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Guillaume Balavoine - One of the best experts on this subject based on the ideXlab platform.
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A metameric origin for the annelid Pygidium
BMC evolutionary biology, 2015Co-Authors: Viktor V. Starunov, Nicolas Dray, Elena V Belikova, Pierre Kerner, Michel Vervoort, Guillaume BalavoineAbstract:Segmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (Pygidium) of annelids are classically described as non-segmental. However, the Pygidium structure and development remain poorly studied. Using different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the Pygidium of Platynereis dumerilii. We establish that the Pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation. We describe a much more complex organization of the Pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the Pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and Pygidium.
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A metameric origin for the annelid Pygidium?
BMC Evolutionary Biology, 2015Co-Authors: Viktor V. Starunov, Nicolas Dray, Elena V Belikova, Pierre Kerner, Michel Vervoort, Guillaume BalavoineAbstract:Background Segmented body organizations are widely represented in the animal kingdom. Whether the last common bilaterian ancestor was already segmented is intensely debated. Annelids display broad morphological diversity but many species are among the most homonomous metameric animals. The front end (prostomium) and tail piece (Pygidium) of annelids are classically described as non-segmental. However, the Pygidium structure and development remain poorly studied. Results Using different methods of microscopy, immunolabelling and a number of molecular markers, we describe the neural and mesodermal structures of the Pygidium of Platynereis dumerilii . We establish that the Pygidium possesses a complicated nervous system with a nerve ring and a pair of sensory ganglia, a complex intrinsic musculature, a large terminal circular blood sinus and an unusual unpaired torus-shaped coelomic cavity. We also describe some earlier steps of pygidial development and pygidial structure of mature animals after epitokous transformation. Conclusions We describe a much more complex organization of the Pygidium of P. dumerilii than previously suggested. Many of the characteristics are strikingly similar to those found in the trunk segments, opening the debate on whether the Pygidium and trunk segments derive from the same ancestral metameric unit. We analyze these scenarios in the context of two classical theories on the origin of segmentation: the cyclomeric/archicoelomate concept and the colonial theory. Both theories provide possible explanations for the partial or complete homology of trunk segments and Pygidium.
Savchuk Oles - One of the best experts on this subject based on the ideXlab platform.
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PLATE 1 in A new brachymetopid trilobite from the Early Permian Shakhtau reef complex of the southwestern Urals, Bashkortostan, Russia
2019Co-Authors: Mychko Eduard, Savchuk OlesAbstract:PLATE 1. Brachymetopus (Conimetopus) alekseevi Mychko, sp. nov., Upper Asselian or Lower Sakmarian, Shakhtau quarry, Sterlitamak town, Ishimbay District, Bashkortostan, Russia. 1a—holotype, spec. PIN RAS, no. 5610/2, external mold; 1b, clatex cast from holotype, dorsal and lateral views. 2a, b—paratype, spec. PIN RAS, no. 5610/6, Pygidium, dorsal and posterior views. 3—paratype, spec. PIN RAS, no. 5610/4, Pygidium, dorsal view. 4—paratype, spec. PIN RAS, no. 5610/3, Pygidium, dorsal view. 5a, b—paratype, spec. PIN RAS, no. 5610/5, Pygidium, dorsal and posterior views. 6—paratype, spec. PIN RAS, no. 5610/7, Pygidium, external mold. The scale bar is 5 mm
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FIGURE 1 in A new brachymetopid trilobite from the Early Permian Shakhtau reef complex of the southwestern Urals, Bashkortostan, Russia
2019Co-Authors: Mychko Eduard, Savchuk OlesAbstract:FIGURE 1. Some members of the genus Brachymetopus from the Permian of Russia. a—B. (Acutimetopus) moelleri (Weber, 1932); reconstruction of cephalon; presumably kept (possibly lost) in the collections of the St. Petersburg Mining University museum, St. Petersburg; Lower Permian, Artinskian, Sarginian (reproduced from Möller [1868, Pl. 1, Fig. 32]). b, c—B. (Conimetopus) alekseevi Mychko, sp. nov.; b—cephalon, spec. TsNIGR no. 5107/2063; c—Pygidium, spec. TsNIGR no. 5107/ 2064; Chelyabinsk Region, Ashinsky District, right bank of the Sim River, the town of Asha, "Kazarmenny Kamen" locality; Lower Permian, Asselian; (reproduced from Weber [1937, Pl. 10, figs 10–11]). d, e—B. (Acutimetopus) caucasicus Likharev, 1944; spec. TsNIGR no. 5217/86, holotype, Pygidium; Krasnodar Region, Mostovsky District, the Malaya Laba River basin, "Nikitinskaya Balka" locality; Upper Permian, Dzhulfian (~Wuchiapingian), Urushten or Nikitin Formations; d—view from the dorsal side, e—view from the end of the posterior margin (reproduced from Weber [1944, Pl 2, Fig. 15]). The scale bar is 1 cm
Westrop, Stephen R. - One of the best experts on this subject based on the ideXlab platform.
