Tremadocian in the context of "Floian"

The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from 486.85 to 471.3 million years ago, until the Dapingian age of the Middle Ordovician. It includes Tremadocian and Floian ages.

The Furongian is the fourth and final epoch and series of the Cambrian. It lasted from 497 to 486.85 million years ago. It succeeds the Miaolingian series of the Cambrian and precedes the Lower Ordovician Tremadocian Stage. It is subdivided into three stages: the Paibian, Jiangshanian and the unnamed 10th stage of the Cambrian.

Eurypterids, often informally called sea scorpions, are a group of extinct marine arthropods that form the order Eurypterida. The earliest known eurypterids date to the Tremadocian stage of the Ordovician period, 480 million years ago. The group is likely to have appeared first during the 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 (or sometime shortly before) 251.9 million years ago.

Although popularly called "sea scorpions", only the earliest eurypterids were marine; many later forms lived in brackish or fresh water, and they were not true scorpions. Some studies suggest that a dual respiratory system was present, which would have allowed for short periods of time in terrestrial environments. The name Eurypterida comes from Ancient Greek εὐρύς (eurús), meaning "wide, broad", and πτερόν (pterón), meaning "wing", referring to the pair of wide swimming appendages present in many members of the group.

Stage 10 of the Cambrian is the still-unnamed third and final stage of the Furongian series. It follows the Jiangshanian and precedes the Ordovician Tremadocian Stage. The proposed lower boundary is the first appearance of the trilobite Lotagnostus americanus around 491 million years ago, but other fossils are also being discussed (see below). The upper boundary is defined as the appearance of the conodont Iapetognathus fluctivagus which marks the beginning of the Tremadocian and is radiometrically dated as 486.85 million years ago.