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.
Yohoia is an extinct genus of megacheiran arthropod from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. The type species, Yohoia tenuis, was described in 1912 by Walcott, who considered it an anostracan crustacean. 711 specimens of Yohoia are known from the Greater Phyllopod bed, where they comprise 1.35% of the community. In 2015, Conway Morris et al. reported another species, Y. utahana, from the Marjum Formation, Utah.
Dinocaridida is a proposed fossil taxon of basal arthropods that flourished in the Cambrian period with occasional Ordovician and Devonian records. Characterized by a pair of frontal appendages and series of body flaps, the name of Dinocaridids refers to the suggested role of some of these members as the largest marine predators of their time. Dinocaridids are occasionally referred to as the 'AOPK group' by some literatures, as the group compose of Radiodonta, Opabiniidae, and the "gilled lobopodians" Pambdelurion and Kerygmachelidae. It is most likely paraphyletic, with Kerygmachelidae and Pambdelurion more basal than the clade compose of Opabiniidae, Radiodonta and other arthropods.
Anomalocaris is an extinct genus of radiodont, an order of early-diverging stem-group arthropods.
Cheloniellida is a taxon of extinct Paleozoic arthropods. As of 2018, 7 monotypic genera of cheloniellids had been formally described, whose fossils are found in marine strata ranging from Ordovician to Devonian in age. Cheloniellida has a controversial phylogenetic position, with previous studies associated it as either a member or relative of various fossil and extant arthropod taxa. It was later accepted as a member of Vicissicaudata within Artiopoda.
Pambdelurion is an extinct genus of panarthropod from the Cambrian aged Sirius Passet site in northern Greenland. Like the morphologically similar Kerygmachela from the same locality, Pambdelurion is thought to be closely related to arthropods, combining characteristics of "lobopodians" with those of primitive arthropods.
Parapeytoia is a genus of Cambrian arthropod. The type and only described species is Parapeytoia yunnanensis, lived over 518 million years ago in the Maotianshan shales of Yunnan, China. Unidentified fossils from the same genus also had been discovered from the nearby Wulongqing Formation.
Megacheira is an extinct class of predatory arthropods defined by their possession of spined "great appendages". Their taxonomic position is controversial, with studies either considering them stem-group euarthropods, or stem-group chelicerates. The homology of the great appendages to the cephalic appendages of other arthropods is also controversial. Uncontested members of the group were present in marine environments worldwide from the lower to middle Cambrian.
Schinderhannes bartelsi is a species of hurdiid radiodont (anomalocaridid) known from one specimen from the lower Devonian Hunsrück Slates. Its discovery was astonishing because previously, radiodonts were known only from exceptionally well-preserved fossil beds (Lagerstätten) from the Cambrian, 100 million years earlier.
Haikoucaris is a genus of megacheiran arthropod that contains the single species Haikoucaris ercaiensis. It was discovered in the Cambrian Chengjiang biota of China.
Occacaris oviformis is an extinct nektonic predatory arthropod from the Lower Cambrian Maotianshan shale Lagerstätte. It bears a superficial resemblance to the Cambrian arthropod, Canadaspis, though, was much smaller, and had a pair of "great appendages", with which it may have grasped prey. It was originally considered to belong to Megacheira, however it is questioned in later study.
Radiodonta is an extinct order of stem-group arthropods that was successful worldwide during the Cambrian period. They may be referred to as radiodonts, radiodontans, radiodontids, anomalocarids, or anomalocaridids, although the last two originally refer to the family Anomalocarididae, which previously included all species of this order but is now restricted to only a few species. Radiodonts are distinguished by their distinctive frontal appendages, which are morphologically diverse and used for a variety of functions. Radiodonts included the earliest large predators known, but they also included sediment sifters and filter feeders. Some of the most famous species of radiodonts are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, Peytoia nathorsti, Titanokorys gainessii, Cambroraster falcatus and Amplectobelua symbrachiata, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.
Kockurus is a problematic genus of Cambrian arthropod, known from the Czech Republic, which bears some resemblance to the eurypterids, aglaspidids and chelicerates. It is diagnostic of a small and low-diversity fauna endemic to the area, which dwelt in brackish waters.
Vladicaris is an arthropod known from the Lower Cambrian Paseky Shale member, Czech Republic, where it occurs alongside other arthropods, namely Kodymirus vagans and Kockurus grandis. It has been attributed to the Crustacea, and if it does indeed belong in the crown group of that subphylum, it would be the earliest known representative of the group. Its position within the Crustacea is unclear – it has been allied with the Phyllocarida and the Branchiopoda.
Cucumericrus ("cucumber-leg") is an extinct genus of stem-arthropod. The type and only species is Cucumericrus decoratus, with fossils discovered from the Maotianshan Shales of Yunnan, China.
Worthenella is a genus of enigmatic arthropod from the Burgess Shale. It known from a single specimen described initially as an annelid by Charles Doolittle Walcott in 1911. The body is elongate and myriapod-like with a head and at least 46 body segments. The head bears poorly preserved appendages, while filamentous branched structures appear to run along the underside of the first 34 trunk segments, with the posterior 8 suggested to have had longer appendages. In 2013, David Legg placed it in the family Kootenichelidae as a sister to Kootenichela, based on the supposed presence of antenniform head appendages. However, this position was questioned in a later study, which argued that the supposed antenniform appendages were actually taphonomic artefacts.
The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Trilobites, in part due to abundance of findings owing to their mineralized exoskeletons, are by far the best recorded, diverse, and long lived members of the clade. Other members, which lack mineralised exoskeletons, are known mostly from Cambrian deposits.
Strabops is a genus of strabopid, an extinct group of arthropods. Strabops is known from a single specimen from the Late Cambrian of the Potosi Dolomite, Missouri, collected by a former professor, Arthur Thacher. It is classified in the family Strabopidae of the monotypic order Strabopida, a group closely related to the aglaspidids with uncertain affinities. The generic name is composed by the Ancient Greek words στραβός, meaning "squinting", and ὄψῐς, meaning "face".
Cheloniellon is a monotypic genus of cheloniellid arthropod, known only by one species, Cheloniellon calmani, discovered from the Lower Devonian Hunsrück Slate of Germany.
Kylinxia is a genus of extinct arthropod described in 2020. It was described from six specimens discovered in Yu'anshan Formation in southern China. The specimens are assigned to one species Kylinxia zhangi. Dated to 518 million years, the fossils falls under the Cambrian period. Announcing the discovery on 4 November 2020 at a press conference, Zeng Han of the Nanjing Institute of Geology and Paleontology, said that the animal "bridges the evolutionary gap from Anomalocaris to true arthropods and forms a key ‘missing link’ in the origin of arthropods," which was "predicted by Darwin’s evolutionary theory." The same day the formal description was published in Nature.
