Agnostida
Peronopsis interstrictus
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Trilobita Order: Agnostida Salter, 1864
Families
Suborder Agnostina Image File history File linksMetadata No higher resolution available. ... Scientific classification or biological classification is a method by which biologists group and categorize species of organisms. ... Digimon, the only known animals. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - trilobites (extinct) Subphylum Chelicerata Arachnida - spiders,scorpions, etc. ... Orders Agnostida Redlichiida Corynexochida Lichida Nektaspida? Phacopida Proetida Asaphida Harpetida Ptychopariida Trilobites are extinct arthropods in the class Trilobita. ... John William Salter (December 15, 1820 - August 2, 1869) was an English naturalist and palaeontologist. ... Superfamily Agnostoidea Agnostidae Ammagnostidae Clavagnostidae Diplagnostidae Doryagnostidae Glyptagnostidae Metagnostidae Peronopsidae Ptychagnostidae Superfamily Condylopygoidea Condylopygidae Suborder Eodiscina Families Suborder Agnostina Superfamily Agnostoidea Agnostidae Ammagnostidae Clavagnostidae Diplagnostidae Doryagnostidae Glyptagnostidae Metagnostidae Peronopsidae Ptychagnostidae Superfamily Condylopygoidea Condylopygidae Suborder Eodiscina Superfamily Eodiscoidea Calodiscidae Eodiscidae Hebediscidae Tsunyidiscidae Weymouthiidae Yukoniidae Agnostida (the agnostids) is an order of trilobite. ... Families Suborder Agnostina Superfamily Agnostoidea Agnostidae Ammagnostidae Clavagnostidae Diplagnostidae Doryagnostidae Glyptagnostidae Metagnostidae Peronopsidae Ptychagnostidae Superfamily Condylopygoidea Condylopygidae Suborder Eodiscina Superfamily Eodiscoidea Calodiscidae Eodiscidae Hebediscidae Tsunyidiscidae Weymouthiidae Yukoniidae Agnostida (the agnostids) is an order of trilobite. ... Superfamily Eodiscoidea Calodiscidae Eodiscidae Hebediscidae Tsunyidiscidae Weymouthiidae Yukoniidae

Agnostida (the agnostids) is an order of trilobite. These small trilobites first appeared toward the end of the Early Cambrian and thrived in the Middle Cambrian. They are present in the loer Cambrian fossil record along with trilobites from Orders Redlichiida, Corynexochida, and Ptychopariida. The last agnostids went extinct in the Late Ordovician. Scientific classification or biological classification refers to how biologists group and categorize extinct and living species of organisms. ... Orders Agnostida Redlichiida Corynexochida Lichida Phacopida Proetida Asaphida Harpetida Ptychopariida Nektaspida (doubtful order) Trilobites are extinct arthropods in the class Trilobita. ... The Cambrian is a major division of the geologic timescale that begins about 542 ± 1. ... Suborders Olenellina Redlichiina Redlichiida is an order within the major extinct arthropod class Trilobita. ... Suborders Suborder Corynexochina Suborder Illaenina Suborder Leiostegiina Corynexochida is an order of trilobite that lived from the Lower Cambrian to the Middle Devonian. ... Suborders Suborder Ptychopariina Superfamily Ellipsocephaloidea Superfamily Ptychoparioidea Suborder Olenina Ptychopariida is a large, heterogenous order of trilobite containing some of the most primative species known. ... In biology and ecology, extinction is the ceasing of existence of a species or group of species. ... The Ordovician period is the second of the six (seven in North America) periods of the Paleozoic era. ...

The Agnostida are divided into two suborders -- Agnostina and Eodiscina -- that are then divided into a number of families. As a group, agnostids have pygidia (tails) that are so similar in size and shape to their cephalons (heads) that it is difficult to distinguish which end is which. Most agnostid species were eyeless. In biological classification, family (Latin: familia, plural familiae) is 1) a rank or 2) a taxon in that rank. ... The pygidium is the posterior body part or shield of crustaceans, some insects, and trilobites. ... The cephalon is the foremost portion of the segmented body of a trilobite. ...

The systematic position of Order Agnostida within Class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group. The challenge to the status has focused on the Agnostina partly because juveniles of one genus have been found with legs greatly different from those of adult trilobites, suggesting they are separately descended from Crustaceans. Other researchers have suggested, based on cladistic analyses, that Eodiscina and Agnostida are closely united, and that the Eodiscina descended from the trilobite Order Ptychopariida. Classes Remipedia Cephalocarida Branchiopoda Ostracoda Maxillopoda Malacostraca The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum. ... Greek clados = branch) or phylogenetic systematics is a branch of biology that determines the evolutionary relationships of living things based on derived similarities. ... Suborders Suborder Ptychopariina Superfamily Ellipsocephaloidea Superfamily Ptychoparioidea Suborder Olenina Ptychopariida is a large, heterogenous order of trilobite containing some of the most primative species known. ...

Scientists have long debated whether the agnostids lived a pelagic or a benthic lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites all suggest a benthic (bottom-dwelling) mode of life. They likely lived on areas of the ocean floor that received little or no light and fed on detritus that descended from upper layers of the sea to the bottom. In contrast, their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracodan-type lifestyle. Furthermore, the orientation of the thoracic appendages appears ill suited for benthic living. The pelagic zone is the part of the open sea or ocean comprising the water column, i. ... In marine geology and biology, benthos are the organisms and habitats of the sea floor; in freshwater biology they are the organisms and habitats of the bottoms of lakes, rivers, and creeks. ... Detritus may refer to: In geology, detritus is the name for loose fragments of rock that have been worn away by erosion. ... It has been suggested that this article or section be merged into Fossil. ...

Agnostina are generally referred to simply as "agnostids" even though they probably should be called "agnostines".

External links

• Agnostida fact sheet
• A Guide to the 8 Orders of Trilobites By Sam Gon III