Araneoid spiders are a very successful group numbering 12,000 to 13,000 known species today, said Jonathan Coddington, an expert on spider systematics at the Smithsonian Institution in Washington, D.C. Though an entire araneoid spider has also been found in amber from this period, this new find provides independent evidence for the great age of that group, said Coddington.
The development of sticky viscid silk is thought to be an important evolutionary innovation as that silk is more effective at snagging passing insects than the non-sticky variety. "It allowed spiders to economize on silk," said Shear. Many spiders have to build a web from scratch again each day. Producing all that silk uses a lot of energy.
Weaving Webs in Deep Time
Though scientists have some evidence that spiders have been weaving webs for hundreds of millions of years, much of it has been circumstantial.
Shear noted that while the araneoid spider fossil contained so-called spinneret organs, adaptations also used by modern spiders to produce silk for webs, it's not possible to be certain that fossilized structures performed the same function that similar-looking structures perform today. But the fact that ancient specimen has comb structures on its feetwhich aid modern spiders' movement on a webadds further evidence.
The oldest confirmed spider fossil was found embedded in ancient rock deposits dating to the mid-Devonian period 380 million years ago, long before the appearance of four-footed vertebrates. This specimen has fully-formed spinneret organs, said Shear, and is strong evidence that spiders were already producing silk at that time. Another small fossil dates to 415 million years ago, but is not complete enough to confirm that it is part of a spider.
Finding spinnerets alone is not proof enough of web building, as primitive spiders must originally have used the silk for another less-complex task, said Coddington. Lining burrows, and creating eggs sacs (which spiders still do with silk) are likely contenders, he said.
"This new find is conclusive evidence that Cretaceous-era spiders were producing viscid silk," said Shear. "We are unlikely to find any older [pieces of web] because this is one of the oldest amber deposits," he said.
Amber deposits only appear in the fossil record in abundance following the proliferation of modern flowering plants during the Cretaceous period.
SOURCES AND RELATED WEB SITES