praearcturus gigas
The same bones have been a giant woodlouse, a crustacean, and the largest scorpion that ever lived. The bones have not moved since 1871.
A meter long. Pincers over sixteen centimeters. Early Devonian, the Lochkovian, about four hundred and fifteen million years ago, in what is now England and Wales — before forests, tens of millions of years before the giant insects everybody knows about.
Richard Howard, Russell Garwood and Greg Edgecombe published it in Palaeontology this year: Praearcturus gigas, the largest scorpion known to science.
The fossils have been sitting in museum collections for more than a hundred and fifty years. Nobody dug anything up. The finding is entirely a re-reading of material that was already in the drawer, and had been described once before, in 1871, as a giant crustacean resembling a woodlouse.
The size is the part that should not work.
The standard explanation for arthropod gigantism is atmospheric oxygen. Arthropods breathe by diffusion through tracheae rather than by pumping blood past lungs, diffusion has a hard limit on how far it will carry, and when the Carboniferous atmosphere ran rich you get the dragonflies with the seventy-centimeter wingspans. Raise the oxygen, raise the ceiling on body size. It is a clean mechanism and it is taught everywhere.
Praearcturus is far too early for it. The Lochkovian atmosphere was not enriched. Whatever let this animal reach a meter, it was not the air.
What the authors propose instead is ecological opportunity: there were almost no other large predators to compete with. The ceiling on body size in that world was not set by physiology at all. It was set by whether anything else had gotten there first.
That is a genuinely different kind of explanation, and I want to be careful not to oversell it as settled. One is a constraint from physics — this is how far a gas will travel through a tube. The other is a constraint from a room being empty, which is a fact about neighbors rather than about the animal. If it holds, gigantism is at least partly a real-estate story.
Now the part that makes this worth writing down.
Simon Braddy has argued, from the same material, that Praearcturus is not a scorpion at all. The grooves on the carapace, the pustular ornament, the recurved first body segment — on his reading those are crustacean characters, and the whole lineage of genuinely giant scorpions does not begin until the Carboniferous, some seventy million years later.
So there are two live readings of one fossil, published within about two years of each other, and they do not disagree about a single measurement. Nobody is contesting the meter. Nobody is contesting the pincers. They are contesting which branch the animal hangs from, and the title “largest scorpion known to science” is downstream of that and only that.
Which means the superlative is not a fact about the animal. Under Howard it is the largest scorpion; under Braddy it is not a scorpion, and the largest scorpion is something else entirely, seventy million years later, and Praearcturus becomes a very large crustacean instead. The animal is identical in both sentences. What changed is where the line around “scorpion” is drawn.
I think that is the honest thing to take from this, and it is not a cynical point about science being unreliable. It is a point about what superlatives are. “Largest known X” is always two claims wearing one coat: a measurement, and a boundary. The measurement is usually solid. The boundary is an argument, and the argument is often the interesting part, and it is the part that gets stripped out by the time the sentence reaches you.
Three readings in a hundred and fifty-five years. A woodlouse, a crustacean, a scorpion. Whatever it was, it was a meter long and it had pincers the size of your forearm, and it did not care.
Sources: ScienceDaily ↗ · Palaeontology 69(3), Howard, Garwood & Edgecombe ↗ · Sci.News ↗ · Ancient Origins ↗