Two new clavate Fragilariopsis and one new Rouxia diatom species with biostratigraphic and paleoenvironmental applications for the Pliocene-Pleistocene, East Antarctica

. Three new pennate diatom taxa, Fragilariopsis clava sp. nov. Duke; Fragilariopsis armandae sp. nov. Frazer, Duke et Riesselman; and Rouxia raggattensis sp. nov. Duke et Riesselman, are described and named from Pliocene-Pleistocene sediments collected from the continental rise adjacent to the Wilkes Land coast of East Antarctica. The stratigraphic occurrence of F. clava and F. armandae at IODP Site U1361 are well-constrained to Marine Isotope Stages G9-G7 (2.76–2.74 Ma) and 101–97 (2.58–2.47 Ma), respectively. The short stratigraphic ranges of F. clava and F. armandae are potentially useful biostratigraphic markers for constraining the age of late Pliocene to Early Pleistocene Antarctic sediments. Rouxia raggattensis is observed in the oldest sample examined at Site U1361 from ∼ 4 . 05 Ma and is more common between 3.0–2.15 Ma. The rise in abundance of R. raggattensis corresponds to a large turnover in diatom species between 3 and 2 Ma associated with Antarctic cooling, suggesting that sea surface conditions were favorable for R. raggattensis during this dynamic time.


Introduction
Documenting the diatom assemblage of the Southern Ocean during the Pliocene and Pleistocene provides a proxy for past environmental conditions and stratigraphic age datums (e.g., Bertram et al., 2018;Taylor-Silva and Riesselman, 2018;McKay et al., 2012;Armbrecht et al., 2018;Barron, 1996;Konfirst and Scherer, 2012;Cortese and Gersonde, 2008).Environmental preferences of fossil taxa are based on the study of extant species, with the assumption that their preferences have remained constant through time (Armand et al., 2005;Crosta et al., 2005Crosta et al., , 1998;;Romero et al., 2005), and their co-occurrence with other taxa and environmental proxies (e.g., Winter et al., 2010).Cosmopolitan diatom species are not useful environmental indicators but can be excellent age indicators if they are well preserved and have wellconstrained first and last stratigraphic appearances.Early literature synthesized the first and last appearances of diatom species relative to paleomagnetic reversals and other chronostratigraphic constraints to create diatom zones, establishing a framework to determine the relative ages of sedimentary sequences (e.g., Ciesielski, 1983;Harwood and Maruyama, 1992;Gersonde and Burckle, 1990;McCollum, 1975;Schrader, 1976).Later work refined the sequence of biostratigraphic events and reconciled published ages to create a more robust biochronology that is applicable across multiple Southern Ocean sites (Cody et al., 2008(Cody et al., , 2012;;Crampton et al., 2016).
55.30 m b.s.f.) and 2.58-2.47Ma (49.57-47.13m b.s.f.), respectively, at IODP Site U1361 on the Wilkes Land continental rise, East Antarctica, and have not previously been identified in Antarctic sediments.Fragilariopsis clava and F. armandae succeed a lineage of Pliocene clavate diatoms, bolstering the biostratigraphic capability of this suite of species.
Rouxia raggattensis was first documented as Rouxia sp.A by Harwood and Maruyama (1992) in Pliocene sediments from Site 751 on the Kerguelen Plateau and subsequently identified in Plio-Pleistocene sediments from ANDRILL Site AND-1B, south of Ross Island on the Ross Sea continental shelf (Riesselman and Dunbar, 2013;Scherer et al., 2007).However, the species could not be formally described from either of these locations due to low abundance and the rarity of complete specimens.Because R. raggattensis is common and well-preserved at Site U1361, it is now possible to fully characterize the species and speculate about its environmental affinities.

U1361A sediment samples
Sample material was collected during IODP Expedition 318 in 2010 from Site U1361 (64.45°S, 143.90°E) at 3454 m water depth (Escutia et al., 2011).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. 1).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 (Cook et al., 2013;Bertram et al., 2018;Wilson et al., 2018).

