sche NiSe 2 von den Verfassern in eva- kuierten, abgeschmolzenen Quarzglas- r6hrchen bei 500 ~ nachsynthetisiert. Im Anschliff ist das kubische NiSe 2 weil3 mit leicht br/iunlichem Stich. Das Reflexionsverhalten ist somit gleich dem des Penroseits. Es bestehen fiir das kubische NiSe 2 zwei genetische Interpretationsm6g- lichkeiten. Es kann sich als stark unter- geordnete Komponente zusammen mit Penroseit und Clausthalit prim/Jr gebil- det haben. Andererseits besteht die M6glichkeit, dab Penroseit (Ni, Cu)S% bei erh6hten Temperaturen mehr Ni in sein Kristallgitter einbauen kann. Bei Abkiihlung erfolgt eine Entmischung zu selbst/indigem NiSe 2 neben nickelfir-

merem Penroseit. Es ist anzunehmen, dab das kubische NiSe z bei friiheren Untersuchungen infolge des iden- tischen Reflexionsverhaltens yon Pen- roseit sowie wegen seines geringm/ichti- gen Vorkommens fibersehen wurde.

Eingegangen am 1. August 1985

1. Vuorelainen, Y., Huhma, A., H/ikli, A. : C. R. Soc. Geol. Finlande 36, 113 (1964)

2. De Jong, W.F., Willems, H.W.V. : Z. an- org. allg. Chem. 170, 241 (1928)

3. Gronvold, F., Jakobsen, E. : Acta Chem. Scand. 10, 1440 (1956)

4. Hiller, J.-E., Wegener, W. : N. Jb. Miner. Abh. 94, 1147 (1960)

5. Franz, E.-D.: N. Jb. Miner. Mh. 1971, 11

Earliest Phytoplankton of Eukaryotic Affinity H.D. Pflug and E. Reitz Geologisch-Pal~iontologisches Institut der Universit/it, D-6300 Giegen

Among the phytoplankton of ancient seas, the acanthomorph acritarchs form a group with a characteristic mor- phology easily recognizable by their spinate wall sculpture. Acritarchs have their maximum diversity in the older Paleozoic (Cambrian to Silurian), then become more rare through the Meso- zoic and finally disappear in the Pleis- tocene. Their taxonomic relationship to modern planktonic organisms is not quite clear, but there is tittle doubt that acritarchs are eukaryotic algae. Many of the forms resemble the cysts of mod- ern dinoflagellates in their principal features, but this does not necessarily mean a close relationship to this group. We are reporting here the detection of acritarchs in the ca. 2000-Ma-old Gun- flint Formation of Ontario. About a dozen specimens belonging to four dif- ferent morphological types (" form spe- cies") have been identified hitherto. Two of these are new. The specimens occur in clear portions of a stromato- litic chert together with Huronispora sp. and Gunflintia sp., two common re- presentatives of cyanobacterian affini- ty. Eomicrhystridium barghoorni Deflan- dre 1968 is a form with many strong cone-shaped spines occupying the en- tire cell surface. The 01rst specimen of

this kind was detected by Deflandre [1] in the Gunflint and assigned to acan- thomorph acritarcha. However, the preservation of his find was so poor that the assignment to the acritarchs

was not generally accepted. Later finds from the Gunflint [2] and from the ca. 2000-Ma-old Frere Formation of Western Australia [3] have been pro- visionally placed to this species [4]. We show here a well-preserved specimen from our Gunflint material (Fig. 1 c). Eomicrhystridium fsp. A (Fig. I a): Cell about 12 gm in diameter. Hollow sick- le-shaped appendages up to 3.5 ~tm in length protrude from the cell wall on a broad base. The basal trunks appear to be interconnected to form a network. Eomicrhystridium fsp. B (Fig. 1 b): Cell about 8 gm in diameter. Appendages are short (ca. 1-1.5 gm) terminally branched (bifurcate?) and cover the cell surface in a spatial arrangement. Eoastrion bifurcatum Barghoorn 1965 [5] is characterized by long spines with terminal branching. In our opinion, the fossil is clearly an acritarch and should be treated separately from Eoastrion simplex which, as Barghoorn and Tyler (1965) have already suggested, is prob- ably a radiating colony of filamental bacteria or fungi (see also [5]). In summary, it can be stated that the Gunflint microfossils are identical to acanthomorph acritarcha in all recog- nizable features of their morphology. The findings suggest that eukaryotic

Fig. 1. Acritarchs from the Gunflint Banded Iron Formation, Schreiber locality, Ontario. a) Eomicrhystridiurn fsp. A, b) Eomicrhystridium fsp. B, c) Eomicrhystridium barghoorni Deflandre 1968

