In geology

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

, the Arenigian (or 'Arenig') refers both to a time interval during the Lower Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

 period and also to the suite of rocks which were deposited during this interval.

History

The term was first used by Adam Sedgwick

Adam Sedgwick

Adam Sedgwick was one of the founders of modern geology. He proposed the Devonian period of the geological timescale...

 in 1847 with reference to the "Arenig Ashes and Porphyries" in the neighbourhood of Arenig Fawr

Arenig Fawr

Arenig Fawr is a mountain located in Snowdonia, North Wales. The mountain, which is the largest in the area, lies close to Llyn Celyn reservoir alongside the A4212 between Trawsfynydd and Bala.-Location:...

, in Merioneth, North Wales

North Wales

North Wales is the northernmost unofficial region of Wales. It is bordered to the south by the counties of Ceredigion and Powys in Mid Wales and to the east by the counties of Shropshire in the West Midlands and Cheshire in North West England...

. The rock-succession in the Arenig district has been recognized by W. G. Fearnsides

William George Fearnsides

William George Fearnsides FRS was a British geologist....

 (“On the Geology of Arenig Fawr and Moel Llanfnant", Q.J.G.S. vol. lxi., 1905, pp. 608–640, with maps). The above succession is divisible into:

1. A lower series of gritty and calcareous sediments, the “Arenig Series" as it is now understood;
2. A middle series, mainly volcanic, with shale, the "Llandeilo
   Llandeilo
   Llandeilo is a town in Carmarthenshire, Wales, situated at the crossing of the River Towy by the A483 on a 19th century stone bridge. Its population is 1,731.The town is served by Llandeilo railway station on the Heart of Wales Line.- Early history :...
   
    Series"; and
3. The shale and limestones of the Bala
   Bala Series
   In geology, the Bala Series is a group of dark slates and sandstones with beds of limestone which occurs in the neighborhood of Bala, Gwynedd, in north Wales....
   
    or Caradoc
   Caradoc
   Caradoc Vreichvras Arm) was a semi-legendary ancestor to the kings of Gwent. He lived during the 5th or 6th century. He is remembered in Arthurian legend as a Knight of the Round Table as Carados Briefbras ....
   
    Stage.

It was to the middle series (2) that Sedgwick first applied the term "Arenig". In the typical region and in North Wales generally the Arenig series appears to be unconformable upon the Cambrian

Cambrian

The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...

 rocks; this is not the case in South Wales.

The Arenig series is represented in North Wales by the Garth Grit and Ty Obry beds, by the Shelve series of the Corndon district, the Skiddaw

Skiddaw

Skiddaw is a mountain in the Lake District National Park in England. With a summit at 931 m above sea level it is the fourth highest mountain in England. It lies just north of the town of Keswick, Cumbria, and dominates the skyline in this part of the northern lakes...

 Slates of the Lake District, the Ballantrae

Ballantrae

Ballantrae is a community in Carrick, South Ayrshire, Scotland. The name probably comes from the Scottish Gaelic Baile na Tràgha, meaning the "town by the beach"....

 Group of Ayrshire

Ayrshire

Ayrshire is a registration county, and former administrative county in south-west Scotland, United Kingdom, located on the shores of the Firth of Clyde. Its principal towns include Ayr, Kilmarnock and Irvine. The town of Troon on the coast has hosted the British Open Golf Championship twice in the...

, and by the Ribband Series of slates and shale in Wicklow

Wicklow

Wicklow) is the county town of County Wicklow in Ireland. Located south of Dublin on the east coast of the island, it has a population of 10,070 according to the 2006 census. The town is situated to the east of the N11 route between Dublin and Wexford. Wicklow is also connected to the rail...

 and Wexford

Wexford

Wexford is the county town of County Wexford, Ireland. It is situated near the southeastern corner of Ireland, close to Rosslare Europort. The town is connected to Dublin via the M11/N11 National Primary Route, and the national rail network...

. It may be mentioned here that the "Llanvirn" Series of H. Hicks

Henry Hicks (geologist)

Henry Hicks, MRCS, FRS was a Welsh physician, surgeon, Member of the Royal College of Surgeons , geologist, President of the Geological Society and Fellow of the Royal Society...

 was equivalent to the bifidus shale and the Lower Llandeilo Series.

Geochronology

In the geologic timescale, the "Arenig" or Arenigian refers to an age of the Lower Ordovician epoch, between 478.6 ± 1.7 and 471.8 ± 1.6 million years ago, contemporary with the more recently proposed Floian by the ICS

ICS

-Communications and technology:* Image Cytometry Standard, a digital multidimensional image file format used in life sciences microscopy* Industrial Control System* Information and Computer Science, the combined field of informatics and computing...

