Acmarhachis

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Acmarhachis
Temporal range: Upper Cambrian
Scientific classification
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Acmarhachis

Resser, 1938
Species
  • A. typicalisResser, 1938 (type)
  • A. acuta(Kobayashi, 1938)
  • A. anhuiensis(Qiu, 1983) synonym Wanagnostus anhuiensis
  • A. apicula Öpik, 1967 synonym Oxyagnostus apicula
  • A. elegans(Lermontova, 1940) [1] synonym Cyclagnostus elegans Lerm., 1940
  • A. karatauensisErgaliev, 1980
  • A. longispinusErgaliev, 1980
  • A. punctatusErgaliev, 1980
  • A. whittingtoniWestrop & Eoff, 2012 [2]
Synonyms

Cyclagnostus, Wanagnostus

Acmarhachis is a genus of trilobite in the order Agnostida, which lived in what are now Australia (Queensland, Tasmania), Canada (British Columbia, Newfoundland, Northwest Territories), China (Anhui), Kazakhstan, Russia (Kharaulakh), and the US (Alaska, Alabama, Nevada, Maryland, Vermont). It was described by Resser in 1938, and the type species is Acmarhachis typicalis. [3]

Contents

Distribution

Description

Like all Agnostida, Acmarhachis is diminutive and the headshield (or cephalon) and tailshield (or pygidium) are of approximately the same size (or isopygous) and outline. As in all Agnostina, Acmarhachis has two thorax segments. When Agnostina are enrolled, a tiny opening between the cephalon and the thorax becomes visible (the cephalothoracic aperture) that uniquely distinguishes this group from the Eodiscina. A feature shared with all Agnostoidea is that the frontal lobe of the central raised area of the cephalon (or glabella) is not laterally expanded compared to the backward lobe, like in the Condylopygoidea. In Acmarhachis the furrow at midline between the front of the glabella and the border is weak or absent. The glabella has a long ogival to subquadrate front lobe, and the furrow defining it at its back is straight. The back lobe of the glabella is accompanied by two triangular basal lobes of moderate to large size. On the pygidium the axis or rhachis reaches the border. It is divided along its length in three lobes. It is shaped a bit like an amphora, the back pointed and strongly constricted across the middle lobe. The middle lobe also carries a node where it contacts the posterior lobe. [2]

Related Research Articles

<i>Agnostus</i> Extinct genus of trilobites

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

<i>Acimetopus</i>

Acimetopus Rasetti, 1966, is a genus of Eodiscinid trilobite belonging to the family Weymouthiidae Kobayashi (1943), Order Agnostida Salter (1864). It lived during the Botomian stage. = late Lower Cambrian Stage 4 ; the upper Botomian boundary corresponds to base of the Middle Cambrian, Miaolingian Series and Wuliuan stage.

Meniscuchus is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the Botomian stage, which lasted from approximately 522 to 516 million years ago. This faunal stage was part of the Cambrian Period. Meniscuchus has been found in the USA, Canada, Russia and Australia.

Semadiscus 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 (Newfoundland), Russia, and the United States. Only the headshield is known, and it may well be that it would be better to include it in Serrodiscus.

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>

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>Lejopyge laevigata</i> Extinct species of trilobite

Lejopyge laevigata is a species of agnostid trilobite belonging to the genus Lejopyge. It existed during the Guzhangian to the Paibian Age of the Cambrian. It has a cosmopolitan distribution and is an important index fossil in biostratigraphy.

Eodiscina

Eodiscina is trilobite suborder. The Eodiscina first developed near the end of the Lower Cambrian period and became extinct at the end of the Middle Cambrian. Species are tiny to small, and have a thorax of two or three segments. Eodiscina includes six families classified under one superfamily, Eodiscoidea.

<i>Mallagnostus</i>

Mallagnostus Howell, 1935, is a trilobite genus belonging to the family Weymouthiidae Kobayashi T. (1943), Order Agnostida Salter (1864) according to Whittington et al. 1997. It lived during the late Lower Cambrian, with remains found in USA, Canada (Newfoundland), Spain, England, Russia, Mongolia, and the early Middle Cambrian as reported from China and Russia (Yakutia).

<i>Galbagnostus</i>

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

Peronopsidae

The Peronopsidae comprise the earliest family of the Agnostina suborder. Species of this family occurred on all paleocontinents. The earliest representatives of this family first occur just before the start of the Middle Cambrian, and the last disappeared just after the start of the Upper Cambrian.

<i>Delgadella</i>

Delgadella is a diminutive trilobite that lived during the late Lower Cambrian and has been found in Russia, Mongolia, Spain, Italy (Sardinia), Portugal, Morocco and Canada (Newfoundland). It can be recognized by its strongly effaced headshield and tailshield, with narrow but distinct furrows and borders along its margins, and three thorax segments.

<i>Geragnostus</i> Extinct genus of trilobites

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>

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.

Eodiscidae

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.

The Hebediscidae Kobayashi, 1944, are a family of trilobites belonging to the order Agnostida that lived during the Lower Cambrian. They are small or very small, and have a thorax of two or three segments. The Hebediscidae include five genera.

The Calodiscidae Kobayashi, 1943 [nom. transl. Öpik, 1975 ex Calodiscinae Kobayashi, 1943] are a family of trilobites belonging to the order Agnostida that lived during the Lower Cambrian. They are small or very small, and have a thorax of two or three segments. The Calodiscidae includes five genera.

<i>Cedaria</i>

Cedaria is a small, rather flat trilobite with an oval outline, a headshield and tailshield of approximately the same size, 7 articulating segments in the middle part of the body and spines at the back edges of the headshield that reach halflength of the body. Cedaria lived during the early part of the Upper Cambrian (Dresbachian), and is especially abundant in the Weeks Formation.

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.

References

  1. Arthropoda - Klass trilobity (Class Trilobita). EV Lermontova, Atlas of the Leading Forms of the Fossil Faunas of the USSR, 1940
  2. 1 2 Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997
  3. Available Generic Names for Trilobites P.A. Jell and J.M. Adrain.
  4. Ergaliev, G.K.; Ergaliev, F.G. (2008). Agnostidy srednego i verkhnego Kembriya Aksayskogo Gosudarstvennogo Geologicheskogo Zakaznika v yushnom Kazakhstane (Kyrshabakty, Malyy Karatau) [Middle and Upper Cambrian Agnostida from the Aksai National Geological Reserve in southern Kazakhstan (Kyrshabakty River, Malyy Karatau Range)]. pp. 1–359.cited inPaleobiological Database. "Acmarhachis whittingtoni" . Retrieved 17 December 2021.
  5. Westrop, S.R.; Eoff, .J.D. (2012). "Late Cambrian (Furongian; Paibian, Steptoean) agnostoid arthropods from the Cow Head Group, western Newfoundland". Journal of Paleontology. 86 (2): 201–237. doi:10.1666/11-034.1.cited inPaleobiological Database. "Acmarhachis whittingtoni" . Retrieved 17 December 2021.