Hibbertopterus is a genus of eurypterid, a group of extinct marine 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. 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. 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.
Description
Like other known hibbertopterid eurypterids, Hibbertopterus was a large, broad-bodied and heavy animal. It was the largest known eurypterid of the suborder Stylonurina, composed of those eurypterids that lacked swimming paddles. A carapace (the part of the exoskeleton which covered the head) referred to the species H. scouleri, from Carboniferous Scotland, measures 65 centimetres (26 in) wide. Since Hibbertopterus was unusually wide relative to its length for a eurypterid, the animal in question would probably have measured around 180–200 centimetres (5.9–6.6 ft) in length. Even though there were eurypterids of greater length (such as Jaekelopterus and Carcinosoma), Hibbertopterus was very deep-bodied and compact in comparison to other eurypterids and the mass of the specimen in question would likely have rivalled that of other giant eurypterids (and other giant arthropods), if not surpassed them. In addition to fossil finds of large specimens, fossil trackways attributed to the species H. wittebergensis from South Africa indicates an animal around 250 centimetres (8.2 ft) in length (the same size attributed to the largest known eurypterid, Jaekelopterus), though the largest known fossil specimens of the species only appear to have reached lengths of 135 centimetres (4.43 ft).
The forward-facing appendages (limbs) of Hibbertopterus (pairs 2, 3 and 4) were specialised for gathering food. The distal podomeres (leg segments) of these three pairs of limbs were covered with long spines, and the end of each limb was covered with sensory organs. These adaptations suggest that Hibbertopterus, like other hibbertopterids, would have fed by a method referred to as sweep-feeding, using its limbs to sweep through the substrate of its environment in search for food. The fourth pair of appendages, though used in feeding like the second and third pairs, was also used for locomotion and the two final pairs of legs (pairs five and six overall) were solely locomotory. As such, Hibbertopterus would have used a hexapodal (six-legged) gait. Although not enough fossil material is known of the other hibbertopterid eurypterids to discuss the differences between them with full confidence, Hibbertopterus is defined based on a collection of definite characteristics. The telson (the posteriormost division of the body) was hastate (e.g. shaped like a gladius, a Roman sword) and had a keel running down the middle, with in turn had a small indentation in its own centre. The walking legs of Hibbertopterus had extensions at their base and lacked longitudinal posterior grooves in all of its podomeres (leg segments). Some of these characteristics, in particular the shape of the telson, are thought to have been shared by other hibbertopterids, which are much less well preserved than Hibbertopterus itself.
Table of species The status of the 10 species listed below follow a 2018 survey by German paleontologists Jason A. Dunlop and Denise Jekel and British paleontologist David Penney and size- and temporal ranges follow a 2009 study by American paleontologists James Lamsdell and Simon J. Braddy unless otherwise noted. The distinguishing features of H. caledonicus, H. dewalquei, H. ostraviensis and H. peachi follow the 1968 description of these species. The descriptors, Norwegian paleontologist Leif Størmer and British paleontologist Charles D. Waterston, did not consider these species to represent eurypterids, though any emended diagnosis of them is yet to be published.
History of research
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