Eodiscina

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Eodiscina
Temporal range: Lower and Middle Cambrian
Eodiscus punctatus head up CRF.jpg
Internal mould of Eodiscus punctatus scanicus , 8mm in length. collected from the lower part of the Menevia Formation (Mid Cambrian Drumian Stage), Mawddachites hicksi Biozone, of Dwrhyd near Nine Wells on the St David's Peninsula, SW Wales.
Scientific classification OOjs UI icon edit-ltr.svg
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class:† Trilobita (?)
Order:† Agnostida
Suborder:† Eodiscina
Kobayashi, 1939
Superfamily:† Eodiscoidea
Raymond, 1913
Families

Eodiscina is trilobite suborder. The Eodiscina first developed near the end of the Lower Cambrian period (late Atdabanian) and became extinct at the end of the Middle Cambrian. Species are tiny to small, and have a thorax of two or three segments. [1] Eodiscina includes six families classified under one superfamily, Eodiscoidea.

Contents

Taxonomy

Cladogram of the relations between the families of the Eodiscina, according to Jell, 1975 Cladogram Eodiscina.jpg
Cladogram of the relations between the families of the Eodiscina, according to Jell, 1975

The Eodiscina are mostly considered the more primitive suborder of the Agnostida, and the Agnostina the more advanced. [3] Some scholars do not consider the Agnostina true trilobites, and consequently rejected the idea that they were related to the Eodiscina. [1] [4] Consequently, these scientists have proposed to elevate the group to ordinal level, which would thus be called Eodiscida Kobayashi, 1939. [5]

Origin

The oldest known eodiscoid is Tsunyidiscus . The glabella of Tsunyidiscus is extremely similar to that of Dipharus clarki, and distinct from all other eodiscoids. D. clarki is thought to represent an immature stage of the redlichioid Bulaiaspis rather than an eodiscoid. This is because of the dominant palpebroocular ridges, extremely long librigenae, and free pleural tips on the pygidium of variable numbers of segments. In short: Tsunyidiscus has probably developed through paedomorphosis from Bulaiaspis. Alternatively Sinodiscus changyangensis has been considered the most primitive of the Eodiscina.

Many characters that are divergent from other trilobites may be explained by paedomorphosis, such as the small number of thorax segments and proparian facial sutures, that occur in all trilobites during larval development, while the Eodiscoid abatochroal eye is not unlike early development stages of the holochroal eye, where lenses are also separated. [1]

Relations within the Agnostida

Three lineages are thought to have evolved from the monotypical family Tsunyidiscidae. First the Calodiscidae. Second the Yukoniidae, who sprouted the Eodiscidae. And finally the Hebediscidae, that themselves gave rise to the Weymouthiidae, which contain Tannudiscus, the probable ancestor of the peronopsid genus Archaeagnostus, [6] the earliest of the Agnostina. [1]

Differences with some other simplified trilobites

Agnostina and a few other trilobites also have very few thorax segments. All Agnostina lack eyes and only have two thorax segments, characters they share with some of the later Eodiscina. Eodiscina however always have an articulating half-ring. This seals the opening in the axis between cephalon and the anterior thorax segment that is created when the animal was enrolled. Agnostida do not have an articulating half-ring, resulting in an opening between the thorax and the cephalon when enrolled, called cephalothoracic aperture. Thoracocare (Corynexochida) has only two thoracic segments at maturity, but has a very wide subquadrate glabella that touches the frontal border along its entire width and lacks a pygidial border. [4] [7] Taklamakania , Pseudampyxina and Nanshanaspis (Raphiophoridae) have 3 thorax segments, but with subtriangular cephalon and pygidium, genal spines, and Taklamakania with a frontal glabellar spine. [8]

Description

Pagetia taijiangensis, 3mm, from Kaili, Guizhou, China Pagetia tajianensis kaili 1.jpg
Pagetia taijiangensis , 3mm, from Kaili, Guizhou, China

Like all Agnostida, members of the Eodiscina are relatively small and isopygous with the cephalon and pygidium of approximately similar size and outline. Thorax consisting of two or three fulcrate segments. Cephalon strongly parabolic in outline with maximum width (tr.) usually anterior to genal angle and either occulate or lacking eyes. The border is convex. Hypostome is natant. Rostral plate is lacking or uncalcified. Facial sutures are present in early Eodiscina, but are lost in some later representatives. When present, the facial sutures are proparian. Outline of the pygidium closely matches that of the cephalon, is usually segmented, with axis almost extending to the border, and in some species the pleural region is segmented. Examples of the protaspid growth stage are known in a few species. [1]

Eye

If present, the eyes of Eodiscina are unique among trilobites. They have up to 70 small lenses per eye, not touching but separated from each other by the so-called interlensar sclera. Each lens has an individual cornea, that is limited to the lens surface. The interlensar sclera is not deeper than the lenses. This type of eye is called abathochroal. A second type of eye, called schizochroal, is unique to the Phacopina suborder, and has up to several hundreds of relatively large lenses (0.05-0.5 mm), that do not touch either, but the corneal membrane extends into the sclera outside the lens, and the sclera is deeper than the lens. The most usual type of trilobite eye, called holochroal, has usually thousands of small hexagonal lenses without an interlensar sclera, and a common cornea. [9] Many later Eodiscina however, lost their visual organs altogether, and this may have resulted from living in an environment with very low visibility. [1]

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 trilobites 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">Redlichiida</span> Extinct order of trilobites

Redlichiida is an order of trilobites, a group of extinct marine arthropods. Species assigned to the order Redlichiida are among the first trilobites to appear in the fossil record, about halfway during the Lower Cambrian. Due to the difficulty to relate sediments in different areas, there remains some discussion, but among the earliest are Fallotaspis, and Lemdadella, both belonging to this order. The first representatives of the orders Corynexochida and Ptychopariida also appear very early on and may prove to be even earlier than any redlichiid species. In terms of anatomical comparison, the earliest redlichiid species are probably ancestral to all other trilobite orders and share many primitive characters. The last redlichiid trilobites died out before the end of the Middle Cambrian.

