1,720,985 research outputs found
Shell microstructures in lopingian brachiopods: Implications for fabric evolution and calcification
No full textThe study of the shell microstructure of brachiopods is fundamental to understand their evolutionary history and their biomineralization process. Here, species of forty Lopingian brachiopods genera, representative of twenty-seven different families, are investigated using the Scanning Electron Microscope. The investigated specimens come from different paleogeographic localities in the Palaeotethys/Neotethys oceans. The studied brachiopods show a large variability of the shell fabric, which is mainly related to the organization of its structural units: laminae, fibers and columns, possibly crossed by pseudopunctae or punctae. For the Strophomenata, the laminar fabric of Productida is crossed by pseudopunctae with taleolae and the laminae are often organized in packages, with the blades oriented about perpendicular to each other; this feature is less evident in the laminar Orthotetida, which bear pseudopunctae without taleoae. For the Rhynchonellata, fibrous fabrics are either impuctate in the Spiriferida, most Athyridida and Rhynchonellida, or with punctae, as observed in the Orthida, Terebratulida and in the Neoretziidae (Athyridida). The fibers show a range of sizes and shapes also in the same specimens and the transition to the columnar layer is different than in Strophomenata. The arrangement of the structural units revealed that the disposition of the organic membranes, on which biomineralization took place, was highly variable among the taxa. On the other hand, two distinctive features are analogous among distantly related groups, i.e. the Strophomenata and the Rhynchonellata: the presence of a columnar tertiary layer underlying the secondary fabric and the alternations between fibers/laminae of the secondary layer and columns of the tertiary layer. This suggests that there are common factors controlling the development and evolution of the shell fabric in all rhynchonelliformean brachiopods that can be linked to their taxonomical position, to their environmental requirements and to constraints imposed by their low-energy life-style. This should be taken into account to understand how these calcifying organisms responded and will respond to environmental and climate change in past and future times
A preliminary study on the ornamentation patterns of ganoid scales in some mesozoic actinopterygian fishes
No full textGanoid scales are one of the most common remains of vertebrates in the fossil record of Paleozoic and Mesozoic. Their knowledge is important for the understanding of the paleobiology and evolution of actinopterygian fishes. The distinctive feature of these scales is the outermost shiny layer of ganoine, an hyper-mineralized enamel-like tissue. During the Mesozoic, ganoid scales show a great variety of shapes and very different patterns of ornamentation made of ganoine. The surface of scales may be from smooth to very ornamented, with a relief arranged in tubercles, ridges, grooves or a combination of them. Here we present a preliminary study on the squamation of some Mesozoic non-teleost actinopterygians in order to test the association between body shape and ganoine ornamentation. Using several morphological descriptors, we built an empirical morphospace to compare ganoid scales ornamentation. The use of a quantitative method to describe scales allows to test if there is a meaningful correlation between body shape and ganoine ornamentation in non-teleost actinopterygians. We found an important association between body shape and scale ornamentation in the taxa under investigation. In particular, deep-bodied fishes bear a more complex and variegate ornamentation than fusiform ones. This finding has important implications in our understanding of non-teleost actinopterygians paleobiology. Since swimming activity is a primary function for fish and this is performed by the body acting as an integrated unit, the correlation between body shape and ganoine ornamentation could be related to hydrodynamics. A quantitative test of this hypothesis, however, would be desirable
A clade-level morphospace for the eocene fishes of Bolca: Patterns and relationships with modern tropical shallow marine assemblages
No full textThe world-famous Eocene Konservat-Lagerstätte of Bolca, northern Italy, has been celebrated since the 16th century for its exquisitely preserved fishes, which provide the earliest evidence of a modern tropical shallow marine fish assemblage. Due to the presence of many of the earliest representatives of several modern families of reef-associated fishes, the remarkable taxonomic and morphological richness of this fish assemblage has traditionally been attributed to the presence of a coral reef system. Several studies have suggested that the broad morphological diversification of modern fish lineages may be due to the availability of greater ecological opportunity for the survivors of the end-Cretaceous mass extinction. This hypothesis was supported by a major expansion of the morphospace occupied by the fish lineages that crossed the Cretaceous-Paleogene transition, occupying regions previously inhabited by extinct Cretaceous groups. In this context, we used a geometric morphometric approach in order to describe the clade-level structure of the morphospace of the Bolca fish assemblage, and to compare the patterns of morphospace occupation and morphological diversification between the Eocene and modern tropical shallow marine fish assemblages. Although the similarity in morphospace area and the overlapping of the different convex hulls seem to indicate a relative stasis in the overall morphospace occupation from the Eocene to Recent, some differences at the clade level can be detected. In particular, the representatives of several lineages, including anguilliforms, “perciforms”, syngnathiforms and tetraodontiforms, seem to have undergone significant changes in terms of morphospace occupation, body shape variation and contribution to the overall disparity from the Eocene to today
