This odd appendage, which is absent in modern spiders, can be found in vinegaroons, a group of nightmarish scorpion-looking creatures that lives today.

The spider species was frozen in amber during the middle of the Cretaceous geological period - when dinosaurs ruled the Earth - in what is now Myanmar. Alongside modern spider traits such as a silk-producing structure called a spinneret, it possessed a remarkably primitive feature: a whip-like tail covered in short hairs that it may have used for sensing predators and prey.

An worldwide team involving University of Kansas paleontologist Paul Selden studied the mid-Cretaceous critter, which is trapped in a piece of amber from Myanmar.

These are tiny arachnids, measured about 2.5 millimeters body length, excluding the almost 3-millimeter-long tail. The chimerarachne yingi isn't an actual spider, but a distant relative that crawled around southeast Asia for millions of years.

Other species of insects, including millipedes and modern spiders, were also found alongside the four chimaera fossils.

Those with arachnophobia might want to look away now, as scientists have unearthed an incredible 100m-year-old spider that actually had a tail.

However, the 100 million-year-old spider fossils could change some of the theories regarding the evolution of the spiders.

The creature, which has been named Chimerarachne yingi, boasts a odd mix of features that we see on modern-day arachnids.

"In the last few years, this kind of amber has become much more available and because of its age - it's a hundred million years old or older - it lets us see really far back into the past", he said.

For now, researchers disagree on the exact placement of the part-spider, part-scorpion critter. And it's not known what the tail would have been used for or if the spider was venomous.

They introduced their discovery, dubbed Chimerarachne yingi, in a pair of papers published Monday in the journal Nature Ecology and Evolution. Spiders use silk for a variety of reasons, including augmenting their living space and protecting their eggs.

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.

Palaeontologist Professor Paul Selden, of Kansas University, said: "We can only speculate that, because it was trapped in amber, we assume it was living on or around tree trunks".

"Taken together, Chimerarachne has a unique body plan among the arachnids and raises important questions about what an early spider looked like, and how the spinnerets and pedipalp organ may have evolved".

"When you find the missing link, you just create two new gaps where previously, there was one".

Very much like today's black widows and huntsman spiders, C. yingi had silk-producing spinnerets.

The dorsal view of the spider encased in amber.

Prof Selden said: "There's been a lot of amber being produced from northern Myanmar and its interest stepped up about 10 years ago when it was discovered this amber was mid-Cretaceous".