Sample material was collected during IODP Expedition 318 in 2010 from Site U1361 (64.45° S, 143.90° E) at 3454 m water depth . The site is located on a levee along the continental rise of Wilkes Land, an area of the East Antarctic margin that is currently influenced by winter sea ice and Antarctic Bottom Water formation (Fig. ). The Wilkes Land margin is of interest because the East Antarctic Ice Sheet is grounded below sea level in the adjacent Wilkes Subglacial Basin, rendering it susceptible to the impact of relatively warm waters intruding onto the continental shelf . Sediment was sampled at approximately 10 cm resolution from 90.64–29.54 m b.s.f., and lithologies range from varied-colored clay to greenish-gray diatom-rich or -bearing silty clay . The interval is dated to approximately 3.9–1.7 Ma based on biostratigraphic datums, and paleomagnetic reversal stratigraphy is used to date an iceberg-rafted debris mass accumulation record .

To support the comparison of our new taxa to established Fragilariopsis species, F. clementia is discussed in Sect.  (Fig. ). New light micrographs of F. clementia were produced from IODP sites U1541 and U1543 material to provide example specimens from the Pacific sector of the Southern Ocean. Sites U1541 (54.28° S, 125.47° W) and U1543 (54.59° S, 76.73° W) were drilled during Expedition 383 in 2019 at 3604 m water depth on the East Pacific Rise and 3860 m water depth about 110 nmi southwest of the Chile coast , respectively. Two slides from Site U1541C core 11 were examined for F. clementia: U1541C-11H3 20–21 cm and U1541C-11H5 20–21 cm. Micrographs from Site U1543A core 23 were made from four slides: U1543A-23H4 120 cm, U1543A-23H5 120 cm, U1543A-23H6 120 cm, and U1543A-23HCC 18–24 cm. These slides were prepared aboard the JOIDES Resolution during Expedition 383 following the smear slide procedure outlined in . The approximate ages of specimens in Fig.  are based on the age models from and .

The new taxa were characterized using light microscopy and scanning electron microscopy. For diatom assemblage analysis with light microscopy, quantitative settled slides were prepared following the method of . For each slide, 35–50 µg of bulk sediment was weighed for samples with high biogenic silica (>15 wt  % BSi) and 60–70 µg for samples with low biogenic silica. The weighed sediment was placed in a glass test tube with 2 mL of 10 % hydrogen peroxide and two drops of saturated sodium hexametaphosphate and left for a minimum of 8 h in a 40 °C oven.

Duplicate slides were prepared by affixing two 22×40 mm glass cover slips to a clean glass microscope slide with a drop of rubber cement and placing them on a PLEXIGLAS platform approximately 2 cm above the outlet of a 2 L square B-Ker2 settling chamber, partially filled with Elix pure water. Once the slide with cover slips was positioned, the water in the settling chamber was topped up to 2 L and gently agitated. A drop of Kodak Photo-Flo surfactant dispersing agent was added to the sample tube before the sample slurry was dispersed into the chamber. The sample settled out of the water column for a minimum of 6 h before the valve was opened, allowing the water to drain at ∼2 drops per second overnight. Next, the slide and attached cover slips were removed from the chamber and allowed to dry, after which each cover slip was removed from the rubber cement and mounted sediment-side-down on a fresh glass slide using a few drops of Norland Optical Adhesive #61 (refractive index =1.56).

