Image caption Most of the found species are completely new to science

More than 700 new species of ancient insect have been discovered in 50-million-year-old amber.

The discoveries come from some 150kg of amber produced by an ancient rainforest in India.

Scientists say in the journal PNAS that many insects are related to species from far-away corners of the world.

This means that, despite millions of years in isolation in the ocean, the region was a lot more biologically diverse than previously believed.

The amber, dubbed Cambay amber, was found in lignite mines in the Cambay Shale of the Indian state of Gujarat.

Jes Rust from the University of Bonn in Germany led an international team of researchers from India, Germany and the US.

According to a predominant theory of continents' formation, at first there were only two so-called supercontinents on Earth. The one in the north was called Laurasia and the other one, located more towards the south, Gondwana.

Drifting away

When Gondwana split up into several smaller pieces in the mid-Jurassic, some 160 million years ago, most of its parts stayed in the southern hemisphere, but one started drifting towards the north.

We have complete, three-dimensionally preserved specimens that are 52 million years old and you can handle them almost like living ones Jes Rust, University of Bonn, Germany

Having shifted for at least 100 million years at a remarkable rate of 15-25cm per year, the plate eventually collided with Asia and became what we know today as the Indian subcontinent. In the process, the Himalayas were formed.

It has long been believed that drifting in complete isolation would have contributed to a potentially unique plant and animal life, found only in the region.

But the mostly tropical climate of India is known to be unfavourable to the preservation of fossils and not much has been found to confirm this hypothesis of what biologists call "endemism". But the present study says the vertebrate fossil record discovered so far reveals little endemism.

Most of the recently discovered bugs also show links to modern insects as well as those that lived millions of years ago in different parts of the world, including Asia, Australia, and even South America.

The lead author Dr Rust told BBC News that this could be explained by land-bridge connections - possibly small islands that formed before the collision with Asia, in the Eocene - between the Indian "ferry" and other landmasses.

"It is possible for plants to drift hundreds of kilometres on open ocean currents, and in the case of insects, some can fly," said Dr Rust.

Image caption The researchers found the amber at the Vastan and Tadkeshwar Lignite mines in western India

"There are those that are only able to fly during mating, but they can fly at least a few kilometres.

"Not many are able to cross open seaways, but [they can] drift with plant material. Then there are also very tiny insects and they sometimes simply get blown away, up to the jet stream."

Rainforest's age

The study says the resin that later became Cambay amber originated from an ancient tropical rainforest.

"The Indian amber is from the Lower Eocene and was likely produced by flowering hardwood trees called Dipterocarpaceae, [trees] that predominate in the forests of southeast Asia today," Paul Nascimbene of the American Museum of Natural History in New York, told BBC News.

To determine where the amber came from, the scientists chemically fingerprinted it.

They also analysed the wood anatomy of fossilised branches and trunks on the site.

What we have here from India is the earliest fossil evidence of a modern type of tropical rainforest in Asia David Grimaldi , American Museum of Natural History

"Fossil wood samples were also recovered in association with the amber, [and they] showed that these samples preserved details of the wood's microstructure, pointing to dipterocarps as the probable source," added Dr Nascimbene.

The team also said that it was able to determine the age of the modern rainforest.

Up until now, many experts used to suggest that this type of tropical rainforest, found today all over the southeast Asia, first originated in the Miocene some 20 or 25 million years ago.

But the recent discovery challenged that idea.

David Grimaldi from the American Museum of Natural History and another co-author told BBC News that the rainforest is at least 60 million years old.

"What we have here from India is the earliest fossil evidence of a modern type of tropical rainforest [of the Dipterocarpaceae family] in Asia," he said.

"Before, we just had no idea to how ancient the dipterocarp forests that occur in southeast Asia today really are; there really was no indication."

Dr Grimaldi explained that one problem with determining the modern rainforest's age was the lack of information - fossil deposits are simply very uncommon in tropical regions.

Image caption Amber looks like yellow-brownish rocks

"Most of the fossil outcrops are in drier, very eroded areas of the northern hemisphere or southern parts of southern hemisphere, but not so much in the equatorial belt.

"Most of the fossil evidence from tropical South America indicated that rainforests were no later than Miocene, no more than 25 million years old.

"And secondly, people for a century or more had always thought of the tropics as a place where species are evolving very rapidly.

"And perhaps as a result they thought it was a very recent type of ecosystem.

"But in reality, they're like an ancient cauldron - they're very ancient ecosystems, at least ancient on land, at least twice [the age we previously thought]."

3-D specimens

But besides the rainforest's age and India's biogeography, the most astonishing part of the discovery was the huge number of perfectly preserved specimens of insects, most of which have never been seen before.

Unlike other types of amber found in deposits in the north, the Indian amber is much softer. This unique property allowed the scientists to completely dissolve the amber using solvents - toluene and chloroform - and extract the ancient insects, plants and fungi.

"We have complete, three-dimensionally preserved specimens that are 52 million years old and you can handle them almost like living ones," said Dr Rust.

"Of course they are very fragile, but it is still astonishing.

"We have several examples where it is possible to get a complete specimen out. And of course this opens a new dimension in investigations of this material.

Image caption Many insects are related to modern and ancient species found in different parts of the world

The researcher said that this amber deposit was the first important one found in India.

Though this natural yellow-brownish substance is quite widespread all over the world, the best-known amber deposits are in the Dominican Republic, Mexico and the Baltic region, where some 80% of the world's known amber is found.

"There are tonnes of amber [in this Indian deposit], and what is interesting about it is that it was produced in the tropics, the most highly diverse areas in respect to species diversity," said Dr Rust.

"And the fossil record of the terrestrial tropics is not so good, because usually all the organic material gets rotten very quickly."

With tonnes of amber at their disposal, the researcher said his team hoped to uncover many more secrets of the peculiar world that existed millions of years ago.