A tiny arachnid discovered in 100-million-year-old amber looks just like a spider, except for one thing: its whip-like tail.

Key points: A new arachnid with a tail and spinnerets has been discovered in 100-million-year-old amber

A new arachnid with a tail and spinnerets has been discovered in 100-million-year-old amber The discovery provides insights into the evolution of spiders and web spinning

The discovery provides insights into the evolution of spiders and web spinning Ancient arachnids had long tails but did not have spinnerets

Ancient arachnids had long tails but did not have spinnerets Modern spiders have spinnerets but do not have tails

The amazing fossil fills a critical gap in the arachnid family tree between today's spiders and spider-like arachnids that lived before the dinosaurs, say two international teams of scientists.

They have described four male fossil arachnids exquisitely preserved in amber sourced from northern Myanmar in separate papers in the journal Nature Ecology and Evolution.

"[The new fossils] are like the missing link from older animals and modern spiders," said Professor Paul Selden, an arachnid expert from the University of Kansas and co-author of one of the papers.

Spiders have appendages on their abdomen called spinnerets, which are used to make webs.

The new species, christened Chimerarachne yingi — or 'chimaera spider' — has fangs, male pedipalps used to transfer sperm, four sets of legs, and spinnerets just like spiders.

The long whip-like tail may have been used like an antennae. ( Supplied: Bo Wang )

But its 3mm-long tail, which extends beyond its 2.5mm body, is a throwback to arachnids such as Attercopus, an extinct creature that lived between 200- and 300 million years ago.

Ancient Attercopus, which was not a spider, had a long tail and hairs called spigots involved in silk production on its abdomen, but it did not have spinnerets.

Professor Selden, who helped discover Attercopus 30 years ago, predicted that long-tailed arachnids developed spinnerets further down the line.

"Now we've got this creature which looks just like that," he said.

"They have spinnerets, but still have the tail."

Silk spinning and tails

We don't know much about the life of this tiny arachnid, but it likely lived in the bark of the resin producing trees, Professor Selden said.

He said it may have used its long tail, which is found in other arachnids such as whip scorpions, to sense its environment like an antennae.

But at some stage modern spiders ditched the tail.

"[Today] modern spiders tend to use silk for sensing," he said.

A close-up shows the fossil had four spinnerets on its abdomen like some primitive spiders today. ( Supplied: Diying Huang )

Even though the new animal had spinnerets, it may not have woven a web like a spider, said Diying Huang, who led the other team.

"More likely [it secreted] silk on the the ground for an early warning function like some recent relatives," said Professor Huang from the Nanjing Institute of Geology and Palaeontology in China.

According to his team, the silk-producing anatomy of the new species resembles that of a primitive family of trapdoor spiders found in South-East Asia known as Liphistiidae.

Professor Selden said spinnerets are used for a whole range of reasons such as shoring up burrows, leaving a silk trail to help the arachnid find its way home, wrapping eggs and attaching sperm.

Surprisingly young fossils

Exactly where the new arachnid fits into the family tree is up for debate.

While Professor Selden's team, which studied two of the fossils, thought it could be one of the first true spiders (Aranaeae), Professor Huang's team put it in the same category as ancient arachnids with tails like Attercopus (Uraraneida).

The fossils, which come from amber deposited in the mid Cretaceous period, are 170 million years younger than the other long-tailed arachnids.

That makes them surprisingly young, Professor Selden said.

Is it an ancient arachnid or an early modern spider? ( Supplied: Bo Wang )

Alongside the chimaera arachnids are a host of insects, millipedes and modern spiders.

"They look like these older creatures so it's rather a surprise to see them alongside spiders," Professor Selden said.

"You've effectively got the typically South-East Asian tropical rainforest, but with these sort of living fossils in with them.

"And in fact there's nothing to say that they're still not living there today for what we know because [the region] is not a well studied area."

Robert Raven, a leading spider researcher at the Queensland Museum, said he was "blown away" by the "absolutely fantastic discovery".

"We haven't had [a discovery] like this for a long, long time," said Dr Raven, who was not involved in the research.

"The textbooks have to be rewritten. And [the fossil] is so young."

He thought the presence of spinnerets pulled it to the side of spiders.

"Spinnerets are spiders so all of a sudden we've truly got the missing link," he said.

"The crazy thing is that the most primitive spiders still live on the planet, not far from where these things were found in South-East Asia."