Fragilariopsis clava and Fragilariopsis armandae were counted on an Olympus BX41 microscope with a 10x eyepiece using a 60x objective. Rouxia raggattensis in samples 56.53–29.54 m b.s.f. was counted using the same microscope configuration as for the Fragilariopsis counts, while samples from 84.94–56.53 m b.s.f. were counted on an Olympus BX41 microscope with a 10x eyepiece using a 63x objective . When further identification was needed, the objective was changed to 100x. Images of specimens were taken using a 63x and a 100x objective, and the scale was adjusted to match when images were placed in the same figure. Fields of view (FOV) were tallied, and counts were converted to absolute abundance (AA), reported in valves per gram of dry sediment, using the equation of : 1AA=Number of diatom countsNumber of FOV⋅FOV area (mm2)⋅Beaker area (mm2)Dry sediment mass (g). Light microscope observations are supported by scanning electron microscope (SEM) images produced on a ZEISS Sigma FEG-SEM at the University of Otago Centre for Electron Microscopy. To prepare SEM slides, 2 mL of hydrogen peroxide and a drop of sodium hexametaphosphate were mixed with approximately 50 mg of sediment in a centrifuge tube and left overnight in a 40 °C oven. Sediment was rinsed with Elix pure water and dried before diatoms were separated from the terrigenous fraction by heavy-liquid separation. Material was centrifuged with sodium polytungstate mixed to a density of 2.15 g cm-3 to prevent clay particulates from covering the finer details of the diatom frustules. The sample was rinsed and stored in 50 mg Elix pure water. Several drops of the slurry were dried on a glass slide before the slide was gold-coated for SEM observation.

Fragilariopsis clava, Fragilariopsis armandae, and Rouxia raggattensis morphometric measurements, defined in Fig. , were systematically measured using ImageJ software (Schneider et al., 2012) in order to quantitatively compare the new taxa to pre-existing species in Tables  and . The degree of symmetry between apices is a crucial defining characteristic that separates F. clava from related taxa. We have established a metric that captures the absolute proportional difference of apices to define this characteristic, with values ranging between 0–1: 2absolute proportional difference=|1-width of minor apex(µm)width of major apex(µm)|. An absolute proportional difference of 0 equates to symmetrical apical widths, whereas values approaching 1 indicate a high degree of asymmetry. For clavate Fragilariopsis species, we defined the narrower and wider apices as the minor and major apices, respectively.

Diagnosis. The valve is clavate, heteropolar, and slightly panduriform. Apices are broadly rounded, and the wide and bulbous major apex tapers to a slightly constricted center and an oblong minor apex. Transapical striae are regularly spaced, curving and branching at the apices, with biseriate transitioning to uniseriate towards the major apex and poroids parallel or alternating (Figs. b and c; a, c, and e). Striation is occasionally disrupted by patch hyaline fields. The raphe is straight and threadlike along the edge of the exterior valve face and terminates at apices with slightly deflected ends (Figs. f; d and f). Based on SEM images of well-preserved specimens, areolae are occluded by an unperforated silica membrane and are attached to the pore wall except for a curved slit opening (Figs. b; b). These flap-like occlusions are referred to as foriculae by Figs. 19 and 24 and as volate occlusions by Fig. 31.

Dimensions. The length is 31–66 µm (mean: 48.5 µm), the major apex width is 8–12 µm (mean: 10 µm), and the minor apex width is. 4.7–10.5 µm (mean: 7.6 µm) (Table ). There are 6–12 costae per 10 µm (mean density: 7–9) (Fig. ; Table ).

Holotype. This can be seen in Fig. i, U1361A-6H7 32–34 cm. The type level and age for the specimen is late Pliocene at 55.49 m b.s.f. (CSF-B) in Hole U1361A. Accession # 47688 is curated by the Geology Department, University of Otago, NZ.

Paratype. This can be seen in Fig. t, U1361A-6H7 12–14 cm, and d, U136A-6H7 32–34 cm. Accession # 47689 for specimen (t) and # 47690 for specimen (d) are curated by the Geology Department, University of Otago, NZ.

Locality. The specimens are located at the Wilkes Land continental rise, East Antarctica.

Stratigraphic occurrence. The first and last appearances of Fragilariopsis clava overlap with the Thalassiosira vulnifica–Thalassiosira insignia diatom zone, which defined as ranging from the first appearance of T. vulnifica around 3.1 Ma and to the last appearance of T. insignia around 2.5 Ma.

Age. F. clava was only observed in samples from a narrow depth range, 55.59–55.30 m b.s.f. at Site U1361, correlating to a single interglacial interval, MIS G7 (2.76–2.75 Ma), based on the age model of .

Derivation of name. Fragilariopsis clava is named using the Latin word for club or staff, “clava”, in reference to its club-like shape.

