New microfossils from the Meso-Neoproterozoic Deoban Limestone, Garhwal Lesser Himalaya, India

Abstract

Forty-five additional microfossils are being described from the Meso-Neoproterozoic Deoban Limestone (Formation). Microfossils are preserved in black bedded chert, occurring as thin lenses and bands intercalated with dolostones. The assemblage comprises both filamentous as well as coccoid forms. The assemblage is dominated by cyanobacterial population along with a number of microfossils belonging to other affinities represented by bacterial, algal, fungal and acritarchean forms. Out of the forty-five forms there are forty-four species belonging to 33 genera and one micro-organism is informally described as Form 'A'. Five taxa are described as new genera.

Additional Deoban forms are as follows:

Cyanobacteria- Archaeoellipsoides minor, A. major, Palaeopleurocapsa sp., Scissilisphaera gradata, Scissilisphaera sp., Coniunctiophycus gaoyuzhuangense, Archaeophycus sp., Gloeodiniopsis lamellosa, Palaeomerismopedia misrai gen. & sp. nov., Siphonophycus septatum, S. robustum, S. solidum, S. typicum, Polytrichoides lineatus, Oscillatoriopsis amadeus, O. obtusa, O. breviconvexa, Rhicnonema antiquum, Nostocomorpha sp., Eomicrocoleus crassus, Cyanonema sp., Palaeolyngbya catenata, Circumvaginalis sp., Obruchevella parva, O. valdaica, O. minor and Glomovertella glomerata.

Incertae sedis- Paleosphaeridium zonale, Germinosphaera sp., Myxococcoides chlorelloides, Myxococcoides stragulescence, Palaeococcus indicus gen. & sp. nov., Dumbellina deobanensis gen. & sp. nov., Maithea indica gen. & sp. nov., Eophycomyces herkoides, Bulgenia septata gen. & sp. nov. and Form 'A'.

Acritarchs- cf. Cymatiosphaeroides sp., Micrhystridium sp ., Favosphaeridium favosum, Trachysphaeridium sp., Leiosphaeridia crassa, L. jacutica, Caudosphaera sp. and cf. Ovulum saccatum.

The Deoban Microfossil Assemblage is characterised by dominance of mat building cyanobacterial population, exhibiting evolutionary conservatism. Extensive size variation from specimens of less than one micron to 265 microns in case of coccoids and from less than one micron to 48 microns among filamentous forms indicate the most favourable preservational conditions for silicification of the biota.

There are a number of forms, which show some resemblance with extant chlorophycean, bacterial and rhodophycean forms. Presence of large sized sphaeromorphs ranging in diameter from 105 to 265 µm and rare occurrence of acanthomorph or spinose acritarch (represented by Micrhystridium sp.) and small-sized, moderately developed spirally coiled filaments of Obruchevella support a pre-Vendian age to the Deoban microfossil assemblage.