Cambrian Series 2

Cambrian Series 2 is the unnamed 2nd series of the Cambrian. It lies above the Terreneuvian series and below the Miaolingian. Series 2 has not been formally defined by the International Commission on Stratigraphy, lacking a precise lower boundary and subdivision into stages. The proposed lower boundary is the first appearance of trilobites which is estimated to be around 521 million years ago.[1]


The International Commission on Stratigraphy has not named the 2nd series of the Cambrian yet.[1] The new name will replace the older terms "Lower Cambrian" and "Early Cambrian". The nomenclature used in Siberia uses the term "Yakutian" for this series.[2]


The 2nd series is currently subdivided by the ICS into two stages: Cambrian Stage 3 and Cambrian Stage 4. Both of these stages also lack formal definition.[1] The Siberian nomenclature distinguishes three stages (lowest first): Atdabanian, Botomian and Toyonian.[2] In general most subdivisions of this series rely on biostratigraphy of trilobite zones.[3]


The beginning of the 2nd series of the Cambrian is marked by the appearance of trilobites. Correlating this event on different continents has proven difficult and resolving this is essential for the definition of the lower boundary of this series. Currently the oldest trilobite known is Lemdadella which marks the beginning of the Fallotaspis zone.[3]

The end of the 2nd series of the Cambrian is marked by the first major biotic extinction of the Paleozoic. Changes in ocean chemistry and the marine environment are posited as the most likely cause of these extinctions.[4]


  1. ^ a b c "GSSP Table - Paleozoic Era". Retrieved 18 November 2012.
  2. ^ a b "The 13th International Field Conference of the Cambrian Stage Subdivision Working Group" (PDF). Episodes. 31 (4): 440–441. Archived from the original (PDF) on 2014-11-05. Retrieved 2012-11-23.
  3. ^ a b Yuan, J.L.; Zhu, X.J.; Lin, J.P.; Zhu, M.Y. (22 September 2011). "Tentative correlation of Cambrian Series 2 between South China and other continents" (PDF). Bulletin of Geosciences: 397–404. doi:10.3140/bull.geosci.1274. Retrieved 23 November 2012.
  4. ^ Zhang, Wenhao; et al. (2014). "Mass-occurrence of oncoids at the Cambrian Series 2–Series 3 transition: Implications for microbial resurgence following an Early Cambrian extinction". Gondwana Research. 28: 432–450. doi:10.1016/

The Anomalocaridids comprise a group of very early marine animals known primarily from fossils found in Cambrian deposits in China, United States, Canada, Poland and Australia. They were long thought to be restricted to this Cambrian time range, but the discovery of large Ordovician specimens has extended this somewhat. The later Devonian Schinderhannes shows many anomalocaridid features. Although originally interpreted as an anomalocaridid-like arthropod, some recent studies suggest that it may represent an anomalocaridid: if so it would extend the group's record by some hundred million years: the non-mineralised anomalocaridid structure means they are absent from the intermediate fossil record.Anomalocaridids are the largest Cambrian animals known—some Chinese forms may have reached 2 m (7 ft) in length—and most of them were probably active carnivores.


Ascidiacea (commonly known as the ascidians or sea squirts) is a paraphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic" made of the polysaccharide cellulose.

Ascidians are found all over the world, usually in shallow water with salinities over 2.5%. While members of the Thaliacea and Larvacea (Appendicularia) swim freely like plankton, sea squirts are sessile animals after their larval phase: they then remain firmly attached to their substratum, such as rocks and shells.

There are 2,300 species of ascidians and three main types: solitary ascidians, social ascidians that form clumped communities by attaching at their bases, and compound ascidians that consist of many small individuals (each individual is called a zooid) forming colonies up to several meters in diameter.

Sea squirts feed by taking in water through a tube, the oral siphon. The water enters the mouth and pharynx, flows through mucus-covered gill slits (also called pharyngeal stigmata) into a water chamber called the atrium, then exits through the atrial siphon.


