Researchers studying a 305-million-year-old fossil reveal that it belonged to a never-before-seen arachnid they named Idmonarachne brasieri. This close relative of the earliest spiders, described in Proceedings of the Royal Study B this week, reveals new clues about spider evolution.
A team led by Russell Garwood from the University of Manchester examined the remarkably preserved, three-dimensional, 1.04-centimeter-long (0.4 inch) fossil from the Montceaules-Mines of France using microscopes as well as lab-based and synchrotron-based high-resolution CT scanning. One of the oldest ever reported spiders was previously discovered at this site, along with Late Carboniferous scorpions, harvestmen, and many other invertebrates.
The team found that, while this arachnid is distinctly spider-like, it lacks a key diagnostic character of the spider order, Araneae: spinnerets, appendages on the abdomen that help control silk production. Idmonarachne brasieri likely secreted silk, but it may not have had enough control to create webs.
"We can say almost 100 percent that it's a predator," Garwood tells IFLScience. "Most arachnids are, and this creature has very spider-like fangs, which are very good for eating other animals, so it's a safe bet that is how this species lived." The team didn't find any forelimb specializations, which some spiders use for ambush predation, and without a web, Idmonarachne likely ran down its prey. "But that is very much a guess," he adds.
"Our evolutionary tree for this study suggests that this fossil is a sister group to the spiders, and the sister group to these two is the extinct order of arachnids called the Uraraneida," Garwood explains. Uraraneids lived during the Devonian–Permian hundreds of millions of years ago. Like Idmonarachne, uraraneids also didn't have spinnerets, though they did have spigots, or openings to silk glands. And Idmonarachne lacks the tail-like structure of uraraneids called the flagelliform telson.
While spiders are a diverse, successful lineage that can be traced back 315 million years to the Late Carboniferous, there are still many uncertainties surrounding their origins. These findings suggest that the ancestors of spiders likely developed the ability to secrete silk before losing their flagelliform telson and gaining the ability to spin silk using spinnerets. "Perhaps the key element in spiders' success is actually spinnerets, which opens them up to a wide range of different ways of living," Garwood adds, "and has allowed them to diversify to such a great extent."
The genus is named after wool-dyer Idmon – the father of Arachne, a weaver in Greek and Roman mythology – a reference to its close evolutionary relationship with spiders. The species name honors the late Oxford professor Martin Brasier.
Image in the text: Suggested appearance of Idmonarachne brasieri in life. R.J. Garwood et al., Proc. R. Soc. B 2016