Pulmonoscorpius


Pulmonoscorpius is an extinct genus of scorpion from the Mississippian Era, discovered in 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. 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 indicates that it was fully terrestrial.

Discovery

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 stage of the Mississippian subperiod, around 335.9 – 330.3 million years ago. The genus name derives from Latin pulmonis, meaning "lung", and Ancient Greek σκορπίος , meaning "scorpion". The specific epithet 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 and Little Cliff Shale, but not the Geikie Tuff. 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.

Description

The diet of Pulmonoscorpius is not known directly, but it is probable that it preyed on smaller arthropods, and small tetrapods.
Most complete specimens were in length, while a large, fragmentary specimen is estimated to have been long when alive. The only portions preserved were the outer portions of the cuticle.

Dorsal surface

The prosoma of Pulmonoscorpius is covered by a carapace, where it possess two pairs of eyes. These include a pair of anterior-positioned median eyes and a pair of compound lateral eyes, 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.
The prosoma is followed by a mesosoma, which has seven segments each covered by a tergite. The surface of the carapace and tergites are relatively smooth in juveniles, and become tuberculated as individuals mature.
The metasoma has five segments, not counting the bulbous telson at the end. The last metasomal segment does not exceed the length of the preceding metasomal segment. Metasomal segments are boxy and ornamented by 4 pairs of carinae in adults. The strongest carinae are the dorsal pair, and superior lateral. Inferior lateral, and inferior median carinae are also present. Sexual dimorphism may be present in Pulmonoscorpius, as some specimens have wider metasomal segments.
In Pulmonoscorpius kirktonensis, the vesicle has a pair of strong carinae on its underside. One unique juvenile specimen is observed to lack these carinae, and may belong to a separate species.

Ventral structures

The coxae of each leg converge along the underside of the prosoma. The sternum is characteristically elongated, with a Y-shaped sulcus at the rear.
The underside of the mesosoma possesses a bilobed and laterally elongated genital operculum, followed by pectines with 150–160 narrow teeth, and finally ventral plates. In juveniles, the first ventral plate has a unique median lobation. As with modern scorpions, four pairs of book lungs were present at the corresponding ventral plates.

Appendages

Like other scorpions, Pulmonoscorpius has four pairs of walking legs as well as larger pincer-like pedipalps and smaller pincer-like chelicerae. One characteristic trait of Pulmonoscorpius is the presence of a long spur on each of the apophyses.
The chelicerae and legs were noted be to be similar to those of extant scorpions, except that the coxa and femur in legs 3 and 4 are less elongated, and the tibia of these legs are elongated to about the same length as their femur. In the pedipalps, the femur and patella are lined with carinae. Setae are sparse and clustered, particularly in larger individuals. Setae density is highest on the large finger of the pedipalps. One juvenile specimen has 30% more setae on its fixed finger than any other Pulmonoscorpius kirktonensis specimen. This specimen may belong to a separate species.
Aside from the type species, two other specimens were noted to possibly be distinct: one having juvenile pedipalp fingers with 30% more setal follicles than P. kirktonensis, and one lacking tuberculateventral carinae on the vesicle of its telson.