Agnostida

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Agnostida
Temporal range: Cambrian Stage 3 –Late Ordovician
Itagnostus interstrictus (White, 1874) - 8 mm 1.JPG
Itagnostus interstrictus
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
(unranked):† Artiopoda
Subphylum:† Trilobitomorpha
Class:† Trilobita (?)
Order:† Agnostida
Salter, 1864
Families

Suborder Agnostina

Suborder Eodiscina

Synonyms

Isopygia Gürich, 1907
Miomera Jækel, 1909

Agnostida are an order of extinct arthropods which have classically been seen as a group of highly modified trilobites, though some recent research has doubted this placement. Regardless, they appear to be close relatives as part of the Artiopoda. [1] They are present in the Lower Cambrian fossil record along with trilobites from the Redlichiida, Corynexochida, and Ptychopariida orders, and were highly diverse throughout the Cambrian. Agnostidan diversity severely declined during the Cambrian-Ordovician transition, and the last agnostidans went extinct in the Late Ordovician. [2]

Contents

Systematics

The Agnostida are divided into two suborders — Agnostina and Eodiscina  — which are then subdivided into a number of families. As a group, agnostids are isopygous, meaning their pygidium is similar in size and shape to their cephalon. Most agnostid species were eyeless.

The systematic position of the order Agnostida within the class Trilobita remains uncertain, and there has been continuing debate whether they are trilobites or a stem group. The challenge to the status has focused on Agnostina partly due to the juveniles of one genus have been found with legs differing dramatically from those of adult trilobites, [3] suggesting they are not members of the lamellipedian clade, of which trilobites are a part. Instead, the limbs of agnostids closely resemble those of stem group crustaceans, although they lack the proximal endite, which defines that group. The study suggested that they were likely the sister taxon to the crustacean stem lineage, and, as such, part of the clade, Crustaceomorpha. [4] Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal features, that Eodiscina and Agnostida are closely united, and the Eodiscina descended from the trilobite order Ptychopariida. [5] A 2019 study of adult specimens with preserved soft tissue from the Burgess Shale found that agnostidans shared morphological similarities to trilobites and other related artiopodans like nektaspids, and their placement as stem-crustaceans was unsupported. The study recovered agnostidans as the sister group to other trilobites within the Artiopoda. [1]

Ecology

Scientists have long debated whether the agnostids lived a pelagic or a benthic lifestyle. Their lack of eyes, a morphology not well-suited for swimming, and their fossils found in association with other benthic trilobites suggest a benthic (bottom-dwelling) mode of life. They are likely to have lived on areas of the ocean floor which received little or no light and fed on detritus which descended from upper layers of the sea to the bottom. Their wide geographic dispersion in the fossil record is uncharacteristic of benthic animals, suggesting a pelagic existence. The thoracic segment appears to form a hinge between the head and pygidium allowing for a bivalved ostracodan-type lifestyle. The orientation of the thoracic appendages appears ill-suited for benthic living. Recent work suggests that some agnostids were benthic predators, engaging in cannibalism and possibly pack-hunting behavior. [6]

They are sometimes preserved within the voids of other organisms, for instance within empty hyolith conchs, [7] within sponges, worm tubes and under the carapaces of bivalved arthropods, [8] presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. [8] In the case of the tapering worm tubes Selkirkia , trilobites are always found with their heads directed towards the opening of the tube, suggesting that they reversed in; the absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter. [8]

Related Research Articles

<span class="mw-page-title-main">Trilobite</span> Class of extinct, Paleozoic arthropods

Trilobites are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetida died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 251.9 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 270 million years, with over 22,000 species having been described.

<span class="mw-page-title-main">Nektaspida</span> Extinct order of arthropods

Nektaspida is an extinct order of non-mineralised artiopodan arthropods. They are known from the mid-Cambrian to the upper Silurian. Originally classified as trilobites, which they superficially resemble, they are now placed as close relatives as members of the Trilobitomorpha within Artiopoda. The order is divided into three major families; Emucarididae, Liwiidae, and Naraoiidae.

<span class="mw-page-title-main">Olenellina</span> Extinct suborder of trilobites

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,Ptychopariida, and Eodiscina follow quickly. The suborder died out when the Lower Cambrian passed into the Middle Cambrian, at the end of the stage called Toyonian. A feature uniting the Olenellina is the lack of rupture lines in the headshield, which in other trilobites assist the periodic moulting, associated with arthropod growth. Some derived trilobites have lost facial sutures again, but all of these are blind, while all Olenellina have eyes.

<span class="mw-page-title-main">Cheloniellida</span> Order of arthropods (fossil)

Cheloniellida is a taxon of extinct Paleozoic arthropods. As of 2018, 7 monotypic genera of cheloniellids had been formally described, whose fossils are found in marine strata ranging from Ordovician to Devonian in age. Cheloniellida has a controversial phylogenetic position, with previous studies associated it as either a member or relative of various fossil and extant arthropod taxa. It was later accepted as a member of Vicissicaudata within Artiopoda.

<i>Acidiscus</i>

Acidiscus Rasetti, 1966, is a genus of Eodiscinid trilobite belonging to the family Weymouthiidae Kobayashi T. (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.

<i>Bolboparia</i>

Bolboparia is an extinct genus of eodiscinid agnostid trilobites. It lived during the late Lower Cambrian in what today Canada and the United States.

