16-605: Eodiscidae is a family of agnostid trilobites that lived during the final Lower Cambrian (late Toyonian) and the Middle Cambrian. They are small or very small, and have a thorax of two or three segments. Eodiscidae includes nine genera (see box). The Eodiscids probably descended from the agnostids of family Yukoniidae . Like other agnostids, the body of eodiscids is diminutive, the headshield (or cephalon ) and tailshield (or pygidium ) are of approximately same size, with 2 or 3 thoracic segments in-between, each consisting of
32-626: 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 . 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
48-410: A horizontal inner portion that abruptly passes into an inclined outer portion (fulcrate). The central raised area of the cephalon (or glabella ) is narrow, usually with parallel sides and rounded front. The furrow between the occipital ring and the more frontal parts of the glabella are a pair of pits low on its sides beneath a large median spine extending from the glabella backwards. The distance between
64-458: 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 . Other researchers have suggested, based on a cladistic analyses of dorsal exoskeletal features, that Eodiscina and Agnostida are closely united, and
80-484: The Mandibulata , are unresolved, with some phylogenies recovering Artiopoda as more closely to chelicerates, forming the clade Arachnomorpha , while others recover Artiopoda as more closely related to mandibulates, forming the clade Antennulata . Some studies place them as stem-group euarthropods, with mandibulates and chelicerates more closely related to each other than either is to Artiopoda. Some studies have recovered
96-497: The Artiopoda. 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
112-544: The Cambrian- Ordovician transition, and the last agnostidans went extinct in the Late Ordovician. 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
128-515: The Eodiscina descended from the trilobite order Ptychopariida . 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
144-555: The absence of any moulted carapaces suggests that moulting was not their primary reason for seeking shelter. Artiopodans 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 abundance of findings owing to their mineralized exoskeletons, are by far
160-685: The antennules and at least three sets of post-antennular limbs being incorporated into the head shield, the postantennular limbs having no or little differentiation into distinct morphologies, and broad paratergal folds which contribute to the dorsoventrally flattened look of artiopods. The limbs of artiopods have also been suggested to bear exites , which were described as similar those of the megacheiran Leanchoilia and probably not homologous to those present in crustaceans. The Artiopoda have been considered by many studies to consist of two major clades; one reusing Trilobitomorpha to encompass trilobites, nektaspids , concilitergans and xandarellids, and
176-692: The best recorded, diverse, and long lived members of the clade. Other members, which lack mineralised exoskeletons, are known mostly from Cambrian deposits. According to Stein and Selden (2012) artiopods are recognised by the possession of filiform antennulae , limbs with bilobate exopods , with the proximal lobe being elongate and bearing a lamella, while the distal lobe is paddle-shaped and setiforous (bearing hair-or bristle like structures). The limb endopod has seven podomeres /segments, with first four podomeres bearing inward facing ( endite ) structures, while podomeres five and six are stenopodous (cylindrical and stout). Common plesiomorphies also include
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#1733085303655192-602: The glabella and the border (or preglabellar area) is usually long. This border has dozens of small furrows radiating inward (a so-called scrobiculate border), but this defining feature may be absent in effaced species or destroyed by poor preservation. The axis of the pygidium has four to ten rings. This Agnostida -related article is a stub . You can help Misplaced Pages by expanding it . Agnostida Suborder Agnostina Suborder Eodiscina Isopygia Gürich, 1907 Miomera Jækel, 1909 Agnostida are an order of extinct arthropods which have classically been seen as
208-415: 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, suggesting they are not members of the lamellipedian clade , of which trilobites are
224-546: The other called Vicissicaudata encompassing aglaspidids , xenopods and cheloniellids . There are some taxa, such as Squamacula and the members of "Protosutura", which often are placed near the base of Artiopoda, outside the Vicissicaudata+Trilobitomorpha clade. These relationships are not always recovered. The relationship of Artiopoda with the two major clades of modern arthropods, the Chelicerata and
240-524: 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. They are sometimes preserved within
256-438: The voids of other organisms, for instance within empty hyolith conchs, within sponges , worm tubes and under the carapaces of bivalved arthropods , presumably in order to hide from predators or strong storm currents; or maybe whilst scavenging for food. 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;
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