Evolution of spiders

Among the oldest known land arthropods are Trigonotarbids, members of an extinct order of spider-like arachnids.{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |issn=1936-6426 |doi-access=free }}

Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration through book lungs, and walking on eight legs,{{cite journal|last1=Garwood|first1=Russell|last2=Dunlop|first2=Jason|title=The walking dead: Blender as a tool for paleontologists with a case study on extinct arachnids|journal=Journal of Paleontology|volume=88|issue=4|year=2015|pages=735–746|issn=0022-3360|doi=10.1666/13-088|s2cid=131202472}} with a pair of leg-like pedipalps near the mouth and mouth parts. They lacked the ability to spin silk: there is no evidence for either spigots or spinnerets within the group. An unpublished fossil exists which has distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk, but given the lack of any structures associated silk production, it seems unlikely the structures were associated with silk.

Trigonotarbids are not true spiders, and the trigonotarbids have no living descendants.{{cite journal|title=Fossils Explained: Trigonotarbids|first1=Russell J.|last1=Garwood|first2=Jason A.|last2=Dunlop|year=2010|journal=Geology Today|volume=26|issue=1|pages=34–37|url=https://www.academia.edu/256387|access-date=June 12, 2015|doi=10.1111/j.1365-2451.2010.00742.x|s2cid=247708509 }}

File:Nephila burmanica attacking Cascoscelio incassa in Burmese amber.jpgd attacking Cascoscelio incassus preserved in amber, {{circa|{{Ma|100}}.}} ]]

According to a 2020 study using a molecular clock calibrated with 27 chelicerate fossils, spiders most likely diverged from other chelicerates between 375 and 328 million years ago.{{cite journal |last1=Lozano-Fernandez |first1=Jesus |last2=Tanner |first2=Alastair R. |last3=Puttick |first3=Mark N. |last4=Vinther |first4=Jakob |last5=Edgecombe |first5=Gregory D. |last6=Pisani |first6=Davide |title=A Cambrian–Ordovician Terrestrialization of Arachnids |journal=Frontiers in Genetics |date=11 March 2020 |volume=11 |page=182 |doi=10.3389/fgene.2020.00182|pmid=32218802 |pmc=7078165 |doi-access=free }}

At one stage, Attercopus was claimed as the oldest fossil spider which lived {{Ma|380}} during the Devonian. Attercopus was placed as the sister-taxon to all living spiders, but has now been reinterpreted as a member of a separate, extinct order Uraraneida which could produce silk, but did not have true spinnerets.{{cite journal |last1=Selden |first1=P.A. |last2=Shear |first2=W.A. |last3=Sutton |first3=M.D. |year=2008 |title=Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=52 |pages=20781–20785|issn=0027-8424 |doi=10.1073/pnas.0809174106 |pmid=19104044 |pmc=2634869|doi-access=free}} The discovery of Chimerarachne in early Late Cretaceous (Cenomanian) aged Burmese amber has also demonstrated that taxa existed until the Cretaceous that had both spinnerets, and a whip-like telson.{{cite journal |last1=Wang |first1=Bo |last2=Dunlop |first2=Jason A. |last3=Selden |first3=Paul A. |last4=Garwood |first4=Russell J. |last5=Shear |first5=William A.|last6=Müller |first6=Patrick |last7=Lei |first7=Xiaojie |year=2018 |title=Cretaceous arachnid Chimerarachne yingi gen. et sp. nov. illuminates spider origins |journal=Nature Ecology & Evolution |volume=2 |issue=4 |pages=614–622 |issn=2397-334X |doi=10.1038/s41559-017-0449-3 |pmid=29403075 |s2cid=4239867 |url=https://www.research.manchester.ac.uk/portal/en/publications/cretaceous-arachnid-chimerarachne-yingi-gen-et-sp-nov-illuminates-spider-origins(b82d83bd-6081-4187-acfc-fb7358c33358).html }}{{cite journal |last1=Huang |first1=Diying |last2=Hormiga |first2=Gustavo|last3=Cai |first3=Chenyang |last4=Su |first4=Yitong |last5=Yin |first5=Zongjun |last6=Xia |first6=Fangyuan |last7=Giribet |first7=Gonzalo |year=2018 |title=Origin of spiders and their spinning organs illuminated by mid-Cretaceous amber fossils |journal=Nature Ecology & Evolution |volume=2 |issue=4 |pages=623–627 |issn=2397-334X |doi=10.1038/s41559-018-0475-9 |pmid=29403076 |s2cid=3268135 |url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:40992631}}

