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Araneae (commonly known as spiders) is an order of air-breathing chelicerate arthropods that have eight legs, and chelicerae modified into fangs that inject venom.

Fossil record


Shultz (2007)'s evolutionary family tree of arachnids – † marks extinct groups.

Although the fossil record of spiders is considered poor, almost 1000 species have been described from fossils. Due to the fact that spiders' bodies are quite soft, the vast majority of fossil spiders have been found preserved in amber. The oldest known amber that contains fossil arthropods dates from in the Early Cretaceous period. In addition to preserving spiders' anatomy in very fine detail, pieces of amber show spiders mating, killing prey, producing silk and possibly caring for their young. In a few cases amber has preserved spiders' egg sacs and webs, occasionally with prey attached; the oldest fossil web found so far is 100 million years old. Earlier spider fossils come from a few lagerstätten, places where conditions were exceptionally suited to preserving fairly soft tissues.

The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about in the Silurian period, and had a triangular cephalothorax and segmented abdomen, as well as eight legs and a pair of pedipalps. Attercopus fimbriunguis, from in the Devonian period, bears the earliest known silk-producing spigots, and was therefore hailed as a spider. However these spigots may have been mounted on the underside of the abdomen rather than on spinnerets, which are modified appendages and whose mobility is important in the building of webs. Hence Attercopus and the similar Permian archnid Permarachne may not have been true spiders, and probably used silk for lining nests or producing egg-cases rather than for building webs.

Several Carboniferous spiders were members of the Mesothelae, a primitive group now represented only by the Liphistiidae. The mesothelid Paleothele montceauensis, from the Late Carboniferous over, had five spinnerets. Although the Permian period saw rapid diversification of flying insects, there are very few fossil spiders from this period.

The main groups of modern spiders, Mygalomorphae and Araneomorphae, first appear in the Triassic well before. Some Triassic mygalomorphs appear to be members of the family Hexathelidae, whose modern members include the notorious Sydney funnel-web spider, and their spinnerets appear adapted for building funnel-shaped webs to catch jumping insects. Araneomorphae account for the great majority of modern spiders, including those that weave the familiar orb-shaped webs. The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families.

Family tree
It is now agreed that spiders (Araneae) are monophyletic, in other words members of a group that contains a common ancestor plus all and only its descendants. There has been debate about what their closest evolutionary relatives are, and how all of these evolved from the ancestral chelicerates, which were marine animals. The cladogram on the right is based on J.W. Shultz' analysis (2007). Other views include proposals that: scorpions are more closely related to the extinct marine scorpion-like eurypterids than to spiders; spiders and Amblypygi are a monopyhletic group. The appearance of several multi-way branchings in the tree on the right shows that there are still uncertainties about relationships between the groups involved.

Arachnids lack some features of other chelicerates, including backward-pointing mouths and gnathobases ("jaw bases") at the bases of their legs; both of these features are part of the ancestral arthropod feeding system. Instead they have mouths that point forwards and downwards, and all have some means of breathing air. Spiders (Araneae) are distinguished from other arachnid groups by several characteristics, including spinnerets and, in males, pedipalps that are specially adapted for sperm transfer.