When is a spider not a spider? When it's 305 million years old and can't spin its own silk.
Scientists in the UK have discovered the oldest spider relative yet, dating to well before the age of the dinosaurs.
Found in a cave in France, the remarkably well-preserved Idmonarachne brasieri fossil had eight legs and could secrete silk, just like modern spiders.
But it lacked spinnerets -- appendages on the abdomen that allow them to fashion silk into webs -- meaning it's not a true spider, according to scientists at the University of Manchester.
Other non-spider arachnids from the period, the late Carboniferous, have flagellum -- tail-like appendages at the back -- which Idmonarachne brasieri also lacks.
The evolution of spinnerets is now seen as crucial to spiders' eventual evolutionary success, with now more than 45,000 different species on every continent, barring Antarctica.
"Spinnerets significantly postdate the origin of silk... This innovation could be a key to the spiders' success," the study notes.
The fossil was found preserved in iron-carbon mineral rock, allowing scientists to analyse it with 3D scanners.
The research was published today in journal Proceedings of the Royal Society B.