Remarks. Fragilariopsis is a large genus defined by a flat, usually linear valve face with rounded valve poles; parallel striae with radiation at apices; and a simple raphe positioned between the valve face and the proximal mantle . Fragilariopsis clava has morphological affinities to an established lineage comprising the clavate Fragilariopsis species F. heardensis Bohaty 2003 , F. clementia Gombos 1976 , F. lacrima Gersonde 1991 , F. claviceps Schrader 1976 , and F. efferans Schrader 1976 . The F. clava holotype exhibits a broad valve outline and apices and a constricted center (Fig. i). The smaller paratype exhibits the same broad valve outline, coarse costae, and constricted center as the holotype, but the absolute proportional difference between apices is smaller, so valve heteropolarity is less pronounced (Fig. t). The elongate paratype has a deflected minor apex (Fig. d).

Fragilariopsis clava shares a similar heteropolar outline to F. heardensis and F. clementia, with one inflated major apex and a narrower minor apex. However, F. clava is more robust than F. heardensis, with coarser costae and a broader shape, and F. clava has more bluntly rounded apices than F. clementia (Fig. ). Fragilariopsis clava has a smaller proportional difference (0.13–0.43, n=100) than F. heardensis (0.56–0.87, n=15) and F. clementia (∼0.7) (Table ).

Figure compares the morphologies of F. clava and related species. When first identified F. heardensis in sediments from the Kerguelen Plateau, the authors compared it to other taxa with clavate outlines: Pliocene species F. clementia and F. lacrima and Miocene species F. efferans and F. claviceps. The morphometric differences are most apparent when comparing the proportions of their outlines. All species are heteropolar, but F. clementia, F. lacrima, and F. efferans have a large proportional difference like F. heardensis. Fragilariopsis claviceps clusters with F. clava, demonstrating their shared coarse costae and low proportional difference (Fig. a and c).

Rare F. clava specimens have 11–12 costae per 10 µm (Fig. g). Fragilariopsis c.f. clementia at Site U1361 may be a transitional form between F. clementia and F. clava, with angular apices, coarse costae, and a broad outline resulting in a smaller proportional difference (Fig. e and f; see Sect. 4.1 for further discussion).

Diagnosis. The valve is clavate and heteropolar. Apices are blunt and rounded, and the domed major apex constricts to a medial bulge and narrower minor apex. There are two endmembers: biundulate, longer valves with a slight medial bulge, narrow minor apex, and rounder apices (e.g., Fig. a–d) and shorter valves with a constricted center and blunter apices (e.g., Fig. i–l). Medial striae are transapical, curved, and branched towards the apices and are uniseriate, at times discontinuous (Table 1; e.g., Fig. d). The raphe is straight and threadlike along the edge of the exterior valve face (Fig. b and c). The raphe ends simply at the major apex and deflects towards the valve face at the minor apex.

Dimensions. The length is 32.85–86.40 µm (mean: 57.27 µm), the major apex width is 8.09–10.70 µm (mean: 9.40 µm), and the minor apex width is 2.23–5.16 µm (mean: 3.59 µm) There are 10–13 costae per 10 µm (Table ).

Holotype. This can be seen in Fig. f, U1361A-6H1, 12–14 cm. The type level and age for the specimen is late Pliocene at 47.13 m b.s.f. (CSF-B) in Hole U1361A. Accession # 47691 is curated by the Geology Department, University of Otago, NZ.

Paratype. This can be seen in Fig. k, U1361A-6H2 2–4 cm. Accession # 47692 is curated by the Geology Department, University of Otago, NZ.

Locality. The specimens are located at the Wilkes Land continental rise, East Antarctica.

Stratigraphic occurrence. The range of F. armandae bridges the Thalassiosira vulnifica–T. insignia and T. vulnifica diatom zones. defined the lower boundary of the T. vulnifica diatom zone as the last appearance of T. insignia around 2.5 Ma and the upper boundary as the last appearance of T. vulnifica around 2.2 Ma.

Age. F. armandae is observed in samples from depths of 49.57–47.13 m b.s.f. which correspond approximately to MIS 101 and 97 (∼2.58–2.47 Ma).

