Thoracocare

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Thoracocare
Temporal range: Lower Middle Cambrian
(Albertella and Glossopleura-zones)
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Thoracocare max enlarged.jpg
Thoracocare minuta, 2 mm along the midline, Spence Formation
Scientific classification
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Thoracocare

Robison and Campbell, 1974
species
  • T. minutaRobison and Campbell, 1974 (type) synonym Vistoia minuta
  • T. idahoensis(Resser, 1939) synonym Tonkinella idahoensis

Thoracocare is a minute to very small (1.7 – 3.6 mm long) trilobite, that lived during part of the Middle Cambrian in what are today the states of Idaho, Nevada and Utah. It is the only trilobite known with just two thorax segments outside most members of the Agnostida order. It can be distinguished from Agnostida by the very wide subquadrate glabella (about half the cephalon), parallel-side or widening forward in the largest specimen, with the full front side touching the border. Two species are known, one, T. idahoensis, only from pygidia. [1]

Thorax frontal part of an animals body, between its head and abdomen

The thorax or chest is a part of the anatomy of humans and various other animals located between the neck and the abdomen. The thorax includes the thoracic cavity and the thoracic wall. It contains organs including the heart, lungs, and thymus gland, as well as muscles and various other internal structures. Many diseases may affect the chest, and one of the most common symptoms is chest pain.

Agnostida order of arthropods (fossil)

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. The last agnostids went extinct in the Late Ordovician.

Contents

Etymology

Thoracocare is derived from the Greek θώραξ (thorax) meaning "breastplate" and ἀκαρής (akares) meaning "tiny".

Distribution

Ecology

Thoracocare minuta occurs in the same deposit as Pagetia rugosa , Pentagnostus bonnerensis , Elrathina spencei, Oryctocare sp., Oryctocephalus walcotti, and Ogygopsis typicalis. Scholars consider that Thoracocare minuta may have lived in the water column (a pelagic lifestyle) rather than on the seafloor.

<i>Pagetia</i> genus of trilobites

Pagetia is a genus of very small, agnostid trilobites, assigned to the family Eodiscidae, and that had a global distribution during the Middle Cambrian. The genus contains 55 currently recognized species, each with a limited spatial and temporal distribution.

<i>Pentagnostus</i> genus of trilobites (fossil)

Pentagnostus is a genus of trilobites restricted to the Middle Cambrian. Its remains have been found in Siberia, Australia, North America, Scandinavia, and Kazakhstan.

Description

Thoracocare is a very small to minute trilobite (1.7 – 3.6 mm long in adulthood), with an elliptical body outline. The headshield (or cephalon) and tailshield (or pygidium) are of approximately same size (or isopygous) with 2 thoracic segments between them. The central raised area of the cephalon (or glabella) is broad, subrectangular, tapering slightly to the back in larger specimens and touching the frontal border, and without transverse furrows. Fixed cheeks are subtriangular. Free cheeks are rarely present. Dorsal sutures are proparian. The pygidium is subcircular, about 1⅔× as wide as long. The segmentation of the pygidial axis (or rhachis) and pleura is indiscernible or very weak. The rhachis is about ⅓ of the pygidial width anteriorly, tapering evenly backwards. There is no pygidial border. There may be small spines extending from the frontal corners of the pygidium.

Differences with other Corynexochida

All other known corynexochoid genera have at least five thorax segments. Other early (Middle Cambrian) species also differ by having less wide glabellas and better defined pygidial furrows.

Differences with Agnostida

In Thoracocare minuta the glabella is more than ⅓ of the width of the cephalon, is parallel sided or expands forward and reaches the frontal border. It also lacks a pygidial border.

Related Research Articles

<i>Agnostus</i> genus of trilobites (fossil)

Agnostus is a genus of agnostid trilobites that lived during the upper Middle Cambrian–lower Upper Cambrian. It is the type genus of the family Agnostidae. It is subdivided into two subgenera, Agnostus and Homagnostus.

<i>Acimetopus</i>

Acimetopus is a genus of trilobites that lived during the Botomian stage.

<i>Dicerodiscus</i>

Dicerodiscus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the early part of the Botomian stage, in China. Four species have been assigned to it. Dicerodiscus is unique for an eodiscoid in having conspicuous and curved spines that are attached anteriorly, and at their base are directed outward perpendicular to the midline, before gradually bending further backwards.

Acadagnostus is a genus of trilobite from the Middle Cambrian, with 7 species currently recognized. The type species A. acadicus has the widest distribution known from any peronopsid and has been found in North America, Greenland, England, Western Europe, Eastern Europe, Central Asia, the Altai Mountains, the Siberian shield, China, and Australia.

Acmarhachis is a genus of trilobite in the order Agnostida, which lived in what are now Australia, Canada, China (Anhui), Kazakhstan, Russia (Kharaulakh), and the US. It was described by Resser in 1938, and the type species is Acmarhachis typicalis.

