Spider origins

Spiders were among the earliest animals to live on land, probably evolving about 400 million years ago. 

A segmented spider from Malaysia

Mike Gray © Australian Musuem

Spiders probably evolved about 400 million years ago from thick-waisted arachnid ancestors that were not long emerged from life in water. The first definite spiders, thin-waisted arachnids with abdominal segmentation and silk producing spinnerets, are known from fossils like Attercopus fimbriungus. This spider lived 380 million years ago during the Devonian Period, more than 150 million years before the dinosaurs.

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 (like cockroaches, giant silverfish, slaters and millipedes). 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 (Opisthothelae) 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.

By the Jurassic Period (191 - 136 million years ago), when dinosaurs roamed the earth, the sophisticated aerial webs of the orb weaving spiders had developed to trap the rapidly diversifying hordes of flying insects. Similarly, the diversification of hunting spiders in litter, bark and foliage niches would have progressed in response to new prey-capture and habitat opportunities.

Despite this the spider fossil record is relatively poor. During the Tertiary Period the rich record of amber spider fossils - complete spiders trapped in clear, sticky, tree resins - show us that a spider fauna basically similar to that of the present day existed more than 30 million years ago.

Living fossils

Amazingly, segmented mesothelid spiders have survived in eastern Asia (China to Indonesia) from late Palaeozoic times to the present day. These large, impressive spiders live in soil burrows with trapdoors in forested areas and caves.Segmented spiders are very similar to their ancestors and have virtually remained unchanged . They are not found in Australia.

The largest 'spider' ever?

A 300 million year old, half metre long, fossil arachnid, Megarachne servinei, was originally described as a spider, but is now thought more likely to represent another type of spider-like ancient arachnid. Its unique features include the enormous size, massive shovel-like jaws and ribbed, shield-like covering over the abdomen. An arachnid of this size must have fed on large prey like cockroaches and giant millipedes. But why did this massive predator need such an impressively armoured body - were there even bigger arachnid predators about?

 

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Dr Mike Gray , Senior Fellow
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