Happy birthday you prehistoric penis-worm.

Adam Spencer
16 hours ago
5 min read

One year on, the Grand Canyon's giant penis worm gets a sober second look. NerdNews asks ‘how does it measure up’?

(First published on my Substack where you can get #NerdNews, marvellous maths and general geekery.)

('One Hungry penis Worm': created by the author via Dall-E)

A year ago this week the internet melted down over a science story with bite. Giant prehistoric penis worms discovered in, where else, the Grand Canyon.

I mean you can’t be surprised that the science internet collectively lost its composure when the story broke of the uncovering of a 500-million-year-old penis worm.

Headlines, reddit threads and twitter posts throbbed, engorged full of puns, double-entendres and eye-watering references to "rings of razor-sharp teeth."

Of course there were Alien memes, Star Wars throws, and, I’m sure, a great many people typing "penis worm" into work computers whilst praying incognito really meant incognito

So a year on, metaphorically the morning after, NerdNews asks what did the scientists actually find down there? Was this really the evolutionary bombshell that some people claimed? Or should everyone put their pants back on and calm down?

The story has teeth.

Let’s make one thing clear. Prehistoric penis worms are a thing.

And we know that thanks to Cambridge PhD student Giovanni Mussini and colleagues, who floated down the Colorado River in a dinghy in 2023, chipped shale off the canyon walls, and headed back to the lab to dissolve it in acid and free any fossils locked inside.

They pulled out more than 1,500 small carbonaceous fossils (SCFs in the paleontology trade). Two-thirds of them, a remarkable 967 specimens belonged to a single new species christened Kraytdraco spectatus.

Star Wars fans hear will have a geeky little heart flutter as they recognise the name of the krayt dragon of Star Wars fame. Yes the enormous fanged reptile that lurks beneath the sand dunes of Tatooine has been immortalised. And fittingly so.

Kraytdraco’s calling card was an eversible pharynx. No that’s not a misprint for ‘reversible. Being eversible the throat it could turn inside out, exposing hundreds of teeth along its length. Sturdy outer teeth that probably scraped food off the seabed. Finer, feathery fangs on the inside, perfect perhaps for filtering smaller particles.

While the teeth themselves were microscopic (the order of 10 to 120 millionths of a metre) they are significant.

"These were cutting-edge technologies for their time, integrating multiple anatomical parts into high-powered feeding systems." — Giovanni Mussini, University of Cambridge.

So far so good. Fascinating science and a glimpse to a bridge from the ancient to modern world.

Smile for the camera.

(Adapted Mussini et al., "Evolutionary escalation in an exceptionally preserved Cambrian biota from the Grand Canyon," Science Advances 11(30), 2025, used under CC BY 4.0. DOI: 10.1126/sciadv.adv6383. Composite layout by NerdNews.) — (Adapted Mussini et al., "Evolutionary escalation in an exceptionally preserved Cambrian biota from the Grand Canyon," *Science Advances* 11(30), 2025, used under CC BY 4.0. DOI: 10.1126/sciadv.adv6383. Composite layout by NerdNews.)

Here we have the mouth of a penis worm, decoded. Down the middle is the throat itself, running from the front of the mouth (top) to deep inside (bottom), with the different tooth types colour-coded to show where each one sat. Flanking it, left and right, are the real fossils: the actual half-billion-year-old teeth, each no bigger than a tenth of a millimetre. Sturdy spikes near the front for raking food off the seabed, finer feathery combs deeper down for sifting it. This is the whole apparatus the worm turned inside out to feed.

Does size matter?

It’s actually not at all juvenile to wonder, about a penis worm, "how big was Kraytdraco?".

The problem is, it really seems to be a case of, depends who you asked.

The Smithsonian plumped for about an inch. CNN fattened this out to around 4 inches. Nat Geo got very excited and claimed 6-8.

So we’ve got one ancient animal but an eight-fold range of estimates as to exactly how long our penis worm stretched.

And to be completely honest, is anyone surprised that perhaps the length of the penis worm might have been … ahem … overestimated?

Yes size does matter … a bit.

see, nobody dug up a whole worm. Mussini and his team uncovered sub-millimetre to millimetre-scale fragments, mostly isolated teeth and small pieces of body wall. The full-body length is a reconstruction.

It didn't stop the coverage getting gloriously overheated, mind you.

"Among the most freakish: penis worms that turned their mouths inside out, revealing a throat lined with hairy teeth." — CNN, July 2025.

And it is also important because it turns out we still have penis worms today. Yep add that to your list of things I learned today!

We currently know of around 20 different species of PWs ranging in length from only a couple of millimetres to a near 40cm monster of a penis worm, named Halicryptus higginsi, found in 1999 off Point Barrow Alaska.

So any information about the dimensions of the ancient ancestor can help us, perhaps understand the evolutionary path from there to here. But to be honest, in that regard, knowing how it’s amazing teeth worked is much more illuminating.

Goldilocks gets a grilling.

Also more interesting than the size claims, were the deeper arguments made about where Kraytdraco used to hang. Was the Bright Angel seabed a “Goldilocks zone” for evolution? Did the rich spoils of oxygen, sunlight, nutrient abundance, water that was not too shallow, not too deep but ‘just right’ (hence Goldilocks) create conditions wherein something as radical as KD had a chance to develop such a pimped up set of gnashers?

This is more interesting and therefore more difficult to substantiate. With the sober window of a year to look back, it seems the experts are divided.

Karma Nanglu, a palaeontologist who had no hand in the work, made the awkward observation that loads of Cambrian sites are biodiverse, so proving this one was a hotspot of genuine evolutionary escalation, rather than just another rich patch that happened to fossilise unusually well, is genuinely hard.

"Whether or not it is uniquely escalated is pretty hard to validate," — Karma Nanglu.

Possibly best filed under ‘awesome theory that still needs some work’ at this stage.

("Goin Down to PW Town": created by the author via Dall-e)

🪱 A good hard look.

So where does that leave us, a year and a thousand sniggers later?

The worm was real, the teeth were spectacular, and the fossil-hunting was properly ingenious.

A half-billion-year-old worm with a retractable mouthful of microscopic knives, prised out of a canyon that millions of tourists stroll past every year, should stop you dead.

We just don't need to inflate it to enjoy it.

Which, when you think about it, is the one lesson a penis worm was always destined to teach us.

Chomp chomp.