Spiders are a common sight all over the world and come in a huge variety of shapes and sizes, from the Samoan moss spider (Patu marplesi) to the Goliath bird-eating spider (Theraphosa blondi) at 28 cm long and 170 g. The World Spider Catalog currently contains over 48,000 accepted species, yet their evolutionary history remains remarkably elusive. So when did spiders first start spinning their webs, and why did they start at all?
Spiders are the largest order within their class Arachnida, which also includes animals like scorpions and ticks. The defining trait of all spiders is their ability to produce silk and weave it using spinnerets – very small, moveable appendages on the tip of their abdomen. So palaeontologists must identify these features that allow modern spiders to produce and weave silk in arachnids from the fossil record. However, this isn’t as easy as it sounds, because spiders are small and delicate, and don’t fossilise well. This is partially why the origin of spiders is so obscure.
For many years, the first spider was thought to perhaps date all the way back to the Devonian Period. The extinct order Uraraneida, including the Devonian-aged Attercopus fimbriunguis, from the USA, and the Permian-aged Permarachne novokshonovi, from Russia, was widely thought to be the first order of spiders, but they are now classified as a sister clade to true spiders. The Uraraneida had spigots, the silk-producing organs of spiders, but they didn’t have spinnerets, which allow for weaving. They also had a flagelliform telson, essentially an appendage that looked a bit like a tail (for example, lobsters have a fan-like telson that helps them swim), which modern spiders have lost. So Uraraneida were spider-like, but because they are missing key features, they aren’t the first true spiders.
The first true spiders discovered so far have been described from the Late Carboniferous of Eastern France. Palaeothele montceauensis had all the characteristics of a true spider, with spinnerets and spigots and lacking a flagelliform telson. It has been attributed to the suborder of spiders Mesothelae, which contains living spiders, the Liphistiidae. Interestingly, a fossil arachnid dubbed Idmonarachne brasieri has also been found in the same locality from the same time period. I. brasieri didn’t have a flagelliform telson or spinnerets, so also couldn’t be considered a true spider. This meant that the first true spiders lived alongside the spider stem-group they presumably evolved from.
So we understand how spiders evolved their web-weaving abilities, but that leaves us with the question of why did they develop this ability in the first place? One possible scenario suggests the spiders covered their deposited eggs in a ‘proto-silk’ to help protect them from threats like predators and the elements. This ‘proto-silk’ eventually evolved into true silk. The other scenario suggests that spiders created a protein mucus membrane that covered their abdomen. This would have helped to shield and moisten the spider’s gills as they evolved into book lungs that can be used in the open air. Certain spiders like Argyroneta aquatica use silk to create a ‘lung’ to live underwater today. This would have greatly increased the range the first spiders could expand into during the terrestrialisation of the Palaeozoic. Of course, it is possible both scenarios were selection pressures that caused spiders to evolve silk spinning. From there, spider silk evolved to be an extremely versatile tool, allowing spiders to flourish and diversify around the world.
Spiders are among the oldest and most successful orders alive today, and whilst people’s fear of spiders is understandable, we hope you now also come to admire these fantastic arthropods for their amazing traits.
 A fossil found in Russia of Permarachne novokshonovi, a relative of modern spiders dating back to the Permian period. Taken from Eskov and Selden (2005).
 Palaeothele montceauensis from the Late Carboniferous of Montceau-les-Mines, France. Taken from Selden (1996).
 Idmonarachne brasieri, from the Late Carboniferous of Montceau-les-Mines, France. Taken from Garwood et al. (2016).
Information References and Further Sources
 Eskov, K.Y., and Selden, P.A. (2005). ‘First record of spiders from the Permian period (Araneae: Mesothelae)’, Bulletin of the British Arachnological Society, 13 (4), pp. 111-116. Accessed 10th August 2020. Click Here.
 Garwood, R. J., Dunlop, J. A., Selden, P. A., Spencer, A. R. T., Atwood, R .C., Vo, N .T. and, Drakopoulos, M. (2016). ‘Almost a spider: a 305-million-year-old fossil arachnid and spider origins’, Proceedings of the Royal Society B: Biological Sciences, 283 (1827), p. 20160125. Accessed 10th August 2020. Click Here.
 Selden, P. A. (2000). ‘Palaeothele, replacement name for the fossil mesothele spider Eothele Selden non Rowell’, Bulletin of the British Arachnological Society, 11 (7), pp. 292. Accessed 10th August 2020. Click Here.
 Selden, P. A. (1996) ‘First fossil mesothele spider, from the Carboniferous of France’, Revue Suisse de Zoologie, Vol. hors série 2, pp. 585-596. Accessed 10th August 2020. Click Here.
 Selden, P. A., Shear, W. A., and Sutton, M. D. (2008). ‘Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order’, Proceedings of the National Academy of Sciences, 105 (52), pp. 20781-20785. Accessed 10th August 2020. Click Here.
 Smith, F. A. (2008). ‘Body Size, Energetics, and Evolution’, in Jørgensen, S. E., and Fath, B. D., Encyclopedia of Ecology, Vol 1., pp. 477-482. Oxford: Elsevier. Accessed 10th August 2020. Click Here.
 Vollrath, F., and Selden, P. (2007). ‘The Role of Behaviour in the Evolution of Spiders, Silks and Webs’, Annual Review of Ecology, Evolution, and Systematics, 38, pp. 819-849. Accessed 10th August 2020. Click Here.
 World Spider Catalog (2020). World Spider Catalog. Version 21.5. Natural History Museum Bern. Accessed 10th August 2020. Click Here.