A 305 million-year old arachnid was similar to early spiders

Different views of Idmonarachne Brasieri scans (Image Russell J. Garwood et al.)
Different views of Idmonarachne Brasieri scans (Image Russell J. Garwood et al.)

An article published in the journal “Proceedings of the Royal Society B: Biological Sciences” describes the discovery of a fossil of a arachnid dating back some 305 million years ago. Named Idmonarachne brasieri, it shows similarities with modern spiders and its exam could provide a lot of information on the evolution of these animals, known to a limited extent because of the scarcity of fossils of their ancestors.

The fossil of Idmonarachne brasieri was discovered in a fossil deposit close to Montceau-les-Mines, France, decades ago, but for a long time it wasn’ identified. It’s a very well preserved specimen but the front half was embedded in rock and for a long time any type of detailed examination would’ve been destructive.

Today technologies unthinkable not many years ago are available and the fossil of Idmonarachne brasieri was the subject of detailed investigations by a team of researchers from The University of Manchester, Berlin’s Museum für Naturkunde, the University of Kansas and Imperial College London who worked together with the Muséum National d’Histoire Naturelle (Natural History Museum) in Paris and the British Diamond Light Source. The result was a series of exams which include a three-dimensional scan of the fossil.

These exams allowed to analyze the similarities but also the differences between Idmonarachne brasieri and spiders. It also seems related to the uraraneids (Uraraneida), arachnids that today are extinct, for other similarities. For this reason, the researchers created a new taxonomic genus named after Idmon, the mythological father of Arachne.

Spiders body’s back half consists of a bulbous structure while Idmonarachne brasieri has segmentation on its abdomen, more similar to those of more ancient arachnids such as pseudoscorpions. Idmonarachne brasieri has no spinnerets either, which are the organs that secrete silk used to create spider webs. The uraraneids were able to produce silk but not to create webs. From this point of view, Idmonarachne brasieri was similar to the uraraneids.

Understanding the various relations is a complex task and, as happens with many taxonomic groups, each new fossil allows to understand them better filling various gaps in the evolutionary tree. In this case, a fossil from the late Carboniferous period can help to understand the evolution from primitive spiders that secreted silk but were still unable to build webs to modern spiders.

Spiders and arachnids in general are not among the favorite human beings’ favorite animals but have a history of a few hundred million years and are spread all over the world. You may not like them but they too have their place in the history of life on Earth and deserve to be studied.

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