Expedition 383 samples
To support the comparison of our new taxa to established Fragilariopsis species, F. clementia is discussed in Sect.4.1 (Fig. 14).New light micrographs of F. clementia were produced from IODP sites U1541 and U1543 material to pro-

Diatom analysis
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 Warnock and Scherer (2014).
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 56.53-29.54m b.s.f. was counted using the same microscope configuration as for the Fragilariopsis counts, while samples from 84.94-56.53m b.s.f. were counted on an Olympus BX41 microscope with a 10x eyepiece using a 63x objective (Taylor-Silva and Riesselman, 2018).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 Warnock and Scherer (2014) A1, and Cody et al. (2008).The black lines denote the estimated age range in which more than three sites record a presence of a species.To test the robustness of this estimate, it was compared to the average range model from Cody et al (2008), a probabilistic determination that calculated an average position of biostratigraphic events, for F. clementia, F. lacrima, F. claviceps, and R. diploneides.The dashed line indicates when a species is present at one to three sites.The question mark over the dashed red line shows when rare F. efferans specimens are identified in the Pliocene (Harwood and Maruyama, 1992;Censarek and Gersonde, 2003).The "x" denotes Marine Isotope Stage G7, the single interglacial period in which F. clava was identified at Site U1361.
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.

Diatom morphometric measurements
Fragilariopsis clava, Fragilariopsis armandae, and Rouxia raggattensis morphometric measurements, defined in Fig. 2, were systematically measured using ImageJ software (Schneider et al., 2012) in order to quantitatively compare the new taxa to pre-existing species in Tables 1 and 2. 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: absolute proportional difference = 1 − width of minor apex (µm) width of major apex (µm) . (2) An absolute proportional difference of 0 equates to symmetrical apical widths, whereas values approaching 1 indi- cate a high degree of asymmetry.For clavate Fragilariopsis species, we defined the narrower and wider apices as the minor and major apices, respectively.

Results
Three new fossil diatom species are described from Pliocene and Pleistocene material collected at IODP Site U1361.New taxa are differentiated from one another and from established species based on morphometric measurements.
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.5b and c; 6a, 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 dehttps://doi.org/10.5194/jm-43-139-2024flected ends (Figs.5f; 6d 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.5b; 6b).These flap-like occlusions are referred to as foriculae by Cox (2004, Figs. 19 and 24) and as volate occlusions by Mann (1980, Fig. 31).
Holotype.This can be seen in Fig. 4i, 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.
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 Harwood and Maruyama (1992) 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, 55.59-55.30m b.s.f. at Site U1361, correlating to a single interglacial interval, MIS G7 (2.76-2.75Ma), based on the age model of Patterson et al. (2014).
Figure 7 compares the morphologies of F. clava and related species.When Bohaty et al. (2003) 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. 7a and c).
Rare F. clava specimens have 11-12 costae per 10 µm (Fig. 15g).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. 15e 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. 8ad) and shorter valves with a constricted center and blunter apices (e.g., Fig. 8i-l).Medial striae are transapical, curved, and branched towards the apices and are uniseriate, at times discontinuous (Table 1; e.g., Fig. 8d).The raphe is straight and threadlike along the edge of the exterior valve face (Fig. 9b and c).The raphe ends simply at the major apex and deflects towards the valve face at the minor apex.
Holotype.This can be seen in Fig. 8f, 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.
Locality.The specimens are located at the Wilkes Land continental rise, East Antarctica.https://doi.org/10.5194/jm-43-139-2024Remarks.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. 7e).Frag- ilariopsis 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. 8i and j) from short specimens of F. clava (e.g., Fig. 4q), 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 Bohaty et al. (2003, shown in Fig. 15) is blunt like the wide apex of F. armandae, but the sides of the valve face do not constrict when narrowing to the https://doi.org/10.5194/jm-43-139-2024smaller 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 1 and Fig. 7).See Sect.4.1 for further discussion.Diagnosis.The valve is long and bilobate (Fig. 10a-f).One apex is cuneate and spatulate (Fig. 10p-x), and the other is narrow and rounded (Fig. 10i-o).Apices are connected by a long, semi-convex central hyaline bar, perforated by rows of marginal pores (Fig. 10a-f).The raphe is simple and threadlike with simple ends (Figs.10; 11b-d and h-i).

Rouxia raggattensis
Information about the striae, alveoli, and silica structure was determined by SEM imaging (Fig. 11).Striae are alveolate, consisting of a single elongate chamber with a wider semicircular medial end (keyhole-shaped; Fig. 11b-e).The elongated section is perforated by two rows of fine pores on the Site U1361 specimen (Fig. 11e) 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. 11e).Alveoli towards the tip of the apex are smaller and have no circular end (Fig. 11c and d), and, in the central bar, alveoli are oblong.The exterior marginal ridge is narrower than the interior valve face (Fig. 11g-i).The pores on the edge of the exterior valve face align with the center of the alveoli on the interior valve face (Figs.11g-i; 12a 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 12b).
Holotype.This can be seen in Fig. 10a, 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.
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 (Harwood and Maruyama, 1992, 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 Schrader (1973) 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. 10y; Table 2).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. 12c-f).Interior views of Rouxia antarctica also have similar features to R. raggattensis (Fig. 11g and h).

