Deccan Volcanism And Mass Extinction

What role did Deccan Volcanism play in the end Cretaceous mass extinction? 

There is widespread agreement that an asteroid struck what is now the Yucatan Peninsula region of Mexico 66.05 million years ago. The resulting environmental catastrophe precipitated an abrupt mass extinction, wiping out 70% of all species. In the rock record, a thin clay layer preserves evidence of the extraterrestrial origin of this impact event. It is referred to as the K-Pg boundary layer, K being Cretaceous and Pg referring to the Paleogene. The biota below the boundary layer is very different from fossil assemblages above it.

The earth at that time was also experiencing a major episode of basalt volcanism. This Large Igneous Province (LIP) is known as the Deccan Volcanic Province or the Deccan Traps. Volcanism occurred both before and after the impact event. Geologists have been divided on whether most the lava erupted after the asteroid hit or before it with different implications for the role of volcanism in the mass extinction. There was also an uncertain and incomplete assessment of the volumes of lava involved.

A new paper by Vivek S. Kale and coworkers titled “Spatio-temporal volume recalibration shows Deccan volcanism caused Terminal Cretaceous Mass Extinction” leaves no room for doubt about which side of the divide this group stands. The researchers examine over 80 lava sections across the Deccan Volcanic Province and recalculate the lava volume, assigning packages of lava a time bracket based of absolute dating, magnetic signatures, and fossils. They find that around 70% of the Deccan lava erupted in a time span of 300,000 years before the mass extinction.

Their results are summarized in this infographic. Source: Vivek S. Kale and coworkers, GSA Bulletin, November 2025.

The volume estimates of previous workers and their study is shown to the left. Also presented are marine paleo-environmental indicators represented by a paleo-temperature curve and carbon isotope curves spanning the mass extinction.

A short explanation of these curves will be useful.

For estimating temperature, geologists use the ratio of Oxygen 18/Oxygen 16 preserved in the shell (CaCO3) of planktonic organisms like foraminifera or calcareous nanoplankton. During warm phases, vigorous evaporation results in more of O18 escaping into the atmosphere, enriching the ocean slightly in the lighter O16. Calcium carbonate shell growth incorporate O18 and O16 without preference, maintaining the same O18/O16 ratio of sea water .

Warmer seas will result in shells having a lower O18/O16 ratio, than shells that grow in cooler water. The temperature is estimated from these measured oxygen isotope ratios.

The increase in sea surface temperature observed here, known as the Late Masstrichtian Warming Event (LMWE), has been linked to increased CO2 emissions during Deccan Volcanism.

Carbon isotope trends through time are presented by measuring the C13/C12 ratio of the target sample. ‘Bulk’ carbon means any carbon from hard parts as against carbon from organic material. It might mean just ground up limestone made up of a mixture of shells and lime mud. Jurassic onward, the proliferation of planktonic foraminifera has allowed geologists to select shells of just one or two species for their measurements. This makes it easier to screen for post depositional alteration which could reset the original oceanic C13/C12 ratio due to reaction with fluids of a different composition. It is more difficult to recognize these effects in a mixed sample, especially one with fine calcium carbonate mud.

Variation in C13/C12 ratio through time is generally regarded as an indicator of primary productivity. Photosynthesizing organisms have a strong preference for the lighter C12 to build organic tissue. The C13/C12 ratio of organic matter is strongly depleted compared to the sea water C13/C12 ratio. Similar to oxygen intake, growing shells preserve the C13/C12 ratio of the ocean. During environmental crises, reduced primary productivity enriches the ocean in the lighter isotope, also depressing the C13/C12 ratio of shells growing under these conditions.

The modest (relative to earlier Phanerozoic mass extinctions) carbon isotope excursion coincident with the K-Pg boundary suggests a decrease in primary productivity due to the extinction of some photosynthesizing groups. In contrast, the amplitude range of the isotope excursions seen during the Late Maastrichtian Warming Event can also be explained by a non-biological driver.

