INDIRECT evidence indicates that scorpions, which first appeared in the middle Silurian, were originally aquatic organisms like their eurypterid relatives1. Living scorpions have four pairs of book-lungs, each pair situated above a sternite on the ventral surface of the mesosoma (anterior abdomen) and each book-lung opening to the outside through a stigma which perforates the sternite. By contrast, most Palaeozoic scorpions had five abdominal plates, homologues of abdominal appendages, which were apparently sutured onto the body wall only along their anterior edges. It has been suggested that there were gills above the abdominal plates2,3and that all scorpions with abdominal plates were aquatic and respired through gills2,4. The only good example, however, of a Palaeozoic scorpion with gill-like structures preserved is the Lower Devonian Waeringoscorpio 5,6. Portions of book-lungs have now been discovered in two specimens of a fossil scorpion with abdominal plates from a Lower Carboniferous limestone in Scotland, providing the first direct evidence of book-lungs and also the earliest evidence of air-breathing in a Palaeozoic scorpion. Unlike Recent scorpions, the fossil lung lamellae have ribs of thicker cuticle along their posterior margins, supporting the homology of these structures with the Limulus book-gill3,7. As Silurian and Devonian scorpions were aquatic1,2 the presence of book-lungs in a Carboniferous scorpion indicates that the transition from aquatic to terrestrial environments was achieved by the direct transformation of book-gills into book-lungs.