Caption below under next figure: Image from “Giant claw reveals the largest ever arthropod” (2007), Biology Letters
The arthropods we are familiar with today tend toward the small side compared to the some of the giants found in the fossil record. From the Late Palaeozoic, 2m long millipedes and dragonflies with 75cm wingspans are known. Even marine arthropods obtained large sizes with examples including Ordovician trilobites and Siluro-Devonian eurypterids (sea scorpions). In part the Late Paleozioc pattern might be explained by increased atmospheric oxygen levels. Work on extant groups like arthropods and mollusks seem to support a pattern between oxygen and body size.
A new individual of eurypterid described by Braddy et al. appearing in Biology Letters may shatter the old record of 250cm length of Acutiramus bohemicus. The new individual is a Jackelopterus rehnaniae from the Early Devonian Willwerath Lagerstatte of Germany. The description and estimated length are based on a single discovered claw (chelicera). Using chelicera/total length ratios from two closely related genera, Acutiramus and Pterygotus, the authors estimate the length (without chelicerae) to be between 233-259cm and 333-359 with the additional length of the chelicerae. The tremndous size of such an animal suggests a top predatory position pontentially feeding on other arthropods and early vertebrates (Inverts 1 Verts 0)
From the Telegraph: Markus Poschmann with the giant sea scorpion fossil
P.S. the species was not from the deep sea but rather a reef predator