Plate Tectonics And Coral Reef Distribution And Diversity

Plate tectonics drive tropical reef biodiversity dynamics- Fabien Leprieur, Patrice Descombes, Theo Gaboriau, Peter F. Cowman, Valeriano Parravicini, Michel Kulbicki, Carlos J. Melian, Charles N. de Santana, Christian Heine, David Mouillot, David R. Bellwood & Loıc Pellissier

Interesting work!

Tropical coral reefs require sunlit shallow sea water to thrive. This is provided by coastlines and continental shelf areas. Plate motions has shifted continents. The geography of shallow sea habitats in which corals thrive has accordingly shifted. This study reconstructs this dynamic over the past 140 million years of the breakup of Gondwanaland and the movement of continents since. It is summarized beautifully in this graphic.

Source: Leprieur et al 2016

The optimum locations of coral habitats moved as the configuration of the Tethyan ocean changed. The fossil coral record shows that coral diversity was maximum in the Western Tethys in the Eocene (55 -33 ma), then shifted to the Arabian Peninsula and Western Indian Ocean during the Late Eocene - Oligocene (37-15 ma). Finally as the Tethys Ocean closed due to the India Asia collision, coral biodiversity hotsposts shifted since the middle Miocene to the Indo- Australian Archipelago.

There is a further step. The study models species diversification based on the distribution of estimated paleo-bathymetry and the residence time of habitat (both controlled by plate tectonics). The reasoning is that the frequency of evolution of new species (biodiversity) depends on the distribution of optimal habitat and how long these habitats remain in place. Large long lasting shallow continental shelves will offer an opportunity for populations of ancestral species to disperse over broad areas and diverge into new species. Tectonically complex areas like western Tethys (in the Eocene) and Indo-Australian Archipelogo in Pliocene-Quaternary have sea floor topography and shelf configurations which fragments habitats based on changes in water depth, current and wave strength and direction and nutrient availability. Populations get split and isolated in such regions leading to genetic divergence and speciation. This is mimicked well by the simulations.  Over time, rich diversity can evolve in such long lasting optimas by the process of dispersal and isolation. The locus of evolution of biodiversity can then shift as plate motions move ideal habitats across the globe. New species can arise from parapatric speciation (adjacent to the range of the ancestor) or sympatric speciation (within the range of the ancestor). The study finds that for either modes of speciation, their simulation matches the observed distribution of fossil and extant coral diversity.

The graphic below captures the comparison between fossil coral diversity and simulated coral diversity. Eocene, Miocene and Quaternary observed diversity (left side)  matches the model results (right side).

Source: Leprieur et al 2016

The scientists caution that local fluctuations like sea water temperature and acidity and ecological complexity will also play a role in evolution of  diversity and call for further validation of their work. But overall, its a nice demonstration of the role of plate tectonics in controlling coral habitats and diversity.

Papers- Precambrian Crustal Evolution Of Peninsular India

Over the last year or so I've collected quite a few papers on the subject of Precambrian crustal evolution and sedimentary basins of Peninsular India. I'm sharing the list with links. Many of them are available open access from various outlets.

I got my teeth cut in field geology and carbonate sedimentology during my M.S. thesis research. This involved mapping a small area of the Mesoproterozic Cuddapah basin in South India.  Peninsular India is a vast repository of these Precambrian rocks. These papers provide very interesting perspectives on various aspects of their geology. Perhaps the big leap in the past decade or so has been -finally!- the increased availability of accurate geochronology that has made it possible for geologists to start piecing together their complex polyphase history.

1) Precambrian Crustal Evolution of Peninsular India: A 3 billion year odyssey ;  Joseph G. Meert,, Manoj K. Pandit, Vimal R. Pradhan, Jonathan Banks, Robert Sirianni, Misty Stroud,Brittany Newstead, Jennifer Gifford

2) Proterozoic orogenic belts and rifting of Indian cratons: Geophysical constraints ; D.C. Mishraa, M. Ravi Kumar

3) The Archean and Proterozoic History of Peninsular India: Tectonic Framework for Precambrian Sedimentary Basins in India; Joseph G. Meert and Manoj K. Pandit

4) Precambrian Basins of India: Stratigraphic and Tectonic Context: Rajat Majumdar and Patrick G. Eriksson eds.- Collection of Paper from the Lyell Collection Geological Society of London Memoirs : Behind Paywall

5) An overview of the Palaeoproterozoic geology of Peninsular India,and key stratigraphic and tectonic issues: Dilip Saha and Rajat Mazumdar

6) Morphodiversity, complexity and macroevolution: Revealed by the megascopic life of the Palaeo-Neoproterozoic Vindhyan Supergroup, India: Poornima Srivastava

7) Stratigraphy and correlation of the Neoproterozoic deposits of central and western India: An overview: S. Kumar

8) The Central India Tectonic Zone: Geophysical perspective on continental amalgamation along a Mesoproterozoic suture:  K. Naganjaneyulu and M. Santosh

Happy reading!

Review- Human Dispersals Out Of Africa

Human Dispersal Out of Africa: A Lasting Debate -Saioa López, Lucy van Dorp and Garrett Hellenthal

Here is a good review of the fossil, archaeological and genetic data that has spawned various theories of how anatomically modern humans dispersed out of Africa and colonized the world.

The graphic below summarizes the dispersal scenarios-

Source: Lopez et al. 2016

One big lacunae is the lack of a Pleistocene skeletal record in the Indian subcontinent.

