A recent discovery, reported in the journal Biology Letters on 21 November 2007 by scientists from Bristol (UK), Mainz (Germany) and New Haven (USA), has established that the extinct sea scorpions, known as eurypterids, were probably the largest arthropods ever to have lived.
Arthropods are invertebrates that also include the insects, spiders, crustaceans and trilobites.
The group reports the finding of a large fossil claw of the eurypterid species Jaekelopterus rheniae, a sea scorpion, which is about 400 million years old. The claw is a whopping 46cm long. By comparing the claw with smaller complete eurypterids, the team have established that the beast would have been about 2.5m long from the tip of its head to its tail, not including the length of the articulated arms that the claws were attached to.
Dr Andrew Ross from the Palaeontology Department at the Natural History Museum, says, ‘there has long been debate whether the eurypterids or Arthropleura were the largest arthropods that ever lived. But this new finding suggests that eurypterids were the largest arthropods to have evolved.’
It has been proposed that high oxygen levels during the Carboniferous Period (about 300 million years ago) would have led to gigantism in arthropods living on land in the coal swamps. The extinct Carboniferious millipede Arthropleura could reach up to 2m in length. Arthropleura is heavily armoured so probably grew large to avoid being eaten by amphibians. However, the giants were the exception, not the rule.
Oxygen levels would not have influenced the giant eurypterids such as this sea scorpion, which lived in water 100 million years before the giant millipedes evolved. Predation pressures probably led to their huge size in this case.
According to Dr Ross, ‘the bigger you are, the less likely that you are going to be eaten by something bigger than you. Also, when you are living in water you are not as heavy relatively as you are in air, so aquatic eurypterids did not have the same challenges as their cousins, the true scorpions, which crawled out to live on land.’
The largest complete eurypterid at the Natural History Museum is 1.1m long and is on display in From the Beginning.