Image caption The legs would have spanned up to 15cm, front to back (scale-bar: 5mm)

Scientists say a fossilised spider from the Inner Mongolian region of China is the biggest yet found.

The female, which lived about 165 million years ago, belongs to a collection of spiders well known today - the golden orb weavers.

These creatures make webs from a very tough and distinctively golden silk.

The researchers tell the journal Biology Letters that Nephila jurassica, as they have called their specimen, would have had a leg span of some 15cm.

"She is the largest known fossil spider," said Professor Paul Selden from the University of Kansas, US.

"Her body is not the biggest, but if you add in her long legs then she's the largest," he told BBC News.

Today's Nephila species are found around the globe in tropical and sub-tropical regions.

Until this new fossil turned up in Inner Mongolia, the most ancient example from this grouping, or genus, was about 35 million years old.

So, this discovery pushes the existence of the Nephila back to the Jurassic Period, making them the longest ranging spider genus known.

No-one can say for sure how this particular arachnid met her end, but she may have succumbed to a natural catastrophe.

The spider was encased in volcanic ash at the bottom of what would have been a lake. Perhaps the ash fall from an eruption pulled her from her web and smothered her. Whatever the circumstances of the spider's end, the preservation of detail today is exquisite.

"You see not just the hairs on the legs but little things like the trichobothria which are very, very fine. They're used to detect air vibrations. There's a very distinct group of them and they're a very distinct size which is typical of this genus, Nephila," Professor Selden explained.

Image caption A modern female golden orb weaver with a small male in close attendance

Nephila females today weave some of the largest orb webs known, up to 1.5m in diameter. The great prowess of the females stands in stark contrast to the rather diminutive males of the genus. Their small form makes the females look like giants.

This disparity in size is an example of what biologists refer to as extreme sexual dimorphism.

Professor Selden and his colleagues are keen to find out whether this characteristic holds true for the ancient Nephila, too.

"The previous oldest Nephilid is a male from the Cretaceous Period found in Spain. That male is normal sized, whereas in the present day the females are giants," the Gulf-Hedberg distinguished professor of invertebrate paleontology at Kansas said.

"So, it looks like we may have this dimorphism going back this great length of time. We'd like to find a male in the deposit to confirm this. All the evidence would suggest the male would be normal size, but we haven't yet located one."

Jonathan.Amos-INTERNET@bbc.co.uk