Ancient spider fossil discovered in the US was adorned with spiny armour

The newly discovered spider named Douglassarachne acanthopoda is described in a paper published in the Journal of Paleontology.

Spiders evolved more than 150 million years before the dinosaurs. The oldest known fossil of these creepy crawlies is an Attercopus fimbriungus specimen which is 380 million years old.

"D. acanthopoda comes from the famous Mazon Creek locality in Illinois and is about 308 million years old," says lead author of the new study Paul Selden from the University of Kansas and the Natural History Museum of London.

"This compact arachnid had a body length of about 1.5 cm and is characterized by its remarkably robust and spiny legs - quite unlike any other arachnid known, living or extinct," Selden adds.

D. acanthopoda lived during the latter stages of the Carboniferous period (359-299 million years ago). This period of Earth's history saw the evolution of the first lizards and is nicknamed the "Age of Amphibians."

Today's coal deposits are from the massive prehistoric forests which grew in the Carboniferous. Having oxygen levels about 50% greater than today meant the period also saw the emergence of massive arthropods, including a spider the size of a cat, a dragonfly with a 1-metre wingspan, and a millipede the length of a car.

While not quite so big, D. acanthopoda does provide a new insight into the bizarre animals of the time.

"Spiders were a rather rare group, only known at that time from primitive lineages, and they shared these ecosystems with various arachnids which have long since died out," says co-author Jason Dunlop from Berlin's Museum of Natural History.

"D. acanthopoda is a particularly impressive example of one of these extinct forms. The fossil's very spiny legs are reminiscent of some modern harvestmen, but its body plan is quite different from a harvestman or any other known arachnid group."

The palaeontologists concluded that the ancient spider belongs to a new group of arachnids.

"Unfortunately, details such as the mouth parts cannot be seen, which makes it difficult to say exactly which group of arachnids are its closest relatives," Selden says. "It could belong to a wider group, which includes spiders, whip spiders and whip scorpions.

"Whatever its evolutionary affinities, these appear to come from a time when arachnids were experimenting with a range of different body plans. Some of these later became extinct, perhaps during the so-called 'Carboniferous Rainforest Collapse,' a time shortly after the age of Mazon Creek when the coal forests began to fragment and die off."