Biostratigraphic and paleoceanographic applications
4.1 Clavate Fragilariopsis evolution, regional occurrences, and environmental affinities 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. 14).The taxon has a wide geographical distribution and two morphological endmembers (Fig. 14).The major apex of the elongate endmember has parallel sides, whereas shorter species may be convex (Gombos, 1976).Both endmembers are present in Fig. 14, which presents F. clementia specimens by site and gives an estimated age based on published age models (Gombos, 1976;Gersonde and Burckle, 1990;Censarek and Gersonde, 2002; highlights the marginal ridge (dashed arrow i), rows of pores (dashed arrow ii), and ovular impressions (dashed arrow iii).Bohaty et al., 2003;Iwai and Winter, 2002;Harwood and Maruyama, 1992).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., Iwai and Winter, 2002;Gombos, 1976;Gersonde and Burckle, 1990).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., Bohaty et al., 2003).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 70.04-57.19m.b.s.f.(3.17-2.8Ma) (Taylor-Silva and Riesselman, 2018), this occurrence is much younger than the established taxon ranges for both morphological endmembers (Cody et al., 2008;Crampton et al., 2016).Constrained optimization has been used to model the last appearance of F. clementia, with the average range model placing the datum between 4.6-4.0Ma and the total range diploneides (red squares) (see Fig. 2).(a) The apex length is compared to the length of the raphe on the apex, (b) the length of one apex is compared to the length of the other, (c) the apex length is compared to its width, and (d) the total specimen length is compared to the transapical width.Rouxia raggattensis spatulate apices are shown as open circles, whereas the rounded apices are solid circles.The apices on a single specimen have a nearly symmetrical length for both R. raggattensis and R. diploneides, but the R. raggattensis spatulate apex is wider than its rounded apex, and, in general, R. diploneides is smaller and has shorter apices than R. raggattensis.model placing it around 2.37-2.34Ma (Cody et al., 2008;Crampton et al., 2016).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 Bohaty et al. (2003) 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. 17).Fragilariopsis c.f. clementia is next observed at Site U1361 during a prolonged period of productivity from 2.84-2.8Ma (58.14-57.19m 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 (∼ 2.6-2.47Ma), while characteristically robust F. clava specimens are present at Site U1361 during MIS G7 at ∼ 2.73 Ma (Fig. 17) 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.Bohaty et al. (2003) originally identified two morphotypes for F. heardensis: the holotype (Plate 2 Fig. 1a and b; shown in Fig. 15) and the specimens that more closely resemble F. armandae (Plate 2 Figs.2-3 and Plate 3 Fig.9; example in Fig. 15).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 0.2-0.3Ma older than the holotype F. heardensis presented by [100][101].The age difference between the appearance of F. clava and F. heardensis at Site U1361 is also about 0.2-0.3Ma.
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, re-lated to changing climate conditions (Crampton et al., 2016).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. 16).Miocene and early Pliocene taxa thrived in relatively warm open-ocean conditions impacted by the ACC.Temperatures were warmer and CO 2 concentrations higher during the Miocene and early-to mid-Pliocene  2003) specimen ( 9) has a similar valve outline.(Pagani et al., 2009;Seki et al., 2010;Sangiorgi et al., 2018;Zachos et al., 2008).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 (Taylor-Silva and Riesselman, 2018).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 (Barron, 1996;Orsi et al., 1995).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 (Crampton et al., 2016).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 age constraints
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. 18).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 3.6-3.0Ma (McKay et al., 2012;Riesselman and Dunbar, 2013;Sjunneskog and Winter, 2012) (Fig. 18).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 (Konfirst et al., 2011;Scherer et al., 2007).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 4.3-4.05Ma 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 %).

