The discovery at Rhynie, near Aberdeen, dates back about 400 million years.
The male and female private parts belonged to an ancestor of the modern daddy-long-legs.
Preserved within the male fossil was a penis two-thirds the length of its body and a female was also found to have a long egg-laying organ known as an ovipositor.
The fossils of the harvestmen arachnids were among many animals and plants trapped in silica deposited by hot springs between 400 and 412 million years ago.
"
The fossils have the oldest known
arachnid respiratory system, suggesting harvestmen's ancestors had long since crawled out of the sea and learned to breathe
"
New Scientist
Along with crane flies, harvestmen are popularly known as "daddy-long-legs".
Like spiders, they belong to the arachnid family, but do not spin webs.
New Scientist magazine said the ancient genitalia were very similar to those of modern day harvestmen.
Since their discovery in 1912, the silica formations have yielded a series of remarkably preserved insects and other arthropods.
A team led by Jason Dunlop from Humboldt University in Berlin will present the harvestmen data this week at a conference in Aberdeen.
New Scientist said: "As well as genitals, the fossils have the oldest known arachnid respiratory system, suggesting harvestmen's ancestors had long since crawled out of the sea and learned to breathe.
"Today's harvestmen also possess a very similar pair of large, branching
trachea."
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