When David Siveter examined rocks collected from rural west England in the 1990s, he didn't expect to find the world's oldest-known penis.
When the University of Leicester palaeontologist and his colleagues created a 3D model of the fossilised remains of a tiny 1mm-long crustacean they saw what he calls "a very long prolongation coming from the posterior part of the body region".
The discovery was possible because the crustacean, called an ostracod, was completely buried by fine volcanic ash some 425 million years ago. As the ash hardened, it preserved the ostracod's body shape and all its appendages exceptionally well.
"That phrase 'exceptional preservation' means a lot to palaeontologists because it means there is soft tissue preserved as well as the hard parts of the animal," Professor Siveter tells ABC RN's What The Duck!? podcast.
"This fossil had all the appendages intact. We had the gills that the animal used to breathe. We had the eyes and we had the penis, which was lovely, because it's very rare that you can [identify the sex of] fossils."
Professor Siveter and his colleagues unveiled the phallic find in the journal Science in 2003.
The previous record holder for oldest-known penis (and female genitals) belonged to harvestman arachnids (also known as daddy-long-legs) that lived 400 million years ago. The males were well endowed, with a penis two-thirds the length of their body.
An ostracod's penis can be relatively big too; up to a third of the animal's total volume. It's why the fossilised creature was dubbed Colymbosathon ecplecticos, which is Greek for "outstanding swimmer with large penis".
Their sperm can also be large for their size. Some ostracods, which are only around a millimetre long, produce sperm that is 10mm long, Professor Siveter says.
To ejaculate, ostracods need help from special organs, he adds: "They're called 'Zenker's organs', and they are used to pump these huge sperms out of the body into the female."
So that's the earliest-known penis. What about the earliest-known erection?
A 99-million-year erection
The harvestman may have been knocked off the oldest penis perch, but it does have the honour of having the oldest-known erection.
An ancient harvestman (Halitherses grimaldii) was encased in golden amber about 99 million years ago during the Cretaceous period in what is now Myanmar.
Its penis had a "slender, distally flattened" shaft with a "heart-shaped" head. The part where the sperm would be ejaculated, called the stylus, was "twisted at the tip", the study's authors wrote in The Science of Nature in 2016.
US scientist and author Emily Willingham says the amber preserved the ancient animal while its curved penis was "hydraulically raised".
"It's this tiny little erection. It's like a millimetre long.
"You could tell it was specialised for its purpose."
Unlike many spiders, which use modified legs to deliver sperm to females, modern harvestmen have a penis that delivers sperm inside a female's body through an opening near her mouth.
They usually tuck their penis away most of the time, and only extend it for mating.
So when did penises evolve?
Scientists don't know exactly when the first penis evolved, but they can trace its ETA in Earth's history.
"The Earth is about four-and-half billion years old, and life started relatively quickly at around four billion years [ago]," Emily Mitchell, a zoologist at the University of Cambridge, says.
Those first living things, organisms comprising a single cell, reproduced by splitting in two.
"It's only when you get the evolution of animals around 600 million years ago … that you start getting a huge diversity in terms of reproduction," Dr Mitchell says.
But the fossil record is sparse when it comes to early examples.
Even with whole animals being fossilised, it was extremely rare to find their reproductive parts preserved too — especially the soft, squishy bits.
For this to happen, an organism must be enclosed by ash or sand or sap before its soft parts decay, but not so much that it gets squashed as it's being covered.
And finding animals in the act? That's even rarer, according to Marissa Betts, a geologist and palaeontologist at the University of New England.
"For reproduction to be captured in the fossil record is a very unique circumstance," Dr Betts says.
Check out What the Duck?!'s new series Sex is Weird to find out more about the oldest-known penis, and subscribe to the podcast for more.