, based on a section in Sweden (Diabasbrottet quarry

Diabasbrottet quarry

The Diabasbrottet quarry , located on Mt. Hunneberg, Västergötland, Sweden, is the location of the Global Boundary Stratotype Section and Point which marks the boundary between the Floian and Tremadocian stages of the Early Ordovician...

) and with the same boundaries. The Arenigian and Floian are the upper part of the Lower Ordovician and follow the Tremadocian (Gasconadian

Gasconadian Stage

The Gasconadian Stage is the first stage of the Ordovician geologic period in North America and of the Lower Ordovician Canadian Epoch, coming immediately after the Late Cambrian Trempealeauan and preceding the middle Canadian Demingian Stage...

 in North America) which is the lower part. Either is followed by the Middle Ordovician ICS Dapingian or by the Llanvirnian of older chronologies. The Arenigian and equivalent Floian are represented in North America by the upper three stages of the Canadian

Canadian Epoch

The Canadian is the Lower or Early Ordovician in North America. The term is common in the older literature and has been well understood for more than a century...

 which is followed by the Middle Ordovician Whiterockian which is the lower part of the now shortened Chazyan.

Events

The Arenig group was deposited during a sudden worldwide rise in sea level resulting in widespread marine transgression. The early Ordovician surge in marine diversity also began around this time.

Brachiopod fauna

Incertae sedis

Incertae sedis

, is a term used to define a taxonomic group where its broader relationships are unknown or undefined. Uncertainty at specific taxonomic levels is attributed by , , and similar terms.-Examples:*The fossil plant Paradinandra suecica could not be assigned to any...

brachiopod

Brachiopod

Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...

s of the Floian

• Eurorthisina
• Tegulella

Acrotretida

Acrotretida

The Acrotretida are a group of sessile, suspension-feeding brachiopods known from the Cambrian period and surviving until the present day. Their shell has a round outline, and is usually phosphatic in recent specimens.-Referemnces:...

 of the Floian

• Acanthambonia
• Cyrbasiotreta
• Eoconulus
• Issedonia
• Karnotreta
• Lurgiticoma
• Multispinula
• Mylloconotreta
• Numericoma
• Orthisocrania
• Polylasma
• Pomeraniotreta
• Pseudocrania
• Scaphelasma
• Schizotreta
• Torynelasma

Lingulida

Lingulida

Lingulida is an order of brachiopods....

 of the Floian

• Acanthorthis
• Aulonotreta
• Dictyobolus
• Ectenoglossa
• Elliptoglossa
• Monobolina
• Paldiskites
• Paterula
• Pseudolingula
• Quasithambonia
• Rafanoglossa
• Volborthia

Orthida

Orthida

Orthida is an extinct order of Brachiopods which appeared during the Early Cambrian period and became very diverse by the Ordovician, living in shallow-shelf seas. Orthids are the oldest member of the subphylum Rhynchonelliformea , and is the order from which all other brachiopods of this group...

 of the Floian

• Acanthorthis
• Acanthotoechia
• Anchigonites
• Angusticardinia
• Anomalorthis
• Antigonambonites
• Apomatella
• Astraborthis
• Crossiskenidium
• Dalmanella
• Desmorthis
• Diparelasma
• Eodiorthelasma
• Eosotrematorthis
• Estlandia
• Euorthisina
• Famatinorthis
• Fasciculina
• Ferrax
• Ffynnonia
• Fistulogonites
• Glossorthis
• Glypterina
• Hesperonomiella
• Incorthis
• Jaanussonites
• Ladogiella
• Lomatorthis
• Monorthis
• Munhella
• Nereidella
• Neumania
• Nocturneilla
• Notoscaphidia
• Oligorthis
• Orthambonites
• Orthidiella
• Orthidium
• Orthis
• Oslogonites
• Panderina
• Paralenorthis
• Paurorthina
• Paurorthis
• Phragmorthis
• Platystrophia
  Platystrophia
  Platystrophia is an extinct genus of brachiopod that lived from the Ordovician to the Silurian in Asia, Europe, North America, and South America.-Sources:* Fossils by David Ward -External links:...
  
  
• Platytoechia
• Polytoechia
• Pomatotrema
• Prantlina
• Productorthis
• Progonambonites
• Protohesperonomia
• Protoskenidiodes
• Pseudomimella
• Raunites
• Rhynchorthis
• Shoshonorthis
• Sinorthis
• Taphrorthis
• Tarfaya
  Tarfaya
  - References :CitationsBibliography* Didier Daurat, , France: Édition Dynamo, 1954....
  
