First Spiders, 430, 390, 305, 240 mya,
Spider evolutionary milestones

Spiders and all the other arachnids are part of a larger and primal group of arthropods known as the Chelicerata, a subphylum of arthropods, which first appear in the fossil record in the Cambrian represented by chasmataspidids (a sort of primitive sea scorpions) and then later in the Ordovician era by Eurypterids (Sea Scorpions). Another member of the Chelicerata, the Xiphosura (horseshoe crabs) split off by the late Ordovician (Lunataspis aurora fossils show this). Some sea scorpions migrated to land and joined early primitive myriapods (millipedes and centipedes, other types of arthropods) and all sorts of worms crawling around mud of the later Silurian. Actually, by the Llandovery Epoch (444-428mya in the Silurian) trigonotarbids (a sort of creature half-way between a sea scorpion and a spider) fossils can be found, so perhaps the sea scorpions had already radiated into early forms of land scorpions (such as Proscorpius) and protospiders (the trigonotarbids and others) earlier in the Silurian.

When it comes to fossil evidence, those trigonotarbids are the earliest arachnid form of Chelicerata for which we have good fossil evidence, but Life was still a long way away from producing modern spiders. There are 140 million years between the fossil debut of arachnids (in the trigonotarbids, about 430mya) and the first modern spiders who had spinnerets to control their silk weaving (the sub-Order Mesothelae, such as Palaeothele montceauensis from 305mya—represented today by the Family Liphistiidae), right at the very end of the Carboniferous, the late Pennsylvanian. The earliest spiders with spinnerets at the end of their abdomens (as opposed to Palaeothele montceauensis and other Mesothelae, who have spinnerets in the middle of the lower part of their abdomens) appear in the early middle (Anisian) age of the Triassic around 245mya. The oldest known of these Opisthothelae (modern spiders that have spinnerets at the end of their abdomen, and aren’t Mesothelae) is Rosamygale grauvogeli, a spider found fossilized in rocks of the Vosges Mountains of France.

While primitive modern spiders in the Mesothelae suborder were crawling around from the late Carboniferous through the Permian, another type of proto-spider, an older type, looking very much like modern spiders (unlike the trigonotarbids), but having only silk spigots rather than spinnerets, and also having a tail at the end of its abdomen, was thriving. This sort of arachnid is the order Uraraneida. The earliest fossil of a protospider from this order is the Attercopus fimbriunguis, which lived in the Devonian around 390mya. Another protospider was Idmonarachne brasieri, a species that lived 305mya, and lacked the “tail” (the flagelliform telson). Another sort of protospider that evolved after the evolution of spinnerets is the Chimerarachne, which had silk spinnerets rather than spigots, but had the flagelliform telson (the “tail”). Some of these have been fossilized in amber as recently as 100mya. So, various sorts of protospiders (the Uraraneida and Chimerarache) begin appearing in the fossil record 390mya, and were still going strong 100mya, but primitive modern spiders (Mesothelae) appear about 305mya, and modern spiders (Opisthothelae) appear about 245mya, implying that proto-spiders and spiders coexisted for at least 140 million years.

  Links about the first spiders:

1. You can read the 2016 article about Idmonarachne brasieri from 305 mya at the Royal Society’s Proceedings. You can also read about that proto-spider at Scientific American.

2. The Open Access article by Paul Selden, Heidi Anderson, and John Anderson gives an overview of fossil spiders as the situation stood in 2009.

3. Paul Anthony Selden shares his 1996 paper [pdf] describing Eothele montceauensis, an example of a Mesothelae spider that lived in the Carboniferous Period.

4. To read about a currently existing species of the Mesothelae, here is an article about a Liphistius from Myanmar.

5. A 2005 paper [pdf] by Eskov and Selden about the earliest fossil funnel-web weaver (a Mesothelae of the Permian Period).

6. A 2008 paper by Selden, Shear, and Sutton about the origin of spinnerets.

7. Michael Byrne at Vice has an article with cool photographs of 100 million year-old proto-spiders stuck in amber. This is a Chimerarachne protospider, more closely related to true spiders than a Uraraneida, but not close enough to be considered a true spider.

8. A Trigonotarbid Arachnid from the Lower Devonian of Tredomen, Wales. By Jason A. Dunlop and Paul A. Selden. A very early arachnid.

This work by Eric & Chun-Chih Hadley-Ives is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.