Fossilised spider relative could not spin silk

The discovery of an ancient, tarantula-like creature in a French mine has bridged a gap in spider lineage.

The fossilised creature, which lived 305 million years ago and was published in Proceedings of the Royal Society B: Biological Sciences, is the closest relative of modern-day spiders found so far.

With eight legs, a bloated abdomen and a pair of fangs, it looks a lot like a common spider.

But close examination of Idmonarchne brasieri (named in honour of Oxford geologist Martin Brasier who died in 2014) reveals telling differences from modern-day “true” spiders, which are able to spin webs using silk-spinning organ called spinnerets.

Russell Garwood from the University of Manchester and colleagues imaged the ancient creature, which was fossilised in siderite, an iron carbonate mineral, using microtomography – a high-resolution form of CT scanning that doesn’t destroy the specimen.

Along with microscope images, the researchers created a 3-D computer model of the fossil. This showed a creature without spinnerets.

They were confident that they didn’t snap off prior to fossilisation, as the computer model would have rendered a hole where they once attached.

Spider evolution, the researchers claim, went so: earlier spider-like creatures couldn’t produce silk, but sported a pair of spikes protruding from their behind.

They gained the internal machinery to make silk but lost the spikes – which is where I. brasieri fits in.

They then grew spinnerets. This is illustrated by another specimen, found in the same area I. brasieri was discovered – Montceau-les-Mines – and described in 1996 by Paul Seldon from London’s Natural History Museum, and co-author on the current study.

Being able to spin silk, the researchers write, “defines the true spiders, significantly postdates the origins of silk, and may be a key to the group’s success”.

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