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A fossil daddy long legs has been discovered with a 99-million-year erection after it died while having sex with a female.

The ancient arachnid was fully aroused when it became trapped in oozing resin which entombed its body.

The resin later turned to amber, preserving the harvestman's impressive member for posterity.

Found in Burma, the fossil is the first to capture such a penis and is among the best preserved ever discovered.

The creature, called Halitherses grimaldii, sported an organ that grew to nearly half its body length when erect.

Unlike male spiders and scorpions, which use modified legs to transfer sperm to females in packets, it inserted its penis into a genital opening next to the female's mouth.

The discovery was reported last Thursday in the journal The Science of Nature.

Careful 3-D scans and photographs of the fossil show that its penis's distinctive shape, which has a heart-shaped head to its twirled tip, is different from those of other species - placing it in its own previously undiscovered family.

(Image: Getty Images)

Study leader Jason Dunlop, of the Berlin Museum for Natural History, told National Geographic said the fossil raised questions about why no female was found beside it.

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He speculated the arachnid might have been cruelly separated from its mate, or the giant erection might have been triggered by its death throes.

He said: "It might be the case that the animal was struggling as it was trapped in the tree resin and that this caused the blood pressure to shoot up and the penis to become squeezed out accidentally."

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