Standards have been empirically developed to describe various morphological characters of eurypterids. The standards pertain to the following characters: 1) shape of the prosoma; 2) shape of the metastoma; 3) shape of the eyes; 4) position of the eyes; 5) types of prosomal appendages; 6) types of swimming leg paddles; 7) structure of the doublure; 8) differentiation of the opisthosoma; 9) structure of the genital appendages; 10) shape of the telson; and 11) types of ornamentation.For the first time, a uniform, standardized taxonomy is proposed for classification and identification of most genera. The taxonomy is based on the observation that most higher taxonomic levels for arthropods are based on the structure and arrangement of the appendages. Details of the taxonomy rely on the morphological standards proposed here.The order Eurypterida Burmeister, 1843, is here defined by the presence of only six pairs of prosomal appendages, the first pair being the chelicera, the next five pairs being the gnathobasic, uniramus legs. Suborders are characterized by the gross morphology of the chelicera. Superfamilies and families are characterized by the use of a single character complex, specifically the structure and arrangement of the second through sixth pairs of prosomal appendages. Genera are recognized by more specific standards.A new classification of the order Eurypterida is proposed. Three new superfamilies, Kokomopteroidea, Megalograptoidea, and Brachyopterelloidea, and five new families, Brachyopterellidae, Adelophthalmidae, Lanarkopteridae, Erieopteridae, and Hardieopteridae, are proposed.

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Exceptionally abundant specimens of Conularia aff. desiderata Hall occur in multiple marine obrution deposits, in a single sixth-order parasequence composed of argillaceous and silty very fine sandstone, in the Otsego Member of the Mount Marion Formation (Middle Devonian, Givetian) in eastern New York State, USA. Associated fossils consist mostly of rhynchonelliform brachiopods but also include bivalve molluscs, orthoconic nautiloids, linguliform brachiopods and gastropods. Many of the brachiopods, bivalve molluscs and conulariids have been buried in situ. Conulariids buried in situ are oriented with their aperture facing obliquely upward and with their long axis inclined at up to 87 degree to bedding. Most specimens are solitary, but some occur in V-like pairs or in radial clusters consisting of three specimens, with the component specimens being about equally long or (less frequently) substantially different in length. The compacted apical end of Conularia buried in situ generally rests upon argillaceous sandstone. With one possible exception, none of the examined specimens terminates in a schott (apical wall), and internal schotts appear to be absent. The apical ends of specimens in V-like pairs and radial clusters show no direct evidence of interconnection of their periderms. The apical, middle or apertural region of some inclined specimens abuts or is in close lateral proximity to a recumbent conulariid or to one or more spiriferid brachiopods, some of which have been buried in their original life orientation. The azimuthal bearings of Conularia and nautiloid long axes and the directions in which conulariids open are nonrandom, with conulariids being preferentially aligned between 350 and 50 degree and with their apertural end facing north-east, and nautiloids being preferentially aligned between 30 and 70 degree. Otsego Member Conularia were erect or semi-erect, epifaunal or partially infaunal animals, the apical end of which rested upon very fine bottom sediment. The origin of V-like pairs and radial clusters remains enigmatic, but it is probable that production of schotts was not a regular feature of this animal's life history. Finally, conulariids and associated fauna were occasionally smothered by distal storm deposits, under the influence of relatively weak bottom currents.

The record of Ordovician Eurypterida from New York State, USA, is shown to be largely false. Twenty-nine species in 17 genera are here recognised as pseudofossils, reducing by more than 75% the total number of named Ordovician eurypterid taxa. Consequently, 10 families now have their first occurrence either later in the Ordovician or in the Early Silurian. The implications for eurypterid palaeoecology, diversity and evolution are not as straightforward as would be expected from such a drastic taxonomic revision. All Ordovician eurypterids are now known to occur in shallow-water, near-shore shales or fine-grained carbonates. Diversity measures indicate that the end-Ordovician extinction event appears to have had less effect on eurypterids than previously known, and their turnover is level in the Ordovician.

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