Ordovician eurypterids

Megalograptus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Megalograptus have been recovered in deposits of Katian (Late Ordovician) age in North America. The genus contains five species: M. alveolatus, M. ohioensis, M. shideleri, M. welchi and M. williamsae, all based on fossil material found in the United States. Fossils unassigned to any particular species have also been found in Canada. The generic name translates to "great writing" and originates from the mistaken original belief that Megalograptus was a type of graptolite, often given names ending with -gr

Pentecopterus is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils have been registered from the Darriwilian age of the Middle Ordovician period, as early as 467.3 million years ago. The genus contains only one species, P. decorahensis, that was previously considered the oldest known eurypterid, surpassing all other Ordovician eurypterids known at the time, such as Brachypterus, in age by almost 9 million years; however, newer discoveries have yielded evidence for eurypterids that were older still (?Carcinosoma aurorae dated back to the Tremadocian). The generic name deriv

Carcinosoma (meaning "crab body") is a genus of eurypterid, an extinct group of aquatic arthropods. Fossils of Carcinosoma are likely restricted to deposits of late Silurian to early Devonian (Llandovery to Lochkovian) age,'''' although an Early Ordovician specimen has been classified as ?Carcinosoma aurorae, which would make this the earliest known eurypterid genus. Classified as part of the family Carcinosomatidae, which the genus lends its name to, Carcinosoma'' contains several species from North America and Great Britain.

Brachyopterus was one of the earliest known eurypterid genera, having been recovered from Middle Ordovician deposits in Montgomeryshire, Wales. Though other species have been assigned to it in the past, Brachyopterus is today recognized as containing one valid species, B. stubblefieldi.

Echinognathus is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Echinognathus, E. clevelandi, is known from deposits of Late Ordovician age in the United States. The generic name is derived from the Neo-Latin echino- ("spiny") and the Greek gnáthos ("jaw"), in reference to a spiny endognathary (used to handle food) appendage part of the fossil type material.

Onychopterella ( , from Ancient Greek: ὄνῠξ (ónyx), "claw", and πτερόν (pteron), "wing") is a genus of predatory eurypterid ("sea scorpion"), an extinct group of aquatic arthropods. Fossils of Onychopterella have been discovered in deposits from the Late Ordovician to the Late Silurian. The genus contains three species: O. kokomoensis, the type species, from the Early Pridoli epoch of Indiana; O. pumilus, from the Early Llandovery epoch of Illinois, both from the United States; and O. augusti, from the Late Hirnantian to Early Rhuddanian stages of South Africa.

Eocarcinosoma is a genus of eurypterid, an extinct group of aquatic arthropods. The type and only species of Eocarcinosoma, E. batrachophthalmus, is known from deposits of Late Ordovician age in the United States. The generic name is derived from the related genus Carcinosoma, and the Greek () meaning 'dawn', referring to the earlier age of the genus compared to other carcinosomatid eurypterids.