Super-scorpion lifts lid on prehistoric creepy-crawlies

Ask people about the biggest animals that have lived on earth and they will probably name something with a backbone. But the discovery of a giant sea scorpion that was longer than a man demonstrates that it is not always necessary to have a spine to be big.

The sea scorpion's fossilised claw, unearthed in a quarry in Germany, reveals a beast that was around 2.5 metres (eight feet) long, showing that the invertebrates were able to grow just as big as some of the largest vertebrates.

Scientists estimated that the sea scorpion, Jaekelopterus rhenaniae, lived around 390 million years ago during a period when it was fairly commonplace for invertebrates to grow into large animals, said Simon Braddy, of the University of Bristol, yesterday, as news of the remarkable discovery spread round the world.

"We have known for some time that the fossil record yields monster millipedes, super-sized scorpions, colossal cockroaches and jumbo dragonflies but we never realised, until now, just how big some of these ancient creepy-crawlies were," Dr Braddy said.

A close relative of Jaekelopterus was the true scorpion Brontoscorpio which lived about 400 million years ago and grew about one metre in length. It was also aquatic but it crawled around on the seabed rather than swimming like Jaekelopterus.

The largest known land invertebrate was a 3m-long relative of the centipedes and millipedes known as Arthropleura, which evolved from crustacean-like ancestors during the Carboniferous period between 350 and 290 million years ago.

This heavily segmented creature lived on plant material and, as a vulnerable herbivore, it was protected by a heavily armoured exterior skeleton. Scientists who have analysed its fossilised jaw believe that it had a powerful bite.

The largest flying insect so far discovered was Meganeuropsis, a primitive dragonfly with a wingspan of 71 centimetres (28 inches), which lived about 250 million years ago, long before the evolution of the first flying vertebrates – the pterosaurs, or flying reptiles, and the birds.

About 50 million years before this dragonfly existed, the ancestors of the mayflies grew much bigger than today. One fossilised specimen called Bojophlebia unearthed in the Czech Republic had a wingspan of 45 centimetres.

One of the theories to explain gigantism in prehistoric invertebrates is that the concentration of oxygen in the air was higher than it is today. Animals that breathe through their skin, rather than using the efficient lungs and gills of vertebrates, were helped them to have larger bodies, according to the theory.

However, another important feature must have been the lack of vertebrate competitors. When insects learnt to fly, there were no other animals occupying that particular ecological niche, which is why the mayfly and the dragonfly could reach such large proportions.