Carbon dioxide of volcanic origin is isotopically lighter than sea water. Pulses of volcanism may result in a large enough pool of lighter C12 dissolving in the ocean and shifting sea water C13/C12 ratio to the depleted values measured in Late Cretaceous shells.

Kale and coworkers argue that increased volume of lava eruption and emissions of CO2 and other gases in the Late Maastrichtian caused global warming and destabilized the biosphere. Ecosystems were already tottering when the asteroid delivered the knockout punch.

Just how stressed was the ecosystem? The most visible indicator of environmental changes impacting an ancient biosphere is the fossil record of that time period. Their paper however does not present any analysis of the Late Maastrichtian biota.

The several studies (Ref. 1, 2, 3) where from Kale and coworkers source the paleo-temperature and isotope record all unambiguously conclude that the LMWE had a limited impact on marine life. Species diversity did not change much. Fossil community structure remained stable, or where it did change, it reflected migration and range shifts. An example of this is L. Woelders and coworkers study of a Late Cretaceous section from the South Atlantic where they find fluctuating abundances of benthic foraminifera and dinoflagellates tracking climate and sea level cycles.

That is not to say that organisms were unaffected by this warming event. Other studies across the marine realm have documented biotic stress. Vincente Gilabert and coworkers find fragmentation of shells, abundance of low oxygen tolerant taxa, and dwarfing of some species, in their study of planktonic foraminifera of the Caravaca section in Spain. Similarly, Gerta Keller and coworkers study on planktonic foraminifera also reveals species dwarfism and abundance of high stress opportunistic species in the late Maastrichtian of the Indian Ocean and South Atlantic.

Nicholas Thibault and Dorothee Husson in a survey of the Late Cretaceous ocean point out that nanoplankton species numbers dropped during the LMWE, but diversity rebounded and increased with no significant extinction in the last 140,000 years of the Cretaceous. Michael Henehan and coworkers infer an increase in ocean acidity coinciding with the LTME based on reduced preservation of calcium carbonate shells on the sea floor. Shell preservation and sea water carbonate saturation return to pre event values in the last 200,000 years before the K-Pg impact event.

Do these observations on plankton species signal some sort of a beginning of the end of ecosystems across the entire marine biosphere? Since Deccan Volcanism spanned several hundred thousand years, could there have been a mass extinction if there had been no asteroid hit? It is conceivable that the prolonged volcanism could have resulted in a ratcheting of environment stress, eventually crossing tolerance thresholds.

This proposition has been hard to test because the asteroid crashed the Cretaceous party denying us an extended view of a volcanism only unfolding of history.

Still, we do have a good 300,000 year record before the impact of sedimentary deposition that coincided with the most voluminous phase of Deccan volcanism. Finer scale resolution of marine sections in the past few years allow us to examine global biotic trends from this time span.

As mentioned above, the impact of ocean warming seems to be quite limited to changes in shell size, temporary species abundance shifts, and adjustments by migration to favorable locales. Importantly, there is no signal of elevated extinction in diverse groups such as planktonic foraminifera, radiolarians, nanoplankton, ammonoids, bivalves, and gastropods in the final few hundred thousand years of the Cretaceous, when volcanism was at its peak (Ref 1,2,3,4).

A closer look at the environmental proxy data is revealing. After a warming phase, the paleo-temperature curve shows a 150 thousand year cooling trend in the terminal Maastrichtian until the K-Pg boundary. Mirroring this cooling trend is a distinct shift towards heavier carbon isotope values. This may be due to increasing primary productivity signaling a general amelioration of ocean conditions.

What could cause such a cooling despite ongoing voluminous volcanism and CO2 emissions? The answer may be enhanced weathering of all that fresh basalt which kept sequestering carbon dioxide during silicate weathering reactions.

There are two lines of observation that support increased weathering during end Cretaceous times.

