Pulmonoscorpius is an extinct genus of scorpion from the Mississippian (Early Carboniferous) of Scotland. It contains a single named species, Pulmonoscorpius kirktonensis. It was one of the largest scorpions to have ever lived, with the largest known individual having an estimated length exceeding 70 cm (28 inches). Pulmonoscorpius retains several general arthropod features which are absent in modern scorpions, such as large lateral eyes and a lack of adaptations for a burrowing lifestyle. It was likely an active diurnal predator, and the presence of book lungs indicate that it was fully terrestrial.[1][2]
Fossils of Pulmonoscorpius kirktonensis have been found at the East Kirkton Quarry, West Lothian in Scotland. Rock layers exposed at the quarry date back to the Carboniferous, specifically the Viséan and Serpukhovian stages of the Mississippian (Early Carboniferous) subperiod, around 336.0 – 326.4 million years ago.[1] The name derives from "Latin pulmonis, a lung, and Greek skorpios, a scorpion."[1] The species name "kirktonensis" refers to the East Kirkton Quarry.
Pulmonoscorpius was described in 1994 based on 16 complete specimens and over 300 additional fragments from East Kirkton. In each specimen, only the outer layer of hyaline cuticle is preserved, estimated to only be 15-18 μm thick in the largest specimen. Scorpion cuticle is present in the East Kirkton Limestone (lower exposed unit) and Little Cliff Shale (middle exposed unit), but not the Geikie Tuff (upper exposed unit). Although cuticle could be found in a variety of shale and carbonate facies, it is most easily prepared out of finely-laminated limestone, which can be dissolved away with dilute hydrochloric acid while leaving the organic cuticle unharmed. Almost all material is completely flattened, so three-dimensional reconstructions are mostly hypothetical.[1]
The diet of Pulmonoscorpius is not known directly, but it is probable that it preyed on smaller arthropods, and small tetrapods (new arrivals).[3]
Most complete specimens were 13–280 mm (0.51–11.02 in) in length, while a large, fragmentary specimen is estimated to have been 70 cm (28 inches) long when alive. The only portions preserved were the outer portions of the cuticle.[1]
Pulmonoscorpius possess two pairs of eyes on the prosoma (the head and legs segment, also known as a cephalothorax). These include a pair of anterior-positioned median eyes (near the center of the prosoma) and a pair of compound lateral eyes (on the edge of the prosoma), with each lateral eye bearing between 40 and 60 lateral ocelli. In modern scorpions, lateral eyes are strongly reduced, but those of Pulmonoscorpius are large, similar to other basal scorpions and most other arthropods.[1] The prosoma is covered by a carapace (a large plate).