Maniraptoran Dinosaurs Show No Decline In Disparity Before Mass Extinction

Its hard to unravel and unpack complex phenomenon like patterns of faunal turnover during mass  extinctions. The methods chosen, the materials (fossils) available for study and the granularity of the study influences the results.

My last post was about a modeling study that concluded that for 40 million years before the mass extinction,  extinction rates exceeded the evolution of new species in many dinosaur lineages. Dinosaur diversity was declining towards the end of  the Cretaceous. Only a few herbivorous dinosaurs showed an increase in diversity.

Now a different study by Derek W. Larson, Caleb M. Brown , David C. Evans published in Current Biology focuses on just one sub group  of dinosaurs and comes to a different conclusion. This is an analysis of over 3000 teeth  of bird-like maniraptoran dinosaurs from 18 stratigraphic units in Western N. America which shows that there was no decline in disparity in different maniraptoran lineages for the last 18 million years before the mass extinction. The maniraptors were largely omnivorous but may have included specialized carnivorous and as well as herbivorous species.

Disparity is a measure of the range of variation of morphology, in this case teeth shape. This in turn is something of a proxy for the range of ecologic niches being exploited.

The authors remark-

Overall, these inferred ecomorphological patterns indicate that toothed maniraptorans, including toothed birds, were ecologically diverse and stable leading up to the end-Cretaceous mass extinction, at least in North America.

An interesting angle explored  in the paper is why toothed bird-like maniraptors and some lineages of toothed birds died out, while their physiologically and morphologically close relatives, the ancestors of modern birds, the Neornithes,  survived the mass extinction.

In the fallout of the bolide impact that marks the end of the Cretaceous, terrestrial food webs that relied on photosynthesis would have collapsed. However, seed banks derived from plants, including relatively abundant angiosperms, could have been a common, nutrient-rich resource that would have persisted among the detritus, which itself has been suggested as a key food source related to species survival across the boundary. With clearing of vegetation, either during a global firestorm or widespread burning of dead ground cover, exposed seeds in these areas worldwide could have been exploited by granivores. This pattern is observed in modern fire succession communities, where granivorous birds are the first avians to re-occupy disturbed habitats due to food resource accessibility. A persistent seed bank, which can remain viable for more than 50 years in modern temperate forests, most likely outlasted the catastrophic ecosystem collapse caused by a bolide impact and associated infrared thermal pulse, acid rain, darkness, and impact winter. Toothed maniraptorans, with their feeding systems adapted to faunivorous or potentially folivorous diets, would have been restricted to food chains dependent on living plant matter and unable to access these seed banks. Therefore, dietary specialization toward granivory in some lineages of crown group birds may have been one of the key factors in their survival through the end-Cretaceous mass extinction.

Although very similar to maniraptors, the Neornithes had evolved a unique dietary specialization. The presence of a beak, which served as a crushing apparatus, may have helped these ancestors of modern birds exploit seeds as a food source.

Decline In Dinosaur Diversity 40 Million Years Before Asteroid Impact

This is based on modeling of speciation and extinction rates.

Manabu Sakamotoa, Michael J. Benton,and Chris Vendittia

Whether dinosaurs were in a long-term decline or whether they were reigning strong right up to their final disappearance at the Cretaceous–Paleogene (K-Pg) mass extinction event 66 Mya has been debated for decades with no clear resolution. The dispute has continued unresolved because of a lack of statistical rigor and appropriate evolutionary framework. Here, for the first time to our knowledge, we apply a Bayesian phylogenetic approach to model the evolutionary dynamics of speciation and extinction through time in Mesozoic dinosaurs, properly taking account of previously ignored statistical violations. We find overwhelming support for a long-term decline across all dinosaurs and within all three dinosaurian subclades (Ornithischia, Sauropodomorpha, and Theropoda), where speciation rate slowed down through time and was ultimately exceeded by extinction rate tens of millions of years before the K-Pg boundary. The only exceptions to this general pattern are the morphologically specialized herbivores, the Hadrosauriformes and Ceratopsidae, which show rapid species proliferations throughout the Late Cretaceous instead. Our results highlight that, despite some heterogeneity in speciation dynamics, dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event.

The paper is under a paywall so I won't comment on the details of the analysis. Ed Yong has written a good summary of the paper in The Atlantic.

I heard this research being covered on BBC News and out came the inevitable statement.. " dinosaurs may have been on their way out"... meaning even if there had been no asteroid impact the dinosaurs would have gone extinct soon. I am not  sure the researchers actually think this way. The dinosaurs were a very long lived group, originating in the Triassic about 230 million  years ago, with all the non-avian groups going  extinct 66 million years ago. In that time frame there were boom and  bust  phases. There were earlier episodes of rapid diversification , then decline and then recovery. The overall pattern seems to be a late Triassic to mid Jurassic phase of diversification, followed by a slowing down or small declines and small expansions of different groups thereafter.

Who knows, maybe the late Cretaceous decline would have given way to another period of recovery and diversification. Or, the branches of the dinosaur tree may have gotten pruned with many groups going extinct, but with some groups continuing to prosper. Dinosaurs were not one entity. They were a varied group.  The extinction of all non-avian dinosaurs 66 million years ago may create an illusion that their evolutionary fates were always co-dependent, but that need not be so. This analysis shows that even when many dinosaurs groups were declining in the late Cretaceous, a few herbivorous groups were diversifying.  All this came to an abrupt end because of the asteroid impact. Only the avian lineage of dinosaurs survived.

The one conclusion that can be definitely drawn from the dinosaur story is that random events and chance have played a huge role in shaping the history of life.