Textulariinid benthic foraminifera represent an important component of Mesozoic assemblages and are found in the marine realm from carbonate platform to deep-water settings. However, the taxonomic classification of several taxa and their phylogenetic relationship are uncertain, because several species are described in thin section and the diagnostic features of the wall texture (e.g., the presence of pores) and of the primary aperture (position, morphology) are often not visible. Vice versa, problems with the determination also arise when species are described from washed residues, and the isolated type specimens are compared with morphotypes encountered in thin section studies. The species belonging to the Duotaxis-Tetrataxis morphogroup and to the genus Trochammina described from Triassic levels are among the taxa that require taxonomic revision. The type species of the genus Tetrataxis (i.e., Tetrataxis conica Ehrenberg) was described from Carboniferous strata and shows a calcareous microgranular double-layered wall typical of fusulinids, whereas Late Triassic Tetrataxis species (T. inflata Kristan, T. humilis Kristan, T. nanus Kristan-Tollmann) are characterized by an agglutinated wall typical of the textulariinids. The range of Carboniferous and Triassic species is also separated by a significant stratigraphic gap. It has been suggested, but not yet supported by taxonomic studies, that these species should rather be placed in the genus Duotaxis Kristan, which currently comprises two species both described from the Triassic. Similar discrepancies exist for the genus Trochammina. Its type species, Trochammina inflata Montagu (type level: Recent) has a finely agglutinated and organic cemented imperforate wall and its primary aperture is an arch covered by a narrow lip in extraumbilical-umbilical position. In contrast, the holotype of the only Triassic species, Trochammina alpina Kristan (type level: Rhaetian) described from washed residues, has an agglutinated wall that is likely cemented by calcite, and the primary aperture is fully umbilical in position and has no arch. The other "Trochammina" species described from Triassic levels (e.g., Trochammina almtalensis Koehn-Zaninetti, Trochammina tabasensis Bronnimann, Zaninetti, Moshtaghian and Huber) were described from thin sections. The aim of this study is to re-illustrate the type specimens of the Triassic species of Duotaxis, Tetrataxis and Trochammina previously known only from the original drawings or low-quality microphotographs, with high-resolution stereo-, optical, and Scanning Electron Microscope (SEM) images. Furthermore, we compare these images with specimens from thin sections and washed residue samples collected from several Tethyan localities (Valle Agricola and Mt. Sparagio in Southern Italy, Mt. Messapion in Greece, Western Black Sea Shelf in Romania, Fonsjoch in Austria) and from different depositional environments (carbonate platform to shelf) with the aim to evaluate their external and internal morphologic variability. Results of this study will produce a taxonomic revision of several textulariinid lineages and will better constrain the species stratigraphic ranges across the Triassic-Jurassic boundary interval, with the ultimate goal of providing new information to more accurately estimate the biodiversity loss across the end-Triassic mass extinction and the evolution of new taxa during the following recovery phase.
Taxonomic revision of some textulariinid benthic foraminifera of the Triassic-Jurassic boundary interval
Francesca Falzoni;
2023
Abstract
Textulariinid benthic foraminifera represent an important component of Mesozoic assemblages and are found in the marine realm from carbonate platform to deep-water settings. However, the taxonomic classification of several taxa and their phylogenetic relationship are uncertain, because several species are described in thin section and the diagnostic features of the wall texture (e.g., the presence of pores) and of the primary aperture (position, morphology) are often not visible. Vice versa, problems with the determination also arise when species are described from washed residues, and the isolated type specimens are compared with morphotypes encountered in thin section studies. The species belonging to the Duotaxis-Tetrataxis morphogroup and to the genus Trochammina described from Triassic levels are among the taxa that require taxonomic revision. The type species of the genus Tetrataxis (i.e., Tetrataxis conica Ehrenberg) was described from Carboniferous strata and shows a calcareous microgranular double-layered wall typical of fusulinids, whereas Late Triassic Tetrataxis species (T. inflata Kristan, T. humilis Kristan, T. nanus Kristan-Tollmann) are characterized by an agglutinated wall typical of the textulariinids. The range of Carboniferous and Triassic species is also separated by a significant stratigraphic gap. It has been suggested, but not yet supported by taxonomic studies, that these species should rather be placed in the genus Duotaxis Kristan, which currently comprises two species both described from the Triassic. Similar discrepancies exist for the genus Trochammina. Its type species, Trochammina inflata Montagu (type level: Recent) has a finely agglutinated and organic cemented imperforate wall and its primary aperture is an arch covered by a narrow lip in extraumbilical-umbilical position. In contrast, the holotype of the only Triassic species, Trochammina alpina Kristan (type level: Rhaetian) described from washed residues, has an agglutinated wall that is likely cemented by calcite, and the primary aperture is fully umbilical in position and has no arch. The other "Trochammina" species described from Triassic levels (e.g., Trochammina almtalensis Koehn-Zaninetti, Trochammina tabasensis Bronnimann, Zaninetti, Moshtaghian and Huber) were described from thin sections. The aim of this study is to re-illustrate the type specimens of the Triassic species of Duotaxis, Tetrataxis and Trochammina previously known only from the original drawings or low-quality microphotographs, with high-resolution stereo-, optical, and Scanning Electron Microscope (SEM) images. Furthermore, we compare these images with specimens from thin sections and washed residue samples collected from several Tethyan localities (Valle Agricola and Mt. Sparagio in Southern Italy, Mt. Messapion in Greece, Western Black Sea Shelf in Romania, Fonsjoch in Austria) and from different depositional environments (carbonate platform to shelf) with the aim to evaluate their external and internal morphologic variability. Results of this study will produce a taxonomic revision of several textulariinid lineages and will better constrain the species stratigraphic ranges across the Triassic-Jurassic boundary interval, with the ultimate goal of providing new information to more accurately estimate the biodiversity loss across the end-Triassic mass extinction and the evolution of new taxa during the following recovery phase.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
