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Posterior Margin (posterior + margin)
Selected AbstractsHow to make a zone of polarizing activity: Insights into limb development via the abnormality preaxial polydactylyDEVELOPMENT GROWTH & DIFFERENTIATION, Issue 6 2007Robert E. Hill Early in vertebrate limb development, a program initiates that polarizes the limb along the antero-posterior axis. The mesenchyme at the posterior margin is ultimately responsible for the asymmetry due to a region called the zone of polarizing activity (ZPA). The ZPA produces and secretes the molecule SHH, which coordinates the patterning of the resulting digits. Preaxial polydactyly (PPD) is a commonly occurring limb abnormality; investigating the genetic basis of this defect has provided insights into our understanding of digit patterning. PPD disrupts limb asymmetry by producing an ectopic ZPA at the opposite margin of the limb bud. Mutations in the long-range, limb-specific regulatory element of the Shh gene are responsible for the defect. Genetic analysis of this limb abnormality provides an important approach in understanding the mechanisms that control digit patterning. [source] On the tail of Errivaspis and the condition of the caudal fin in heterostracansACTA ZOOLOGICA, Issue 2009Elga Mark-Kurik Abstract Articulated caudal fins of heterostracans are preserved in very rare cases. Their morphology and internal structure are completely unknown for the majority of species. One of the few preserved caudal fins belongs to the Early Devonian pteraspidid Errivaspis waynensis from the Welsh Borderland, UK. There are two different reconstructions of the tail: (1) strongly asymmetric, with a considerably longer ventral lobe, and a rather concave posterior margin (e.g. White 1935); and (2) almost symmetrical with an only slightly longer ventral lobe and a small notch in the posterior margin (e.g. Blieck 1984). A new reconstruction of the Errivaspis caudal fin, based on a detailed study of White's specimens is presented here. The ventral lobe is longer than the dorsal one but not so much as in White's reconstruction, and the notch in the posterior margin of the fin is less deep. The comparatively massive ventral lobe and ,en echelon' arrangement of three digitations, supporting the fin web and branching off from this lobe, indicate the hypocercal condition of the asymmetrical tail. The same internal structure is present in other heterostracans with nearly symmetrical caudal fins (e.g. Doryaspis, Nahanniaspis, Dinaspidella and Athenaegis) where all, or most, of the intermediate digitations branch off from the ventral lobe. [source] Early development and segment formation in the centipede, Strigamia maritima (Geophilomorpha)EVOLUTION AND DEVELOPMENT, Issue 2 2004Ariel D. Chipman Summary Geophilomorph centipedes exhibit a number of unique characteristics that make them of particular developmental and evolutionary interest. Segment numbers in geophilomorphs are higher than in any other centipedes, ranging from 27 to 191. They may be constant within a species, presenting in extreme form the "counting" problem in development, or they may vary,a situation that provides us with the opportunity to study naturally occurring variation in segment numbers. All their segments are generated during embryogenesis, a situation unlike that in the more basal centipede orders, which generate only a fraction of their 15 trunk segments in the embryo and develop the rest postembryonically. Here we provide a foundation for further developmental studies of the Geophilomorpha, building on the one study that has been conducted to date, on the coastal species Strigamia maritima. Development begins with the migration of nuclei to the surface of the egg, which then condense to form an embryonic rudiment of more than 20,000 cells, covering an entire hemisphere. During early development, the embryo can be divided into two distinct areas: a large terminal disc of apparently undifferentiated tissue and the germ-band, which has a clear anteroposterior axis and differentiated segments. The germ-band forms from the anterior of the terminal disc and extends anteriorly as the disc contracts. New segments are formed at the posterior margin of the germ-band. Once the process of segmentation ends, the germ-band folds and sinks into the yolk. We note that the classic description of centipede development, by Heymons more than a century ago, contains a fundamental error in the identification of the axes and hence in the interpretation of early segmentation. [source] A NEW SPECIES OF FORMOSATETTIX TINKHAM (ORTHOPTERA:TETRIGIDAE) FROM HUBEI PROVINCEINSECT SCIENCE, Issue 4 2000ZHENG Zhe-min Abstract, A new species of the genus Formosatettix Tinkham 1937, Formosatettix tiantangensis sp. nov. from Dabie Mountains in Hubei Province is described. The new species is allied to Formosatettix henanensis Liang, but differs in: 1) width of vertex about 1.6 times the width of an eye; 2) length of antennae about 1.6,1.7 times the length of anterior femora; 3) upper margin of pronotum distinctively arched; 4) ventral margin of posterior pronotal process arched, lateral carinae curvingigly arched and 5) posterior margin of subgenital plate of female concave, median keel present on the plate. The type specimens are deposited in the Institute of Zoology, Shaanxi Normal University and Department of Biology, Huanggang Normal College, Hubei Province respectively. [source] Functional