Amber is considered a gem, and pieces containing intact insects are among those that jewelry makers prize the most.
The newfound amber piece is roughly cylindrical and measures about 0.7 by 0.3 of an inch (18 by 7.5 millimeters).
It comes from fossil-rich amber deposits discovered in the Aragon region of northeastern Spain about seven years ago (map of Spain).
According to the authors, the new find is not only the oldest but also the first known substantial fossilized section of spider web.
The partial web includes 26 strands of silk, some of them joined by cross links, Grimaldi says, which are typical of orb-weaving spider webs.
The only sample comparable to the find is a piece of Lebanese amber 130 million years old that holds a single strand of spider silk (read "Dinosaur-Era Spiderweb Found in Amber").
Both specimens formed during the early Cretaceous period, a time when creatures such as iguanadons were on the scene but well before the arrival of Tyrannosaurus rex.
While there was no spider present in the latest find, the amber did contain a mite, a beetle, a leg from a wasp, and a fly caught by the spider's silk.
The discovery, Grimaldi says, helps researchers understand the evolution of both spiders and their prey.
For example, a long history of hanging webs would mean spiders have been influencing the evolution of flying insects for millions of years.
Jonathan Coddington is a spider expert with the Smithsonian Museum of Natural History in Washington, D.C., who was not involved with the study.
He agrees that even without a spider present, there is little doubt that the webbing is spider silk, because of the tiny, characteristic beads of glue spaced out along several of the strands.
"There is just nothing else that could do it," he said.
The paper's authors propose that the webbing in the amber is from a single section caught by the sap as it dripped slowly from a tree branch.
Coddington disagrees with that detail.
"A glob like that is probably not going to get a whole section with that much prey at once," he said.
Instead he imagines that the web was suspended close alongside a hanging glob of sap. As wind occasionally slapped the web against the resin, the web would lose a chunk along with its trapped prey.
But Coddington supports the paper's basic point about the time frame for the origins of orb weaving.
Another fossil and amber expert, George O. Poinar, Jr., of Oregon State University in Corvallis, is more doubtful.
He says the new report is interesting, but that many authorities "just assume spiders have made webs since [the animals] first appeared" more than 400 million years ago.
Still, he says, amber such as this latest discovery does preserve vital information on spider evolution. By one count, 500 extinct spider species have been found in amber from Europe's Baltic region alone.
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