Biceratopsidae is an extinct family of redlichiid trilobites, with species of small to average size. Species of belonging to this family lived during the Toyonian stage (Olenellus-zone), 522–513 million years ago, in the former continent of Laurentia, including what are today the south-western United States and Canada. It contains the subfamilies Biceratopsinae and Bristoliinae.


The Biceratopsinae is an extinct subfamily of redlichiid trilobites within the family Biceratopsidae, with species of small to average size. Species belonging to this subfamily lived during the Toyonian stage (Upper Olenellus-zone), 516-513 million years ago, in the former continent of Laurentia, including what are today the South-Western United States and Canada.

Cambrian Stage 3

Cambrian Stage 3 is the still unnamed third stage of the Cambrian. It succeeds Cambrian Stage 2 and precedes Cambrian Stage 4, although neither its base nor top have been formally defined. The plan is for its lower boundary to correspond approximately to the first appearance of trilobites, about 521 million years ago, though the globally asynchronous appearance of trilobites warrants the use of a separate, globally synchronous marker to define the base. The upper boundary and beginning of Cambrian Stage 4 is informally defined as the first appearance of the trilobite genera Olenellus or Redlichia around 514 million years ago.

Cambrian Stage 4

Cambrian Stage 4 is the still unnamed fourth stage of the Cambrian and the upper stage of Cambrian Series 2. It follows Cambrian Stage 3 and lies below the Wuliuan. The lower boundary has not been formally defined by the International Commission on Stratigraphy. One proposal is the first appearance of two trilobite genera, Olenellus or Redlichia. Another proposal is the first appearance of the trilobite species Arthricocephalus chauveaui. Both proposals will set the lower boundary close to 514 million years ago. The upper boundary corresponds to the beginning of the Wuliuan.


Corynexochida is an order of trilobite that lived from the Lower Cambrian to the Late Devonian. Like many of the other trilobite orders, Corynexochida contains many species with widespread characteristics.

The middle region of the cephalon (the glabella) is typically elongate, with the sides often spreading forward (pestle-shaped). Some species have glabellae that are effaced, meaning they are smooth and show little detail. The glabellar furrows (when not effaced) typically have a splayed arrangement. In most species, the hind pair on either side of the cephalon become spines that point sharply backwards, and the spinose tips of the anterior pairs of thoracic segments tend to become more and more forward directed toward the pygidium. The eyes are typically large. Pygidia are typically large, competing in size with the cephalon in some species.

The tips of the thoracic segments of many Corynexochida species are spine-like (though in some species they are flush with the sides and smooth). The thorax can have 2-12 segments (rarely more), but they more typically have 7-8.


Crustaceans (Crustacea ) form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimps, prawns, krill, woodlice, and barnacles.

The crustacean group is usually treated as a class under subphylum Mandibulata and because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the Pancrustacea clade other than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans.

The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 3.8 m (12.5 ft) and a mass of 20 kg (44 lb). Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.

Most crustaceans are free-living aquatic animals, but some are terrestrial (e.g. woodlice), some are parasitic (e.g. Rhizocephala, fish lice, tongue worms) and some are sessile (e.g. barnacles). The group has an extensive fossil record, reaching back to the Cambrian, and includes living fossils such as Triops cancriformis, which has existed apparently unchanged since the Triassic period. More than 7.9 million tons of crustaceans per year are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as widely fished, but may be the animals with the greatest biomass on the planet, and form a vital part of the food chain. The scientific study of crustaceans is known as carcinology (alternatively, malacostracology, crustaceology or crustalogy), and a scientist who works in carcinology is a carcinologist.

End-Botomian mass extinction

The end-Botomian mass extinction event, also known as the late early Cambrian extinctions, refer to two extinction intervals that occurred during Stages 4 and 5 of the Cambrian Period, approximately 513 to 509 million years ago. Estimates for the decline in global diversity over these events range from 50% of marine genera up to 80%. Among the organisms affected by this event were the small shelly fossils, archaeocyathids (an extinct group of sponges), trilobites, brachiopods, hyoliths, and mollusks .


Hurdiidae is a clade of anomalocaridan. It is the most long-lived anomalocaridan clade, lasting from the Cambrian period to the Devonian period.


The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet") are worm-like taxa with stubby legs.

