Researchers have discovered the fossilised remains of a daddy longlegs from way back in the Cretaceous period - and the poor guy has been sporting a boner for the past 99 million years.

The half-a-millimetre penis, which Stephanie Pappas over at LiveScience describes as having "a heart-shaped tip and a bit of a twist at the end", was visibly erect when the daddy longlegs was encased in amber in Myanmar. And as humorous as it is, this rare penis sighting could teach scientists a whole lot about arachnids, and where this individual fits in the family tree.

While a lot of people think that daddy longlegs are spiders, they're actually an order of arachnids called Opiliones, or the harvestmen. And we don't know a whole lot about their evolution, given the fact that only 38 fossilised harvestmen have ever been found - and none of them had a visible penis for scientists to inspect.

That doesn't sound like a big deal, but the penis is actually a really useful tool for researchers to identify harvestman species with, seeing as they're so unique. In fact, most spiders don't even have penises, they have organs called 'pedipalps' instead, and some species, such as the orb-web spiders, can even detach their pedipalps to make a speedy getaway.

But harvestmen have an extendable penis more similar to mammals', which is why it's so rare to see it. "It was very surprising to see the genitals, as they are usually tucked away inside the harvestman's body," said lead researcher Jason Dunlop, a curator at the Museum für Naturkunde in Berlin.

Jason Dunlop et al., The Science of Nature

"These penis details (shape, form of the tip, etc.) are very important for saying where this amber species fits in the harvestman family tree," he explained. "In fact, we couldn't find an exact match in terms of penis shape with any living species."

The fossilised harvestman has now been classified as belonging to an extinct species called Halitherses grimaldii, but based on the morphology of its penis, researchers now believe this species belonged to a harvestmen family of its very own, which is sadly now extinct.

So onto the really important question - why was the harvestman so excited in the first place? It's possible that he was mating right before the sap hit him, and his female partner managed to escape, or maybe she simply wasn't preserved, as Pappas explains over at LiveScience.

Or maybe the poor guy was just really terrified about his impending doom. And we all know, fear can do strange things to the body.

"In harvestmen, the penis is sometimes pushed out by increasing blood pressure," said Dunlop. "Maybe as the animal struggled when it got caught in the sticky tree resin, its blood pressure rose and the penis was pushed out accidentally?"

Either way, we have to admit we feel bad for the guy. Imagine if your body survived 99 million years of global warming and cooling, two world wars, and all kinds of crazy societal change, just to end up being written about as the harvestman that "died with a stiffy". We feel terrible.

The research has been published in The Science of Nature.