genital morphology of armored spiders (Arachnida: Araneae: Tetrablemmidae)JOURNAL OF MORPHOLOGY, Issue 9 2008Matthias BurgerArticle first published online: 18 JUN 200 Abstract This study describes the female genitalia of the tetrablemmid spiders Brignoliella acuminata, Monoblemma muchmorei, Caraimatta sbordonii, Tetrablemma magister, and Ablemma unicornis by means of serial semi-thin sections and scanning electron microscopy and compares the results with previous findings on Indicoblemma lannaianum. Furthermore, the male palps and chelicerae are briefly described. The general vulval organization of females is complex and shows similarities in all of the investigated species. The copulatory orifice is situated near the posterior margin of the pulmonary plate. The opening of the uterus externus lies between the pulmonary and the postgenital plate. Paired copulatory ducts lead to sac-like receptacula. Except for A. unicornis, the male emboli of all investigated species are elongated and thread-like. However, they are too short to reach the receptacula. Hence, the spermatozoa have to be deposited inside the copulatory ducts. The same situation was also found in I. lannaianum. Females of this species store sperm encapsulated in secretory balls in their receptacula. The secretion is produced by glands adjoining the receptacula. The presence of paired fertilization ducts and spermatozoa in the uterus internus suggested that fertilization takes place internally in I. lannaianum. Secretory balls in the receptacula are found in all of the investigated species in this study, showing that sperm are stored in the same way. The place of fertilization may also be identical since dark particles, presumably spermatozoa, are located in the uterus internus of all investigated species except for T. magister. However, fertilization ducts are only found in B. acuminata and M. muchmorei. A sclerotized central process with attached muscles is present in A. unicornis, M. muchmorei, C. sbordonii and T. magister. Only in A. unicornis does the central process show an internal lumen and hold spermatozoa. In the other species, it could be used to lock the uterus during copulation in order to prevent sperm from getting into it as suggested for certain oonopid species. The uterus externus of all investigated species shows a sclerotized dorsal fold with attached muscles, previously described as "inner vulval plate." Contractions of the muscles lead to a widening of the dorsal fold, thus creating enough space for the large oocytes to pass the narrow uterus externus. The males of all investigated species have apophyses on their chelicerae. At least in B. acuminata and A. unicornis, where females have paired grooves on the preanal plate, these apophyses allow males to grasp the female during copulation as described for I. lannaianum. © 2008 Wiley-Liss, Inc. [source] Colonization by barnacles on fossil Clypeaster: an exceptional example of larval settlementLETHAIA, Issue 4 2008ANA G. SANTOS The presence of c. 1450 individuals of the balanid barnacle Balanus crenatus Bruguičre encrusting the test of a clypeasteroid sea urchin from the Late Miocene of the Guadalquivir Basin (southwestern Spain) allows proposing a settlement pattern linked to the growth of the encrusting organism. The possible influence of dip angle was controlled by dividing the test into four concentric zones ranging from lowest margin to apex (0,15°, 15,30°, 30,50° and 0°). Contour diagrams were prepared to identify areas of highest barnacle density as well as size categories distribution in relationship to the pitch of the sea urchin test. The orientation of balanid tests was recorded and plotted on rose diagrams from 0° to 180°. Four size categories of barnacles were distinguished: (1) < 1 mm, (2) 1,2 mm, (3) 2,3 mm and (4) 3,4 mm; these correspond to a growth sequence ranging from post-larval forms to juveniles. Two areas of maximum settlement density are situated on the posterior margin of the test, on aboral as well as oral surfaces. The aboral surface shows the maximum number of barnacles. Two groups of individuals are defined on the basis of their location, that is, those encrusting the posterior medium part of the urchin, and those located on the anterior half. The results suggest that larval settlement was initially controlled by the availability of free space and afterwards by an intensification effect. Orientation and dip of the test may have played a secondary role in the settlement of the larvae. Substrate colonization seems to have been closely related to the biostratinomic history of the sea urchin test and although several scenarios are possible, our data are congruent with a synchronous settlement of both surfaces (aboral and oral) by one spat or several. [source] Shell structure, ontogeny and affinities of the Lower Cambrian bivalved problematic fossil Mickwitzia muralensis Walcott, 1913LETHAIA, Issue 4 2004UWE BALTHASAR Exceptionally preserved carbonate- and shale-hosted Mickwitzia muralensis from the Lower Cambrian Mural Formation, southern Canadian Rocky Mountains, complement one another to yield an unusually complete account of its ontogeny, ecology and phylogenetic relationships. The shell of M. muralensis is composed of dense phosphatic layers interspersed with porous organic-rich layers. At the insertion of shell-penetrating tubes, shell layers deflect inwards to produce inwardly pointing cones. The tubes are