Late Ordovician extinctions

mass extinction event at the end of the Ordovician period and the beginning of the Silurian period in the Paleozoic era, around 444 million years ago

Agnostida are an order of extinct arthropods which have classically been seen as a group of highly modified trilobites, though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopoda. They are present in the Lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders, and were highly diverse throughout the Cambrian. Agnostidan diversity severely declined during the Cambrian-Ordovician transition, and the last agnostidans went extinct in the Late Ordovician.

Endoceras, from Ancient Greek ἔνδον (éndon), meaning "inside", and κέρας (kéras), meaning "horn", is an extinct genus of large, straight-shelled cephalopods that gives its name to the Nautiloid order Endocerida. The genus lived during the middle and upper Ordovician 470 to 443 million years ago. The cross section in the mature portion is slightly wider than high, but is narrower laterally in the young. Sutures are straight and transverse. Endoceras has a large siphuncle, located close to the ventral margin, composed of concave segments, especially in the young but which may be tubular in the a

glaciation

Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. The earliest species occurred in the second half of the Lower Cambrian, and the last species did not survive the Ordovician–Silurian extinction event.

Aglaspidida is an extinct order of marine arthropods known from fossils spanning the Middle Cambrian to the Upper Ordovician. Initially considered chelicerates, modern anatomical comparisons demonstrate that the aglaspidids cannot be accommodated within this group, and that they lie instead within the Artiopoda, thus placing them closer to the trilobites, being placed in the artiopod subgroup Vicissicaudata.

Endocerida, from Ancient Greek ἔνδον (éndon), meaning "inside", and κέρας (kéras), meaning "horn", is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian. Their shells were variable in form. Some were straight (orthoconic), others curved (cyrtoconic); some were long (longiconic), others short (breviconic). Some long-shelled forms like Endoceras attained shell lengths close to . The related Cameroceras is anecdotally

Endoceratidae, from Ancient Greek ἔνδον (éndon), meaning "inside", and κέρας (kéras), meaning "horn", is a family of large to very large straight-shelled nautiloid cephalopods belonging to the order Endocerida that lived during the Middle and Late Ordovician. They include the largest known Paleozoic invertebrates, represented by Endoceras and Cameroceras.

thumb|left|Sacabambaspis, the best known arandaspid, from the Ordovician of Bolivia. Shows the characteristic, frontally positioned eyes, like car head lamps Arandaspida is a taxon of very early, jawless prehistoric fish which lived during the Ordovician period. Arandaspids represent some of the oldest known vertebrates. The group represents a subclass within the class Pteraspidomorphi, and contains only one order, the Arandaspidiformes. The oldest known genus of this group is Sacabambaspis found in South America.

Pleurocystites (meaning rib bag or side bladder) is a genus of rhombiferan echinoderm (a cystoid) that lived in the Late Ordovician. Its fossils are known from Europe and North America. Pleurocystites grew to a height of 2 centimeters (3/4 of an inch) and fed on plankton.

Scenella is an extinct genus of fossil invertebrate animal which is generally considered to be a mollusc; at various times it has been suggested that this genus belongs with the gastropods, the monoplacophorans, or the helcionellids, although no firm association with any of these classes has been established. An affinity with the hydrozoa (as a flotation device) has been considered, although some authors oppose this hypothesis. A gastropod affinity is defended on the basis of six pairs of internal muscle scars, whilst the serially-repeated nature of these scars suggests to other authors a mono

Ceraurus is a genus of cheirurid trilobite of the middle and, much more rarely, the upper Ordovician. They are commonly found in strata of the lower Great Lakes region. These trilobites have eleven thoracic segments, a very small pygidium and long genal and pygidial spines.