Spider evolution
From Wikipedia, the free encyclopedia
Today's 40,000 described species of spiders are members of the hugely diverse arthropod animal class. They first appeared about 400 million years ago and evolved from crab-like ancestors.
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[edit] Early forms
Trigonotarbids, spider-like arachnids, were among the oldest known land arthropods. Like spiders, they were terrestrial, respired through book lungs, and walked on eight legs with two additional legs adapted to use around their mouth. However, they were not true spiders, not even ancestral to them, but represented independent offshoots of the Arachnida.
[edit] True spiders
True spiders (thin-waisted arachnids) evolved about 400 million years ago, and were among the first species to live on land. They are distinguished by abdominal segmentation and silk producing spinnerets. The first known fossil spider, Attercopus, lived 380 million years ago during the Devonian. Attercopus is placed as sister-taxon to all living spiders, on the basis of characters of the spinneret and the arrangement of the patellatibia joint of the walking legs. Graeophonus, another genus of early spider, lived over 300 million years ago during the Carboniferous.
Most of the early segmented fossil spiders belonged to the Mesothelae, 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 weaving 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. Additional genetic evidence suggests, by examining the drift of genes thought to be used to produce the web-spinning behavior, suggest 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, mite, wasp's leg, and 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.