  
• Treioria
• Trematorthis
• Valcourea
• Virgoria

Paternida of the Floian

• Dictyonites

Pentamerida

Pentamerida

Pentamerida is a brachiopods order in the class Rhynchonellata.Pentamerida have narrow hinge lines and looped brachidia....

 of the Floian

• Acanthorthis
• Acanthoglypha
• Boreadocamara
• Camerella
• Doloresella
• Hesperotrophia
• Idiostrophia
• Imbricatia
• Karakulina
• Liricamera
• Lycophoria
• Porambonites
• Rectotrophia
• Rosella
  Rosella
  A rosella is one of five to eight species of colorful Australian parrots in the genus Platycercus. Platycercus means "broad-tailed" or "flat-tailed", reflecting a feature common to the rosellas and other members of the broad-tailed parrot tribe...
  
  
• Rugostrophia
• Stenocamara
• Syntrophia
• Syntrophinella
• Xenelasma

Strophomenida

Strophomenida

Strophomenida

Strophomenida is a large, extinct order of articulate brachiopods that existed from the lower Ordovician to the lower Jurassic period. It was the largest known order of brachiopods, encompassing over 400 genera, including the largest and heaviest of known brachiopod shells...

 of the Floian

• Acanthorthis
• Ahtiella
• Aporthophyla
• Borua
• Calyptolepta
• Christiania
• Ingria
  Ingria
  Ingria is a historical region in the eastern Baltic, now part of Russia, comprising the southern bank of the river Neva, between the Gulf of Finland, the Narva River, Lake Peipus in the west, and Lake Ladoga and the western bank of the Volkhov river in the east...
  
  
• Leptestia
• Petroria
• Plectambonites
• Reinversella
• Schedophyla
• Taffia
• Tourmakeadia

Trimerellida of the Floian

• Dinobolus

Upper

The following is a list of Actinocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

• Leurorthoceras
• Nybyoceras
  Nybyoceras
  Nybyoceras is an actinocerid genus assigned to the Armenoceratidae and similar to Armenoceras except for having a siphuncle close to the ventral side of the shell.-Morphology:...
  
  
• Actinoceras
  Actinoceras
  Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.- Morphology:...
  
  
• Wutinoceras
  Wutinoceras
  Wutinoceras, orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.and the type genus of the Wutinoceratidae. -Distinguishing Characters:...
  
  
• Ormoceras
  Ormoceras
  Ormoceras is an actinocerid genus of the family Ormoceratidae which is found in North America from the late Chazyan though the early Cincinnatian of the Middle and Upper Ordovician, but continued through the Devonian worldwide....
  
  
• Adamsoceras
  Adamsoceras
  Adamsoceras is an actinocerid of the family Wutinoceratidae, with spheroidal siphuncle segments like Ormoceras but having a reticular canal system like Wutinoceras. Adamsoceras has a slender, gently expanding, orthoconic shell that is slightly broader than high, i.e...
  
  
• Georgina

Orthocerida

Orthocerids

Orthocerida

Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...

 of the Floian

• Eobactrites

Barrandeocerida of the Floian

• Plectoceras

Ellesmerocerida

Ellesmerocerida

The Ellesmerocerida is a order of primitive cephalopods belonging to the subclass Nautiloidea with a widespread distribution that lived during the Late Cambrian and Ordovician.-Morphology:...

 of the Floian

• Amsleroceras
• Apocrinoceras
• Avoceras
• Bakeroceras
• Baltoceras
  Baltoceras
  Baltoceras is a member of the Ellesmerocerida, included in the family, Baltoceratidae. The shell of Baltoceras is slender with a subcircular cross section, straight transverse sutures, and a large siphuncle in contact with the venter...
  
  
• Catoraphinoceras
• Cochlioceras
  Cochlioceras
  Cochlioceras is an extinct baltoceratid genus from the lower and middle Ordovician of what are now Europe, the U.S , and China, having existed for approximately 14 million years, from about 478 to 464 mya. -Taxonomy:...
  
  
• Copiceras
• Clelandoceras
• Cumberloceras
• Cyclostomiceras
• Cyrtobaltoceras
  Cyrtobaltoceras
  Cyrtobaltoceras is an extinct cephalopod genus known from the upper Lower Ordovician Fort Cassin Formation at Valcour, N.Y. that's included in the Nautiloid family Baltoceratidae-Taxonomy:...
  
  
• Desioceras
• Diaphoroceras
• Diastoloceras
• Dwightoceras
• Dyscritoceras
• Ectocycloceras
• Endorioceras
  Endorioceras
  Endorioceras is an extinct genus of actively mobile carnivorous cephalapod of the family Baltoceratidae that lived in what would be North America during the Ordovician from 490—479 mya, existing for approximately .-Taxonomy:...
  
  
• Eocyckistomiceras
• Eothinoceras
• Hemichoanella
  Hemichoanella
  The Genus Hemichoanella is a small, extinct, orthoconic nautiloid cephalopod from the Lower Ordovician of Western Australia assigned to the orthoceratoid family, Baltoceratidae....
  
