Vinod Chandra Tewari

Wadia Institute of Himalayan Geology, Dehra Dun 248 001, Uttarakhand, India and International Centre for Theoretical Physics, Miramare 34100 Trieste, Italy

1. Introduction

A comparative study of possible genesis of life on planet Earth and Mars reveals that similar conditions would have been present on Mars supporting a probable occurrence of unicellular Martian microfossils and stromatolites. The genetic aspect of eukaryotes or eukaryogenesis, eukaryotic evolution and probable geological and recent astrobiological possibilities from Mars and Europa has been discussed by many astrobiologists in Europe and U.S.A. (Chela Flores, 1998, 2007; Seckbach,1994; and the references therein). Westall and Southam (2006) and Westall and Walsh (2003) hypothesise that primitive life (prokaryotes) could have existed on Mars. Recently Schopf et al. (2008) have suggested that search for fossilized evidence of ancient life in rocks from other planets is likely to be constrained severely by the amount of rock available for study. Precambrian fossil record on the planet earth may be used as a model for the search for evidence of ancient life on Mars and other earth like planet.

The molecular evidences indicate that unicellular organisms (prokaryotes) lack nuclear membrane, mitochondrion and the chloroplast and its DNA is normally a single ring shaped chromosome which is not grouped with protein. The multicellular organisms (eukaryotes) have their DNA linked in chromatin, the main organelles are normally in its cytoplasm (Chela Flores, 1998). Seckbach (1994b) and Tewari (1993, 1999) studied the multicellular red algae (rhodophytes) Cyanidium caldarium and red brown algaeVendotaenid Krolotaenia gniloskayi Tewari from the Ediacaran Krol Formation of the Lesser Himalaya, India for its genetic aspects. The Ediacaran multicellular fossils of the Lesser Himalaya India are found in the Krol basin (Fig. 1) in the northwestern part and recently discovered diversified unicellular-multicellular microfossils including sponges come from the Buxa Dolomite, northeastern Lesser Himalaya (Figs. 1 and 2). Ediacaran fossils occur above the highest Marinoan glaciation (=Blaini diamictites) and below the lowest Cambrian deposits (=Tal Formation) globally (Tewari, 2007; Tewari and Sial, 2007). The present chapter deals with the important aspect of the Proterozoic unicellular and multicellular microfossils from the Buxa Dolomite of the NE Lesser Himalaya, India and its global and astrobiological implications.

Classification Himalaya India Map
Figure 1. Geological map of India showing occurrences of the fossiliferous formations.

2. Diversified Unicellular and Multicellular Microfossils from Buxa Dolomite

A diversified assemblage of organic-walled microfossils comprising 27 taxa of the benthic and planktonic forms (cyanobactera, acritarchs and Vase shaped microfossils) has been recovered in petrographic thin sections from the lenses and bedded chert belonging to Buxa Dolomite exposed near Igo Bridge, Daring - Basar road in West Siang District of Arunachal Pradesh (Figs. 2, 5 and 6). In

Geological Location Himalaya
Figure 2. Geological and location map of the Buxa Dolomite in the NE Lesser Himalaya.

this assemblage 13 cyanobacterial remains belonging to Chroococcaceae, Nostocaceae and Oscillatoriaceae, 13 taxa of Acritarchs belonging to Sphaeromorphida, Scaphomorphida and Sphaerohystricho-morphida subgroups of acritarchs and one VSM (Vase shaped microfossils) have been reported in this paper. The cyanobacterial remains are Huroniospora psilata; Eosynechococcus moorei; Paratetraphycus giganteus; Glenobotrydion aenigmatis; Myxococcoides minor; Palaeoanacystis suketensis; Oscillatoriopsis breviconvexa, O. robusta, O. rhomboidalis; Palaeolyngbya contenada; Siphonophycus typicum, S. rugosum; Polythrycoides lineatus; Obruchevella parva; Volyniella valdaica; Vetronostocale amoenum and Vetronostocale equale sp. nov. The 13 taxa of acritarchs are Margominuscula rugosa, M. simplex; Leiosphaeridia visingsa; Granomarginata vetula; Lophosphaeridium rarurm, L. jansoniusii; Trachysphaeridium robustum; Micrhystridium lanatum, M. ampliatum; Baltisphaeridium cerinum; Archaeohystrichosphaeridium semireticulatum; A. cel-lulare; Vandalosphaeridium reticulatum; Trachyhystricho-sphaera aimica; Gorgonisphaeridium pindyium; Meghhystrichosphaeriium perfectum (Kolosova) comb nov.; Navifusa segmentatus and N. bacillaris. The single genus of VSM viz. Melanocyrillium hexodiadema has been recorded.