Derivation of name. Fragilariopsis armandae is named in honor of the late Dr. Leanne Armand in recognition of her seminal contributions to the field of Southern Ocean diatom micropaleontology and her deep and selfless commitment to the advancement of our community.

Remarks. The holotype of F. armandae exhibits discontinuous poroids and a blunt major apex that constricts to a slight medial bulge and a narrow minor apex. The paratype has a lower absolute proportional difference than the holotype and a valve face with nearly straight sides as they taper to the minor apex. The minor apex width, absolute proportional difference, and medial bulge distinguish F. armandae from the most similar clavate Fragilariopsis species, F. clava and F. heardensis. Fragilariopsis armandae clusters between F. clava and F. heardensis when their absolute proportional difference and minor apex width is compared (Fig. e). Fragilariopsis armandae has a robust clavate valve shape similar to F. clava but, unlike F. clava, it is uniseriate, and it has discontinuous poroids and finer costae. The blunter, almost flat apices of F. armandae further separate the shorter endmember of this species (e.g., Fig. i and j) from short specimens of F. clava (e.g., Fig. q), which are similarly proportioned. The longer endmember of F. armandae is distinguishable from long specimens of F. clava by its medial bulge and the arrangement of its poroids.

Fragilariopsis armandae is differentiated from F. heardensis by its lower absolute proportional difference, slightly coarser striation, and discontinuous poroids. The major apex of the F. heardensis holotype from shown in Fig.  is blunt like the wide apex of F. armandae, but the sides of the valve face do not constrict when narrowing to the smaller apex. Three specimens identified as F. heardensis by Bohaty et al. (2003) and displayed in their Plate 2 (Figs. 2 and 3) and Plate 3 (Fig. 9) have valve shapes, coarser costae, wider minor apices, and longer lengths more similar to F. armandae than F. heardensis (Table  and Fig. ). See Sect. 4.1 for further discussion.

Synonymy. Synonyms are Rouxia sp. A in , Plate 17 Fig. 11, and Rouxia sp. A of Harwood and Maruyama in , Fig. 5.

Diagnosis. The valve is long and bilobate (Fig. a–f). One apex is cuneate and spatulate (Fig. p–x), and the other is narrow and rounded (Fig. i–o). Apices are connected by a long, semi-convex central hyaline bar, perforated by rows of marginal pores (Fig. a–f). The raphe is simple and threadlike with simple ends (Figs. ; b–d and h–i).

Information about the striae, alveoli, and silica structure was determined by SEM imaging (Fig. ). Striae are alveolate, consisting of a single elongate chamber with a wider semicircular medial end (keyhole-shaped; Fig. b–e). The elongated section is perforated by two rows of fine pores on the Site U1361 specimen (Fig. e) and by three to four rows on specimens from AND-1B (Riesselman and Dunbar, 2013, Fig. 5 panels 7 and 8). The distribution of pores at the semicircular end is more disorganized and irregular (Fig. e). Alveoli towards the tip of the apex are smaller and have no circular end (Fig. c and d), and, in the central bar, alveoli are oblong. The exterior marginal ridge is narrower than the interior valve face (Fig. g–i). The pores on the edge of the exterior valve face align with the center of the alveoli on the interior valve face (Figs. g–i; a and b). Faint circular impressions run parallel to the pores on the external valve face and align with the medial ends of the interior alveoli (Figs. g–h; b).

Dimensions. The length is 120–201 µm (mean 161.5 µm), the medial width is 3.1–5.2 µm (mean 4.15 µm), the spatulate apex length is 17.5–26.1 µm, the spatulate raphe length is 14.4–22.3 µm, the spatulate apex width is 5.1–8.2 µm, the rounded apex length is 17.8–25.8 µm, the rounded raphe length is 15.3–23.0 µm, and the rounded apex width is 3.4–7.1 µm. There are 12–15 costae per 10 µm (Table ).

Holotype. This can be seen in Fig. a, U1361A-6H7 32–34 cm. The age and type level for the holotype specimen is late Pliocene at 55.49 m b.s.f. (CSF-B) in Hole U1361A. Accession # 47693 is curated by the Geology Department, University of Otago, NZ.