  
• Irianoceras
• Kyminoceras
• Lawrenceoceras
• Meikeloceras
• Microbaltoceras
  Microbaltoceras
  Microbaltoceras is an extinct genus of actively mobile carnivorous cephalapod of the family Baltoceratidae, essentially a Nautiloid that lived in what would be North America during the Ordovician from 490—479 mya, existing for approximately ....
  
  
• Monogonoceras
• Ogygoceras
• Pictetoceras
• Protocycloceras
• Quebecoceras
  Quebecoceras
  Quebecoceras is an extinct genus from the nautiloid order Ellesmerocerida that lived during the Gasconadian Stage at the beginning of the Early Ordovician....
  
  
• Rangeroceras
  Rangeroceras
  Rangeroceras is an extinct nautiloid cephalopod genus that lived in what is now western North America during the latter part of the Early Ordovician....
  
  
• Rhabdiferous
• Rioceras
  Rioceras
  Rioceras is an extinct orthocerid cephalapod genus of the family Baltoceratidae that lived in what would be North America during the Early Ordovician from 480—472 mya, existing for approximately 8 million years.-Taxonomy:...
  
  
• Rudolfoceras
• Smithvilloceras
• Somalinautilus
• Somalinautilus
• Somalinautilus
• Vassaroceras
• Veneficoceras
  Veneficoceras
  Veneficoceras is a member of the rod-bearing Baltoceratidae, an extinct cephalopod family with characteristics of the orthoceratoid Dissidocerida, found in Cassinianage, Lower Ordovician, limestone in western Utah....
  
  
• Ventroloboceras

Endocerida

• Campendoceras
  Campendoceras
  Campendoceras is a proterocameroceratid genus from the Lower Ordovician of NW Australia and possibly Estonia that can be recognized by its slender, weakly endogastric, longiconic and annulate shell and large marginal siphuncle that contains endocones....
  
  
• Dartonoceras
• Kaipingoceras
• Kugeloceras
• Lobosiphon
• Manitouoceras
• Mcqueenoceras
  Mcqueenoceras
  Mcqueenoceras is an early endocerid, a nautilus from the Ordovician period similar in overall form to Clitendoceras, from which it may have been derived. It lived during the later stages of the lower Ordovician. McQueenoceras, like Clitendoceras, has ventral siphuncle but the endocones are thicker...
  
  
• Mysticoceras
• Notocycloceras
• Oderoceras
• Parapiloceras
• Phragmosiphon
• Platysiphon
• Pliendoceras
• Retroclitendoceras
• Stenosiphon
• Subpenhsioceras
• Utoceras
• Yorkoceras
• Vaginoceras
• Chisiloceras
• Cyrtovaginoceras
• Tallinnoceras
• Juaboceras
• Penhsioceras
• Ventrolobendoceras

Lower
The following is a list of Endocerid genera whose fossils are geochronologically found first in lower Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

• Allopiloceras
• Choreanoceroides
• Escharendoceras
• Lebetoceras
• Loxochoanella
• Sewardoceras
• Telleroceras
• Clitendoceras
  Clitendoceras
  Clitendoceras is a member of the Endocerida from the Lower Ordovician with an elongate shell with a slight downward, endogastric, curvature and a siphuncle that lay along the ventral margin. Common for endocerids, the chambers are short and the septa close spaced with sutures sloping forward...
  
  
• Coreanoceras
• Cotteroceras
  Cotteroceras
  Cotteroceras is a proterocameroceratid from the Lower Ordovician of North America and Siberia characterized by a long straight and compressed shell with very short camerae and long body chamber. Sutures are straight and oblique, sloping dorsoventrally toward the apex. The siphuncle is large but...
  
  
• Piloceras
• Endoceras
  Endoceras
  Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in its siphuncle.Endocerida comprises a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian . Their shells varied in form...
  
  
• Proterovaginoceras
  Proterovaginoceras
  Proterovaginoceras is a medium to large sized endocerid from the Early and Middle Ordovician included in the family Endoceratidae....
  
  
• Cyrtendoceras
• Anthoceras
  Anthoceras
  Anthoceras is a straight, annulated, proterocamerioceratid from the Lower Ordovician, found in North America, NW Australia, and Siberia. The cross section is circular, the siphuncle moderately large, and marginal...
  
  
• Chaohuceras
• Proterocameroceras
  Proterocameroceras
  Proterocameroceras is an early Endocerid from the upper Lower Ordovician belong to the Proterocameroceratidae, characterized by a rather large, straight, longiconic shell, short chambers, generally straight sutures, and large ventral siphuncle with short septal necks, thick complex connecting...
  