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FIGURE 10. Cedarina clevensis n in The Marjuman trilobite Cedarina Lochman: thoracic morphology, systematics, and new species from western Utah and eastern Nevada, USA
2018Co-Authors: Adrain, Jonathan M., Peters, Shanan E., Westrop, Stephen R.Abstract:FIGURE 10. Cedarina clevensis n. sp., from the Lincoln Peak Formation Marjuman; Crepicephalus Zone), Cleve Creek, Schell Creek Range, White Pine County, eastern Nevada. All magnifications are x 15. Juvenile specimens in A-C and E are questionably assigned. A, B, E. Cranidium, SUI 111860, dorsal, right lateral, and anterior views, x 30 (CC 1). C. Cranidium, SUI 111861, dorsal view, x 25 (CC 1). D. Left librigena, SUI 109006, external view, x 10 (CLC 67.5 T m). F, G, I, J. Thoracic segment, SUI 109008, dorsal, ventral, anterior, and left lateral views (CLC 67.5 T m). H, K, N, Q. Pygidium, SUI 109009, left lateral, dorsal, posterior, and ventral views (CLC 67.5 T m). L, O, R, S. Pygidium, SUI 109010, dorsal, posterior, ventral, and right lateral views (CLC 67.5 T m). M, P, T, U. Pygidium, holotype, SUI 109011, dorsal, posterior, left lateral, and ventral views (CLC 67.5 T m)
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FIGURE 5. Cedarina schachti n in The Marjuman trilobite Cedarina Lochman: thoracic morphology, systematics, and new species from western Utah and eastern Nevada, USA
2018Co-Authors: Adrain, Jonathan M., Peters, Shanan E., Westrop, Stephen R.Abstract:FIGURE 5. Cedarina schachti n. sp., from the Weeks Formation (Marjuman; Cedaria Zone), northern House Range, Millard County, western Utah, USA. Dorsal exoskeleton lacking Pygidium, holotype, FMNH PE 57116, dorsal view, x 12
Stephen R. Westrop - One of the best experts on this subject based on the ideXlab platform.
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Bartonaspis new genus, a trilobite species complex from the base of the Upper Cambrian Sunwaptan Stage in North America
Canadian Journal of Earth Sciences, 2007Co-Authors: Stephen R. Westrop, Jonathan M. AdrainAbstract:Despite being reported widely from basal Sunwaptan strata, Dellea? punctata Palmer, 1965, is known from only four figured cranidia. New material from Nevada, Utah and Oklahoma, including librigenae and pygidia, indicate that specimens assigned previously to D.? punctata are best interpreted as a complex of related species that are assigned to a new genus, Bartonaspis. This genus is diagnosed by punctate sculpture, a pair of fossulae at the anterior corners of the glabella, and by a subelliptical Pygidium with a short convex axis composed of three segments. Three new species, B. fredericksoni, B. palmeri, and B. wilsoni, are described and B. punctata (Palmer) is restricted to its holotype. Bartonaspis is known only from the Irvingella major Zone, and its first appearance defines the base of the Sunwaptan Stage. Dellea Wilson, 1949, is related to Bartonaspis and is differentiated from it primarily by pygidial morphology. In contrast to Bartonaspis, the Pygidium of Dellea has an axis composed of four, rather...
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A systematic revision of Thaleops (Trilobita: Illaenidae) with new species from the middle and late Ordovician of Oklahoma and New York
Journal of Systematic Palaeontology, 2004Co-Authors: Lisa Amati, Stephen R. WestropAbstract:Synopsis Illaenus Dalman, 1827 has long served as a wastebin for effaced trilobite species and the problem has been compounded in North America by the addition of Thaleops Conrad, 1843 and Nanillaenus Jaanusson, 1954. This study takes the first step toward defining the relationships of Laurentian taxa within this group using cladistic methods. Parillaenus Jaanusson, 1954 forms a monophyletic group based on pygidial morphology that is distinct from Illaenus. Ouranalysis indicates that Illaenus is paraphyletic and requires major revision. Thaleops is a large group of species whose monophyly is supported by an unambiguous apomorphy: the presence of long articulating facets on the Pygidium that are oriented at a high angle to the anterior margin of the Pygidium. All but one basal member of the clade possesses a second apomorphy, posteriorly positioned palpebral lobes. Nanillaenus is not monophyletic and is synonymised with Thaleops. Several Laurentian species originally identified as Illaenus are reassigned t...
Shanchi Peng - One of the best experts on this subject based on the ideXlab platform.
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morphology and ontogeny of hunanocephalus ovalis trilobite from the cambrian of south china
Gondwana Research, 2014Co-Authors: Xingliang Zhang, Shanchi PengAbstract:article i nfo The juvenile morphology and ontogeny of the Cheiruroidid trilobite Hunanocephalus ovalis Lee, 1963 from the lower Cambrian Shuijingtuo Formation in Hubei Province, South China is presented. The new material com- prises a relatively complete meraspid ontogenetic series (degree 0 to 10), which reveals more details on their morphological changes such as the contraction and disappearance of the pronounced posteromedial notch in the Pygidium and the addition of the trunk segments, which are all documented for the first time and can also be used as developmental markers defining their ontogenetic phases. The trunk segmentation schedule of H. ovalis is also discussed, which is similar to the other early Cambrian oryctocephalid trilobites, i.e. as the boundary between the thorax and Pygidium migrated posteriorly there is no change in the trunk segment number; the processes of liberation of the thoracic segment and segment insertion into the Pygidium are separated from one another, implying that the control of trunk exoskeletal segment appearance and articulation might be decoupled in these trilobites.