Locality. The specimens are located at the Wilkes Land continental rise, East Antarctica.

Age. Shipboard biostratigraphy from IODP Exp. 318 Site U1361 Hole A identified R. raggattensis apices (as Rouxia sp. spatulate, long heteropolar) in sample 10HCC 19–24 cm (94.94–94.99 m b.s.f. CSF-B; Escutia et al., 2011), placing the species in sediment as old as 4.06 Ma . The last occurrence of R. raggattensis at Site U1361 was identified in sample 5H1 2–4 cm (37.53 m b.s.f. CSF-B), which corresponds to ∼2.06 Ma. Rouxia raggattensis has previously been documented as Rouxia sp. A Harwood and Maruyama in the late Pliocene (3.6–3.0 Ma) sediments from ANDRILL Site AND-1B (see Sect. 4.2 for more detail).

Derivation of name. Rouxia raggattensis is named for the Raggatt Basin on the Kerguelen Plateau, where David Harwood and Toshiaki Maruyama first documented this species from late Pliocene sediments collected during ODP Leg 120 from Site 751A Plate 17 Fig. ll.

Remarks. Complete specimens of R. raggattensis are rarely preserved because the long narrow central bar is fragile; however, apical fragments are common. The holotype displays distinct and clear examples of the spatulate and rounded apices attached by a narrow semi-convex central bar. Rouxia diploneides has a similar valve outline to R. raggattensis, where the two apices taper below the end of the raphe to a narrower central bar. However, R. diploneides is isopolar, the valve center is only slightly narrower than the apices, and the total length is shorter than R. raggattensis (Fig. y; Table ). Fragmented R. raggattensis apices are distinguishable from R. diploneides because the apices have a narrower taper to the central area than R. diploneides. The alveoli and interior structure of Rouxia raggattensis are comparable to other Rouxia species. The keyhole-shaped alveoli are notable on apices of Rouxia antarctica, Rouxia isopolica, and Rouxia leventerae (Fig. c–f). Interior views of Rouxia antarctica also have similar features to R. raggattensis (Fig. g and h).

Clavate Fragilariopsis species diversified in the late Pliocene in the Southern Ocean. Fragilariopsis clava appears to have evolved from F. clementia based on transitional forms at Site U1361. The evolutionary relationships between F. clava, F. armandae, and F. heardensis are more difficult to discern.

Fragilariopsis clementia is most commonly observed in Southern Ocean sediments dated between 7–4 Ma (total age range: 10–3 Ma; see Fig. ). The taxon has a wide geographical distribution and two morphological endmembers (Fig. ). The major apex of the elongate endmember has parallel sides, whereas shorter species may be convex . Both endmembers are present in Fig. , which presents F. clementia specimens by site and gives an estimated age based on published age models . Specimens selected to be included in a publication serve as a representation of what authors identified as F. clementia at a given location. Fragilariopsis clementia in the vicinity of the Drake Passage and the Weddell Sea appears to present most frequently in an elongate form with rounded apices e.g.,. In comparison, F. clementia from the Indo-Pacific presents with a convex valve face and a wider, more angular, and cuneate board apex e.g.,. It is possible that the observed regional differences are due to author bias, change in F. clementia species concepts over time, or an evolution of the taxon. However, the sites exhibiting more elongate F. clementia are sourced from papers authored by different scientists and published in different decades. While out of the scope of this paper, a deeper investigation into F. clementia morphologies may be fruitful if original site material was revisited to make observations using a light microscope and SEM.