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

<i>Tsunyidiscus</i> Genus of trilobites

Tsunyidiscus is a trilobite belonging to the Suborder Eodiscina. Tsunyidiscus appeared near the end of the Lower Cambrian, during the late Atdabanian stage of geologic time and some collections suggest it may have survived into the Botomian. The genus is very small, oculate and isopypous with a narrow dome-shaped glabella and a narrow bullet-shaped pygidial axis. Thorax consists of three segments. Tsunyidiscus is the only genus currently attributed to the family Tsunyidiscidae.

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

Ptychagnostus is a member of the agnostida that lived during the Cambrian period. Ptychagnostidae generally do not exceed one centimetre in length. Their remains are rarely found in empty tubes of the polychaete worm Selkirkia. The genus probably ranged throughout the water column. It has two glabellar lobes, and three pygidial lobes.

<span class="mw-page-title-main">Crustaceomorpha</span> Clade of arthropods

Crustaceomorpha is a proposed clade of arthropods that includes crustaceans and numerous extinct groups. Synapomorphies for the clade are that the larval antenna is a feeding or locomotory organ, and there are six endopodal podomeres in post-antennal limbs.

The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segments. The word cephalon derives from the Greek κεφαλή (kephalē), meaning "head".

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.

<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.

<span class="mw-page-title-main">Eodiscina</span>

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>Galbagnostus</i>

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

<i>Sphaeragnostus</i>

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, while the tailshield although effaced, has a clear furrow parallel to its border, and a short, convex, subcircular axis. It lived during the Ordovician.

<span class="mw-page-title-main">Crustacean</span> Subphylum of arthropods

Crustaceans belong to the subphylum Crustacea, and form a large, diverse group of arthropods including decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, opossum shrimps, amphipods and mantis shrimp. The crustacean group can be treated as a subphylum under the clade Mandibulata. It is now well accepted that the hexapods emerged deep in the Crustacean group, with the completed group referred to as Pancrustacea. The three classes Cephalocarida, Branchiopoda and Remipedia are more closely related to the hexapods than they are to any of the other crustaceans.

Tannudiscus Pokrovskaya (1959) is a genus of Eodiscinid trilobites belonging to the family Weymouthiidae Kobayashi T. (1943), Order Agnostida. It lived during the late Lower Cambrian, with remains found in Canada (Newfoundland), China (Gansu), The United Kingdom (England), and the Russian Federation.

<span class="mw-page-title-main">Artiopoda</span> Extinct group of arthropods

The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997 to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Trilobites, in part due to their mineralising exoskeletons, are by far the most diverse and long lived members of the clade, with most records of other members, which lack mineralised exoskeletons, being from Cambrian deposits.

<i>Retifacies</i> Species of arthropod

Retifacies abnormalis is an extinct arthropod that lived in the lower Cambrian. Its fossil remains have been found in the Maotianshan Shales of Yunnan, China. It is a member of the Artiopoda, and closely related to Pygmaclypeatus.

References

  1. 1 2 Moysiuk J, Caron JB (January 2019). "Burgess Shale fossils shed light on the agnostid problem". Proceedings. Biological Sciences. 286 (1894): 20182314. doi:10.1098/rspb.2018.2314. PMC   6367181 . PMID   30963877.
  2. ↑ Barnes BD, Sclafani JA, Zaffos A (April 2021). "Dead clades walking are a pervasive macroevolutionary pattern". Proceedings of the National Academy of Sciences of the United States of America. 118 (15). doi: 10.1073/pnas.2019208118 . PMC   8053996 . PMID   33827921. S2CID   233184869.
  3. ↑ Müller KJ, Walossek D (1987). "Morphology, ontogeny, and life habit of Agnostus pisiformis from the Upper Cambrian of Sweden". Fossils and Strata. 19: 1–124.
  4. ↑ Bergström J, Hou XG (2005). "Early Palaeozoic non-lamellipedian arthropods". In Koenemann S, Ronald AJ (eds.). Crustacea and Arthropod Relationships. Taylor and Francis Group. pp. 75–93. ISBN   978-1-4200-3754-8.
  5. ↑ Cotton TJ, Fortey RA (2005). "Comparative morphology and relationships of the Agnostida". In Koenemann S, Jenner RA (eds.). Crustacea and Arthropod Relationships. Vol. 16. Taylor and Francis Group. pp. 95–136. ISBN   978-1-4200-3754-8.
  6. ↑ McMenamin MA (October 2010). "Cambrian cannibals: agnostid trilobite ethology and the earliest known case of arthropod cannibalism". Geological Society of America Abstracts with Programs. 42 (5): 320.
  7. ↑ Fatka O, Vokáč V, Moravec J, Šinágl M, Valent M (January 2009). "Agnostids entombed in hyolith conchs". Memoirs of the Association of Australasian Palaeontologists. 37: 481–489.
  8. 1 2 3 Chatterton BD, Collins DH, Ludvigsen R (2003). "Cryptic behaviour in trilobites: Cambrian and Silurian examples from Canada, and other related occurrences". In Lane PD, Siveter DJ, Fortey RA (eds.). Trilobites and Their Relatives. Special Papers in Palaeontology. Vol. 70. The Palaeontological Association. pp. 157–173. ISBN   978-0-901702-81-4.
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