  
• Thylacoceras

Upper
The following is a list of Endocerid genera whose fossils are geochronologically found first in upper Arenig strata. These genera may survive into later portions of the Arenig stage, or even into later geological stages. This list should not be thought of in terms of the lifespan of the genera included.

• Allocotoceras
  Allocotoceras
  Allocotoceras is an endocerid from the Lower Ordovician of Australia , included in the Endoceratidae, based on small, straight or gently curved siphuncles....
  
  
• Cassinoceras
  Cassinoceras
  Cassinoceras is of the nautiloid genus belonging to the endocerid family Piloceratidae that comes from the late Early Ordovician of eastern North America and adjacent terratories....
  
  
• Chihlioceras
• Cyclocyrtendoceras
• Cyptendoceras
  Cyptendoceras
  Cyptendoceras is an extinct nautiloid cephalopod included in the family Ellesmeroceratidae that lived in what would be North and South America during the latter part of the Early Ordovician from about 475 - 472 mya, existing for approximately .-Taxonomy:Cyptendoceras was named by Ulrich and Foerste...
  
  
• Kirkoceras
• Lobendoceras
  Lobendoceras
  Lobendoceras is a proterocameraceratid with a rather large, moderately expanded, straight shell with a large marginal siphuncle in which sutures have a broad, deep, ventral lobe and septal necks are subholochoanitic to holochoanitic....
  
  
• Najaceras
  Najaceras
  Najaceras is a genus of straight, slender endocerid known only from the Whiterockian age Oil Creek Limestone of Oklahoma, introduced and named by Rousseau Flower in 1971 and further described by him in 1976....
  
  
• Protocyclendoceras
• Yehlioceras
• Nanno
  Nanno (animal)
  Nanno in cephalopod paleontology, as commonly used, refers to endocerids in which the siphuncle swells to fill the entire apex of the shell leaving septa and camerae to begin a few centimeters forward, having a morphologic rather than a taxonomic connotation.The term is based on the genus Nanno,...
  
   
• Dideroceras
• Lobocyclendoceras
• Meniscoceras
  Meniscoceras
  Meniscoceras is a straight and slender Chazyan endocerid described by Rousseau Flower in 1941 The genus was originally included in the Proterocameroceratidae but later placed with its predecessor, Najaceras, in the Najaceratidae.The siphuncle in Mensicoceras is large with short septal necks and...
  
  
• Paracyclendoceras
• Cacheoceras
• Perkinsoceras
  Perkinsoceras
  Perkinsocerasis an endocerid genus from the Middle Ordovician of Champlain Valley established by Flower in 1976, which he added to his Allotrioceratidae based on certain similarities to Williamsoceras and Cacheoceras which had been added previously.Perkinsoceras is characterized by a large ventral...
  
  
• Williamsoceras
  Williamsoceras
  Williamsoceras is an endocerid that Rousseau Flower added to his Allotrioceratidae on the basis of having a vertical partition within the siphuncle, known as a ventral process, with inter-connecting tubule-like structures along its margin where intercepted by endocones...
  
  
• Manchuroceras
• Schmidtoceras

Intejocerida

Intejocerida

Intejocerida is the name given to a group of generally straight shelled nautiloid cephalopods originally found in Lower and Middle Ordovician sediments in the Angara River basin in Russia; defined in the Treatise as an order, and combined there with the Endocerida in the...

 of the Floian

• Bajkaloceras
  Bajkaloceras
  Bajkaloceras is a straight-shelled orthoceroid, and possibly a member of the Intejocerida, from the Angara River basin in central Russia, named by Balashov in 1962...
  
  
• Evencoceras
• Intejoceras
• Rossoceras

Oncocerids of the Floian

• Phthanoncoceras
• Valhalloceras

Nautiloids of the Floian

• Buttsoceras
• Centroonoceras
  Centroonoceras
  Centroonoceras is a middle Ordovician cyrtoconic nautiloid cephalopod, otherwise similar to the orthoconic Sactorthoceras and also included in the Sactorthoceratidae...
  
  
• Gangshanoceras
• Geisonoceras
• Glenisteroceras
• Michelinoceras
  Michelinoceras
  Michelinoceras is the oldest known genus of the Michelinocerida, more commonly known as the Orthocerida, characterized by long, slender, nearly cylindrical orthocones with a circular cross section, long camerae, very long body chambers, and a central or near central tubular siphuncle free of...
  
  
• Orthoceras
  Orthoceras
  Orthoceras is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros ....
  
  
• Oxfordoceras
• Rhynchorthoceras
  Rhynchorthoceras
  Rhynchorthoceras is a Middle Ordovician genus characterized by a rapidly expanded, weakly annulate orthocone, like the orthoconic section of Ancistroceras, but with only a curved, cyrtoconic apex instead of juvenile whorls....
  
