Hibbertopterus

Hibbertopterus is a genus of eurypterid, a group of extinct aquatic arthropods. Fossils of Hibbertopterus have been discovered in deposits ranging from the Devonian period in Belgium, Scotland and the United States to the Carboniferous period in Scotland, Ireland, the Czech Republic and South Africa. The type species, H. scouleri, was first named as a species of the significantly different Eurypterus by Samuel Hibbert in 1836. The generic name Hibbertopterus, coined more than a century later, combines his name and the Greek word πτερόν (pteron) meaning "wing".

Hibbertopterus was the largest eurypterid within the stylonurine suborder, with the largest fossil specimens suggesting that H. scouleri could reach lengths around 180–200 centimetres (5.9–6.6 ft). Though this is significantly smaller than the largest eurypterid overall, Jaekelopterus, which could reach lengths of around 250 centimetres (8.2 ft), Hibbertopterus is likely to have been the heaviest due to its broad and compact body. Furthermore, trackway evidence indicates that the South African species H. wittebergensis might have reached lengths similar to Jaekelopterus.

Like many other stylonurine eurypterids, Hibbertopterus fed through a method called sweep-feeding. It used its specialised forward-facing appendages (limbs), equipped with several spines, to rake through the substrate of the environments in which it lived in search for small invertebrates to eat, which it could then push towards its mouth. Though long hypothesised, the fact that eurypterids were capable of terrestrial locomotion was definitely proven through the discovery of a fossil trackway made by Hibbertopterus in Scotland.^[1] The trackway showed that an animal measuring around 160 centimetres (5.2 ft) had slowly lumbered across a stretch of land, dragging its telson (the posteriormost division of its body) across the ground after it. How Hibbertopterus could survive on land, however briefly, is unknown but it might have been possible through either its gills being able to function in air as long as they were wet or by the animal possessing a dual respiratory system, theorised to have been present in at least some eurypterids.

Though sometimes, and often historically, treated as distinct genera, the hibbertopterid eurypterids Cyrtoctenus and Dunsopterus have been suggested to represent adult ontogenetic stages of Hibbertopterus.^[2][3][4] The features of fossils associated with these genera suggest that the sweep-feeding strategy of Hibbertopterus changed significantly over the course of its life, from simpler raking organs present in younger specimens to specialised comb-like organs capable of trapping prey (rather than simply pushing it towards the mouth) in adults.

Restoration of H. scouleri.

The size of H. scouleri, the largest species known from fossil remains, compared to a human.

Outdated 1872 reconstruction by Henry Woodward of H. scouleri with a telson and appendages based on those of Eurypterus (with swimming paddles and unspecialised walking appendages).

The two fossil fragments referred to Cyrtoctenus wrightianus.

Fossil tergites of H. scouleri, drawn by Henry Woodward in 1866–1878.

Restoration of Cyrtoctenus wittebergensis

Fossil spines of Dunsopterus stevensoni

More fossil spines and fossil ornamentation referred to Dunsopterus stevensoni

Fossil trackway discovered in Scotland, attributed to Hibbertopterus.