Rouxia raggattensis regional and environmental occurrences
The maximum abundance of R. raggattensis coincides with a dynamic period (2.8-2.4Ma) of instability in Antarctic and https://doi.org/10.5194/jm-43-139-2024Southern Ocean conditions (Barron, 1996;Hillenbrand and Cortese, 2006;Naish et al., 2009;Crampton et al., 2016;Tauxe et al., 2015;Bertram et al., 2018) during which Northern Hemisphere glaciation intensified (Balco and Rovey, 2010;Jansen and Sjoholm, 1991;Lisiecki and Raymo, 2005;Naafs et al., 2013;Rea and Schrader, 1985;Shackleton et al., 1984).By summing the rate of diatom extinction and origination, Crampton et al. (2016) 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-iceadapted species are marginalized when open-water niches south of the Polar Front are lost to expanding sea ice (Crampton et al., 2016).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 (Armand et al., 2005).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 (Armand et al., 2005;Cefarelli et al., 2010).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 (Lisiecki and Raymo, 2005;Rohling et al., 2014;Woodard et al., 2014).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 (Patterson et al., 2014;Rohling et al., 2014) 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.

Conclusions
This   2.47 Ma), as F. heardensis.We speculate that F. armandae and F. heardensis evolved from the same branching point that separated uniseriate clavate Fragilariopsis species from biseriate species (e.g., F. clava).Although present throughout the Pliocene-Pleistocene sediments examined for this study, R. raggattensis sp.nov.was more abundant from about 3-2 Ma when the Southern Ocean underwent significant cool-ing.Rouxia raggattensis appears to be associated with sea ice conditions based on its strong correlation with F. sublinearis, an extant Southern Ocean species.We speculate that R. raggattensis first appeared at Site U1361 between 4.3-4.05Ma and last occurred between 2-2.2 Ma.

Figure 1 .
Figure 1.ODP and IODP site locations.Map of Antarctica shows the location of Southern Ocean sites that document the presence of clavate Fragilariopsis species (red dots).Site U1361 is distinguished with a yellow dot.Further detail about each site is given in Appendix A TableA2.

SpeciesFigure 2 .
Figure 2. Measuring the dimensions of the valve face provides quantitative comparison between species.Measurements of R. raggattensis, F. armandae, and F. clava are given in Tables 1 and 2 and displayed graphically in Figs.7 and 13.

Figure 5 .
Figure 5. Fragilariopsis clava scanning electron micrographs.All specimens from U1361A-6H7 32-34 cm.(a) Interior valve face with (cd) close-up views of the constricted center and minor apex and (b) detail of the flap-like occlusions (arrow).Panels (a), (c), and (d) display the discontinuous areolae within striae.Panel (e) shows the exterior view of the minor apex, and panel (f) highlights the raphe (arrow) terminating along the valve edge.

Figure 6 .
Figure 6.Fragilariopsis clava scanning electron micrographs.All specimens from U1361A-6H7 32-34 cm.The exterior valve face is displayed in panels (a), (c), and (e).The raphe (arrows) runs along the outer edge of the valve face, ending at the major apex as seen in panels (d) and (f), which are close-ups of panels (c) and (e), respectively.Panel (b) highlights preserved perforated membranes (arrows) from the specimen shown in panel (a).

Figure 7 .
Figure 7. F. clava morphometrics.Fragilariopsis clava (blue circles) and F. armandae (green squares) morphometric measurements are compared to other clavate species in panels (a-d): F. heardensis (pink line and markers), F. clementia (solid green line), F. lacrima (dashed green line), F. efferans (solid black line), and F. claviceps (dashed black line).(a) The minor apex width is compared to the total specimen length, (b) the major apex width is compared to the total specimen length, (c) the minor apex width is compared to the major apex width, and (d) the number of costae per 10 µm is compared to the total specimen length.Panel (e) displays the number of costae per 10 µm to the absolute proportional difference of F. clava, F. armandae, and F. heardensis.Examples of F. heardensis are shown from U1361 (pink triangles) and Bohaty et al. (2003, pink crosses) (pink crosses).We suggest that specimens identified as F. heardensis inBohaty et al. (2003) Plate 2 (Figs.2 and 3) and Plate 3 (Fig.9) (green crosses) should be reclassified as F. armandae or F. c.f. armandae.Fragilariopsis clava is distinguished from other Pliocene species in panels (a), (c), and (e) by its broader minor apex and coarse costae.

Figure 9 .
Figure 9. Fragilariopsis armandae scanning electron micrographs.All specimens from U1361A-6H1 12-14 cm.(a) Exterior valve face with irregular, uniseriate areolae.The raphe runs along the outer edge of the valve face, ending at the major apex as seen in panel (b) and the minor apex in panel (c), which are close-ups of panel (a).

