Spider Tail

Up to 100 million years ago, a species of tiny, eight-legged creepy-crawler scuttled across the tropical rainforest of present-day Myanmar. Measuring a fraction of an inch, this ancient proto-spider sported arachnid-like features like legs and silk-producing spinnerets, but they also had one particularly unique characteristic: a long, hairy tail.

The creatures would have likely lived in the bark of resin-producing trees during the mid-Cretaceous period. But at some point, tree sap poured over the bodies of some of them, encasing and preserving their corpses for amber miners to uncover millions of years later. Eventually, some of these amber slabs found their ways into the hands of paleontologists like Paul Selden, who wrote a paper on the findings published February 5 in Nature Ecology & Evolution.

So why did this spider relative have a tail, and what was it doing encased in amber? And could this spider still be around today?

Ancient Relatives

Uncovered from a quarry in northern Myanmar, the four male specimens aren't quite "spiders." With a name drawing inspiration from a mythological hybrid animal, Chimerarachne yingi is a kind of proto-spider, linking primitive crawlers to the eight-legged creatures of today.

The proto-spider would have looked similar to spiders in the modern Liphistiidae family, which today live in Southeast Asia, China, and Japan. This family contains about 100 species of medium-size spiders that have back plates and spinnerets.

"It's the most primitive living spider," says Selden, who is also the director of the Paleontological Institute at the University of Kansas. "From the back, it would have looked quite similar."

The proto-spider had several arachnid-like characteristics, including fangs, pedipalps (or feelers), and four walking legs. Silk-producing spinnerets would have protruded from its rear, though it's unclear how the species would have used the silk, exactly. Spiders didn't start weaving webs in tree branches until their insect prey began taking to the skies.

Spiders have also used their silk to make trails to guide them back to their homes and lines for diverging or trapping prey, among other purposes. This proto-spider, the researchers say, likely lined its burrow with silk.

"It's weaving a web, but it's on the ground," Selden says. The silk was "almost certainly for wrapping its eggs in, as well."

Tailed Discovery

Researchers have known for more than a decade that modern spiders evolved from uraraneids, a family of tailed creatures resembling arachnids; other specimens Selden has uncovered range from 290 to 380 million years old. The tailed finds are relatively young by comparison, and never before has there been a fossil showing spiders with tails.

The hairy tail, which measures about a tenth of an inch, would have been used to help the spider sense its environment. Co-author Bo Wang says the tails could have evolved into obscurity when the creatures switched from roaming around to look for food to becoming sit-and-wait predators. The tail would have been unnecessary and would have eventually been lost. There are no tailed spiders today, and arachnids wouldn't need sensory flagellum because they can feel vibrations in their webs.

Selden says it's possible the tailed proto-spiders could still live in the rainforests of Myanmar today. Those areas aren't very well studied, and since the creature is to tiny, it could easily go unspotted. It's unknown if the proto-spiders were venomous or not, but zoologist Gonzalo Giribet says they likely wouldn't be harmful to people.

Amber Coffins

The specimens are all adult males, so Selden says it's possible they became caught in resin when they left their burrows to go looking for females.

Amber has already built up its reputation as a time capsule for ancient species. Just this month, a 99-million-year-old bird was found in a cloudy slab of Burmese amber, and other specimens including dinosaur tails, ticks, other bird parts, and more ancient spiders have also been discovered.

And the list of discoveries may go on.

"Already, we've got some other primitive spiders from this amber," Selden says.