Абстракт:

A metabentonite deposit in the upper Elkhorn formation (Richmond subseries, Cincinnatian series, Upper Ordovician) near Manchester, Adams County, Ohio, has yielded some of the most complete eurypterids known anywhere. The chitinous exoskeletons are preserved in an inflated state with the chitin still flexible and retaining what seems to be original "scorpionid" color pattern. Many of the specimens are completely articulated, and some have been largely freed from the matrix. Males, females, and various growth stages are known. The association is typically marine biocoenose with many characteristic upper Richmond marine invertebrates present (brachiopods, Bryozoa, pelecypods, trilobites). The general aspect of the organisms is that of mixopteroid eurypterids having exaggeratedly spinose grasping appendages. The whole body was scaly, porous and generally heavily hirsute, owing to the presence of many sensory bristles. Although referable to Megalograptus Miller, 1874 (based on an eurypterid leg originally mistakenly considered to be a giant graptolite), the species is new, and a new family is required. In several respects this is the most primitive eurypterid known, although by no means the oldest. It is at the same time curiously specialized: its body terminates in a horizontally opposable pair of scimitar-like cereal blades lying beneath the short stout telson which was capable of considerable vertical movement. Such a protective or copulatory device as the cereal blades is hitherto unknown among fossils, and among Recent forms only the earwigs (Insecta) are comparable. The systematics of the study include a reanalysis of the Caster and Kjellesvig-Waering (1955) families Megalograptidae and Mixopteridae. Under the former monotypic family all known species are considered: Megalograptus welchi Miller, the type species (Liberty formation); M. alveolatus (Shuler) (Bays formation of Virginia); M. ohioensis C. and Kj.-W., the main object of this paper (Elkhorn formation); M. shideleri, nov. sp. (Saluda formation); M. williamsae, nov. sp. (Waynesville formation), and a new form from the Whitewater formation too poorly represented for taxonomy. Stormer's (1934, 1955) Carcinosomatidae is emended to include four genera, two of which are proposed as new: Carcinosoma Claypole; PARACARCINOSOMA: P. scorpionis (Grote and Pitt); Rhinocarcinosoma Novojilov: and EOCARCINOMA, E. batrachophthalmus, nov. gen., nov. sp. A reclassification of the Eurypterida is proposed in which the Eurypterida are considered an order under the Class Merostomata. Two suborders, based on the structure of the chelicerae, are proposed: Eurypterida and Pterygotida. Three superfamilies are recognized for the suborder Eurypterida, namely Mixopteracea, Eurypteracea and Stylonuracea. The controversial habitat of the Eurypterida is reviewed with the conclusion that the eurypterids are found to have lived in three main marine ecological environments and remained in their ecological niches throughout their existence. The two more marine zone members reached extinction much earlier (essentially pre-Middle Devonian) that those of the more brackish water zone which continued to at least Middle Permian.