Poseidonella Holzmann & Siemensma, 2025
Diagnosis: Agglutinated monothalamous foraminifera with an oval to elongate amphora-shaped test: broadest at mid-length, tapering posteriorly into a rounded base and anteriorly into a short tubular or funnel shaped aperture. Although bilaterally symmetrical, most tests appear slightly asymmetrical in lateral view, with one side exhibiting a more pronounced curvature than the other.
Type species: Poseidonella transaquatica Siemensma & Holzmann, 2025
Species assigned: Poseidonella transaquatica
Poseidonella transaquatica Siemensma & Holzmann, 2025
Diagnosis: As for the genus. Poseidonella is currently monotypic.
Type locality: An emerged rock in a small pool connected to a minor tributary of the Gave du Marcadau, Marcadau Valley, near Pont d’Espagne, Cauterets, France, at 1600 m altitude (42◦53′19.5″ N, 0◦06′49.9″ W).
Description: All observed tests were oval, elongate-oval, or amphora-shaped in profile, and nearly circular to slightly compressed in cross-section, with the posterior region tapering into a rounded termination. All tests were bilaterally symmetrical in general structure, but most appeared slightly asymmetrical in lateral view, with one side more curved than the other. The test wall is thin and composed of fine, flat mineral grains, resulting in a moderately dense agglutinated wall structure. Under transmitted light using a Nikon Diaphot microscope, the wall appeared dark brownish, whereas it appeared light yellowish and more transparent under an Olympus BX51, reflecting differences in illumination and contrast settings.
The aperture is terminal and located at the anterior end, featuring a short tubular or funnel-shaped collar. Test length was highly variable, ranging from 138 to 571 μm (mean = 304 μm, n = 18); test width ranged from 83 to 273 μm (mean = 138 μm, n = 18). Aperture diameter varied between 23 and 49 μm, and neck length ranged from 5 to 21 μm.
No cytoplasm, inclusions, or nuclear structures were observed, as all examined specimens were empty (dead) tests.
Differential diagnosis: In general morphology, Poseidonella trans aquatica closely resembles other described non-marine agglutinated monothalamids, particularly Lacogromia spp., sharing features such as bilateral symmetry and an elongate-oval to lanceolate outline. However, it is readily distinguished from all known non-marine species by its well developed tubular to conical collar surrounding the aperture, although this feature is not consistently present in all empty tests. The only observed species with a tubular (but not conical) neck is ‘Undetermined Lacogromia BAH’, found in an underground lake in Ponor Kovaci, a cave in Bosnia and Herzegovina (Siemensma and Holzmann, 2023).
Unlike other non-marine monothalamids, P. transaquatica branches within a predominantly marine clade (Clade E), reinforcing its transitional ecological significance.
Molecular characteristics: The partial 18S rDNA sequences of P. transaquatica are 1075–1077 nucleotides in length, with a GC content of 43 %. Pairwise sequence distances range from 0.005 to 0.01.
