Sphaeragnostus

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Sphaeragnostus
Temporal range: Ordovician
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Sphaeragnostus cingulatus linedrawing.jpg
Sphaeragnostus cingulatus, linedrawing
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Trilobita
Order: Agnostida
Family:Sphaeragnostidae
Kobayashi, 1939
Genus:Sphaeragnostus
Howell & Resser in Cooper & Kindle, 1936
Species
  • S. similaris(Barrande, 1872) (Type) synonym Agnostus similaris
  • S. cingulatus(Olin, 1906) synonym Agnostus cingulatus, S. cerus, S. gaspensis europeensis, S. subcircularis [1]
  • S. gaspensis gaspensisHowell & Resser in Cooper & Kindle, 1936
  • S. gaspensis quxianensisJu in Qiu et al, 1983
  • S. nudatusKoroleva, 1982
  • S. orientalisPetrunina

Sphaeragnostus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It can be recognized by having two thorax segments, a totally effaced headshield (or cephalon), while the tailshield (or pygidium) although effaced, has a clear furrow parallel to its border, and a short, convex, subcircular axis. It lived during the Ordovician (upper Tremadoc to Ashgill). [2]

A genus is a taxonomic rank used in the biological classification of living and fossil organisms, as well as viruses, in biology. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus.

Fossil Preserved remains or traces of organisms from a past geological age

A fossil is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved in amber, hair, petrified wood, oil, coal, and DNA remnants. The totality of fossils is known as the fossil record.

Arthropod phylum of animals

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

Contents

Taxonomy

It is difficult to relate Sphaeragnostus to other agnostids, and it is undecided whether this genus should be assigned to the Agnostoidea, the Condylopygoidea, or a new monotypic superfamily. [1]

Distribution

Description

Like all Agnostida, Sphaeragnostus is diminutive and the cephalon and pygidium are of approximately the same size and outline (or isopygious). Like all Agnostina, Sphaeragnostus has only two thorax segments. The cephalon is externally totally effaced and lacks border furrows, axial furrows, and the glabellar node. The pleural regions of thorax segments are very small, while the axis is very wide (perpendicular to the midline) and remarkably short (along the axis). The pygidium has well-defined border furrows and narrow borders. Axial furrows define a short, broad, convex, subcircular, and unfurrowed axis, that bears both axial and terminal nodes. It lacks a median postaxial furrow. If there are spines on the ”rear corners” of the pygidium, these are minute. [2]

Pygidium

The pygidium is the posterior body part or shield of crustaceans and some other arthropods, such as insects and the extinct trilobites. It contains the anus and, in females, the ovipositor. It is composed of fused body segments, sometimes with a tail, and separated from thoracic segments by an articulation.

Differences with Leiagnostus

Cephalons of Sphaeragnostus are extremely effaced and look much like Leiagnostus species. Pygidia of Sphaeragnostus however, differ from other highly effaced genera in a smooth but easily recognizable subcircular axis.

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>Acidiscus</i>

Acidiscus is an extinct genus of eodiscinid agnostid trilobites. It lived during the Botomian stage of the Cambrian period.

<i>Acimetopus</i>

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

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.

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.

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

Pseudonaraoia is a genus of small marine arthropods within the family Naraoiidae, that lived during the late Middle Ordovician period. The only species presently known is Pseudonaraoia hammanni.

<i>Weymouthia</i> (trilobite) genus of arthropod (fossil)

Weymouthia is an extinct genus of eodiscinid agnostid trilobites, which lived at the end of the Lower Cambrian, in what are now the eastern United States, England, Siberia and China.

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

Trinodus is a very small to small blind trilobite, a well known group of extinct marine arthropods, which lived during the Ordovician, in what are now the Yukon Territories, Virginia, Italy, Czech Republic, Poland, Denmark, Sweden, Svalbard, Ireland, Scotland, Wales, Iran, Kazakhstan and China. It is one of the last of the Agnostida order to survive.

Marocconus

Marocconus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It is still debated if it lived at the very end of the Lower Cambrian or at the very beginning of the Middle Cambrian. Marocconus notabilis is the only known species in this genus.

<i>Mallagnostus</i>

Mallagnostus 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 USA, Canada (Newfoundland), Spain, England, Russia, Mongolia, and the lower Middle Cambrian according to fossils from China and Russia (Yakutia).

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

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

<i>Thoracocare</i>

Thoracocare is a minute to very small 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, 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.

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.

<i>Cyclopyge</i> (trilobite) genus of arthropods (fossil)

Cyclopyge is a genus of small to average size trilobites that lived during the Ordovician. Like all members of the family Cyclopigidae, it has very large convex eyes, that cover most of the free cheeks, and in some species touch each other. The eyes almost touch the large glabella. The occipital ring has merged with the rest of the glabella. The glabella does not extend into a frontal thorn. The cephalon lacks genal spines. The 6 thorax segments have short pleurae. The pygidium is rather large, and often rather effaced. These are features that also occur in other Cyclopygidae, and are indications of a pelagic lifestyle.

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.

Plutonides is a genus of trilobite, an extinct group of marine arthropods. Species occur in the middle Middle Cambrian of Russia (Siberia), Mongolia, England and Wales, Sweden, eastern Canada, and the Czech Republic. The frontal lobe of the central raised area of the headshield overhangs the short frontal border and it is slightly pointed, rather than rounded or truncate. This character is shared with Anabaraspis, but in Anabaraspis there is a wide area in front of the glabella without a differentiated border and preglabellar field.

<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. 1 2 3 Ahlberg, P. (1989). "Agnostid trilobites from the Upper Ordovician of Sweden and Bornholm, Denmark" (PDF). Bulletin of the Geological Society of Denmark. 37: 213–226.
  2. 1 2 Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997


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