The scope of the Lobopodian concept varies from author to author. Its most limited sense refers to a suite of mainly Cambrian panarthropod taxa with flexible non-segmented limbs – for example Aysheaia, Hallucigenia and Xenusion. A broader definition of lobopodia would also incorporate the extant phyla Onychophora and Tardigrada. The broadest definition proposes the (monophyletic) Superphylum Lobopodia to encompass all three panarthropod phyla.The oldest near-complete fossil lobopodians date to the Lower Cambrian; some are also known from Ordovician, Silurian and Carboniferous Lagerstätten. Some bear toughened claws, plates or spines, which are commonly preserved as carbonaceous or mineralized microfossils in Cambrian strata.


The Mesonacinae comprise an extinct subfamily of trilobites that lived during the Botomian, found in North-America, Greenland and North-Western Scotland. The two genera in this subfamily are Mesonacis and Mesolenellus.


The Miaolingian is the third Series of the Cambrian period, and was formally named in 2018. It lasted from about 509 to 497 million years ago and is divided into 3 stages: the Wuliuan, the Drumian, and the Guzhangian. The Miaolingian is preceded by the unnamed Cambrian Series 2 and succeeded by the Furongian series.


Nectocaris pteryx is a species of possible cephalopod known from the "early Cambrian" (Series 2) Emu Bay Shale and Chengjiang biota, the "middle Cambrian" (Mioalingian, Wuliuan) Burgess Shale.

Nectocaris was a free-swimming, predatory or scavenging organism. This lifestyle is reflected in its binomial name: Nectocaris means "swimming shrimp" (from the Ancient Greek νηκτόν, nekton, meaning "swimmer" and καρίς, karis, "shrimp"; πτέρυξ, pteryx, means "wing"). Two morphs are known: a small morph, about an inch long, and a large morph, anatomically identical but around four times longer.

The closely related Ordovician taxon Nectocotis is a second genus, closely resembling Nectocaris, but possessing an internal skeletal element.


Olenellidae is an extinct family of redlichiid trilobite arthropods. Olenellids lived during the late Lower Cambrian (Botomian/Toyonian) in the so-called Olenellus-zone in the former paleocontinent of Laurentia plus parts of what became the Famatinian orogen in what is now Argentina. This family can be distinguished from most other Olenellina by the partial merger of the frontal (L3) and middle pair (L2) of lateral lobes of the central area of the cephalon, that is called glabella, creating two isolated slits.


Olenellina is a suborder of the order Redlichiida of Trilobites that occurs about halfway during the Lower Cambrian, at the start of the stage called the Atdabanian. The earliest trilobites in the fossil record are arguably Olenellina, although the earliest Redlichiina and Eodiscina follow quickly. The suborder died out when the Lower passed into the Middle Cambrian, at the end of the stage called Toyonian. A feature uniting the Olenellina is the lack of rupture lines (or sutures) in the headshield, which in other trilobites assist the periodic moulting (or ecdysis), associated with arthropod growth. Some derived trilobites have lost facial sutures again (some Eodiscina, all Agnostina, and a few Phacopina), but all of these are blind, while all Olenellina have eyes.


The Olenelloidea are a superfamily of trilobites, a group of extinct marine arthropods. They lived during the late Lower Cambrian and species occurred on all paleocontinents.


Radiodonta is a clade of stem-group arthropods that was successful worldwide during the Cambrian period, and included the earliest large predators known. Some of the most famous species included in Radiodonta are the Cambrian taxa Anomalocaris canadensis, Hurdia victoria, and Peytoia nathorsti, the Ordovician Aegirocassis benmoulai and the Devonian Schinderhannes bartelsi.


Yukoniidae is a family of trilobites, belonging to the Eodiscina, small trilobites with headshield and tailshield of equal size and shape, and with two or three thorax segments.

Cenozoic era
(present–66.0 Mya)
Mesozoic era
(66.0–251.902 Mya)
Paleozoic era
(251.902–541.0 Mya)
Proterozoic eon
(541.0 Mya–2.5 Gya)
Archean eon (2.5–4 Gya)
Hadean eon (4–4.6 Gya)


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