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Evolution of spiders

The evolution of spiders has been going on for at least 400 million years, since the first true spiders (thin-waisted arachnids) evolved from crab-like chelicerate ancestors. Today, there are over 40,000 described spider species within the diverse phylum of arthropods.

Major developments in spider evolution include the development of spinnerets and silk secretion, as well as different adaptions for its use.

Early spider-like arachnids

Among the oldest known land arthropods are Trigonotarbids, members an extinct order of spider-like arachnids.

Sharing many superficial characteristics with spiders, Trigonotarbida were terrestrial, respired through book lungs, and walked on eight legs with two additional legs adapted to use around their mouth. Arguments still remain open as to whether they possessed the ability to create silk. This had been popular thought for quite some time, until an unpublished fossil was described with distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk.

Regardless Trigonotarbida are not considered true-spiders. They are generally accepted as an independent early offshoot within the Arachnida clade, and not directly ancestral to modern spiders.

Emergence of true spiders

At one stage the oldest fossil spider was believed to be Attercopus 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.
Microscopic structure of spider silk, possibly the most important adaption developed during the evolution of these arachnids

The oldest true spiders are thus Carboniferous in age, or about 300 million years. 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 primitive 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.

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 250 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 the Triassic of France and belongs to the modern family Hexathelidae. Megarachne servinei from the Permo-Carboniferous was once thought to be a giant mygalomorph spider and, with its body length of 1 foot (34 cm) and leg span of above 20 inches (50 cm), the largest known spider ever to have lived on Earth, but subsequent examination by an expert revealed that it was actually a middling-sized sea scorpion.

By the Jurassic, 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 136 million years ago.

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.[1] The ability to weave orb webs is thought to have been "lost", and sometimes even re-evolved or evolved separately, in different breeds of spiders since its first appearance.

A complete list of fossil spiders, down to species level, can be found at the American Museum of Natural History's website.[1]