Although F. clementia was previously identified at Site U1361 between 70.04–57.19 m.b.s.f. (3.17–2.8 Ma) , this occurrence is much younger than the established taxon ranges for both morphological endmembers . Constrained optimization has been used to model the last appearance of F. clementia, with the average range model placing the datum between 4.6–4.0 Ma and the total range model placing it around 2.37–2.34 Ma . In the course of our work documenting new clavate Fragilariopsis taxa from Site U1361, we have re-examined the original sides and determined that these younger specimens should be reclassified as F. c.f. clementia. Specimens have angular apices and valve outlines that resemble the F. clementia specimens identified by in mid-Pliocene sediments collected from the Kerguelen Plateau in the Indian Ocean, but they also have a broad minor apex and coarse striae similar to the morphology of F. clava sp. nov. The first occurrence of F. c.f. clementia at Site U1361 at 70.04 m b.s.f. (∼3.17 Ma) corresponds to the mid-Pliocene warm interval at MIS KM3 (Fig. ). Fragilariopsis c.f. clementia is next observed at Site U1361 during a prolonged period of productivity from 2.84–2.8 Ma (58.14–57.19 m b.s.f.). The last occurrence occurs at 54.24 m b.s.f. (∼2.72 Ma) as one specimen.

It is more difficult to determine when F. heardensis and F. armandae evolved in relation to F. clava. Both species are present at Site U1361 between MIS 101–97 (∼2.6–2.47 Ma), while characteristically robust F. clava specimens are present at Site U1361 during MIS G7 at ∼2.73 Ma (Fig. ) alongside endmembers with finer costae. The paper that formally describes F. heardensis, Bohaty et al. 2003, is the only literature that explicitly lists F. heardensis in its diatom assemblage at ODP Site 1138 on the Kerguelen Plateau in the Indian sector of the Southern Ocean. originally identified two morphotypes for F. heardensis: the holotype (Plate 2 Fig. 1a and b; shown in Fig. ) and the specimens that more closely resemble F. armandae (Plate 2 Figs. 2–3 and Plate 3 Fig. 9; example in Fig. ). The latter morphotype shares a constricted center and possible medial bulge with F. armandae. However, one specimen (Bohaty et al., 2003 Plate 3 Fig. 9) is biseriate in the center, and all specimens have 9–10 costae, characteristics more comparable to F. clava than F. armandae. We suggest these specimens could be reclassified as F. armandae or F. c.f. armandae. All three specimens are from the same sample (138-1138A-8R-CC) dated about 3 Ma, approximately 0.2–0.3 Ma older than the holotype F. heardensis presented by Bohaty (138-113A-8R-5, 100–101 cm). The age difference between the appearance of F. clava and F. heardensis at Site U1361 is also about 0.2–0.3 Ma.

The Fragilariopsis heardensis holotype is a similar age to trace occurrences of F. efferans or F. c.f. efferans recorded in the late Pliocene by older literature records, although the diatom taxon is typically observed in middle Miocene sediments ∼14.5–11 Ma;. Because F. efferans has not been found in Late Miocene (10–6 Ma) sediments , we suggest that traces of F. efferans in the Pliocene are either reworked Middle Miocene specimens or misidentified Pliocene clavate species. identified a specimen as F. c.f. efferans from ODP Site 328 located in the outer Malvinas Basin at ∼2.9–2.8 Ma, which has the characteristic morphology of F. heardensis. Other examples of Pliocene F. efferans come from late Pliocene sediment (3.8–2.6 Ma) collected from ODP sites 747 and 748 on the Kerguelen Plateau and from mid-Pliocene sediments (∼4.5 Ma) from ODP Site 1092 . Fragilariopsis heardensis is less common than F. clementia, but its geographical distribution expands to cover a similar area to F. clementia (Fig. ) if alternative identifications from older literature are considered.

The origination and extinction of F. clava, F. armandae, F. heardensis, and transitional forms occurred during a period of pronounced diatom species turnover between 3–2 Ma, related to changing climate conditions . Although clavate Fragilariopsis species are not common in Southern Ocean sediments, they are observed in trace to rare amounts in the region affected by the Antarctic Circumpolar Current (ACC) (Fig. ). Miocene and early Pliocene taxa thrived in relatively warm open-ocean conditions impacted by the ACC. Temperatures were warmer and CO2 concentrations higher during the Miocene and early- to mid-Pliocene . Pliocene occurrences of F. c.f. clementia at Site U1361 have been indirectly associated with warmer subantarctic waters circulating southward, closer to the Antarctic continental margin . The Wilkes Land continental rise is situated to the east and south of the Kerguelen Plateau and is a place where regional bathymetry constricts the eastward-flowing ACC to a more southerly position . The onset of cooler sea surface temperatures and increased sea ice in the late Pliocene may have marginalized clavate Fragilariopsis species and facilitated their extinction and the extinction of many other diatom species . We speculate that F. clava, F. armandae, and F. heardensis evolved as the clavate Fragilariopsis lineage attempted to adapt to cooling climate conditions in the Southern Ocean, ultimately unsuccessfully.

