, appendages around the mouth (it is assumed that these were chelicerae) and large eyes. These animals crawled along the bottom of the ancient seas in the middle Cambrian, a little over 500 million years ago. The first mollisonia were discovered in the famous Canadian Burgess Shale, a kind of “treasury” that preserved a huge variety of animals of that time.

However, a new amazing discovery is not associated with excavations and expeditions. To accomplish it, it was enough for American scientists to look at the already existing finds of Mollisonia symmetrica from the Smithsonian Institution and the Museum of Comparative Zoology at Harvard University.

Nervous system preserved in fossilized chelicerate ancestor

“At first glance, these fossils do not look particularly interesting. <…> Outwardly, they are completely ordinary, ”said Javier Ortega-Hernández , author of a new article in Nature Communications . From the outside, only a simple exoskeleton can be distinguished in Mollisonia, consisting of a shield covering the head, trunk segments, and another shield at the back. Something like the outer covers of modern woodlice, only narrower and longer.

However, when the scientist placed the fossil under a microscope, he still had to be surprised . “There was something strange inside this animal, in the fossil itself,” continues Ortega-Hernandez. It turned out that in the samples of Mollisonia symmetrica , which were unsightly on the outside , the nervous system was preserved.

Fossilized nerve tissue looks like ink stains, because over many millions of years, living cells have turned into films of organic carbon. Behind the large eyes of Mollisonia symmetrica , optic nerves are visible, which are part of a thicker bundle. It passes through the head section of the animal and reaches for the tail. Smaller nerves extend from the lower side of this cord to both sides. Above the nervous system of mollisonia is the digestive system, which does not allow distinguishing some details.

Fossilized mollisonia have something resembling large head ganglia. They may turn out to be a primitive semblance of the brain or something of a completely different nature.

The structure of the nervous system of Mollisonia symmetrica is generally consistent with what scientists expected to see in a primitive chelicerate. The animal lacks the specialized features described in later members of this group. The authors of the study believe that the anatomy of the limbs of mollisonia and new data on the nervous system contradict each other. Therefore, they believe that at the first stages the evolution of chelicerae had a mosaic character – it successively changed first one, then the other organs of these animals.


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