The first observation is the recovery of calcium carbonate saturation level of the ocean. Weathering releases divalent cations like calcium and magnesium in to the ocean resulting in an excess positive charge (alkalinity) in sea water. The charge imbalance promotes dissociation of the poorly soluble CO2 gas (represented in equations as carbonic acid -H2CO3) and the formation of more stable negatively charged bicarbonate (HCO3) or carbonate (CO3) anions, raising carbonate saturation state. A decrease in dissolved gas leads to a draw down of atmospheric CO2 in to the ocean by air-sea gas exchange. Alkalinity can also be described as the buffering capacity of the ocean against excessive CO2 buildup.

Today, rapid atmospheric CO2 increase is a major concern for ecosystem and societal health. Research and small scale demonstrations to enhance ocean alkalinity are gaining importance as we try to replicate on human time scales what nature does over much longer time spans.

The other independent support for enhanced weathering comes from another isotope system, the osmium 187/osmium188 ratio. Continental crust is enriched in Rhenium187 which decays to Osmium187. Rocks like basalt derived directly from partial melting of the mantle have much lower Rhenium187 and consequently Osmium187. There is a progressive decrease in the Osmium187/Osmium188 ratio in Late Maastrichtian ocean sediments. This is consistent with weathering and increased supply of non-radiogenic Os188 from a fresh basalt source. Some of the decline can also be explained by a reduced supply of Os187 as large areas of radiogenic continental crust were blanketed by lava.

Late Cretaceous marine ecosystem health can also be assessed by looking at the difference in the C13/C12 ratio between surface dwelling planktonic and bottom dwelling benthic foraminifera shells. This isotope gradient is maintained by the biological pump, a transfer of organic carbon and nutrients from the sea surface to depths. Organic carbon is enriched in the lighter isotope compared to planktonic shells (as explained earlier). This organic matter sinks to the ocean depths and oxidizes, enriching the Dissolved Organic Carbon (DIC) pool in C12. Shells of benthic foraminifera (bottom dwelling) have a lower C13/12 ratio than planktonic calcifiers.

A noticeable C13/C12 gradient is present (Ref. 1, 2) in the latest Cretaceous ocean. It converges immediately above the K-Pg boundary, indicating a disruption of the marine carbon cycle only after the asteroid impact.

Environmental perturbations need not precipitate a terminal decline of ecosystems. My reading of the fossil record and the different proxies is that the effects of the LMWE were transient. Key biogeochemical cycles remained intact through the Late Maastrichtian implying that volcanism did not greatly diminish pelagic ecosystem function. The end Cretaceous marine biosphere shows recovery and resilience and not signs of a ‘critically damaged’ system.

David E. Fastovsky and Antoine Bercovici perceptively ask in their review of the terrestrial record of the K-Pg extinction- “how does one weaken an ecosystem in terms of its ability to withstand catastrophic, that is to say, very short-term events? In other words, this is not a ‘straw that broke the camel’s back’ situation, where preceding events play a big role in the eventual failure. Since large swaths were simply obliterated in the asteroid’s wake, and more distant regions experienced acute shock, it wouldn’t have mattered if ecosystems were weakened or hale and hearty.

I expressed a similar sentiment earlier in the post. To attribute a causal role to Deccan volcanism, there have to be signs that significant global extinction events occurred prior to the asteroid strike. The data just doesn’t point in that direction.

Earlier in earth history, the end Permian (251.9 million years ago) and the end Triassic (201.4 million years ago) mass extinctions have been more convincingly linked to massive environmental damage triggered by sustained igneous activity. The impact on the environment due to Deccan volcanism appears muted in comparison.

One reason could be that during the Permian and Triassic igneous episodes, magma intruded through thick sedimentary basins filled with limestone, organic rich shale, and sulfur bearing evaporites. Interestingly, the mass extinction coincided with the emplacement of thick sills, tabular bodies of magma injected parallel to the strata. As a result, a surge of volatiles emanating from baking sediment amplified volcanic emissions, overwhelming the earth’s natural buffering capacity. Both these mass extinctions are estimated to have unraveled in just tens of thousands of years.

