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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.

The late Paleogene to early Neogene salamander Chelotriton paradoxus is redescribed on the basis of new and excellent material from two deposits, the late Oligocene Enspel locality and the Miocene Randeck Maar. C. paradoxus is characterised by the following features: (1) skull outline broad and parabolic, (2) snout abbreviated, with nasal only half the length of the frontal, (3) quadratojugal with a series of 3-5 spikes on the lateral margin, and (4) osteoderms larger than in other genera. Like extant Echinotriton, its trunk ribs bear extensive spikes (epipleural processes), with an elongated one on the 3rd rib. The Enspel sample of C. paradoxus shows greater variation in the size and number of quadratojugal spikes, whereas the best-preserved Randeck specimen bears exceptionally tall tubercles on all dermal bones and osteoderms and has a larger quadratojugal. Based on preliminary taphonomical data, we suggest that C. paradoxus probably led a predominantly aquatic life.

Pterygotid eurypterids have traditionally been interpreted as active, high-level, visual predators; however, recent studies of the visual system and cheliceral morphology of the pterygotid Acutiramus contradict this interpretation. Here, we report similar analyses of the pterygotids Erettopterus, Jaekelopterus and Pterygotus, and the pterygotid sister taxon Slimonia. Representative species of all these genera have more acute vision than A. cummingsi. The visual systems of Jaekelopterus rhenaniae and Pterygotus anglicus are comparable to that of modern predatory arthropods. All species of Jaekelopterus and Pterygotus have robust crushing chelicerae, morphologically distinct from the weaker slicing chelicerae of Acutiramus. Vision in Erettopterus osiliensis and Slimonia acuminata is more acute than in Acutiramus cummingsi, but not to the same degree as in modern active predators, and the morphology of the chelicerae in these genera suggests a grasping function. The pterygotids evolved with a shift in ecology from generalized feeder to specialized predator. Pterygotid eurypterids share a characteristic morphology but, although some were top predators, their ecology differs radically between genera.