Acetabularia moldavica Barattolo & Ionesi & Ţibuleac 2019, sp. nov.
Creators: name: Barattolo, Filippo
name: Ionesi, Viorel
name: Ţibuleac, Paul
Publisher: name: Zenodo
Year: 2019
Subjects: Biodiversity
Taxonomy
Plantae
Chlorophyta
Ulvophyceae
Dasycladales
Polyphysaceae
Acetabularia
Acetabularia moldavica

description: Acetabularia moldavica sp. nov.
Figs. 4–8.
Etymology: From the Moldova Region.
Type material: Holotype: fertile cap (Fig. 4F), MP-UAIC 8228/ PD.1. Paratypes: specimens contained on the surfaces of five thin marl layers, MP-UAIC 8228/ PD.1–5.
Type locality: The outcrop is located 2.5 km WSW from Pătrăuţi village (Suceava County) along the Bradul stream incision, at 411 m asl, coordinates 47°43’12” N, 26°09’53” E (Fig. 2B).
Type horizon: A clay seam, about 40 cm thick, within a sequence of sand and sandstone beds, twenty metres above the horizon Pătrăuţi I (Fig. 2); Sarmatian (middle Miocene).
Diagnosis.— Discs made of 10–24 mucronate fertile rays with evidence of both corona superior and inferior. Each ray, quite flat in transverse section, contains 14–35 mineralized gametangia (cysts). The mineralized cyst layer, 20–25 μm thick, is made of fibrous aragonite crystals set perpendicular to the wall surface. A delicate extracellular calcareous layer sometimes envelopes the fertile rays (Table 1).
Description. —General features of the calcareous skeleton: The alga is known only by its fertile caps (Figs. 4–7). No trace possibly related to the stem has been found on the rock surface. The fertile disks are slightly concave, the central upper part being sunken (Fig. 4A, D, G) while the lower part is protruding (Fig. 4E, F). Some thin extracellular mineralized layers envelope the basal part (primary lateral), superior and inferior coronae, and the space between the fertile rays (Fig. 5D, E). Occasionally a delicate extracellular sheet covers the upper and lower sides of the disk, and a thin radial rib marks the boundary between gametophores (Figs. 5B, 6B).
Primary laterals, upper and lower corona: According to level at which the structure is broken and the side exposed, it is possible to observe the mineralized walls and cavities belonging to corona superior and inferior (Fig. 5C, D). The coronal structure of Fig. 7B 1 (fragment of a cap in upper view), is probably broken at the basal part (primary lateral) level, and therefore prominences (arrows) correspond to mineralized walls among the basal parts (furrows). The pore in each furrow connects the basal part with the corona inferior below.
Fertile rays (gametophores): The gametophore develops beyond the basal part, and is about 1.2–3.2 mm long and 0.5–1.0 mm wide distally. The outer end is pointed (mucronate; Fig. 4A–G), but this character decreases as size increases (Fig. 5A 1) and is not evident in Figs. 5B, 6B. The transverse distal sections are elliptical in small sized specimens and rectangular in large ones, both are arranged horizontally. A cap contains 10–24 gametophores.
Gametangia (cysts): Fertile rays are estimated to contain 14–35 mineralized gametangia (cysts), closely packed so that their outlines are roughly polygonal to quadrangular. Cysts are usually bound together at the cyst-cyst contact (Fig. 7C– E). The spaces between cysts appear to be uncalcified (Fig. 7A), but very occasionally micro-spherulitic crystal aggregates have been observed under SEM (Fig. 8). Mineralized cysts are 0.26–0.38 mm in diameter. The wall is 20–25 μm thick, fibrous, made by acicular crystals arranged orthogonal to the wall surface (seemingly original aragonite). Each gametangium has a circular opening, 50 μm wide, closed by a mineralized lid, that is commonly lost.
Reconstruction of the thallus: Figure 9 depicts the possible appearance of the alga in axial (Fig. 9A, B) and apical view (Fig. 9C). Extracellular mineralization is drawn in black, and the intracellular mineralized cyst wall is dark gray (Fig. 9A, B). Cyst structure is shown in Fig. 10A.
Comparisons.— Acetabularia moldavica sp. nov. differs from all other fossil Acetabularia species by the presence of mineralized gametangia. Moreover, the number of fertile rays in a cap is very low (10–24), whereas it is much higher in A. miocenica (90–100), A. chiavonica (78), and A. transylvana (90–100).
Remarks.— The sediments of Moldavian Platform represent lacustrine environments evolving into peat bog facies (observed to alternate cyclically), while being connected to the Carpathian Orogen by temporary land bridges (Ionesi et al. 1993; Ţibuleac 1999; Ionesi 2006).
On the contrary, there is no consensus regarding reverse brackish-marine evolution, despite its being reported within the Moldavian Platform (Ţibuleac 1999; Ionesi 2006).
Dasycladaceans are euryhaline organisms and prefer shallow water conditions ranging from mangrove swamps to coral reefs environments (Valet 1979). The pelitic facies containing A. moldavica sp. nov. suggests a soft substrate and in-situ fossilisation. The alga probably populated environments equivalent to a mangrove swamp. Acetabulariaceans quoted by Génot et al. (2002) probably required marine conditions.
Stratigraphic and geographic range.— Type locality and horizon only.
description: Published as part of Barattolo, Filippo, Ionesi, Viorel & Ţibuleac, Paul, 2019, A new polyphysacean alga from the Miocene of Romania and its biomineralization, pp. 85-100 in Acta Palaeontologica Polonica 64 (1) on pages 88-90, DOI: 10.4202/app.00537.2018, http://zenodo.org/record/10626616