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Definition: Suggested appearance of Idmonarachne brasieri. Source: Proceedings of the Royal Society BIt was “almost a spider.” That’s how the researchers referred to the ancient, 1.5-cm creature found in Montceau-les-Mines, France.

Using computed tomography reconstruction, U.K., U.S., and German researchers recently learned more about the creature’s anatomy. They’ve given the fossil creature the scientific name Idmonarachne brasieri and dated to around 305 million years ago. Their research was published today in Proceedings of the Royal Society B.   

According to BBC News, the fossil escaped identification for decades, having been previously stored in a box at Paris’ Museum National d’Histoire Naturelle. With computer tomography, the researchers were able to glimpse the portion of the fossil embedded within the Carboniferous-age stone.

While morphologically similar to spiders, the researchers noted the specimen’s lack of a couple key diagnostic characters of the spider order Araneae. Namely, the absence of spinnerets on the posterior’s underside. Additionally, it lacked a tail-like appendage that was found in spider-related creatures from the Devonian-Permian. This led researchers to classify I. brasieri as a sister group to uraraneids, both spider stem groups.

Prior to developing spinnerets, spider stem groups developed the ability to produce silk, which emerges from microscopic spigots. “This would have been followed by the loss of flagelliform telson (the tail-like appendage), and then the ability to spin silk using spinnerets,” the researchers wrote. “This last innovation defines the true spiders, significantly post-dates the origins of silk, and may be a key to the group’s success.”

“Our creature probably split off the spider line after (Attercopus), but before true spiders appeared,” Russell Garwood, a co-author of the study from the University of Manchester, told BBC News. “The earliest known spider is actually from the same fossil deposit—and it definitely has spinnerets. So what we’re actually looking at is an extinct lineage that split off from the spider line some time before 305 million years ago, and those two have evolved in parallel.” 

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