Wednesday, March 29, 2017

Exploring India's Paleogeography And Fossils Using The Paleobiology Database Navigator

I was directed to the Paleobiology Navigator by a tweet from @avinashtn .

Great fun! The Paleobiology Database is being maintained by an international non-governmental group of paleontologists. Contributing members add to it fossil occurrences from scientific publications.  The Paleobiology Database Navigator is a web mapping application managed by the University of Wisconsin-Madison that allows you to explore the geographic context of these fossil locations. You can filter the data based on age, taxonomy and geography. You can also generate diversity trends for the selected set.

I played around a bit with India specific fossil locations.

Paleozoic versus Mesozoic Basins

The figure below shows the distribution of fossil localities for the Paleozoic Era. India is shown as it is today and in its Paleozoic geography.

Source: Paleobiology Navigator

You can clearly see that fossils in Peninsular India are predominantly located in one narrow band in the center and east of the country. These are the Permian Gondwana basins. They are, starting from the westernmost and going eastwards, Satpura Basin, Son Valley Basin, Damodar Valley Basin and the Ranjganj Basin.  These are continental interior basins comprising river, lake and swamp environments. Most of India's coal deposits come from these basins. These basins are rich in plant fossils, and reptile and amphibians remains.

Now take a look at India's geographic position (arrow) during the Permian (298-252 million years ago). Peninsular India occupies an interior location within Gondwanaland, far away from any ocean. Tectonic stability through most of the Paleozoic meant lack of crustal movements. During this time, peninsular India was an erosional landscape until the Permian basin formation in the east.

The one Paleozoic fossil location in Rajasthan shown here represents early Permian marine sediments formed by the flooding of the western region by an arm of the Tethys sea.

And this database has still not added one important fossil location. This is the early Cambrian age locality near Jodhpur where sediments of the Nagaur Group are exposed. They contain trilobite trace fossils.  No basin development and sedimentation took place in Peninsular India from Mid-Cambrian to Permian times (530 million years to 298 million years). 

In contrast, look at the northern edge of India, where the Himalaya stand today. That margin was submerged under the Tethyan ocean. A thick pile of marine sediment accumulated right through the Paleozoic, forming the fossil rich Tethyan Sedimenary Sequence of the Himalaya.

Continental configurations changed in the Mesozoic (252 million to 66 million years ago). The figure below shows Mesozoic fossil locations and the Cretaceous paleogeography of India.

Source: Paleobiology Navigator

There is now a wide swath of fossil localities across Peninsular India. The dotted lines trace important linear depressions where sediments were deposited. The east west oriented Narmada rift zone (NRZ; Jurassic and Cretaceous) and the NW-SE oriented Pranhita Godavari zone (PGR; Triassic to Cretaceous) are important fossil repositories.  The eastern India basins continued accumulating sediment. To the west are the basins which formed in Gujarat and Rajasthan (Jurassic and Cretaceous). The Kutch rift (KR) is outline by dotted lines. And to the south east in Tamil Nadu, marine flooding of the eastern continental margin in the Cretaceous resulted in the deposition of richly fossiliferous sedimentary sequences.

All these basins ultimately owe their origin to the forces exerted on the crust as India pulled away (arrow) from Gondwanaland.  Seaways formed along these rifts and crustal depressions. The Mesozoic, especially the Jurassic and Cretaceous, was a time of global high sea levels. The western margin saw marine incursions from the nascent Indian Ocean, while the eastern margin was submerged by the waters of the newly formed Bay of Bengal.  River and lake systems also developed in more continental interior locations. The northern margin (Himalaya) was mostly a marine environment through the Mesozoic.

Marine versus Continental Interior Basins in Mesozoic Central India

The distribution of terrestrial organisms versus marine organisms can tell us about the extent of marine flooding into Peninsular Central India in the Mesozoic.

I created these maps by using localities of dinosaur fossils (above) to map the distribution of terrestrial sedimentary environments. I used localities of invertebrate marine organisms, namely,  brachiopods, echinoderms and ammonoids  to delimit the extent of marine environments along the Central Indian basins (below).

 Source: Paleobiology Navigator

You can see that terrestrial environments were present right across the Narmada rift zone, the Pranhita Godavari rift basin and in the western Indian basins also. In the western basins, some of the dinosaur fossils have been found in marginal marine settings comprising coastal and estuarine environments.

Deeper water marine environments as evidenced by brachiopod, echinoderm and ammonoid localities are however restricted to Gujarat, Rajasthan and western Madhya Pradesh. The Cretaceous Bagh Beds in Madhya Pradesh is the eastern most limit of Mesozoic marine flooding into Central India. Seaways did not extend into eastern parts of the Narmada rift basins.

Global and Indian Dinosaur Diversity Patterns

I used the Stats tool to create graphs of dinosaur diversity. The number of Genus per Stage is being used as a measure of diversity. Geologic time is subdivided in to bins. An Age is a bin spanning a few million years. Stage represents rock layers deposited in an Age. So, a diversity measure has been created by counting the number of dinosaur genus reported from successive bundles of rock layers, each representing a few million years of time.

Source: Paleobiology Navigator

The global diversity pattern shows episodes of diversification and decline in the Triassic, Jurassic and the Cretaceous. There appears to be a trend of increasing diversity through time with peak diversity in the Mid-Late Cretaceous. The Late Cretaceous extinction of dinosaurs forms the right side boundary.

The diversity measures in India show some differences with global trends. The number of Genus sampled are less. This is due to regional versus global sample. A smaller locale will generally have less of the total observed variation. The trends in diversity with time also is different from the global trajectories. There are a couple of reasons for this. First, this is a preservation artifact. Mesozoic terrestrial basins in India were receiving sediment only episodically. Depositional phases were interrupted by erosional hiatuses. Rock sections thus have been removed as well.   There was little to no sedimentation from Mid-Jurassic to Mid-Cretaceous in the Narmada rift basins. Hence, no fossils either. The lost diversity from this interval is irretrievable.

The second reason gives more hope. A couple of years ago, Dr. Dhananjay Mohabey of the Geological Survey of India gave a talk in Pune on Late Cretaceous dinosaurs of India. He mentioned that there are roomful of dinosaur fossils in government archives that are yet to be studied and catalogued. There is scope then to enhance our understanding of at least late Cretaceous dinosaur diversity of India.

I have barely scratched the surface. There are many more stories and patterns and trends in the Indian fossil record waiting to be teased out from this database. Dive in!

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