In central Peninsular India, Deccan volcanism intersected older sedimentary basins only at the northern and southern fringes. Slow cooling shallow subsurface sills facilitating effective heat transfer to surrounding crust are absent. Instead, eruptions were fed by magma ascending rapidly along thin conduits (dikes) aligned in swarms. More relevant is that only sterile granitic rocks underlie the regions where the most voluminous eruptions took place.

Volatile bearing sedimentary rocks did play a role in the end Cretaceous mass extinction. But by a quirk of fate, it was the asteroid that found them. The Yucatan region of Mexico where the asteroid hit is underlain by hydrocarbon rich sediments, and sulfur rich salt. The impact released soot and sulfate aerosols in the atmosphere resulting in global cooling and reducing photosynthesis for weeks to months. Eventually, the dust settled upon a world utterly transformed. It was the beginning of a new world, one where flowering plants, song birds, and mammals diversified and flourished. From time to time, chance events have opened up entirely new avenues for evolution to explore.

It is pertinent to stress again the paramount importance of rates of processes in the context of our developing climate crises. Although in absolute terms humans may not emit more than past large volcanic events like the Cretaceous Deccan Volcanism or the Siberian Volcanism of late Permian times, we are emitting CO2 at a rate many times faster than these natural events. Carbon dioxide is building up in the atmosphere too quickly for weathering and ocean ecosystems to offset over the next few hundred years.

Lastly, Jurassic onward, the proliferation and evolution of plankton shell building organisms such as foraminifera and coccolithophores have played an important role in regulating ocean chemistry and as a buffer against atmosphere CO2 changes. This occurs by various processes. For one, the heavy shells act as ballast, exporting attached organic matter to depths before their degradation consumes oxygen in the uppermost oceanic layer. Much of this carbon gets buried, becoming a long term carbon sink.

The other important function is sustaining the ocean alkalinity balance. Precipitation of calcite and aragonite (CaCO3) removes positive charge from the ocean, in the form of divalent Ca cations, reducing alkalinity (see previous section on weathering). This alkalinity is recovered when calcareous skeletons, falling through the water column like an ocean snow, start dissolving when they sink below a threshold depth, releasing divalent Ca ions. This process known as carbonate compensation maintains the ocean’s capacity to absorb atmospheric CO2.

The cooling trend observed in the final 150,000 years of the Cretaceous suggests that the rates of emissions during Deccan volcanism likely remained relatively low. Silicate weathering on land and enhanced ocean alkalinity coupled to weathering runoff and carbonate compensation would have substantially offset Deccan CO2 emissions, moderating their environmental impact.

In the pre-Jurassic ocean, most carbonate skeleton producing organisms lived in the water column or on the sea bed of shallow continental shelves where the sea water was saturated with calcium carbonate. Skeletons did not dissolve in this environment and the biological contribution to ocean alkalinity was limited. This changed when the more geographically widespread plankton evolved biomineralization and their skeletons began settling down to depths where the ocean water is undersaturated with calcium carbonate. The calcareous plankton ecosystem has made post Jurassic oceans more resistant to external shocks, albeit over times scales of thousands of years.

By now readers may be wanting to ask- What about the dinosaurs? Did they persist right until the asteroid hit, or did they go extinct earlier? The avian branch of dinosaurs did survive. The following section is about the non-avian groups and the terrestrial fossil record.

There would have been an extirpation of local populations of dinosaurs and other fauna and flora living in areas directly affected by Deccan Volcanism. But what about the global record?

Terrestrial environments such as lakes and rivers which can preserve dinosaur fossils are patchily distributed. Frequent erosion also removes sediment, making the reconstruction of organic diversity and evolutionary trends much more difficult than for biota living in deep sea environments.

Despite these limitations, we do have some sedimentary sections which inform us about dinosaur life and the terrestrial biosphere of the Late Maastrichtian.

