Tremadocian

System (period)	Series	Stage (age)	Lower boundary, Ma
Silurian	Llandovery	Rhuddanian	443,8±1,5
Ordovician	Upper	Hirnantian	445.2±1.4
		Katian	453.0±0.7
		Sandbian	458.4±0.9
	Middle	Darriwilian	467.3±1.1
		Dapingian	470.0±1.4
	Lower	Floian	477.7±1.4
		Tremadocian	485.4±1.9
Cambrian	Furongian	Stage 10	older
Subdivisions and "golden spikes" according to IUGS as of September 2023

Rock from the Skiddaw Group, of Tremadocian in Cumbria, England, UK.

The Tremadocian is a lower stage of the Lower Ordovician series and the whole Ordovician system, corresponding to the Tremadocian age of the Early Ordovician epoch of the Ordovician period. It lasted from approximately 485.4 Ma to around 477.7 Ma (million years ago). It is preceded by the still unnamed tenth age of the Cambrian and is followed by the Floian age of the Early Ordovician. The earliest planktonic graptolites are known from this stage.

It was named after the village Tremadoc in Wales, UK, by Adam Sedgwick in 1852. It is the only original stage of the Ordovician which was not rejected by the International Commission on Stratigraphy (ICS) in 2008 and is still use by the International Union of Geological Sciences (IUGS). Despite this, the GSSP of the Tremadocian (see below) was established far away from the British Isles, where other Ordovician stages, later rejected by ICS, were distinguished. In British chart, Tremadoc stage corresponds to Tremadocian.

Definition

The GSSP of the Tremadocian stage, the Lower Ordovician series and the whole Ordovician system was ratified in 2000. The lower boundary is fixed within Bed 23 in Green Point Section, western Newfoundland, Canada, and related to the first appearance of the conodont species Iapetognathus fluctivagus.

Tremadocian life

There is no consensus among researchers when the Great Ordovician Biodiversification Event (GOBE) began. Some believe it started as early as the Furongian (the last epoch of the Cambrian), while others attribute GOBE to an increase in biodiversity shortly after the start of the Tremadocian. The evolution of living organisms still took place mainly in the seas, and some primitive Cambrian forms still existed at the beginning of the Ordovician. As a result of increasing competition for food resources and the number of predators, life began to spread from shallow waters to deep areas, and drifting organisms such as graptolites and cephalopods reached the coasts of other continents.

Ellesmeroceras scheii

Sponges such as Amsassia are found in deep-sea shelf sediments. Brachiopods (Ottenbyella, Eurytreta, Schizambon, Wahwahlingula, Conotreta) lived in shallow waters. Graptolites spread throughout the seas. Tremadocian genera include Tetragraptus, Bryograptus, Clonograptus and Adelograptus. Shelled cephalopods such as Burenoceras, Clarkoceras, Ellesmeroceras and Levisoceras, hunted in the water column. Gastropods (Ozarkispira, Gasconadia, Peelerophon, Sinuites) also reached large numbers.

Aegirocassis

The dawn of arthropods continued, and trilobites were especially numerous. Various ptychopariid (Plethopeltis, Larifugula, Yukonaspis, Phylacterus), asaphid (Kainella, Eurekia, Elkanaspis, Calvinella), corynexochid (Leiostegium, Tatonaspis, Pseudocalymene, Missisquoia), and olenid (Apoplanias, Parabolinella, Pareuloma, Highgatella) trilobites are known from the Tremadocian. Agnostids (Geragnostus, Anglagnostus, Micragnostus, Lotagnostus), close relatives of trilobites, also lived in the seas. Tariccoia, a nektaspidid, have been discovered in the Tremadocian of Morocco. Caryocaris, a relative of modern crustaceans, had a shell to protect it from predators. Aegirocassis, the largest animal of its time, fed on plankton, which it filtered with the help of its frontal appendages. Pseudoangustidontus, a close relative, was smaller.

Among the chordates, the most numerous were conodonts, which had teeth and resembled modern lampreys and hagfishes. Tremadocian conodonts include Drepanodus, Cordylodus, Oneotodus, Teridontus, Clavohamulus and other genera.

External links

• https://paleobiodb.org/classic/displayTimescale?interval=Tremadocian