Agnostida is an order of arthropod which first developed near the end of the Early Cambrian period and thrived during the Middle Cambrian. They are present in the Lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders. The last agnostids went extinct in the Late Ordovician.
Trilobites are a group of extinct marine arachnomorph arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period, and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetids died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 300 million years.
Agnostus is a genus of agnostid trilobites that lived during the upper Middle Cambrian–lower Upper Cambrian. It is the type genus of the family Agnostidae. It is subdivided into two subgenera, Agnostus and Homagnostus.
Acidiscus is an extinct genus of eodiscinid agnostid trilobites. It lived during the Botomian stage of the Cambrian period.
Acimetopus is a genus of trilobites that lived during the Botomian stage.
Bolboparia is an extinct genus of eodiscinid agnostid trilobites. It lived during the late Lower Cambrian in what today Canada and the United States.
Oodiscus is an extinct genus of small size trilobite, with three known species. It lived during the Toyonian it what are now Canada and the USA.
Tsunyidiscus is a genus of eodiscinid agnostid trilobites. Tsunyidiscus occurs near the end of the Lower Cambrian, during the late Atdabanian stage period and some collections suggest it may have survived into the Botomian. They are very small, have eyes, and equal sized head and tail shields, with a narrow dome-shaped glabella and a narrow bullet-shape pygidial axis. It has a thorax of three segments. Tsunyidiscus is the only genus currently attributed to the family Tsunyidiscidae.
Ptychagnostus is an extinct genus of blind trilobite that lived during the Cambrian period, part of the order Agnostida. Ptychagnostidae generally do not exceed ten millimetres in length. Their remains are sometimes found in the otherwise empty tubes of the polychaete worm Selkirkia. It probably lived in the water column. Its major characteristics are lack of eyes, two lobes on the glabella, and three lobes on the pygidium.
Yukoniidae is a family of trilobites, belonging to the Eodiscina, small trilobites with headshield and tailshield of equal size and shape, and with two or three thorax segments.
Crustaceomorpha is a proposed clade of arthropods that includes crustaceans and numerous extinct groups. Synapomorphies for the clade are that the larval antenna is a feeding or locomotory organ, and there are six endopodal podomeres in post-antennal limbs.
Acmarhachis is a genus of trilobite in the order Agnostida, which lived in what are now Australia, Canada, China (Anhui), Kazakhstan, Russia (Kharaulakh), and the US. It was described by Resser in 1938, and the type species is Acmarhachis typicalis.
Eodiscina is a suborder of trilobite arthropods. The Eodiscina first developed near the end of the Lower Cambrian period and became extinct at the end of the Middle Cambrian. Species are tiny to small, and have a thorax of two or three segments. Eodiscina includes six families classified under one superfamily, Eodiscoidea.
Ptychagnostidae is a family of agnostid trilobites from the 5th Stage to the Paibian Age of the Cambrian. The family includes several important index fossils.
Weymouthia is an extinct genus of eodiscinid agnostid trilobites, which lived at the end of the Lower Cambrian, in what are now the eastern United States, England, Siberia and China.
Trinodus is a very small to small blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician, in what are now the Yukon Territories, Virginia, Italy, Czech Republic, Poland, Denmark, Sweden, Svalbard, Ireland, Scotland, Wales, Iran, Kazakhstan and China. It is one of the last of the Agnostida order to survive.
Galbagnostus is an extinct genus of agnostid trilobite. It lived during the Lower and Middle Ordovician.
The Peronopsidae comprise the earliest family of the Agnostina suborder. Species of this family occurred on all paleocontinents. The earliest representatives of this family first occur just before the start of the Middle Cambrian, and the last disappeared just after the start of the Upper Cambrian.
Condylopyge is a genus of very small trilobites, that lived during the latest Early and the early Middle Cambrian, in what are today Canada, Czech Republic, England and Wales, France, Germany, Italy, Morocco, the Russian Federation, Spain, Turkey and Sweden. It can easily be distinguished from all other Agnostida because the frontal lobe of the central raised area of the headshield is wider than the rear lobe. It looks like Pleuroctenium but the frontal glabellar lobe does not fold around the rear lobe, as it does in Pleuroctenium.
The Hebediscidae are a family of trilobites belonging to the order Agnostida that lived during the Lower Cambrian. They are small or very small, and have a thorax of two or three segments. The Hebediscidae include five genera.
