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Agglutinated or porcelaneous tests: where to draw the line?

Published in Journal of foraminiferal research. 2016, vol. 46, no. 3, 333–344
Abstract The simple, tubular foraminifer Glutameandrata vallieri n. gen., n. sp. is formally described from Spring Creek, one of the most fossiliferous Late Triassic localities of North America (Martin Bridge Formation, Wallowa terrane, Oregon, USA). Uncoiled in the final stage, this new benthic form displays an initial meandrospiroid arrangement, a coiling mode typically observed in coeval porcelaneous foraminifers. Yet, we demonstrate through a comparative study of diagenetic results that the wall of G. vallieri n. gen., n. sp. is, in fact, finely agglutinated. Agglutinated meandrospiroid foraminifers, here encompassed by the family Glutameandratidae n. fam., are sporadically but continuously observed in Triassic-Recent rocks. Their potential phylogenetic relationships and their significance to define the relative taxonomic importance of wall characteristics versus coiling mode are discussed. For over a century, optical differences in the wall appearance of agglutinated and porcelaneous foraminifers have been used for taxonomic distinction, with moderate success in the fossil record. Parameters and processes controlling these optical differences remain partly unclear and the exact boundary between these two types of walls is still indefinite. Calcite-cemented calc-agglutinated tests and porcelaneous agglutinating tests only differ in fine details of the microstructure and composition of the wall, related to differences in the wall morphogenesis. With diagenesis, these details are progressively obliterated, often hindering correct taxonomic assignments. We here provide criteria allowing the distinction between porcelaneous and calcareous agglutinated tests in the fossil, up to a high level of thermal maturation. The walls of porcelaneous and agglutinated foraminifers are not equally affected by diagenetic effects and subtle structural differences in the wall outline persist up to a significant level of test alteration.
Keywords ForaminifersTaxonomySysthematicPorcelaneousAgglunitatedPhylogenesisDiagenesisTests compositionTriassic
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Research group Sedimentology, Biostratigraphy and Micropaleontology
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RIGAUD, Sylvain, MARTINI, Rossana. Agglutinated or porcelaneous tests: where to draw the line?. In: Journal of foraminiferal research, 2016, vol. 46, n° 3, p. 333–344. https://archive-ouverte.unige.ch/unige:85957

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Deposited on : 2016-08-15

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