Discovery and name
East Kirkton Quarry, near Bathgate in West Lothian, had yielded very few fossils until 1984, when the collector Stan Wood discovered a previously unsuspected community of land animals in its limestone. Careful bed-by-bed excavations followed from 1985 to 1992, and the results were published together in one volume of the Transactions of the Royal Society of Edinburgh: Earth Sciences, dated 1993.
In that volume Andrew Jeram described the East Kirkton scorpions and named most of the material as a new genus and species, Pulmonoscorpius kirktonensis. The name is usually cited as Jeram 1994, although the journal volume is dated 1993. It combines the Latin pulmo, 'lung', with scorpius, 'scorpion', so it means 'lung scorpion'; kirktonensis means 'from Kirkton'.
Body and anatomy
Most of the fossils are fragments of cuticle, the tough outer skeleton, with rarer articulated specimens. When the cuticle is freed from the rock it is exquisitely preserved, so Jeram could describe these scorpions in more detail than any other Carboniferous scorpion fauna. The overall body plan was that of a scorpion: a pair of pincers, four pairs of walking legs, a broad body and a segmented tail ending in a sting.
The name records its most important feature: book lungs, stacks of thin air-breathing plates like those of living scorpions. Early fossil scorpions also had compound lateral eyes made of many small lenses, while living scorpions have only a few simple eyes on each side of the head. Pulmonoscorpius had large lateral eyes of this older kind. A detail used to classify it is the position of its last pair of leg bases, which touch the small breastplate on its underside.
How it lived
Book lungs mean Pulmonoscorpius breathed air and lived on land, not in water. That matters, because the same deposit also contains rare fragments of an aquatic scorpion, a reminder that not all scorpions of this age had moved onto land.
No fossil shows what Pulmonoscorpius ate. Every living scorpion is a predator, so palaeontologists reasonably assume it was too. An animal of its size could have tackled millipedes, harvestmen and other arthropods, and possibly small four-legged vertebrates, but this is inference, not evidence. Jeram found that most specimens are juveniles, so young scorpions were far more common in the deposit than giant adults.
Its world
East Kirkton was a shallow lake set among volcanic cones a few hundred metres high, surrounded by a fire-prone forest of seed plants. The lake water had unusual chemistry, and at times it was heated by local hot springs; both factors kept out most normal aquatic life. The lake existed for only a few tens of thousands of years before it filled with volcanic ash. Uranium-lead dating published in 2025 placed the fossil beds at 341 million years old, give or take 3 million years, in the early to middle Visean Age, older than previously thought.
Its neighbours included some of the earliest land-living tetrapods: Westlothiana, Balanerpeton, Eucritta, Silvanerpeton, the legless Ophiderpeton and Kirktonecta. Large eurypterids such as Hibbertopterus, early harvestmen and millipedes lived there too. High oxygen levels in the Late Palaeozoic air are often linked with giant land arthropods of the Carboniferous, but how high oxygen was at East Kirkton's time, and whether it affected this scorpion's size, has not been measured.
Size and records
Most Pulmonoscorpius fossils come from young animals. The largest individual is known from fragments and has been estimated at about 70 cm (28 in) long, roughly the length of an adult's arm, or about 40 per cent of the height of a 1.8 m (5 ft 11 in) person. That makes it one of the largest scorpions known.
Because the estimate is scaled up from pieces, it should be treated as approximate. The juvenile specimens, which are much smaller and more complete, give a clearer picture of the body shape but not of its maximum size.
Relatives and legacy
Pulmonoscorpius belongs to an extinct group of early scorpions that Jeram placed in the infraorder Mesoscorpionina. Within that group, it belongs to a set of forms with the same leg-base arrangement that ranged from the Late Devonian to the Late Carboniferous. Its closest living relatives are modern scorpions, although exactly how Carboniferous scorpions relate to living families is still studied.
Over time, scorpions lost the many-lensed compound side eyes seen in early forms like Pulmonoscorpius, keeping just a few simple eyes. Its home, East Kirkton, is now one of the most important sites for understanding how animals, including early tetrapods, first built communities on land.
Myths and misconceptions
Pulmonoscorpius is often confused with the sea scorpions (eurypterids). It was a true scorpion that breathed air, while the eurypterids found beside it at East Kirkton, such as Hibbertopterus, belong to a different group. It is also sometimes pictured stalking through a steaming swamp. The East Kirkton lake sat in a forest of seed plants among volcanic hills, and hot springs heated the water only at times.
Many websites state that it preyed on early tetrapods. That is possible, but it is a guess based on its size and on the fact that tetrapods lived nearby; no fossil records such an attack. Its size is also often credited to high oxygen, which is plausible but has not been tested for this animal.