656 Naturwissenschaften 72 (1985) �9 Springer-Verlag 1985

Fig. 2. Sphaeromorph acritarchs (Prasinophyta) newly detected in the middle Belt Super- group (Montana/Idaho), a) cluster of Leiosphaeridia specimens, b, c) Leiosphaeridia sp. Eisenack 1968, d) Nueellosphaeridium sp. Timofeev 1963, e) Trachysphaeridium sp. Timofeev 1966, f) Montanella behensis Pflug 1965. a)-c) from St. Regis Formation (ca. 1300 Ma), locality 16 km southeast of Superior, 1.5 km west from Northern Pacific Railroad, NW- Montana, d)-e) from Striped Peak Formation (ca. 1100 Ma), locality at US Highway 10 A, 2 km southeast of Clark Fork, NE Idaho

\\

I1"\ I I \1 I ; &

/ @ /

2060 1500 1066 '700' 560 I 360 260 lcIo Time [Me]

�9 1500

O00

500

,400 7~

.300 ' 3 Q)

.200 o.. t j )

.100

-50

Fig. 3. Diversity of eukaryotic phytoplankton species in the last 2000 Ma plotted on loga- rithmic scales. Abscissa: time in Ma, ordinate: number of known species, - acritarchs and prasinophytes, - modern phytoplankton (after [1~14])

phytoplankton was in a diversified evo- lutionary stage in Gunflint times. Con- sequently their origin must go back to times earlier than 2000 Ma ago. In the younger Proterozoic, subsequent to the Gunflint, another group of eu- karyoric organisms, the Prasinophyta, become the dominant elements in the

phytoplankton spectra. These unicellu- lar organisms (Fig. 2) are commonly placed among the green algae. The old- est representatives have been reported from the 1680-Ma-old Daihongyu For- marion of China [6], and from the lower Rhiphean of the southern Ural Mountains (ca. 1400-1600Ma) [7].

Further finds o f prasinophytes have been described in the about 1400-Ma- old Roper Group in the Northern Ter- ritory of Australia [8]. Similar micro- biota are also known from the Beltian series of Idaho/Montana, where they occur in the Chamberlain Shale For- marion, the lower part of the sequence, about 1400 Ma in age [9]. We detected corresponding microbiota in the mid- dle, approximately 1300- to l l00-Ma- old part of the Beltian (Fig. 2). Some of these representatives can be traced into the upper part (ca. 1000 Ma) [10]. In the younger Proterozoic, subse- quently to 1000 Ma, eukaryotic phy- toplankton increases considerably in number and diversity. Microbiota are known from occurrences all over the world [11, 12]. It seems from the available evidence that the evolution of the eukaryotic plankton took a periodical course (Fig. 3). Two maxima of diversity are indicated in the Phanerozoic, and a more indistinct one in the Proterozoic. The reasons for this apparent periodici- ty are unknown. Climatic and paleo- geographic changes may have contrib- uted to the phenomenon.

Received July 12 and September t0, 1985

1. Deflandre, G. : C. R. Acad. Sci. Ser. D 266, 2385 (1968)

2. Hofmann, H.J. : Geol. Surv. Can. Bull. 189 (1971)

3. Walter, M.R., Goode, A.D.T., Hall, W.D.M. : Nature 261, 221 (1976)

4. Hofmann, H.J., Schopf W., in: Earth's Earliest Biosphere, Its Origin and Evolu- tion, p. 321 (W. Schopf, ed.). Princeton, N.J. : Princeton Univ. Press 1983

5. Barghoorn, E.S., Tyler, S.A.: Science 147, 563 (1965)

6. Lei-Ming, Y. : Acta Paleont. Sinica 24, 112 (1985)

7. Jankauskas, T.V.: Dokl. Acad. Sci. USSR, Earth Sci. Sect. 147, 1465 (1979)

8. Peat, C.J., et al.: Bur. Min. Res., J. Austr. Geol. Geophys. 3, 1 (1978)

9. Horodyski, R.J.: J. Palaeont. 54, 649 (3980)

10. Pflug, H.D. : Palfiont. Z. 39, 10 (3965) 11. Timofeev, B.V.: Proterozoic Sphaero-

morphes. Acad. Sci. USSR, Inst. Pre- cambrian Geol. Geochronol. Leningrad: Nauka 3969

12. Vidal, G., Knoll, A.H.: Geol. Soc. Am., Mem. 161,265 (1983)

13. Tappan, H., Loeblich, A.R. : Geol. Soc. Am., Spec. Pap. 127, 247 (1971)

14. Tappan, H. : The Paleobiology of Plant Protists. San Francisco: Freeman 1980

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