The oldest reported spiders date to the Carboniferous Period, or about {{Ma|300}}. Most of these early segmented fossil spiders from the Coal Measures of Europe and North America probably belonged to the Mesothelae, or something very similar, a group of spiders with the spinnerets placed underneath the middle of the abdomen, rather than at the end as in modern spiders. They were probably ground-dwelling predators, living in the giant clubmoss and fern forests of the mid-late Palaeozoic, where they were presumably predators of other primitive arthropods. Silk may have been used simply as a protective covering for the eggs, a lining for a retreat hole, and later perhaps for simple ground sheet web and trapdoor construction. They co-existed with a range of spider-like forms which had some, but not all, the characters associated with the true spiders.{{cite journal |last1=Garwood |first1=Russell J. |last2=Dunlop |first2=Jason A. |last3=Selden |first3=Paul A. |last4=Spencer|first4=Alan R.T. |last5=Atwood |first5=Robert C. |last6=Vo |first6=Nghia T. |last7=Drakopoulos|first7=Michael |year=2016 |title=Almost a spider: A 305 million year-old fossil arachnid and spider origins |journal=Proceedings of the Royal Society B: Biological Sciences |volume=283 |issue=1827 |pages=20160125 |issn=0962-8452 |doi=10.1098/rspb.2016.0125 |pmid=27030415 |pmc=4822468}}

As plant and insect life diversified so also did the spider's use of silk. Spiders with spinnerets at the end of the abdomen (Mygalomorphae and Araneomorphae) appeared more than {{mya|250}}, presumably promoting the development of more elaborate sheet and maze webs for prey capture both on ground and foliage, as well as the development of the safety dragline. The oldest mygalomorph, Rosamygale, was described from the Triassic of France. Megarachne servinei from the Permo-Carboniferous was once thought to be a giant mygalomorph spider and, with its body length of {{cvt|34|cm|foot}} and leg span of above {{cvt|50|cm|inches}}, the largest known spider ever to have lived on Earth, but subsequent examination by an expert revealed that it was actually a relatively small sea scorpion.

By the Jurassic period, the sophisticated aerial webs of the orb-weaver spiders had already developed to take advantage of the rapidly diversifying groups of insects. A spider web preserved in amber, thought to be 110 million years old, shows evidence of a perfect "orb" web, the most famous, circular kind one thinks of when imagining spider webs. An examination of the drift of those genes thought to be used to produce the web-spinning behavior suggests that orb spinning was in an advanced state as many as {{mya|136}}. One of these, the araneid Mongolarachne jurassica, from about {{ma|165}}, recorded from Daohuogo, Inner Mongolia in China, is the largest known fossil of a spider.

The 110 million year-old amber-preserved web is also the oldest to show trapped insects, containing a beetle, a mite, a wasp's leg, and a fly.{{cite news |title=Oldest known spider web discovered in amber |date=22 June 2006 |website=Live Science (LiveScience.com) |url=http://www.livescience.com/841-oldest-spider-web-discovered-amber.html |access-date=25 June 2006 }} The ability to weave orb webs is thought to have been "lost", and sometimes even re-evolved or evolved separately, in different species of spiders since its first appearance.

Around half of modern spider species belong to the RTA clade, a group of spiders linked by the shared morphological trait of the retrolateral tibial apophysis (RTA) on the male pedipalp. Despite their modern diversity, there is no unambiguous evidence of the clade from the Mesozoic, though molecular clocks suggest that diversification of the group began in the Late Cretaceous. There appears to be a faunal turnover in the Cretaceous-Cenozoic interval, with the Cretaceous dominated by Synspermiata and Palpimanoidea, as well as enigmatic extinct families like the lagonomegopids, while the Cenozoic is dominated by RTA clade and araneoid spiders.{{cite journal |last1=Magalhaes |first1=Ivan L. F. |last2=Azevedo |first2=Guilherme H.F. |last3=Michalik |first3=Peter |last4=Ramírez |first4=Martín J. |date=February 2020 |title=The fossil record of spiders revisited: Implications for calibrating trees and evidence for a major faunal turnover since the Mesozoic |journal=Biological Reviews |volume=95 |issue=1 |pages=184–217 |doi=10.1111/brv.12559|pmid=31713947 |s2cid=207937170 |issn=1464-7931 |lang=en |url=https://onlinelibrary.wiley.com/doi/abs/10.1111/brv.12559 }}

• Spider taxonomy
• Insect evolution

{{Reflist}}

• {{cite book|title=Spider silk : evolution and 400 million years of spinning, waiting, snagging, and mating|url=https://archive.org/details/spidersilkevolut0000brun|url-access=registration|author1=Brunetta, Leslie |author2=Craig, Catherine L.|location=New Haven|publisher=Yale University Press|year=2010|isbn=978-0-300-14922-7}}
• {{cite book|author=Penney, D.|year=2008|title=Dominican Amber Spiders: a comparative neontological approach to identification faunistics ecology and biogeography|publisher=Siri Scientific Press|location=Manchester|isbn=978-0-9558636-0-8}}
• {{cite book|author1=Penney, D. |author2=Selden P.A.|year=2011|title=Fossil Spiders: the evolutionary history of a mega-diverse order|publisher=Siri Scientific Press|location=Manchester|isbn=978-0-9558636-5-3}}

• [http://www.xs4all.nl/~steurh/engfavor/espin.html Picture of spider fossil]
• Dunlop, J. A., Penney, D. & Jekel, D. (2016). A summary list of fossil spiders and their relatives. World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 16.5.

{{Spider nav}}

{{Evolution}}

Category:Spiders

Spiders