MelanocyrilliumPorifera Ascon

Figure 4. A variety of Demosponges.

2. Text figure of extant Chalina.

3. Text figure of Plate. 1, Fig. 6 showing canal system cf. Chalina.

4. Text figure showing Ascon type of water canal system as found in Chalina.

5. Text figure showing spongin fibers. Note the comparison with sponging fibers in Figs. 2, 5 and 7.

A. Stalk

B. Flagellated chamber

C. Finger like projections

D. Spongocoel

E. Spongin fiber net

F. Osculum

G. Ostium

Figure 4. A variety of Demosponges.

2. Text figure of extant Chalina.

3. Text figure of Plate. 1, Fig. 6 showing canal system cf. Chalina.

4. Text figure showing Ascon type of water canal system as found in Chalina.

5. Text figure showing spongin fibers. Note the comparison with sponging fibers in Figs. 2, 5 and 7.

A. Stalk

B. Flagellated chamber

C. Finger like projections

D. Spongocoel

E. Spongin fiber net

F. Osculum

G. Ostium

The present assemblage of microbiota compares well with the known assemblage from the Latest Proterozoic/Vendian sediments of Northwest and Central Lesser Himalaya, India and its equivalent sediments in other parts of the world (Table 1) The Buxa dolomite is well exposed in Arunachal Pradesh as windows or linear belts from Kameng District in the west to Siang district in the east. Cherty bands, oolites, stromatolites and intraclasts characterize this dolomite. The stromatolites and microbiota are known from Kameng area and the Menga window in the Upper Subansiri District. Recently an assemblage of diversified organic-walled microfossils has been recovered in petrographic thin sections of lenses and bedded chert belonging to the Buxa Dolomite exposed near Igo Bridge, Daring - Basar road in West Siang District of Arunachal Pradesh. This assemblage which shows dominance of the spinated forms and coiled cyanobacterial remains viz. Obruchevella indicates Vendian age for these beds. The non-mineralized sponges are being reported for the first time from the same location. The assemblage of these fossil specimens represents three types of extinct sponge forms.

3. Sponges from the Buxa Dolomite

The present forms are simplest metazoan having numerous small pores on their body surface and hence justify their placement in Phyllum Porifera (Pore bearers). They show cellular grade constructions and loose aggregation of cells bound into soft tissues. Sponges are the most primitive of multicellular animals with a low

Lesser Himalaya
Figure 5. Coccoid assemblage of the Buxa Dolomite, NE Lesser Himalaya, India.

grade of organization. They are incapable of movement being attached to the substratum as plant. In other words sponges are fixed to some submerged object in water. All sponges have skeleton which provides them strength and rigidity. These skeletons are of three type - (i) consisting of soft colloid, varying from a mucous like sol to rather stiff gel; (ii) mineralized skeletons, composed of spicules made of opaline silica or calcium carbonate, and (iii) skeletons comprising of stiff and tough organic material called spongin. The first type of skeleton comprising of mucous like sol or stiff gel is often transient. In some forms the mineral matter in concentric layers covers the original axial thread, which is formed by longitudinal fission of pre-existing thread inside the living sponge cell. We believe that the ancient sponges may not be having the mineralized skeleton made by spicules

Figure 6. Filamentous assemblage of the Buxa Dolomite, NE Lesser Himalaya, India.

and forms with the organic skeleton and morphology similar to existing forms may be found in ancient sediments.

4. Systematic Description of Sponges 4.1. TYPE A

The present specimens are colonial forms having several fingers like structures hence may be finger sponge of the class Demonspongia (Fig. 3, Plate 1, Figs. 1, 4 and 6).

Table 1. Comparative Chart of O.W.M. (cyanobacterial remains and acritarchs and VSM) in present assemblage belonging to Neoproterozoic/ Vendian sediments of the world.

Cyanobacteria

Acritarch o U

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