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

The Phacopina comprise a suborder of the trilobite order Phacopida. Species belonging to the Phacopina lived from the Lower Ordovician (Tremadocian) through the end of the Upper Devonian (Famennian). The one unique feature that distinguishes Phacopina from all other trilobites are the very large, separately set lenses without a common cornea of the compound eye.

<span class="mw-page-title-main">Phacopidae</span> Extinct family of trilobites

Phacopidae is a family of phacopid trilobites that ranges from the Lower Ordovician to the Upper Devonian, with representatives in all paleocontinents.

<span class="mw-page-title-main">Emuellidae</span> Extinct family of trilobites

Emuellidae are a small family of trilobites, a group of extinct marine arthropods, that lived during the late Lower Cambrian of the East Gondwana supercontinent, in what are today South-Australia and Antarctica.

<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.

<i>Huntoniatonia</i> Genus of trilobites

Huntoniatonia is genus of trilobites, an extinct group of marine arthropods of average to large size.

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.

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

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.

Resserops is an extinct genus from a well-known class of fossil marine arthropods, the trilobites. It lived during the middle of the Atdabanian or the early part of the Botomian stage, which lasted from approximately 524 to 518.5 million years ago. This faunal stage was part of the Cambrian Period. It has been found in Spain and southern Morocco. It can be recognised by the sabre-like spines of the headshield that are a smooth continuation of the frontal edge, and the enlarged spines on the 9th segment of the thorax.

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

Yunnanocephalus is a genus of ptychopariid trilobite. It lived during the late Atdabanian and Botomian stages, in what are currently Antarctica, Australia and China. It was a "moderately common" member of the Chengjiang Fauna. Yunnanocephalus is the only genus currently assigned to the Yunnanocephalidae family.

<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.

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>Odontochile</i> Genus of trilobites

Odontochile is a genus of trilobites in the order Phacopida, family Dalmanitidae.

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

Richterops is an extinct genus of trilobite arthropods. The genus lived during the middle of the Atdabanian or the early part of the Botomian stage, which lasted from approximately 524 to 518.5 million years ago. This faunal stage was part of the second half of the Lower Cambrian. It has been found in southern Morocco. It can be recognised by the long spines of the headshield that are a smooth continuation of the frontal edge, and the enlarged spines on the 11th segment of the thorax.

<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.

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

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.

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

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.

References

  1. 1 2 3 4 5 6 Whittington, H. B. et al. Part O, Treatise on Invertebrate Paleontology. Revised, Volume 1 – Trilobita – Introduction, Order Agnostida, Order Redlichiida. 1997
  2. ↑ Jell, P.A. (1975). "Australian Middle Cambrian Eodiscoids with a review of the superfamily". Palaeontographica Abteilung A. 150: 1–97. cited in W.B. Wittington; et al., eds. (1993). Treatise on Invertebrate Paleontology, Part O., Revised.
  3. ↑ Cotton, T.J.; Fortey, R.A. (2005). "5. Comparative morphology and relationships of the Agnostida". In Koenemann, S.; Jenner, R. (eds.). Crustacean Issues 16, Crustacea and Arthropod Relationships. Boca Raton: CRC Press.
  4. 1 2 Shergold, John H. (1991). "Protaspid and early meraspid growth stages of the eodiscoid trilobite Pagetia ocellata Jell, and their implications for classification". Alcheringa: An Australasian Journal of Palaeontology. 15 (1): 65–86. Bibcode:1991Alch...15...65S. doi:10.1080/03115519108619010.
  5. ↑ Westrop, S.R.; Landing, E. (2012). "Lower Cambrian (Branchian) eodiscoid trilobites from the lower Brigus formation, Avalon Peninsula, Newfoundland, Canada". Memoirs of the Association of Australasian Palaeontologists. 42: 209–262.
  6. ↑ Naimark, E.B. (2012). "Hundred species of the Genus Peronopsis Hawle et Corda, 1847". Paleontological Journal. 46 (9): 945–1057. Bibcode:2012PalJ...46..945N. doi:10.1134/S0031030112090018. S2CID   85130465.
  7. ↑ S.M. Gon III. "Evolutionary Trends in Trilobites" . Retrieved 14 February 2013.
  8. ↑ Zhou, Z.; Webby, B.D.; Yuan, W. (1995). "Ordovician trilobites from the Yingan Formation of northwestern Tarim, Xinjiang, northwestern China". Alcheringa: An Australasian Journal of Palaeontology. 19 (1): 47–72. Bibcode:1995Alch...19...47Z. doi:10.1080/03115519508619098.
  9. ↑ Sam Gon III. "The Trilobite Eye" . Retrieved 16 November 2012.
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