photo credit: Digital visualization of Idmonarachne brasieri gen. et sp. nov. based on laboratory and synchrotron scans of the fossil. R.J. Garwood et al., Proc. R. Soc. B 2016
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 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 the fossil is distinctly spider-like, it lacks a key diagnostic character of the spider order, Araneae: spinnerets, appendages on the abdomen that helps spiders control silk production. This new arachnid likely secreted silk, but it may not have had enough control to create elaborate webs.
They named it Idmonarachne brasieri. “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 specializations of the forelimbs, which some spiders use for ambush predation, and without a web, the arachnid likely ran down its prey. “But that is very much a guess,” Garwood adds.
“Our evolutionary tree for this study suggests that this fossil is 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. And like Idmonarachne, uraraneids also lacked spinnerets, though they did have spigots, or openings to silk glands. Additionally, Idmonarachne lacks the tail-like structure of the 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.
The genus is named after wool-dyer Idmon – the father of Arachne in Greek and Roman mythology – referring to its close evolutionary relationship with spiders. The species name honors the late Oxford professor Martin Brasier.