  
• Stereoplasmoceras
• Tajaroceras
  Tajaroceras
  Tejaroceras is an extinct slender cephalopod from the uppermost Lower Ordovician of western North America, belonging to the Orthocerid family Troedssonellidae....
  
  
• Wardoceras
  Wardoceras
  Wardoceras is an extinct nautiloid genus from the late Early Ordovician of Western Utah, assigned to the orthocerid family, Michelinoceratidae...
  
  

Tarphycerida

Tarphycerida

The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...

• Deltoceras
• Pseudancistroceras
• Seelyoceras

Lower

• Alaskoceras
• Moreauoceras
• Pilotoceras
• Shumardoceras
• Pycnoceras
• Campbelloceras
  Campbelloceras
  Campbelloceras: a tarphyceratid with a circular whorl section, only slightly impressed, and a siphuncle that is close to the venter in all growth stages. Differs from Tarphyceras in that the rate of expansion is greater, the siphuncle is proportionally largers, and an impression is shallower...
  
  
• Aphetoceras
  Aphetoceras
  Aphetoceras is a genus of Tarphycerida cephalopod within the Estonioceratidae; loosely coiled without an impression along the dorsal margin; early whorls barely teaching, separating then diverging in the final mature whorl; weakly ribbed in some. The cross section of Aphetoceras is higher than...
  
  

Upper

• Aethoceras
• Bentoceras
• Centrotarphyceras
• Clytoceras
• Cycloplectoceras
• Eichwaldoceras
• Eurystomites
• Hardmanoceras
  Hardmanoceras
  Hardmanoceras is a Tarphycerid genus belonging to the Trochilidae from the upper Lower Ordovician to possibly the lower Middle Ordovician, found in Western Australia. Hardmanoceras is like Discoceras but prominently ribbed and with a depressed whorl section. The body, or living, chamber is long,...
  
  
• Holmiceras
  Holmiceras
  Holmiceras is one of two ancestral lituitids from the late Early Ordovician . The other being Ancistroceras.Holmiceras begins with about 1.5 to 2 loosely coiled whorls followed by a rapidly expanding orthocone, much like Ancistroceras except for the apical whorls and that Holmiceras is known only...
  
  
• Pionoceras
• Trocholitoceras
• Wichitoceras
• Tarphyceras
  Tarphyceras
  Tarphyeras is a genus of tarphyceratid with whorls rounded in cross section, having a deeply impressed dorsum and a ventral to subcentral siphuncle, known from the Lower Ord of North America...
  
  
• Litoceras
  Litoceras
  Litoceras is a trocholitid genus that has been found the Lower and early Middle Ordovician of Newfoundland. Whorls in Litoceras have a broadly rounded cross section with the width greater than the height and somewhat resembles the tarphyceratid Pionoceras from the same time, except for the...
  
  
• Curtoceras
  Curtoceras
  Curtoceras is a genus in the tarphycerid family Trocholitidae found widespread in the late Early and Middle Ordovician of North America and northern Europe. Curtoceras has a shell that is gradually expanded, with half the fully mature body chamber divergent from the preceding volution. Whorl...
  
  
• Arkoceras
• Estonioceras
  Estonioceras
  Estonioceras is an extinct genus of tarphyceridan nautiloid from the Ordovician of Europe.-Sources:* Dinosaur Encyclopedia by Jayne Parsons* Fossils by David Ward-External links:* in the Paleobiology Database...
  
  
• Tragoceras

Trilobite fauna

Trilobites of the Floian

• Canningella
• Gogoella
• Macrogrammus
  Macrogrammus
  Macrogrammus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Priceaspis
  Priceaspis
  Priceaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Thymurus
  Thymurus
  Thymurus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  

Agnostida

Agnostida

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...

 of the Floian'

• Galbagnostus
• Geragnostella

Asaphida

Asaphida

Asaphida is a large, morphologically diverse order of trilobite found in strata dated from the Middle Cambrian to the Silurian. The order contains six superfamilies , but no suborders...

 of the Floian

• Phthanoncoceras
• Ampyx
  Ampyx
  Ampyx has several meanings; in hair care, an ampyx is a headband, often made of metal. In Greek mythology, there were a number of figures with the name Ampyx, Amycus or Ampycus ....
  
  
• Ampyxoides
• Anebolithus
  Anebolithus
  Anebolithus is a Trinucleus species of Trilobite found in Gilwern Hill, Powys, Wales.-Features of Trinucleid Trilobites:-External references:http://www.kgg.org.uk/anebolithush.html...
  
  
• Asaphus
  Asaphus
  Asaphus is a genus of Ordovician trilobites....
  
