Tuesday, April 15, 2014

Metazoan Embryos From Terminal Neo-Proterozoic Early Cambrian Himalayas

In Current Science Sabhyasachi Shome et al  and in Journal of Geological Society of India V.K Mathur et al report fossilized cellular remains in phosphatic chert sediments from the Krol Group (Terminal Neo-Proterozoic ~ 590 -543 mya) and Tal Group (Early Cambian ~ 540 mya) from the Lesser Himalayan sequences exposed in the Kamlidhar syncline north west of Mussorie. These they interpret as metazoan (multicellular animals) eggs and embryos. The findings are significant because they are some of the earlier known examples of body fossils of animals and will add to our understanding of the timing and nature of early animal evolution.

The Krol Group and the succeeding Tal Group represents sedimentation taking place in shallow to mid shelf environments along a passive continental margin that would in the future after Gondwanaland split up become the northern edge of the Indian continent facing the Tethys ocean.

The stratigraphic sequence is depicted in the image below with the position of the reported metazoan remains marked in red. 

Modified from:  Jiang et al 2003

The Krol Group has been interpreted as representing an evolving carbonate platform that shows a change from an early sloping ramp style geometry to a rimmed shelf to an open flat shelf profile over the course of tens of millions of years. This platform was north north-west facing i.e. the shoreline was to the south and the open deep ocean towards the north. Only the facies representing the shallower environments have been preserved in the study area. The deeper water equivalents are likely to be found in the High Himalayas in what are called the Tethyan Sequence exposed at great heights. The reported embryos are preserved in phosphatic chert lenticular bodies in a sandstone from the lowermost unit of the Krol sedimentary sequence. Younger units also contain evidence of multicellular animal life in the form of Ediacaran biota and sponge spicules.

The Tal Group represents more restricted conditions with algal buildup and lagoonal facies with deposits of phosphatic limestone, black shale and fine sandstone. From the sediments, phosphatized globular to sub-oval metazoan eggs with distinctively ornamented covering and polar lobe forming embryos have been found. They are associated with Small Shelly Fossils, a name given to a diverse array of fossils forms which are probably the dis-articulated remains of organisms like sponges, brachiopods, echinoderms, significant because they represent one of the earliest examples of metazoan biomineralization i.e. the ability to secrete hard skeletons from calcium carbonate.

The picture below from the Krol Group sediments shows a CT scan of the metazoan embryos at a two cell division stage. These have been interpreted as blastomeres representing the blastula stage embryos of animals.

Source: Sabhyasachi Shome et al -2014

And here are some more pictures of interpreted metazoans embryos from an earlier study of the Krol Group.

Source: R. Babu et al 2013 - Open Access- Description - a, Embryo showing cytoplasm, arrow showing air chamber;  b, Embryo showing developmental stage comparable to gastrula, arrow showing multicelled structure; c, Lower enlarged part of (b) (embryo), arrow showing two-layered wall (epidermis and endodermis); d, Embryo, arrow showing cleavages of blastula stage infilling homogenous organic matter (? proteineous in nature); e, Embryo showing cleavages; f, Upper enlarged part of (b) showing multicelled structure;

Some time back a friend asked me whether there is any chance of finding a Burgess Shale like Cambrian fossil bonanza in India. The Burgess Shale fossil locality in the Canadian Rocky Mountains is a Lagerstatte.. which means it represents an event of sudden entombment of the animal and/or plant life. This may be due to a submarine mud and silt flows burying the animal and plant communities in soft sediment leading to a snapshot of life at that particular moment. Rapid burial prevents degradation on the sea floor of organic remains. This means that even details of soft tissue along with hard parts are preserved as impressions on sediment. 

My answer to the question was that in Peninsular India sedimentation stopped by terminal Neo-Proterozoic. There were no Cambrian basins. But along the northern margin of proto-India in a basin which in the future would be deformed and uplifted to become the Himalayas, sedimentation continued in the Paleozoic. Which means Cambrian sediments are present, although I remarked they may be metamorphosed and destroyed. That is not the case however. The Krol and Tal Group do contain a Neo-Proterozoic to Early Cambrian fossil record with Ediacaran biota, sponge spicules and small shelly fossils indicating presence of metazoans. Their deeper water equivalents may be found in the High Himalayas! So, there is always a change of a spectacular fossil find either in or near more familiar places like Mussorie or Nainital where the Krol and Tal Groups are exposed or along the frigid heights of the Himalayan snow giants -for explorers brave enough to venture there.

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