In their work on peat deposits from the Western Interior, U.S., Lauren O’Conner and coworkers estimate end Cretaceous paleo-temperatures based on configuration of preserved organic compounds. Besides the LMWE, they identify shorter warming and cooling phases in the final 100,000 years before the K-Pg boundary, likely driven by Deccan volcanism. However, the impact on the biota seems limited, with no decline in angiosperm diversity until the mass extinction.

Andrew J Flynn and coworkers study in New Mexico on the Naashoibito Member, a dinosaur rich sedimentary deposit, shows that dinosaurs were diverse and formed regionally distinct assemblages during the final few hundred thousand years before the asteroid strike.

The Hell’s Creek Formation spread over the States of North Dakota and Montana also offers insights in to the terrestrial ecosystems (Ref. 1, 2, 3) . Pollen and leaf assemblages attest to a healthy end Cretaceous flora. Both show a substantial and abrupt extinction at the K-Pg boundary. Vertebrate diversity also shows no decline throughout the Late Maastrichtian.

Mammals appear to the exception here. Gregory P Wilson and coworkers detailed survey of the Hell’s Creek Formation reveals two sequential changes in mammalian fauna 650,000 years and 200,000 years before the K-Pg boundary. Apart from changes in mammal assemblages, there is a decline in the abundance of metatherians. The authors link this decline to regional environmental changes, but don’t rule out the effect of Deccan volcanism.

Dinosaur fossils at the Hell Creek section have been found to within a few tens of cm of the K-Pg boundary, representing the last few hundred to a few thousand years of the Cretaceous. Their presence in numbers comparable to lower levels of the sedimentary section negates arguments that dinosaurs were either declining or had gone extinct well before the K-Pg impact event.

The icing on the cake is some recent work by Robert DePalma and coworkers on a debris layer in the Hell’s Creek section. These chaotic beds made up of boulders and fossils was deposited by a tsunami triggered by the asteroid hit. Among other organic remains, the researchers have found a fossil hind limb of an ornithiscian dinosaur. Their work was presented at the EGU 22 General Assembly 2022 in Vienna, Austria. The peer reviewed paper is yet to be published and some scientists have expressed skepticism and called for a more detailed examination of the fossils. But this initial announcement, along with other documented dinosaur fossil finds from North America, does strongly suggest that dinosaurs were well and alive until the very day the world changed 66.05 million years ago.

Harappan Technology, Homo Floresiensis, Foraminifera

Some exciting readings for you- 

1)  Manufacture of synthetic stone in the Bronze Age Harappan Civilization.

Stone beads were important in Harappan culture and trade. They were made out of agate, an amorphous or cryptocrystalline form of silica. The Kutch region was the primary source of these agates. Stone and bead processing Harappan age workshops abound in this region. The tradition continues today. Some of the best ornamental agate is still being sourced from Kutch. The agates precipitate as secondary silica in cavities of the Deccan Basalts and associated silica rich lava.

To perforate these agates, the Harappans needed tools such as drill bits which were harder than the agate. For this, they manufactured a synthetic stone, now called Ernestite, by high temperature sintering of sand and laterite raw materials. Reaction temperature needed to fuse these materials into a cohesive rock would have reached 1100 deg C! Mesozoic sandstone, found all over Kutch, provided the sand, and the laterite came from iron rich weathered layers capping the Deccan Basalt.

A terrific study by M.K. Mahala and coworkers that details the provenance and fabrication of this interesting artificial stone has just been published in Nature Heritage Science. There is a lot of mineralogy and geochemistry described in the paper, but the conclusions are clearly laid out for all to understand.

2) Climate change and the decline of the Hobbit.

Why are small isolated populations of animals vulnerable to extinction?

Inbreeding, small geographic range, limited access to resources, reliance on one or few food sources, a chance catastrophic event, all may be factors making them susceptible to extirpation.

