Subphylum Tunicata Lamarck, 1816
Class Ascidiacea de Blainville, 1824
Remarks. Classification of the fossil Ascidiacea is based on (1) their pouch-like structure, which is similar to recent representatives of this class, (2) the presence of traces of siphons, and (3) imprints of internal organs on the surface of semi-decomposed remains, which are similar to the tunic, pharyngeal basket mantle (branchial sac), atrium, stomach, and the atrial/oral siphons of the present-day tunicates. These morphological features are typical of the family Ascidiidae Herdman, 1882 (Herdman 1882; Fedonkin et al. 2012) and cannot be misled for other group of organisms.
Genus Finkoella gen. nov. Martyshyn
Etymology. After S.V. Finko, an amateur palaeontologist, who provided the holotype and paratype of the type species.
Type species. Finkoella ukrainica sp. nov. Martyshyn.
Diagnosis. A convex or concave, ovate, oval, or rarely oblong, cucumber-shaped, lenticular structure on the lower or upper surface of sandstone/siltstone/mudstone layers. In most specimens, one end is more elongated, and the opposite one is more rounded or almost even. One of the ends may show a protruded, short neck and/or a shallow sinus. A low/shallow circular protrusion/depression can be present in the convex or concave structure, respectively. The protrusion/depression is usually located eccentrically towards one of the ends. Moreover, some shallow/low concavities/convexities may occur on the surface. The edges are usually slightly elevated in convex specimens or depressed in concave forms.
Species. Finkoella ukrainica and F. oblonga described in this paper.
Comparison. No analogues among the Ediacaran and younger fossils.
Distribution. Lomozov and Yampol members, Mogilev Formation, Mogilev-Podolsky Group, upper Ediacaran.
Finkoella ukrainica sp. nov. Martyshyn
Figures 3a–d, 4a–b, 5a–i, 6a–f, 7
v 2018 problematic organism, probably Tunicata.—Nesterovsky et al.: fig. 4c.
Etymology. After the country Ukraine.
Diagnosis. Finkoella showing oval to elliptical, rarely oblong shape, which may show a protruded, short neck and/or a shallow sinus. A low/shallow circular protrusion/depression can be present in the convex or concave structure, respectively. Smaller specimens can be without neck and protrusions/depressions.
Holotype. One of two larger structures (Figs. 3a, b) on the lower surface of the slab IGKNU17p170, i.e. the specimen IGKNU17p170/1 (Fig. 3c). The slab is built of muddy, very fine-grained sandstone and derives from the upper part of the Yampol Member of the Mogilev Formation, Mogilev-Podolsky Group, in the quarry near the DHPS at Bernashivka village, Vinnytsia region.
Other materials. (1) paratype IG KNU17p201/1 (Figs. 3d, 4a, b) and all other (at least 46) specimens on the upper surface of sandstone slab from the upper part of the Yampol Member (Figs. 4a, b, 5); moreover (2) two specimens in slabs IGKNU17p204 (Fig. 7a), IGKNU17p210 (Fig. 7f), IGKNU17p2011 (Fig. 7g), and IGKNU 17p214 (Fig. 7j), and single specimens in slabs IGKNU17p205 (Fig. 7d), IGKNU17p207 (Fig. 7e), IGKNU17p209 (Fig. 7b), IGKNU17p208 (Fig. 7c), and IGKNU17p213 (Fig. 7i), all from the Lomozov Member of the Mogilev Formation in the quarry near the DHPS at Bernashivka village, Vinnytsia region.
Description. The holotype (Fig. 3a–c) is a convex oval structure on the lower surface of a thin bed of micaceous, clayey, very fine-grained sandstone. It is 95 mm long and up to 55 mm wide. A round protrusion, 5 mm in diameter, is located slightly eccentrically towards one of the ends. One of the ends is more elongated, and the opposite one is more rounded. The more rounded end shows a protruded short neck, which is wider at its termination. The termination is unevenly lobate. The margins are sharp and even. The surface of the structure is generally smooth, except for local grain-size corrugations, thin and short grooves and a longer, narrow, shallow groove which runs obliquely through three quarters of the width close to the wider end.
The second fossil is located on the same side of the slab bearing the holotype (Fig. 3a, b). This is a concave, smooth structure in the shape of a simple cucumber. One end of the structure shows a sinus, which is 4 mm deep and 7 mm wide. The opposite end is broken on the edge of the slab and therefore not available. The preserved margins are sharp and locally elevated.