A morphospace for the Eocene fish assemblage of Bolca, Italy: A window into the diversification and ecological rise to dominance of modern tropical marine fishes
No full textThe celebrated Eocene fishes of Bolca, northeastern Italy, provide one of the earliest evidence of a modern tropical shallow marine fish assemblage, in the aftermath of the end-Cretaceous extinction. This fish assemblage has been traditionally interpreted as closely linked to a coral reef system based on a certain similarity in taxonomic composition with modern ecosystems. In this study, we use geometric morphometrics to compare the patterns of morphospace occupation and morphological variation between Eocene and extant tropical shallow water fish assemblages. Morphospace analysis revealed that there are not significant differences in morphospace occupation, and the Eocene fish assemblage shows a greater frequency of deep-bodied morphotypes and a higher morphological disparity compared to the extant tropical marine shallow-water assemblages. Because of the highly reduced reef-building potential of early Eocene coral communities and the extremely scarce evidence of corals in the Bolca area, the broad morphospace occupation and the high morphological richness observed for the Bolca assemblage suggest that an Eocene tropical non-coral reef setting shows higher, or at least similar, morphological diversity than modern coral reef-associated fish assemblages. Therefore, our paleontological evidence suggests that coral reefs may have played a secondary role in shaping the morphological richness of these fossil and extant tropical marine fish assemblages, and are consistent with the hypothesis of the rapid niche-filling and early saturation of the teleost morphospace after the end-Cretaceous extinction
Strontium isotope stratigraphic insights on the end-Permian mass extinction and the Permian-Triassic boundary in the Dolomites (Italy)
No full textThe Dolomites (Southern Alps, Italy) is a most significant region to investigate the evolution of shallow-marine ecosystems during the end-Permian mass extinction (EPME). Shallow-marine ecosystems are complex places from an oceanographic viewpoint and combine high biological productivity and ecological diversity. Therefore, establishing the timing and correlation of globally recognisable events in this region are fundamental to interpreting the biological crisis that followed. We took advantage of the Strontium Isotope Stratigraphy (SIS) concept to correlate our stratigraphic succession, ranging from the upper Bellerophon (Bulla Member, upper Changhsingian) to the lower Werfen (Tesero Member, upper Changhsingian - lower Induan) Formations of the Dolomites, to the Meishan (Zhejiang Province, China), the Global Stratotype Section and Point (GSSP) for the Permian–Triassic boundary. We integrated new and previously published 87Sr/86Sr data from well preserved brachiopods and show that the topmost part of the Bellerophon Formation (the Bulla Member) correlates with beds 24 e-f of the GSSP section, while the Tesero Member, where the Permian-Triassic boundary is located, is characterised by more radiogenic Sr isotope ratios than expected. Differences in Sr isotope ratios may be due to small-scale variations in local paleoenvironmental and paleogeographic conditions/settings. Continental weathering of sedimentary or igneous rocks, distance from terrestrial environment and nearby occurrence of radiogenic rocks might also produce locally variable Sr isotope signatures especially in shallow water environments complicating the application of SIS principles
Brachiopod fabric, classes and biogeochemistry: Implications for the reconstruction and interpretation of seawater carbon-isotope curves and records
No full textShell calcite of modern and ancient brachiopods is an important proxy in reconstructing oceanographic conditions of seawater. Our investigation reveals significant magnesium and carbon isotope relationships between two classes of ancient brachiopods. Brachiopods of the extinct Class Strophomenata constructed two- or three-layer shells with a primary layer of randomized granular calcite, a secondary layer of cross-bladed laminar calcite and, at times, a tertiary layer of large prismatic calcite. Contemporaneous, but extant, Rhynchonellata also have two- or three-layer shells, but their secondary layer consists of calcite fibers instead of cross-bladed laminar calcite. Furthermore, inter-crystalline space is limited in between the fibers of the secondary layer of the Rhynchonellata but higher in the laminar secondary layer of Strophomenata. δ18O values of the two classes of brachiopods from the same stratigraphic interval of the Lopingian Nesen Formation of North Iran show a considerable overlap with no significant difference (p=0.263). In contrast, carbon isotope values are significantly different (p=0.001) between contemporaneous Strophomenata (mean: +2.18‰) and Rhynchonellata (mean: +4.13‰). An evaluation of literature data of Permian (Asselian) and Carboniferous (Visean and Serpukhovian) Strophomenata (mean: +2.32‰, N=59) and Rhynchonellata (mean: +4.94‰, N=105) confirms a similar differentiation in their carbon isotope values.Its higher magnesium contents suggest that the Strophomenata probably secreted shell calcite at a faster rate than their coeval Rhynchonellata. This process may account for the δ13C differentiation observed between specimens of these two brachiopod classes. However, the different amount and composition of the organic matter of the Strophomenata may also play a role in shaping their more negative carbon isotope signature. Thus, the class of brachiopod and their associated different fabric must be adjusted for in their δ13C results and taken into consideration during the reconstruction of Paleozoic global δ13C carbonate-based seawater curves. Invariably, in some earlier studies undetected impacts imposed by the class-fabric on δ13C may account for some of the variation or trends documented in Deep-Time seawater-13C curves reconstructed with brachiopod populations from North America, Europe and Russia. © 2014 Elsevier B.V
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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