Evolution Spider

fossil record

spider preserved in amber

although fossil record of spiders considered poor, 1000 species have been described fossils. because spiders bodies quite soft, vast majority of fossil spiders have been found preserved in amber. oldest known amber contains fossil arthropods dates 130 million years ago in cretaceous period. in addition preserving spiders anatomy in fine detail, pieces of amber show spiders mating, killing prey, producing silk , possibly caring young. in few cases, amber has preserved spiders egg sacs , webs, prey attached; oldest fossil web found far 100 million years old. earlier spider fossils come few lagerstätten, places conditions exceptionally suited preserving soft tissues.

palaeotarbus jerami, trigonotarbid , oldest known exclusively terrestrial arachnid

the oldest known exclusively terrestrial arachnid trigonotarbid palaeotarbus jerami, 420 million years ago in silurian period, , had triangular cephalothorax , segmented abdomen, 8 legs , pair of pedipalps. attercopus fimbriunguis, 386 million years ago in devonian period, bears earliest known silk-producing spigots, , therefore hailed spider @ time of discovery. however, these spigots may have been mounted on underside of abdomen rather on spinnerets, modified appendages , mobility important in building of webs. hence attercopus , similar permian arachnid permarachne may not have been true spiders, , used silk lining nests or producing egg-cases rather building webs. largest known fossil spider of 2011 araneid nephila jurassica, 165 million years ago, recorded daohuogo, inner mongolia in china. body length 25 mm, (i.e., 1 inch).

several carboniferous spiders members of mesothelae, primitive group represented liphistiidae. mesothelid paleothele montceauensis, late carboniferous on 299 million years ago, had 5 spinnerets. although permian period 299 to 251 million years ago saw rapid diversification of flying insects, there few fossil spiders period.

the main groups of modern spiders, mygalomorphae , araneomorphae, first appear in triassic before 200 million years ago. triassic mygalomorphs appear members of family hexathelidae, modern members include notorious sydney funnel-web spider, , spinnerets appear adapted building funnel-shaped webs catch jumping insects. araneomorphae account great majority of modern spiders, including weave familiar orb-shaped webs. jurassic , cretaceous periods provide large number of fossil spiders, including representatives of many modern families.

family tree

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

it agreed spiders (araneae) monophyletic (i.e., members of group of organisms form clade, consisting of last common ancestor , of descendants). there has been debate closest evolutionary relatives are, , how of these evolved ancestral chelicerates, marine animals. cladogram on right based on j. w. shultz analysis (2007). other views include proposals that: scorpions more closely related extinct marine scorpion-like eurypterids spiders; spiders , amblypygi monophyletic group. appearance of several multi-way branchings in tree on right shows there still uncertainties relationships between groups involved.

arachnids lack features of other chelicerates, including backward-pointing mouths , gnathobases ( jaw bases ) @ bases of legs; both of these features part of ancestral arthropod feeding system. instead, have mouths point forwards , downwards, , have means of breathing air. spiders (araneae) distinguished other arachnid groups several characteristics, including spinnerets and, in males, pedipalps specially adapted sperm transfer.