The holotype, or first specimen, of the new arachnid species Chimerarachne yingi. (Bo Wang)

Deep in a tropical rain forest, during a time when dinosaurs walked Earth, four itsy bitsy spiders crawled down a tree, got stuck in some sticky resin and never climbed up again.

Some 100 million years later, blocks of amber containing their fossilized forms wound up on the desks of two scientists in China. Both researchers looked at the perfectly preserved animals and came to the same conclusion: This was an entirely new kind of creature.

They introduced their discovery, dubbed Chimerarachne yingi, in a pair of papers published Monday in the journal Nature Ecology and Evolution. With its curious mix of ancient and modern traits — a long, skinny tail inherited from a distant arachnid ancestor, but a silk-producing organ like those found in spiders today — the tiny chimerarachne, or “chimera spider,” is not a member of the immediate family. But it is one of modern spiders' closest cousins, and it presents intriguing hints at how they evolved.

The C. yingi fossils were uncovered by amber miners in northern Burma, sold to dealers, then purchased by researchers at the Chinese Academy of Sciences. By coincidence, two sets of the fossils became available around the same time, and Bo Wang and Diying Huang — colleagues in the academy's paleobiology lab — began to analyze them almost simultaneously. Neither was aware of what the other was up to until they submitted their studies for publication. Happily, their results were close enough that the journal opted to publish both papers.

Both describe creatures so small they could fit on the tip of a fine-point pen, with eight legs and tiny but formidable fangs. Their hindquarters bear spinnerets, the same organs from which living species spin their silken webs. The males also have modified pedipalps — syringe-like appendages on the fronts of their faces that modern spiders use during mating.

(Ready to learn more than you ever wanted to know about spider sex? Male spiders don't have penises, so they instead deposit their sperm on a ready-made swatch of web, suck up that sperm with their pedipalps and inject it into a female. The whole affair typically ends with the female spider eating her mate.)

Other features of chimerarachne appear much more primitive. Their torsos are segmented, like those of older arachnid groups, and they have long, whip-like tails, called telsons, that seem to be inherited from a more primitive ancestor. This mix of features gave the spiders their name: a reference to a mythical creature with a lion's head and serpent-like tail.

This odd appendage, which is absent in modern spiders, can be found in vinegaroons, a group of nightmarish scorpion-looking creatures that lives today. And it's visible in one of the oldest fossils from a close spider relativeAttercopus fimbriunguis, which dates back 380 million years.

Paul Selden, a paleontologist at the University of Kansas who unveiled that other ancient arachnid and worked with Wang to analyze this latest discovery, said he'd been waiting to find something like this ever since A. fimbriunguis was discovered.

“It seems to be an intermediate form,” Selden said — midway between the spinneret-less A. fimbriunguis and the spiders of today.

Because of this mix of features, the two research groups differ slightly over where C. yingi fits in the spider family tree. But they agree it is a close cousin of the Araneae, or true spider, order.

Scientists have identified many spiders from this lineage in the same amber deposit from the Cretaceous period. Their numbers suggest that even 100 million years ago, chimerarachne was already a “living fossil” — a species that resembles creatures otherwise known only from the fossil record, Selden said. Like today's horseshoe crabs and ginkgo trees, it was a holdover from an earlier period in evolutionary history.

Selden even entertained what he called the “tantalizing possibility these creatures are still around.”

C. yingi wouldn't be the first fossil arachnid to show up in the wild. In the 1880s, scientists working in Madagascar were surprised to see a new type of “assassin spider” crawling about. Until then, that lineage had only been found only in 50-million-year-old amber.

Gonzalo Giribet, an evolutionary biologist at Harvard University who worked on Huang's team, said the new discovery might also shake up the arachnid family tree. Scientists have traditionally used silk spinnerets to distinguish true spiders from other species. Some argue that spinnerets were the key innovation that allowed spiders to become so successful; there are nearly 50,000 known spider species  alive today.

“And now suddenly we have another group that is not a spider that also has those characteristics,” Giribet said. “It challenges our view of how we define 'spider.' ”

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