<Emphasis Type="Italic">Trachyhystrichosphaera</Emphasis>: An age-marker acanthomorph from the Bhander group,upper Vindhyan,Rajasthan |
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| Authors: | Purnima Srivastava |
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| Institution: | (1) Centre of Advanced Study in Geology, University of Lucknow, Lucknow, 226007, Uttar Pradesh, India;(2) Wadia Institute of Himalayan Geology, Dehradun, 248001, Uttarakhand, India |
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| Abstract: | A highly diversified, advanced and exceptionally well preserved microfossil assemblage, dominated by a planktic community,
has been recorded from petrographic thin sections of chert belonging to the Sirbu Shale Formation, Bhander group, upper Vindhyans,
Rajasthan. Recently, it was noticed that the assemblage also contains well preserved, large-sized acanthomorphic acritarchs,
Trachyhystrichosphaera considered to be an age-marker microfossil of the Cryogenian (850-630 Ma). It is reported for the first time from any Indian
microfossil assemblage of Proterozoic succession. The other microfossils of the Sirbu Shale Formation are: well preserved
simple, small and large-sized sphaeromorphs; complex acanthomorphs, cyanobacterial community; especially a very small-sized
but exceptionally well preserved Obruchevella, a form resembling Volvox colonies; cf. vase-shaped microfossils and morphologies, possibly inclining towards fungal affinity, or lichenlike symbiotic
associations of algae and fungi. Till date, Trachyhystrichosphaera has so far not been reported from successions older than the Tonian (1000850 Ma). It is believed that acanthomorphs attained
maximum size in Ediacaran (630-542 Ma), and further decreased in size in the Cambrian. The global paleontological literature
indicates that Trachyhystrichosphaera ranges in age from Tonian-Ediacaran (1000-542 Ma).
The present record of Trachyhystrichosphaera as well as the earlier studies of micro and megascopic life of the Bhander Group in general and the Sirbu Shale in particular
(aided by the absence of any Cambrian fossil record) indicate that in all possibilities, age of the Sirbu Shale should lie
near Cryogenian (850-630 Ma) and the uppermost Bhander group, may incline towards the Ediacaran (630-542 Ma). |
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