A silicoflagellate from the middle Albian of Austral Basin, Argentina

Calcareous nannofossil biostratigraphic investigations of Cretaceous sediments within the southern high latitude Austral Basin, Argentina, have revealed the presence of a fragment of a silicoflagellate possibly belonging to the species Variramus aculeifera (Def landre) McCartney, Wise, Harwood and Gersonde. Although only a fragment was identified in the samples examined from this locality, its presence is important, as it is only the second documented occurrence of an Early Cretaceous silicof lagellate. The specimen was found in a subsurface sample from the informal Palermo Aike Formation at Austral Basin. This stratigraphic level is interpreted as middle Albian on the basis of calcareous nannofossil biostratigraphy. The specimen consists of a fragmented strut with broken spines at the base, which allows the observation of the hollow skeletal structure. The strut is curved and slightly expanded at the widest part, which are characteristics diagnostic of the genus Variramus.


Introduction
Calcareous nannofossil biostratigraphic investigations of Cretaceous sediments within the southern high latitude Austral Basin, Argentina (Pérez-Panera, 2012), have revealed the presence of a fragment of a Cretaceous silicoflagellate possibly related to the species Variramus aculeifera (Deflandre, 1950).Silicoflagellates are almost unknown from Lower Cretaceous sediments, so even though this is a single fragment, it is important because it is only the second documented occurrence of an Early Cretaceous silicoflagellate in the world (McCartney, 2013).The specimen comes from a middle Albian sample from Cañadón Salto borehole, Austral Basin, Argentina.The investigated samples were provided by Petrobras-Energía S.A. and the name and location of the borehole are confidential; however, figure 1 illustrates its general location within the Austral Basin.

Materials and Methods
The sample was prepared according to the gravity settling technique of Bramlette and Sullivan (1961), originally for calcareous nannofossil analysis.A FEI Quanta 200 Scanning Electron Microscope was used for sample examination and photographs.In order to evaluate the composition of both the matrix and the sillicoflagellate, an EDX analysis was completed (Fig. 2), confirming the siliceous composition of the specimen.The sample is deposited in the Museo de La Plata nannopaleontological collection under number MLP-NC 0001.

Stratigraphic provenance
The specimen was recovered from a subsurface sample at 1,200-1,209 m sub-bottom depth (msbd) in the informal Palermo Aike Formation at Austral Basin.This stratigraphic level is interpreted as middle Albian based on calcareous nannofossil biostratigraphy (Pérez-Panera, 2012).The interval between 1,191-1,200 and 1,350-1,352 msbd is correlated with the early to middle Albian Zone CC8 of Sissingh (1977) and the Rhagodiscus asper and Sollasites falklandensis Subzones of Wise (1988).More precisely, the last occurrence of S. falklandensis FIG. 2. Comparison of EDX analysis for the sediment (A) and for the silicoflagellate specimen (B).Note the differences between the silica (Si) and oxygen (O) peaks, which are higher for the silicoflagellate (b), illustrating the opaline nature of its skeleton.
is identified in sample 1,191-1,200 msbd, just above the sample where the silicoflagellate specimen occurs.Thus, on the basis of calcareous nannofossil biostratigraphy, it is possible to constrain the age of the sample containing the silicoflagellate to the middle Albian (Pérez-Panera, 2011).

Systematic Palaeontology
Division Description.The recovered specimen consists of a fragmented strut with broken spines at the base, which allows observation of its hollow skeletal structure.The strut is curved and slightly expanded at its widest part, which are diagnostic characteristics of the genus Variramus.It is possible to recognize the base of three spines that are oriented in two different planes.The end of the strut attaches to a basal spine, which is obscured by the surrounding clay matrix, but it is possible to identify the structure as a spine based on its tapered appearance.The opposite end of the strut is broken, possibly at the point where it connects with the bridge.Although this specimen is only a small fragment of the entire individual, it is here compared to Variramus aculeifera based on the curved and expanded strut and the slightly displaced triple joint at the end, which resembles a sabaton.This specimen looks very similar to the specimen illustrated in plate 3, figures 4 and 5, and to the ends of the specimens in plate 2, figures 5 and 8 of McCartney et al. (1990).Wise, 1988;McCartney et al., 1990McCartney et al., , 2014)); Santonian-Campanian of Canadian archipelago, Arctic Ocean (McCartney et al., 2011a(McCartney et al., , 2011b)).Unconfirmed record from the Maastrichtian of California (Def landre, 1950;Tsumura, 1963, age assignment may be wrong and could be older, see McCartney et al.,1990, 2010, 2011aand McCartney, 2013 for discussion); Campanian-Maastrichtian of Southwest Pacific Ocean, Deep Sea Drilling Project (DSDP) Site 275, (Bukry, 1975); Campanian-Maastrichtian of Arctic Ocean, Alpha Ridge (Bukry, 1981(Bukry, , 1985)), Campanian-Maastrichtian of Tonga Trench, Pacific Ocean (Quinterno et al., 1994).

Discussion
The origin and evolutionary history of silicoflagellates during the Cretaceous is an interesting but poorly understood subject.In the last two decades, important advances had been made (MacCartney, 2013;MacCartney and Wise, 1988;McCartney et al., 1990McCartney et al., , 2010McCartney et al., , 2011aMcCartney et al., , 2011bMcCartney et al., , 2014)), but questions still remain.McCartney et al. (1990) documented an abundant and diverse silicoflagellate assemblage from the lower to middle Albian of the Weddell Sea, indicating an older origin for the group than previously documented.In subsequent studies, McCartney et al., (2010McCartney et al., ( , 2014) ) demonstrated that this Weddell Sea assemblage is even more diverse than previously thought and includes a variety of species and morphologies assigned to four Albian genera: Gleserocha, Schulzyocha, Vallacerta and Variramus.Recent studies (McCartney et al., 2011a(McCartney et al., , 2011b(McCartney et al., , 2014) ) of upper Santonian-lower Campanian samples from the Canadian archipelago showed that silicoflagellates had experienced a radiation by that time, with the addition of five new genera: Cornua, Umpiocha, Corbisema, Lyramula and Arctyocha.These localities contain the best-known Cretaceous silicoflagellate fossil records, and their discovery has led to a significant increase in our understanding of silicoflagellate evolution (McCartney, 2013;McCartney et al., 2014).By the late Maastrichtian, the oldest silicoflagellate lineages, with the exception of the genus Vallacerta, had become extinct.At the Cretaceous/Paleogene boundary, only Corbisema survived the extinction event, and this genus gave rise to all Cenozoic silicoflagellates (McCartney et al., 2014).
Even though these recent advances have shed light on the origin and early evolution of the silicoflagellates, there are still important gaps in our knowledge.The geographic range of the Cretaceous silicoflagellate records is very small and limited to a few regions from the middle to high latitudes.Until now, the only Early Cretaceous silicoflagellates known were from ODP Site 693 (McCartney, 2013), which contains an early Albian moderately diverse assemblage of silicoflagellates.Silicoflagellates were more abundant and diverse by the late Santonian-early Campanian, but there is currently no known record spanning the Cenomanian to Coniacian.This scarcity of information for the early history of this group makes each new record of vital importance.
The identification of this specimen in a sample of middle Albian age from southern Patagonia represents the second record of a silicof lagellate of Early Cretaceous age, providing a new geographic region to look for clues about the evolution and diversification of this group.