Willwerathia is a genus of Devonian arthropod. It is sometimes classified as synziphosurine,^[1][2] a paraphyletic group of horseshoe crab-like fossil chelicerate arthropods,^[2] while some studies compare its morphology to an artiopod.^[3][4] Willwerathia known only by one species, Willwerathia laticeps, discovered in deposits of the Devonian period from the Klerf Formation, in the Rhenish Slate Mountains of Germany.^[1][5]

As a synziphosurine, Willwerathia is unusually large and so far the largest known synziphosurine, with largest carapace measured about 90mm in width.^[1] Prosoma of Willwerathia covered by a vaulted carapace with pointed genal spines, recurved (M-shaped) ophthalmic ridges and pairs of dorsal nodes.^[1] Tergites of the opisthosoma are either incomplete or disarticulated in available fossil materials, making it difficult to reveal the original number of opisthosomal segments.^[1] The opisthosoma of Willwerathia most likely compose of 10 segments, each expressed by a tergite that bore a median dorsal spine and a pair of tergopleurae (lateral extensions).^[1] The opisthosoma subdivided into a wider, most likely 7-segmented preabdomen and a narrower, 3-segmented postabdomen.^[1] tergite of the first opisthosomal segment is reduced in length while the remaining segments possess well-developed tergites with lateral nodes and posteriorly curved tergopleurae. The final segment terminated with a short, teardrop-shaped telson.^[1]

Willwerathia was most likely a bottom-dwelling predator.^[1] The marked articulation surfaces on each of the preabdominal segments suggest that Willwerathia capable to enroll itself in a way similar to Legrandella.^[1] The environment in which Wilwerathia lived in was likely an estuarine to deltaic one, and other animals like the largest eurypterid, Jaekelopterus, are known from the same formation.^[6]

Willwerathia was originally thought to be an eurypterid (sea scorpion), with additional fossils described in 1998 reveal its synziphosurine affinities.^[1] In the redescription done by Anderson et al. 1998, Willwerathia had been grouped under the synziphosurine family Weinberginidae alongside Weinbergina and Legrandella,^[1] a classification which is not supported by phylogenetic analysis.^[2] Willwerathia was regarded as part of the monophyletic Xiphosura sensu stricto (true horseshoe crab) by Lamsdell 2013,^[2][7] but further phylogenetic analysis repeatedly resolving it within a clade compose of Bunodids, Pseudoniscids and Dekatriatan (chasmataspidids, eurypterids and arachnids).^{[8][9][10][11]} Morphology of Silurian-Devonian arthropd Maldybulakia is sometimes compared to that of Willwerathia.^[4][12] However, in 2020, Lamsdell found that Willwerathia bears a strong resemblance to the Cambrian artiopod Falcatamacaris, while it does not bear resemblance to other chelicerates.^[3]