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The fossil record has yielded various gigantic arthropods, in contrast to their diminutive proportions today. The recent discovery of a 46 cm long claw (chelicera) of the pterygotid eurypterid (‘sea scorpion’) Jaekelopterus rhenaniae , from the Early Devonian Willwerath Lagerstätte of Germany, reveals that this form attained a body length of approximately 2.5 m—almost half a metre longer than previous estimates of the group, and the largest arthropod ever to have evolved. Gigantism in Late Palaeozoic arthropods is generally attributed to elevated atmospheric oxygen levels, but while this may be applicable to Carboniferous terrestrial taxa, gigantism among aquatic taxa is much more widespread and may be attributed to other extrinsic factors, including environmental resources, predation and competition. A phylogenetic analysis of the pterygotid clade reveals that Jaekelopterus is sister-taxon to the genus Acutiramus , and is among the most derived members of the pterygotids, in contrast to earlier suggestions.

Stylonurid eurypterids (Arthropoda: Chelicerata) include some of the largest known arthropods -bizarre sweep-feeding hibbertopterids from the Carboniferous to end-Permian. New material of Drepanopterus abonensis, a stylonurid from the Late Devonian (Famennian) of Portishead, south-west England, offers key insights into this genus and its affinities. A redescription utilising the new material enables D. abonensis to be assigned as basal member of the Superfamily Hibbertopteroidea, the large-sweep-feeding forms, possessing a cleft metastoma and blades (modified blunt spines) on their anterior prosomal appendages. D. abonensis also shares characters such as a clavate telson and median ridge on the carapace with the proposed hibbertopteroid sister group the Kokomopteroidea. Hibbertopteroid eurypterids are the most long-ranging stylonurids, surviving the decline and extinction of the other eurypterid families in the Late Devonian, their survival probably because of their sweep-feeding mode of life, which was not in direct competition with their eurypterine relatives and other predators.

The fossil remains of eurypterid cuticles in this study yield long-chain (ϽC 9 to C 22 ) aliphatic components similar to type II kerogen during pyrolysis-gas chromatography/mass spectrometry, in contrast to the chitin and protein that constitute the bulk of modern analogs. Structural analysis (thermochemolysis) of eurypterid cuticles reveals fatty acyl moieties (derived from lipids) of chain lengths C 7 to C 18 , with C 16 and C 18 components being the most abundant. The residue is immune to base hydrolysis, indicating a highly recalcitrant nature and suggesting that if ester linkages are present in the macromolecule, they are sterically protected. Some samples yield phenols and polyaromatic compounds, indicating a greater degree of aromatization, which correlates with higher thermal maturity as demonstrated by Raman spectroscopy. Analysis (including thermochemolysis) of the cuticle of modern scorpions and horseshoe crabs, living relatives of the eurypterids, shows that C 16 and C 18 fatty acyl moieties likewise dominate. If we assume that the original composition of the eurypterid cuticle is similar to that of living chelicerates, fossilization likely involves the incorporation of such lipids into an aliphatic polymer. Such a process of in situ polymerization accounts for the fossil record of eurypterids.