The fossil form of Rhizosolenia observed in this study has been interpreted as a resting spore, and
occurs in two forms; a thin and a thick form, identical in morphology except in respect to the cell diameters (Barron, 1985b; Kitchell et al., 1986). Analysis of both cell morphologies under optical
and electron microscopy in this study has revealed for the first time that cells possess a non- ornamented, hyaline cylindrical portion, to which the ribbed conical section, identified as
Rhizosolenia, is connected by a fine girdle band (Fig. 3.7 and 3.8). In most specimens, the two
valve segments have become unconnected. The two valve segments may be connected in a straight line along the central axis of the cylindrical valve section (Fig. 3.7a and 3.8a), or the two valve segments may be joined at an angle (Fig 3.7b and 3.8b). The cylindrical, hyaline portion of the valve has not been associated with the Rhizosolenia valve segment until now, and when
unconnected resembles valves of Psuedopyxilla. Although superficially resembling Rhizosolenia,
this species differs in a number of important respects. Firstly there appears to be no groove in the valve surface to accommodate the spine of adjacent valves, seen in modern Rhizosolenia. The
heavily ribbed valves also contrast with modern forms, which also possess numerous scale-like girdle bands, unlike the hyaline valves shown here. The relatively heavily silicified nature of the hyaline valve suggests that the cells are likely to be resting spores, consistent with the conclusions of earlier studies (Kitchell et al., 1986). The morphological similarity between this fossil
Rhizosolenia and modern forms of Rhizosolenia/Proboscia suggests it may be a spore of one of
these genera. Valves of this species occur above vegetative cell laminae (dominated by Hemiaulus
spp.) and below resting spore laminae. Alternatively, the Arctic Rhizosolenia may be a heavily
silicified ‘winter’ stage, by analogy with observations of the modern Antarctic diatom Eucampia antarctica. Eucampiaantarctica is known to form a heavily silicified winter stage, which actively
forms chains and undergoes binary cell division when conditions permit growth (Fryxell & Prasad, 1990). This fossil is only known from the Cretaceous/Eocene Arctic cores CESAR 6, Fl-437 and Fl-422 (Barron, 1985b; Dell'Agnese & Clark, 1994) and may therefore have been an endemic Arctic species.
Chapter 3 Diatom Flora and Ecology
Figure 3.7. Optical and BSEI photographs from the CESAR 6 showing valves of Rhizosolenia with; A) the attached cylindrical hyaline valve segment in line with the valve axis and B) offset at an oblique angle.
Figure 3.8. Illustration of the different valve components of Rhizosolenia observed in the CESAR 6.
3.2.18.2 Genus Proboscia Sundström
Valves of this fossil species of Proboscia are long, curved and tubular, composed of 6 hyaline
Chapter 3 Diatom Flora and Ecology
identified in this study, P. cretacea, was originally described as Rhizosolenia by Hajós & Stradner
(1975). However, Jordan and Priddle (1991) demonstrated that the fossil resembles the proboscis of the modern genus Proboscia, which only usually occur in the sediment as broken proboscis.
Furthermore, it was noted that P. cretacea resembles the broken proboscis of modern “winter-
form” Proboscia. The ecology of this fossil species is unknown, although the basal end of each
valve ends in a semilunar grip, which Hajós & Stradner (1975) suggest implies these cells (or cell elements) were connected to another structure. Jordan and Priddle (1991) speculate that chain formation may have involved other structures, analogous to the fossil genus Pyxilla. P. cretacea is
a long ranging (Campanian to Palaeocene), cosmopolitan species, known from both the CESAR 6 and Marca Shale, SW Pacific DSDP Site 275 (Hajós & Stradner, 1975), Seymour Island,
Antarctica, (Martinez-Machiavello, 1987), Emperor Canyon (Fenner, 1991) and ODP Site 758, Indian Ocean (Fourtanier, 1991).
Some modern Rhizosolenia species form macroscopic mats, which can contain up to seven
Rhizosolenia species (Villareal & Carpenter, 1989), that are able to regulate their buoyancy in order
to move between a deep nutrient source and the photic zone (Villareal et al., 1993; Villareal et al.,
1996). Direct evidence of a deep-dwelling habitat come from direct sampling in the Sargasso Sea, where Goldman (1993) obtained samples of Pseudoguinardia recta from 100 m water depth and
subsequently found that it was able to grow in low-light conditions.
Sediment traps in the Gulf of California (Sancetta, 1995) and off the coast of Oregon (Sancetta et al., 1991; Sancetta, 1992) have shown that large fluxes of Rhizosolenia spp. occur in the late
autumn/winter, coincident with the breakdown of stratification. A prominent peak flux of
Rhizosolenia setigera has been recorded in October from Saanich Inlet, British Columbia (Sancetta,
1989b) and the diatom has been found to have a maximum abundance in coastal waters during the autumn (Hobson & McQuoid, 2001). At Station PAPA, northeast Pacific, Takahashi et al. (1989)
recorded a peak flux of Proboscia alata during the autumn over the 1982-1985 period. In the Santa
Barbara Basin a large flux of Rhizosoleniarobusta has been recorded in December (Lange et al.,
2000b). However, sediment traps in the North Pacific gyre show peak flux of Rhizosolenia cf. clevei var. communis during the summer (Scharek et al., 1999b), whilst off northwest Africa a
single flux event of P. alata was recorded in July at 2195 m, accounting for 18% of the total
assemblage (Lange et al., 1998).
SEM studies of laminated sediments from the Gulf of California show that Rhizosolenid diatoms are commonly found concentrated along the top of summer terrigenous laminae (Kemp et al.,
2000). The assemblage is dominated by Rhizosoleniabergonii, but also includes R. setigera, R. styliformis, R. robusta, Proboscia alata and Pseudosolenia calcar-avis. In Mediterranean
Chapter 3 Diatom Flora and Ecology
Rhizosoleniasetigera and R. styliformis, overlie spring/winter bloom mixed assemblages (Pearce et al., 1998). In conjunction with the sediment trap data, these Rhizosolenid laminae are interpreted to
represent “fall dump” export flux, in which Rhizosolenids are regarded as key diatom taxa (Kemp
et al., 2000).
3.3 Class FRAGILARIOPHYCEAE (araphid pennate diatoms) Round