  
• Aspidaeglina
  Aspidaeglina
  Aspidaeglina is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Australopyge
  Australopyge
  Australopyge is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Bergamia
• Bohemopyge
• Borogothus
  Borogothus
  Borogothus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Bumastides
  Bumastides
  Bumastides is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Ceratolithus
  Ceratolithus
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Cloacaspis
  Cloacaspis
  Cloacaspis is an extinct genus of Olenid Ptychopariid trilobite. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago...
  
  
• Cnemidopyge
• Degamella
• Dionide
• Dionidella
• Ellipsotaphrus
• Falanapis
  Falanapis
  Falanapis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Famatinolithus
  Famatinolithus
  Famatinolithus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Globampyx
  Globampyx
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Gog
  Gog
  Gog, Gogg or Gogs may refer to:Biblical:* Gog and MagogPeople:* Anikó Góg, Hungarian triathlete* Gog or "gogledd", a person from North Wales from the Welsh word for "north"...
  
  
• Hanchungolithus
• Hoekaspis
  Hoekaspis
  Hoekaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Hungioides
• Hunnebergia
  Hunnebergia
  Hunnebergia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Isocolus
• Isoteloides
• Lachnostoma
  Lachnostoma
  Lachnostoma is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Lannacus
  Lannacus
  Lannacus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Lapidaria
  Lapidaria (animal)
  Lapidaria is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Liomegalaspides
• Megalaspidella
  Megalaspidella
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Megalaspides
  Megalaspides
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Mendolaspis
  Mendolaspis
  Mendolaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Merlinia
  Merlinia
  Merlinia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Microparia
• Mioptychopyge
• Myttonia
  Myttonia
  Myttonia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Ningkianolithus
  Ningkianolithus
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Niobides
• Ogmasaphus
• Ogyginus
  Ogyginus
  Ogyginus is an extinct genus of Asaphida trilobite that lived during the Ordovician period. Commonly found in Wales.-External links:*...
  
  
• Ogygiocaris
• Opipeuter
  Opipeuter
  Opipeuter is a genus of lizard....
  
  
• Parabasilicus
• Paraptychopyge
  Paraptychopyge
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Plesiomegalaspis
  Plesiomegalaspis
  Plesiomegalaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Presbynileus
• Pricyclopyge
• Psilacella
• Psilocara
  Psilocara
  Psilocara is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Ptychopyge
• Rhombampyx
• Robergiella
• Seleneceme
• Stapeleyella
• Taihungshania
• Thysanopyge
  Thysanopyge
  Thysanopyge is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Trigonocercella
  Trigonocercella
  Trigonocercella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Tungtzuella
  Tungtzuella
  Tungtzuella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Zhenganites
• Zuninaspis
  Zuninaspis
  Zuninaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  

Corynexichida of the Floian

• Bumastus
  Bumastus
  Bumastus is an extinct genus of trilobites which existed from the Early Ordovician period to the Late Silurian period. They were relatively large trilobites, reaching a length of . They were distinctive for their highly globular, smooth-surfaced exoskeleton...
  
  
• Dysplanus
• Ectillaenus
• Panderia
• Phillipsinella
• Pseudocalymene
• Theamataspis

Lichida

Lichida

Lichida is an order of typically spiny trilobite that lived from the Ordovician to the Devonian period.-Morphology:These trilobites usually have 8-13 thoracic segments. Their exoskeletons often have a grainy texture or have tubercles...

 of the Floian

• Apatolichas
  Apatolichas
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Autoloxolichas
• Lichakephalina
  Lichakephalina
  Lichakephalina is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Metopolichas

Odontopleurida

Odontopleurida

Odontopleurida is an order of trilobites, containing the following taxa:Odontopleuridae*Acanthalomina*?Acidaspidella*?Acidaspides*Acidaspis*Anacaenaspis*Apianurus*Archaeopleura*Boedaspis*Borkopleura...

 of the Floian

• Phthanoncoceras
• Ceratocephala
• Diacanthaspis
• Selenopeltis
  Selenopeltis
  Selenopeltis is an extinct genus of trilobite from the Ordovician of Africa.- Sources :* http://www.fossilmuseum.net/Fossil_Galleries/TrilobitesMorocco/Selenopeltis-buchii/Selenopeltis-buchii.htm...
  
  

Phacopida

Phacopida

Phacopida is an order of trilobite that lived from the Ordovician to the Devonian. It is made up of a morphologically diverse group of related suborders....

 of the Floian

• Bathycheilus
• Calymenella
• Ceraurinella
• Colobinion
• Colpocoryphe
  Colpocoryphe
  Colpocoryphe is a genus of trilobites in the family Calymenidae.-Morphology:* Convex Cephalon* Trapezoidal glabellea, frequently distinctly furrowed and surelevated compared to the axis....
  