The paratype (Figs. 3d, 4a, b) is a concave, elongate oval structure on the upper surface of a sandstone slab. It is 56 mm long and up to 25 mm wide. The wider end is tapered and the opposite, narrower end shows a short, indistinct neck. The surface is covered with local bulges of oval outline and irregular, gentle depressions. The structure is mostly sharply bounded along its margin, but locally the boundary is less distinct. From one side, the margin is elevated and locally marked by a double ridge, which is 2 mm wide (Fig. 3d). The ridge is separated by an uneven narrow groove.
Other specimens (Figs. 4, 5, 6) are usually oval, convex or concave structures; some are obtuse from one end. They show variable sizes, with length ranging from 5 to 110 mm and width from 5 to 55 mm. The width versus length of all specimens show strong positive correlation (r = 0.84; Fig. 8). Most of the smaller specimens are usually smooth, without any neck and protrusions, but some of them show slightly elevated/depressed edges and or bulges interpreted as atrial siphon. Only some of them show a short neck (Fig. 7c) interpreted as oral siphon and knob-like protrusions/depressions. One of them shows a narrow side neck, which can be atrial syphon (Fig. 7i).
The described structures may occur in clusters (Figs. 3a, b, 4a, b). Individuals in the cluster may show different size. Longer axis of the fossil shows distinct orientation (rosette in Fig. 4b).
Remarks. The small specimens, which are less than 25 mm wide (see Fig. 8), are considered as representing juvenile organisms (see “Discussion”). The holotype was selected from specimens representing large, fully developed and most completely preserved organism (Fig. 9).
Finkoella oblonga sp. nov.
Etymology. After oblongus (Latin)—elongate, in correspondence to the overall shape.
Diagnosis. Elongated Finkoella showing winding margins and longitudinal furrows/ridges, which tend to run parallel to the margin.
Holotype. Split slab of horizontally to wavy laminated siltstone showing positive, i.e. convex fossil (IGKNU17p202/1) and is counterpart showing negative, i.e. concave part (IGKNU17p202/2). This is the only specimens of the species. It derives from the Lomozov Member of the Mogilev Formation, Mogilev-Podolsky Group, in the quarry near the DHPS at Bernashivka village, Vinnytsia region.
Other material. A split slab with specimen IGKNU17p215 and its counterpart IGKNU17p215/1, Yampol Member (Fig. 10b). Moreover, IGKNU17p216, Lomozov Member (determined as Finkoella cf. oblonga).
Description. The holotype (Fig. 10a) is an elongate, cucumber-like structure, 120 mm long and up to 42 mm wide, which is visible as a positive (convex) part and its negative (concave) counterpart in a horizontally split slab. It shows six longitudinal, low groves and ridges, which show gentle slopes and are 3–4 mm wide. They run parallel to the margin of the structure and turn accordingly at the terminations, so they tend to form concentring rings. They are more continuous on one side than on the other side, where only their fragments are visible. Moreover, the structure shows small corrugations, but without any pattern. Edges of the structure are distinctly elevated/depressed and locally show a double ridge or groove, which is 1.5–3 mm wide.
The specimen KSU17p215 and KSU17p215/1 (Fig. 10b) shows the same shape as the holotype. It is 95 mm long, up to 40 mm wide. Six, more or less complete and an additional one incomplete ridges/grooves are visible. Their course is the same as in the holotype. The edges are sharp and locally elevated. One of the termination is narrower. It forms a dome-like protrusion. The other end is obtuse and seems to partly broken.
The specimen IGKNU17p216 (Fig. 10c, d) is ovate, 38 mm long, up to 21 mm wide. It becomes gradually narrower toward its semi-circular distal. The opposite end is obtuse and uneven. The perimeter is located in one third of the length from the obtuse end. Six low ridges and grooves run parallel to the lateral margins of the structure. They disappear toward the ends. A curved tail of small corrugations emerges from the obtuse end (rm in Fig. 10c). Its margins are diffused. Similar corrugations cover larger part of the fossil body.
Remarks. In the holotype, both terminations of the structure touch the edge of the slab, so a possible neck-like protrusion is not visible, if it was present at all. The neck is also not obvious in KSU17p215. The specimen IGKN17p216 is determined as Finkoella cf. oblonga because its ovate shape differs from the elliptical, cucumber-like shape of the holotype. It is also distinctly smaller than the holotype.