Figure 11 .
Figure 11.Rouxia raggattensis scanning electron micrographs.All specimens from U1361A-6H7 32-34 cm.Panels (b-e) are close-ups of the interior valve face shown in panel (a), and panels (g-i) are close-ups of the exterior valve face in panel (f).Panels (b-c), (e), and (i) are the spatulate apex, and panels (d) and (f) show the rounded apex.Panels (b-d) show the keyhole-shaped alveolar openings which decrease in size towards the apices where the raphe terminates.Panel (e) displays the perforated membrane of the alveolate striae (arrow).Panel (h) highlights the marginal ridge (dashed arrow i), rows of pores (dashed arrow ii), and ovular impressions (dashed arrow iii).

Figure 12 .
Figure 12.Rouxia raggattensis, R. antarctica, R. isopolica, and R. leventerae scanning electron micrographs.All specimens from U1361A-6H7 32-34 cm.Dissolution of R. raggattensis (a) exterior and (b) interior valve face reveals the frustule structure between surfaces.Panel (a) highlights the marginal ridge (dashed arrow i), rows of pores (dashed arrow ii), and ovular impressions (dashed arrow iii).(c) Rouxia antarctica interior view and (d) example of keyhole-shaped alveoli from R. antarctica.White arrows in panels (e) and (f) point to keyholeshaped alveoli in R. isopolica and R. leventerae, respectively.Panel (g) exhibits the exterior view of R. antarctica, and panel (h) is a close-up of an apex which displays features similar to R. raggattensis.

Figure 13 .
Figure 13.R. raggattensis and R. diploneides morphometrics.R. raggattensis morphometrics (blue circles) are compared to those of R.diploneides (red squares) (see Fig.2).(a) The apex length is compared to the length of the raphe on the apex, (b) the length of one apex is compared to the length of the other, (c) the apex length is compared to its width, and (d) the total specimen length is compared to the transapical width.Rouxia raggattensis spatulate apices are shown as open circles, whereas the rounded apices are solid circles.The apices on a single specimen have a nearly symmetrical length for both R. raggattensis and R. diploneides, but the R. raggattensis spatulate apex is wider than its rounded apex, and, in general, R. diploneides is smaller and has shorter apices than R. raggattensis.

Figure 16 .
Figure 16.Relative abundance of clavate species by ODP and IODP site.Site numbers and specifications are given in Appendix Tables A1and A2.Symbols at each site represent the relative abundance given by the literature: large red circles denote a few occurrences, black circles indicate rare abundances, and solid yellow circles show where there were trace or scarce occurrences or single-specimen observations.

Figure 17 .
Figure 17.Abundance of R. raggattensis, F. clava, and other clavate Fragilariopsis species.Absolute and relative abundance of R. raggattensis, F. clava, F. armandae, and other relevant species.(a) The paleomagnetic reversal stratigraphy at Site U1361 (Escutia et al., 2011).(b) Site U1361 shipboard natural gamma radiation (NGR; counts) (Escutia et al., 2011).(c) Absolute abundance of total diatoms counted at Site U1361 and species of interest.(d) Relative abundance of species of interest that were identified during diatom counts.(e) Benthic oxygen isotope (δ 18 O) stack of Lisiecki and Raymo (2005).Low NGR denotes lower clay content which is potentially linked to higher biological deposition.The light-blue shaded areas highlight intervals in which NGR is lower than 34, the threshold used by Tauxe et al. (2015) to indicate times of higher productivity.More positive δ 18 O values indicate cooler conditions.Marine Isotope Stages corresponding to low NGR are given to the far right.The absolute and relative abundance between 84.94-56.53m b.s.f. were completed by Taylor-Silva and Riesselman (2018).
Species age ranges.The age ranges of clavate Fragilariopsis species, R. raggattensis, and R. diploneides were approximated using Site U1361 data, published abundance records from sites noted in Table paper describes three new diatom taxa: Fragilariopsis clava sp.nov.Duke; Fragilariopsis armandae sp.nov.Frazer, Duke et Riesselman; and Rouxia raggattensis sp.nov.Duke et Riesselman.Fragilariopsis clava, distinguishable by a robust clavate valve face with coarse costae, existed briefly at Site U1361 during Marine Isotope Stage G7, around 2.74 Ma.Transitional forms provide evolutionary links between F. clava sp.nov.and F. clementia Gombos.Fragilariopsis armandae is present at Site U1361 during the same Marine Isotope Stages, MIS 101-97 (∼ 2.68-