The main point of distinction: This newly discovered arachnid very likely could produce silk but lacked the spinnerets used by true spiders to, well, spin it, the scientists say. The researchers say it belongs to a “sister group” to the real-deal spiders.
Here’s more from National Geographic on the comparatively clumsy beginnings of spiderly silk production:
“While delicately constructed webs seem synonymous with spiders, we know from the fossil record that the ability to secrete silk came before the ability to carefully control it. Spider relatives called uraraneids, which lived from 385 million years ago through the time of Idmonarachne, could produce silk but could not build webs.”
University of Manchester’s Russell Garwood, who was one of the article’s authors, told the BBC, “This fossil is the most closely related thing we have to a spider that isn’t a spider.” More.
Could produce silk but couldn’t build webs? Okay, let’s keep the file open. We’ve heard accounts like that before. Cambrian eyes (“sophisticated”) and behaviour (“behaviourally sophisticated”) were supposed to be primitive, because … well…
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See also: Stasis: Life goes on but evolution does not happen