interpreted as having hosted setae that were secreted by outer-epithelial follicles. Follicular setae also occurred at the mantle margin, where they were oriented within the plane of the shell as in modern brachiopods. During ontogeny, the initial setae oriented in the plane of the shell occurred before the first shell-penetrative setae. In the juvenile and early-mature stages of shell secretion, a posterior opening was present between both valves and was used for the protrusion of an attachment structure. In the late-mature shell, this opening became fixed in the ventral valve. Based on the posterior margin and the shell microstructure, a close relationship between Mickwitzia and the paterinids is proposed with differences interpreted as heterochronic. The shell-penetrative setal apparatus of M. muralensis is distinct from that previously described of Micrina, though both types are conceivably homologous to adult and juvenile setae of modern brachiopods. [source] BILLENGSELLIDE AND ORTHIDE BRACHIOPODS: NEW INSIGHTS INTO EARLIEST ORDOVICIAN EVOLUTION AND BIOGEOGRAPHY FROM NORTHERN IRANPALAEONTOLOGY, Issue 1 2009LEONID E. POPOV Abstract:, The eastern Alborz Mountains of Iran comprise a significant peri-Gondwanan terrane relevant to the early evolution of late Cambrian , early Ordovician brachiopods incorporated into the emerging benthic biota of the Paleozoic Evolutionary Fauna. A low diversity brachiopod assemblage from the late Tremadocian unit of the Lashkarak Formation contains six new species including the polytoechioideans Polytoechia and Protambonites and the orthoideans Paralenorthis, Ranorthis, Tarfaya and Xianorthis. The fauna preserves the earliest records of Polytoechia, unknown previously outside Laurentia and the Uralian margin of Baltica, and of Paralenorthis and Ranorthis, which were widespread along Gondwanan margins and in Baltica from the Floian (Arenig), plus Xianorthis, known hitherto only from the Floian of South China. The enigmatic Tarfaya has an impunctate shell fabric and setigerous perforations along the posterior margin, indicating placement within the Orthoidea in a new Family Tarfayidae. New species of Polytoechia, Protambonites, Paralenorthis, Ranorthis, Tarfaya, Xianorthis are described. [source] THE FIRST FOSSIL PROSCOPIIDAE (INSECTA, ORTHOPTERA, EUMASTACOIDEA) WITH COMMENTS ON THE HISTORICAL BIOGEOGRAPHY AND EVOLUTION OF THE FAMILYPALAEONTOLOGY, Issue 2 2008SAM W. HEADSArticle first published online: 14 MAR 200 Abstract:,Eoproscopia martilli gen. et sp. nov. is described from the Early Cretaceous (Aptian) Crato Formation Lagerstätte of Cearį State, north-east Brazil. The new taxon is assigned to the extant family Proscopiidae and represents the first occurrence of the group in the fossil record. Eoproscopia is similar to crown group proscopiids in its stick-like habitus, elongate prothorax and absence of the cryptopleuron, but differs in the presence of well-developed wings, the short head with a small, simple fastigium, the prothoracic legs being inserted near the posterior margin of the prothorax, and the absence of spines on the metathoracic tibiae. The discovery of Eoproscopia extends the geological range of the family by approximately 110 myr and confirms the presence of stem-group proscopiids in the Atlantic rift zone of South America during the Early Cretaceous. [source] A New Carcinosomatid Eurypterid From The Upper Silurian Of Northern VietnamPALAEONTOLOGY, Issue 5 2002Simon J. Braddy A new carcinosomatid eurypterid, Rhinocarcinosoma dosonensis sp. nov., and Hughmilleria sp., are described from the Dō Son Formation of the Dō Son Peninsula, northern Vietnam. R. dosonensis is characterized by podomere 7 of prosomal appendage VI produced into an anterodistal spine, a metastoma with a cordate posterior margin, and an indented opisthosomal differentiation (i.e. preabdomen,postabdomen margin is concave). The Dō Son Formation was originally interpreted as Late Devonian (Givetian/Frasnian) in age but the unit containing the eurypterid assemblage is now considered much older (Late Silurian). A deltaic palaeoenvironmental setting is interpreted from biotic associations and sedimentological evidence. [source] Ectopic supernumerary tooth on the inferior nasal conchaCLINICAL ANATOMY, Issue 1 2006Bappaditya Ray Abstract Variations regarding the location of an ectopic tooth in the human nasal cavity, although rare, are documented in the literature, but presence of an ectopic tooth on the inferior nasal concha (INC) has not been reported. We observed an anomalous tooth projecting from the posterior margin of the right INC in two adult female skulls. A small quadrangular tooth projected beyond the posterior margin of the hard palate in one of the skulls and a medium sized conical tooth was observed in the other skull. The affected INC in both skulls were located more inferiorly compared to the opposite side and were in close approximation with the hard palate. No similar findings were noted on the contralateral side nor were there any associated congenital or iatrogenic deformity. The phylogenetic, ontogenetic, and clinical importance of this variant is described. Knowledge of such an anomaly is of paramount importance to otorhinolaryngologists, reconstructive and dental surgeons, and radiologists for identification of such rarities encountered during invasive or non-invasive procedures. Clin. Anat. 19:68,74, 2006. © 2005 Wiley-Liss, Inc. [source] | |||||||||||||