Genus Pharyngomorpha gen. nov.
Etymology. After the pharyngeal basked, the characteristic anatomic part of tunicates, which is probably preserved in the type species of this genus, and after morpha—in relation to morphology the pharyngeal basked.
Type species: Pharyngomorpha reticulata sp. nov.
Diagnosis. Basket-like structures showing ribs and/or grooves arranged in a rectangular net.
Species. Only Pharyngomorpha recticulata—monotypic genus.
Comparison. No analogues among the Ediacaran and younger fossils.
Distribution. Lomozov Member, Mogilev Formation, Mogilev-Podolsky Group, Upper Ediacaran. Also the lower part of the Nagoryany Formation (Dzhurzhevka Member) in Moldova, dated roughly to 555–551 Ma (Francovschi et al. 2021).
Pharyngomorpha reticulata sp. nov.
2021 ?Ausia/Gibbavasis sponge-like organism—Francovschi et al.: fig. 14E
Etymology. After reticulum (Latin)—net, in correspondence to the morphology.
Diagnosis. Elongated Finkoella showing winding margins and longitudinal furrows/ridges, which tend to run parallel to the margins.
Holotype. One specimen in the slab IGKNU17p203 (Fig. 11), i.e. IGKNU17p203/1 (Fig. 11b, c) from the Lomozov Member of the Mogilev Formation, Mogilev-Podolsky Group, in the quarry near the DHPS at Bernashivka village, Vinnytsia region.
Description. The holotype is 37 mm long and up to 19 mm wide. It contains about ten, straight, perpendicular, densely packed parallel ribs separated by narrow furrows. There is 4 ribs per 10 mm. The furrows are less than 1 mm wide. The ribs are crossed by five longitudinal furrows (running along the structure) which are perpendicular to the ribs. The longitudinal furrows are c. 1 mm wide. The ribs in the place of intersection with the longitudinal furrows are still visible but they are much lower than in the inter-ribs stretches. The inter-ribs stretches form rectangles (5.0 × 2.5 mm), which are perpendicularly oriented to the structure, so they show the net-like pattern. The structure is elevated in the marginal part. In the narrower part, short bars occur on the margins.
The smaller specimen IGKNU17p203/1 is located c. 20 mm from the holotype. It is 14 mm long and 12 mm wide and composed of five ribs separated by narrow furrows, which are similar to these in the holotype.
Remarks. It is not excluded that the smaller specimen is a disarticulated part of the holotype. In the vicinity of the holotype, poorly outlined ribs can be visible, which may also derive from fragmentation of the organisms represented by the holotype or other individuals. Both specimens co-occur with Finkoella ukrainica on the same surfaces. ?Ausia/Gibbavasis sponge-like organism (Francovschi et al. 2021: fig. 14E) is so far the best preserved specimen ascribed to Pharyngomorpha reticulate.
Genus Burykhia Fedonkin et al., 2012
Figure 12a, b
Material. One specimen IGKNU17p191a, b (positive and negative) in a thin (3–8 mm) mudstone slab from village Kitaygorod in the Khmelnytsky region. Uppermost Ediacaran, Kanilov (Kanyliv) Group, Studenitsa Formation (Komarovo Member), 7 m below the boundary with the Okunets Formation, ca. 12 m below the Precambrian–Cambrian Boundary. The slab contains Harlaniella on the reverse side.
Description. An incomplete (broken) convex structure having its concave counterpart and an outline of incomplete ellipse, up to 45 mm wide and at least 42 mm long (probably twice longer when would be complete). It shows diagonal, straight, narrow furrows, which are about 0.5 mm wide and 5–8 mm apart. The furrows show slit-like widenings, up to 4 mm long and up to 2 mm wide, 4–5 m apart, which are deeper than the remaining parts of the furrow. In some inter-furrow areas, small, oval depressions, 1–2 mm wide may occur. They have a concave counterpart. They are located in the middle between the furrows, in a line parallel to them.
Remarks. Burykhia hunti Fedonkin et al., 2012 (Fig. 11c, d), the only known species of Burykhia is also known from fragments of the pharyngeal basket. However, the pattern of the furrows and depressions and their counterpart is different: it does not show the slit-like depressions along the furrows, and the structures in Burykhia sp. are arranged diagonally, while in B. hunti they are transverse and longitudinal.