Spiders have existed since at least 380million years. The group's origins lie within anarachnid sub-group defined by the presence ofbook lungs (the tetrapulmonates);[1][2] the arachnids as a whole evolved from aquaticchelicerate ancestors. More than 45,000 extantspecies have been described, organisedtaxonomically in 3,958genera and 114families.[3] There may be more than 120,000 species.[3] Fossil diversity rates make up a larger proportion than extant diversity would suggest with 1,593 arachnid species described out of 1,952 recognized chelicerates.[4] Both extant and fossil species are described annually byresearchers in the field. Major developments in spiderevolution include the development ofspinnerets andsilk secretion.
Among the oldest known landarthropods areTrigonotarbids, members of an extinctorder of spider-like arachnids.[5]
Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration throughbook lungs, and walking on eight legs,[6] with a pair of leg-likepedipalps 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 An unpublished fossil exists which has distinctmicrotubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk, but given the lack of any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.[7]
According to a 2020 study using amolecular clock calibrated with 27chelicerate fossils, spiders most likely diverged from other chelicerates between 375 and 328 million years ago.[8]
At one stage,Attercopus was claimed as the oldest fossil spider which lived380 million years ago during theDevonian.Attercopus was placed as the sister-taxon to all living spiders, but has now been reinterpreted as a member of a separate, extinct orderUraraneida which could produce silk, but did not have true spinnerets.[9] The discovery ofChimerarachne in earlyLate Cretaceous (Cenomanian) agedBurmese amber has also demonstrated that taxa existed until the Cretaceous that had both spinnerets, and a whip-liketelson.[10][11]
The oldest reported spiders date to the Carboniferous Period, or about300 million years ago. Most of these early segmented fossil spiders from the Coal Measures of Europe and North America probably belonged to theMesothelae, 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.[12]
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 andAraneomorphae) appeared more than250 million years ago, 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 theTriassic of France.Megarachne servinei from thePermo-Carboniferous was once thought to be a giant mygalomorph spider and, with its body length of 34 cm (1.1 ft) and leg span of above 50 cm (20 in), the largest known spider ever to have lived on Earth, but subsequent examination by an expert revealed that it was actually a relatively smallsea scorpion.
By theJurassic period, the sophisticated aerial webs of theorb-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 as136 million years ago. One of these, thearaneidMongolarachne jurassica, from about165 million years ago, recorded fromDaohuogo,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 abeetle, amite, awasp's leg, and afly.[13] The ability to weave orb webs is thought to have been "lost", and sometimes even re-evolved orevolved separately, in different species of spiders since its first appearance.
Around half of modern spider species belong to theRTA clade, a group of spiders linked by the shared morphological trait of theretrolateral tibial apophysis (RTA) on the malepedipalp. 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 theLate Cretaceous. There appears to be a faunal turnover in the Cretaceous-Cenozoic interval, with the Cretaceous dominated bySynspermiata andPalpimanoidea, as well as enigmatic extinct families like thelagonomegopids, while the Cenozoic is dominated by RTA clade andaraneoid spiders.[14]
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