  
• Cybelopsis
• Cybelurus
• Diaphanometopus
  Diaphanometopus
  Diaphanometopus is an extinct genus of trilobite in the order Phacopida.-External links:* at the Paleobiology Database...
  
  
• Dindymene
  Dindymene
  Dindymene is an extinct genus of trilobite in the order Phacopida. It contains two species, D. didymograpti, and D. hughesiae.-External links:* at the Paleobiology Database...
  
  
• Eccoptochile
  Eccoptochile
  Eccoptochile is an extinct genus of trilobite in the order Phacopida. It contains one species, E. clavigera.-External links:* at the Paleobiology Database...
  
  
• Ectenonotus
• Encrinurella
• Encrinuroides
• Evropeites
• Gyrometopus
  Gyrometopus
  Gyrometopus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Heliomeroides
• Kanoshia
• Kawina
  Kawina
  Kawina is an extinct genus of trilobite in the order Phacopida. It contains one species, K. wilsoni.-External links:* at the Paleobiology Database...
  
  
• Kolymella
  Kolymella
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Lehua
  Lehua
  Lehua is a small, crescent-shaped island in the Hawaiian islands, only north of Niihau, due west of Kauai. The uninhabited, barren island is a tuff cone which is part of the extinct Niihau volcano....
  
  
• Lyrapyge
  Lyrapyge
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Neseuretus
• Nieszkowskia
• Ormathops
• Ovalocephalus
• Placoparia
• Platycoryphe
• Pliomeridius
• Pliomerops
• Protoencrinurella
• Protopliomerella
• Pseudocybele
• Pterygometopus
• Strotactinus
• Sycophantia
  Sycophantia
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Synhomalonotus
• Toletanaspis
• Xystocrania

Proetida

Proetida

Proetida is an order of trilobite that lived from the Ordovician to the Permian. It was the last order of trilobite to go extinct, finally dying out in the Permian extinction....

 of the Floian

• Acidiphorus
• Bathyuriscops
  Bathyuriscops
  Bathyuriscops is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Benthamaspis
  Benthamaspis
  Benthamaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Biolgina
  Biolgina
  Biolgina is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Bolbocephalus
  Bolbocephalus
  Bolbocephalus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Carolinites
• Celmus
  Celmus
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 471 to 478 million years ago....
  
  
• Ceratopeltis
  Ceratopeltis
  Ceratopeltis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Decoroproetus
• Dimeropygiella
  Dimeropygiella
  Dimeropygiella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Eleutherocentrus
  Eleutherocentrus
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Goniophrys
• Goniotelina
• Grinnellaspis
  Grinnellaspis
  Grinnellaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Ischyrophyma
• Ischyrotoma
• Lutesvillia
  Lutesvillia
  Lutesvillia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Oenonella
  Oenonella
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Petigurus
• Rananasus
  Rananasus
  Rananasus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Raymondaspis
• Telephina

Ptychopariida

Ptychopariida

Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician...

 of the Floian

• Anaximander
  Anaximander
  Anaximander was a pre-Socratic Greek philosopher who lived in Miletus, a city of Ionia; Milet in modern Turkey. He belonged to the Milesian school and learned the teachings of his master Thales...
  
  
• Annamitella
• Balnibarbi
  Balnibarbi
  Balnibarbi is an extinct genus olenid ptychopariid trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago...
  
  
• Bulnibarbi
• Bvalbardites
  Bvalbardites
  Bvalbardites is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Circulocrania
  Circulocrania
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Endymionia
• Etheridgaspis
  Etheridgaspis
  Etheridgaspis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Furcalithus
  Furcalithus
  Furcalithus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Gymnostomix
  Gymnostomix
  Gymnostomix is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Lacorsalina
  Lacorsalina
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Leioshumardia
• Lordshillia
  Lordshillia
  Lordshillia is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Nambeetella
  Nambeetella
  Nambeetella is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Novakella
• Oopsites
• Peraspis
• Phorocephala
• Porterfieldia
• Prosopiscus
• Pytine
  Pytine
  Pytine is an extinct genus of trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Sagavia
• Selenoharpes
• Stegnopsis
• Stenorhachis
  Stenorhachis
  Stenorhachis is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Svalbardites
  Svalbardites
  Svalbardites is an extinct genus of trilobites. It lived during the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....
  
  
• Tasmanocephalus
  Tasmanocephalus
  Tasmanocephalus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Arenig stage of the Ordovician Period, a faunal stage which lasted from approximately 478 to 471 million years ago....
  
  
• Turgicephalus
• Yinpanolithus
  Yinpanolithus
  is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the later part of the Arenig stage of the Ordovician Period, approximately 478 to 471 million years ago....