"It must have lived for about 200 million years side-by-side with spiders, but we've never found a fossil of one of these [before] that's younger than 295 million years", said Dr Garwood, from Manchester's School of Earth and Environmental Sciences.
But it's unique in that it has a "long flagellum, or tail", which is not something found on any living spider.
The team theorized that the tail potentially served as a form of antenna for sensing its environment, particularly because the tail was thin and whip-like.
The new animal, called Chimerarachne after the Greek mythological Chimera, a hybrid creature composed of the parts of more than one animal, lies one step closer to modern spiders on account of its possession of spinning organs.
Chunks of it were purchased by paleontologists, who found in the amber many well-preserved spiders - some of them with long, nearly scorpion-like tails.
Thanks to Mother Nature's great freeze-frame device known as amber, scientists have just discovered a remarkable new species of arachnid from 100 million years ago.
The dorsal view of the spider encased in amber.
The Sun reports, the eight-legged creepy crawlie is estimated to have scuttled along the forest floors as far back as 100 million years ago. In the last few years, we've seen some incredible finds inside amber, including a tick in the middle of a meal, an otherworldly insect, a bug that's jumped out of its skin, mammalian red blood cells, and a dinosaur tail complete with feathers.
Scientists say the spiders were a missing link between the even more ancient uraraneids and primitive living spiders.
In addition to the tail and the fangs, this new spider has male pedipalps, four walking legs, and silk-producing spinnerets at the rear.
Around 15 years ago they discovered that Burmese amber was older, which resulted in a huge amount of new material becoming available for study.
"The ones we recognised previously were different in that they had a tail but don't have the spinnerets", said Paul Selden of the research team.
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.
"All four specimens are adult males, which would have been roving around looking for females at this point in their lives", Selden said.
The creatures are less than a quarter-inch long (5.5 millimeters) including their tails, which account for half that length. "I$3 t is covered in short hairs, but when spiders changed to lifestyles like being sit-and-wait predators, the tail was no longer really needed and became lost".
"Amber is fossilised resin, so for a spider to have become trapped, it may well have lived under bark or in the moss at the foot of a tree".
We do know that the spider has spinnerets, which can be used to produce silk. Some argue that spinnerets were the key innovation that allowed spiders to become so successful; there are almost 50,000 known spider species alive today.
"We haven't found them", he says in the University of Kansas statement, "but some of these forests aren't that well-studied, and it's only a tiny creature".
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