Rouxia raggattensis persisted from at least the mid-Pliocene through to the Early Pleistocene. To better constrain the first appearance of R. raggattensis, we compare the occurrences from Site U1361 to occurrences from ANDRILL core AND-1B (Fig. ). Samples from U1361 were examined down to 94.9 m b.s.f. (4.05 Ma), and possible trace fragments of R. raggattensis were identified, although poor preservation generates some uncertainty in identification. In AND-1B, R. raggattensis was consistently present in three diatom units (VIII, IX, and XI) in trace amounts (maximum relative abundance: 3.67 %) from approximately 3.6–3.0 Ma (Fig. ). Rouxia raggattensis was not identified below an unconformity at 440 m b.s.f. in diatom unit XI, which corresponds to an approximately 0.7 Ma time gap between 3.64 to 4.34 Ma . The unconformity prevents observing the first occurrence of R. raggattensis in the Ross Sea and Wilkes Land area, but we propose R. raggattensis evolved sometime between 4.3–4.05 Ma based on its occurrence in sediments from U1361 and AND-1B.

The abundance of Rouxia raggattensis is not constant at Site U1361. Relative abundance is low from 4.05 until 3.15 Ma (64.94 m b.s.f.), about MIS G21, when the relative abundance exceeds 5 %. The absolute abundance also increases slightly during MIS G21 followed by a dramatic increase at 2.81 Ma (MIS G9). Rouxia raggattensis becomes a significant secondary species, reaching a maximum relative abundance of 26.9 % at 2.74 Ma (55.14 m b.s.f.) and maximum absolute abundance at 2.44 Ma (46.34 m b.s.f.); from ∼2.16 Ma (39.92 m b.s.f.), its abundance rapidly declines. The decrease in abundance of R. raggattensis suggests that environmental conditions along the Wilkes Land margin became progressively less favorable as Pleistocene cooling progressed. Rouxia raggattensis briefly reappears in the youngest U1361 sample examined in this study, 1.76 Ma (31.83 m b.s.f.), in low relative abundances (1.9 %).

The maximum abundance of R. raggattensis coincides with a dynamic period (2.8–2.4 Ma) of instability in Antarctic and Southern Ocean conditions during which Northern Hemisphere glaciation intensified . By summing the rate of diatom extinction and origination, identified five periods of major species turnover in the last 15 Ma, including from 2–3 Ma. Diatom speciation is sensitive to sea ice extent; non-ice-adapted species are marginalized when open-water niches south of the Polar Front are lost to expanding sea ice . However, as other diatom species went extinct as surface conditions cooled, R. raggattensis seems to have thrived, particularly when sea ice was present. At Site U1361, R. raggattensis is strongly correlated (r=0.78) with Fragilariopsis sublinearis, an extant species associated with locations with 7 or more months of sea ice a year . Relative abundances of F. sublinearis greater than 2 % are correlated to places with winter sea ice, such as the Ross Sea, Prydz Bay, Wilkes Land, and the Weddell Sea . Rouxia raggattensis reached a maximum relative abundance (26.92 %) at ∼2.74 Ma (55.14 m b.s.f. in MIS G7), close to a major cooling step recorded in deep-sea temperatures at 2.73 Ma associated with the intensification of Northern Hemisphere glaciation . Furthermore, the relative abundance of R. raggattensis decreased dramatically at ∼2.16 Ma (39.92 m b.s.f.), at about the same time as a reduction in iceberg calving and an inferred reduction in the extent of sea ice. We therefore surmise that R. raggattensis was a sea-ice-affiliated species that preferred surface water conditions dominated by cold temperatures.