Lotagnostus is a genus of very small trilobites in the order Agnostida, which lived on the outer continental shelves worldwide, during the late Upper Cambrian. It was described by Whitehouse in 1936, and the type species is Lotagnostus trisectus, which was originally described as a species of Agnostus by Salter in 1864.

<i>Phalagnostus</i> genus of trilobites (fossil)

Phalagnostus is a genus of small trilobites, in the order Agnostida. It lived during the Middle Cambrian, in what are now Canada, China, the Czech Republic, Denmark, England, France, the Russian Federation, Wales, Sweden, and possibly the United States (Vermont). The headshield is almost entirely effaced and wider than the tailshield. The pygidium is also very effaced, but the ovate pygidial axis is well defined and a border furrow is also present.

Eodiscina suborder of arthropods (fossil)

Eodiscina is a suborder of trilobites, a well known group of marine arthropods. The Eodiscina first developed near the end of the Lower Cambrian period and became extinct at the end of the Middle Cambrian. They are small or very small, and have a thorax of two or three segments. Eodiscina includes six families classified under one superfamily, Eodiscoidea.

<i>Galbagnostus</i> genus of trilobites (fossil)

Galbagnostus is an extinct genus of agnostid trilobite. It lived during the Lower and Middle Ordovician.

Weymouthiidae

The Weymouthiidae are an extinct family of eodiscinid agnostid trilobites. They lived during the late Lower Cambrian and earliest Middle Cambrian in the so-called Olenellus- and Eokochaspis-zones in the former paleocontinents of Laurentia, Avalonia, Gondwana. The Weymouthiidae are all blind and lack free cheeks.

<i>Pleuroctenium</i> genus of trilobites (fossil)

Pleuroctenium is a genus of very small agnostid trilobites whose fossils are found in Middle Cambrian-aged marine strata of Canada, Czech Republic, England and Wales, France, and Sweden. Species of Pleuroctenium can be easily distinguished from all other agnostids because the frontal lobe of the central raised area of the headshield is wider than and folds around the rear lobe.

Diplorrhina is a genus of trilobites, a well known class of extinct marine arthropods. It lived during the early Middle Cambrian in what are now the Czech Republic and the North Siberian plateau. Like all agnostina it has a headshield (or cephalon and tailshield (or pygidium of approximately the same shape and size, and two thorax segments. Like other members of the Peronopsidae family, it lacks a furrow connecting the furrow surrounding the central raise area of the cephalon and the furrow that defines the border of the cephalon. Both the cephalon and the pygidium lack spines. It is difficult to distinguish from many other peronopsids.

<i>Geragnostus</i> genus of trilobites (fossil)

Geragnostus is a genus of very small agnostid trilobites whose fossils are found Ordovician-aged marine strata from Eurasia, North America and Argentina.

<i>Toragnostus</i> genus of arthropods (fossil)

Toragnostus is a genus of trilobites restricted to the late Middle Cambrian. Its remains have been found in the United States, Greenland, Denmark, China, Sweden, the Russian Federation, and Kazakhstan. Its headshield and tailshield are almost completely effaced and it has two thorax segments.

Ovatoryctocara is a genus of small corynexochid trilobites from the Cambrian, that lived in what now are Siberia, China, Greenland and Canada (Newfoundland). Ovatoryctocara can be recognised by the combination of the following characters: the central raised area of the cephalon is approximately cylindrical and has two rows of four triangular or round pits. The thorax only has 5 or 6 segments. The tailshield has an axis of 6 to 12 rings, the pleural furrows are well developed and the border is absent or narrow as a hair.

Eodiscidae family of arthropods (fossil)

Eodiscidae is a family of agnostid trilobites that lived during the final Lower Cambrian and the Middle Cambrian. They are small or very small, and have a thorax of two or three segments. Eodiscidae includes nine genera.

Tannudiscus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the upper Lower Cambrian, with remains found in Canada (Newfoundland), China (Gansu), The United Kingdom (England), and the Russian Federation.

Anabaraspis is a genus of redlichiid trilobite. A. splendens occurs in the uppermost Lower Cambrian and lowest Middle Cambrian of Russia. In Anabaraspis there is a long area in front of the glabella which is not differentiated in a border and a preglabellar field. This is a unique character in the family Paradoxididae. The frontal lobe of the central raised area of the headshield is slightly pointed, rather than rounded or truncate, a character shared with Plutonides, though, in Plutonides it hangs over the short anterior border.

<i>Serrodiscus</i>

Serrodiscus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It has been collected from the Lower Cambrian of Canada, Germany (Silesia), Poland, Russia, the United Kingdom (Wales) and the United States. It is named for the spines on the ventral side of the tailshield (or pygidium, which give it a serrated impression.

References

  1. Robison, R.A.; Campbell, D.P. (1974). "A Cambrian corynexochoid trilobite with only two thoracic segments". Lethaia. 7 (4): 273–282. doi:10.1111/j.1502-3931.1974.tb00903.x.