page_title: Eurypterid
section_title: Classification
hierarchical_section_title: Eurypterid / Classification
caption_attribution_description: English: Life restoration of Fuxianhuia protensa
context_page_description: Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is likely to have appeared first either during the Early Ordovician or Late Cambrian period. With approximately 250 species, the Eurypterida is the most diverse Paleozoic chelicerate order. Following their appearance during the Ordovician, eurypterids became major components of marine faunas during the Silurian, from which the majority of eurypterid species have been described. The Silurian genus Eurypterus accounts for more than 90% of all known eurypterid specimens. Though the group continued to diversify during the subsequent Devonian period, the eurypterids were heavily affected by the Late Devonian extinction event. They declined in numbers and diversity until becoming extinct during the Permian–Triassic extinction event 251.9 million years ago.
  Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions.
context_section_description: Historically, a close relationship between eurypterids and xiphosurans (such as the modern Atlantic horseshoe crab) has been assumed by most researchers. Several homologies encourage this view, such as correlating segments of the appendages and the prosoma. Additionally, the presence of plate-like appendages bearing the "gill tracts" on appendages of the opisthosoma (the blatfüssen) was cited early as an important homology. In the last few decades of the nineteenth century, further homologies were established, such as the similar structures of the compound eyes of Pterygotus and horseshoe crabs (seen as especially decisive as the eye of the horseshoe crab was seen as possessing an almost unique structure) and similarities in the ontogeny within both groups. These ontogenetical similarities were seen as most apparent when studying the nepionic stages (the developmental stage immediately following the embryonic stage) in both groups, during which both xiphosurans and eurypterids have a proportionally larger carapace than adults, are generally broader, possess a distinct ridge down the middle, have a lesser number of segments which lack differentiation and have an underdeveloped telson.
  Due to these similarities, the xiphosurans and eurypterids have often been united under a single class or subclass called Merostomata (erected to house both groups by Henry Woodward in 1866). Though xiphosurans (like the eurypterids) were historically seen as crustaceans due to their respiratory system and their aquatic lifestyle, this hypothesis was discredited after numerous similarities were discovered between the horseshoe crabs and the arachnids. Some authors, such as John Sterling Kingsley in 1894, classified the Merostomata as a sister group to the Arachnida under the class "Acerata" within a subphylum "Branchiata". Others, such as Ray Lankester in 1909, went further and classified the Merostomata as a subclass within the Arachnida, raised to the rank of class.
  In 1866, Ernst Haeckel classified the Merostomata (containing virtually only the Eurypterida) and Xiphosura within a group he named Gigantostraca within the crustaceans. Though Haeckel did not designate any taxonomic rank for this clade, it was interpreted as equivalent to the rank of subclass, such as the Malacostraca and Entomostraca, by later researchers such as John Sterling Kinsgsley. In subsequent research, Gigantostraca has been treated as synonymous with Merostomata (rarely) and Eurypterida itself (more commonly).
  A phylogenetic analysis (the results presented in a cladogram below) conducted by James Lamsdell in 2013 on the relationships within the Xiphosura and the relations to other closely related groups (including the eurypterids, which were represented in the analysis by genera Eurypterus, Parastylonurus, Rhenopterus and Stoermeropterus) concluded that the Xiphosura, as presently understood, was paraphyletic (a group sharing a last common ancestor but not including all descendants of this ancestor) and thus not a valid phylogenetic group. Eurypterids were recovered as closely related to arachnids instead of xiphosurans, forming the group Sclerophorata within the clade Dekatriata (composed of sclerophorates and chasmataspidids). Lamsdell noted that it is possible that Dekatriata is synonymous with Sclerophorata as the reproductive system, the primary defining feature of sclerophorates, has not been thoroughly studied in chasmataspidids. Dekatriata is, in turn, part of the Prosomapoda, a group including the Xiphosurida (the only monophyletic xiphosuran group) and other stem-genera.
caption: Restoration of the primitive Cambrian arachnomorph Fuxianhuia, one of the most basal members of the lineage that would eventually lead to both the chelicerates and the trilobites.
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