Evolution of spiders

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A spider in Baltic amber

The evolution of spiders has been ongoing for at least 380

spinnerets and silk
secretion.

Early spider-like arachnids

Among the oldest known land

Trigonotarbids, members of an extinct order of spider-like arachnids.[5]

Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration through

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 An unpublished fossil exists which has distinct microtubercles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.[7]

Emergence of true spiders

Geratonephila attacking Cascoscelio incassus preserved in amber, c. 100 million years ago

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.[8]

At one stage, Attercopus was claimed as the oldest fossil spider which lived 380 million years ago 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.[9] 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.[10][11]

The oldest reported spiders date to the Carboniferous Period, or about 300 million years ago. 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.[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 (

sea scorpion
.

By the

Mongolarachne jurassica, from about 165 million years ago, 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.[13] 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

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.[14]

See also

References

  1. PMID 25405073
    .
  2. PMID 28431496
    .
  3. ^
    PMID 26925338
    .
  4. S2CID 42371883
  5. ISSN 1936-6426
    .
  6. S2CID 131202472
    .
  7. S2CID 247708509
    . Retrieved June 12, 2015.
  8. PMID 32218802
    .
  9. PMID 19104044
    .
  10. S2CID 4239867
    .
  11. S2CID 3268135
    .
  12. PMID 27030415
    .
  13. ^ "LiveScience.com - Oldest Known Spider Web Discovered in Amber". Live Science. 22 June 2006. Retrieved June 25, 2006.
  14. S2CID 207937170
    .

External links

  • 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.
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