A case in point is the Hobbit or Homo floresiensis, the diminutive hominin discovered on Flores Island, Indonesia, in 2003. The archaeologic record shows it lived on the island for at least one million years. Homo floresiensis is thought to be a descendant from an early Homo species which dispersed from Africa 2 million years ago. The other famous inhabitant of the island that coexisted with the Hobbit and was its main food source is the dwarf elephant Stegodon.

A careful analysis by Michael Gagan and coworkers, published in Nature Communications Earth and Environment, using geochemistry of calcite from cave deposits show how climate change and decrease in water availability may have increased competition for resources and made life challenging for the inhabitants.

Summer rainfall began declining around 76,000 years ago with record low rainfall between 61,000 to 55,000 years ago. Both the Hobbit and Stegodon fossils become rarer during this interval and disappear by 50,000 years ago. Modern humans entered Flores Island around 46,000 years ago, and whether the Hobbit interacted with them on Flores Island is uncertain. 

3) The history of the ocean, as told by tiny beautiful fossils. 

The tiny fossils are planktonic foraminifera living suspended in the upper sunlit portion of the ocean. Their shells are made of calcium carbonate. They occur in huge numbers and have short lives. Their shells drop down and carpet the ocean floor, making them valuable archives for understanding evolution. Scientists make use of them for studying climate change too. Certain changes in the chemical composition of their shell are a function of water temperature. Tim Vernimmen has written a short piece on how foraminifera inform us about past environmental crisis and ocean conditions.

That Day 66 Million Years Ago

Just wanted to share this abstract of a paper detailing an outcrop from Baja California, Mexico, which preserves heterogeneous deposits resulting from the Chicxulub meteorite impact 66 million years ago. 

We report K-Pg-age deposits in Baja California, Mexico, consisting of terrestrial and shallow marine materials re-sedimented onto the continental slope, including corals, gastropods, bivalves, shocked quartz grains, an andesitic tuff with a SHRIMP U-Pb age (66.12 ± 0.65 Ma) indistinguishable from that of the K-Pg boundary, and charred tree trunks. The overlying mudstones show an iridium anomaly, and fungal and fern spores spikes. We interpret these heterogeneous deposits as a direct result of the Chicxulub impact, and a mega-tsunami in response to seismically-induced landsliding. The tsunami backwash carried the megaflora offshore in high-density flows, remobilizing shallow marine fauna and sediment en route. Charring of the trees at temperatures up to >1000°C took place in the interval between impact and arrival of the tsunami, which on the basis of seismic velocities and historic analogues amounted to only tens of minutes at most. This constrains the timing and causes of fires, and the minimum distance from the impact site over which fires may be ignited.

Raging forest fires, a tsunami and its backwash, hundreds of millions of tons of sediment mobilized as gigantic mixed debris flows, ecosystems laid waste.

What a catastrophic time! 

The paper is open access but be aware that it is a preprint, yet to be peer reviewed. 

Forest fire at the K-Pg boundary on the Pacific margin of Baja California, Mexico: timing and causes- Amanada Santa Catharina et.al. 2022.

Mass Extinction, Peopling Of America, Tale Of The Horse

 Sharing some interesting items:

1) What was the impact of Deccan Volcanism on the end-Cretaceous mass extinction? Improved dating of the timing of volcanism shows that volcanism spanned the mass extinction. But what changes occurred to marine environments because of the outgassing wasn't well documented. A new study uses the oxygen isotope ratios in foraminifera shells to estimate ocean temperature changes before and after the mass extinction. The finding is that the oceans warmed well before the extinction but cooled back again. The warming event doesn't appear to correlate with marine extinctions. Rather the mass extinction coincides with evidence for a meteorite impact. 

Here is a figure from the paper on the estimated temperature changes collated using a variety of proxies:

Source: On impact and volcanism across the Cretaceous-Paleogene boundary

Joshua Sokol has written a good summary of the paper:

A Rapid End Strikes the Dinosaur Extinction Debate.

2) Anthropological geneticist Jennifer Raff has pieced together the genomic story of the peopling of the American continents in this really insightful article. Do read it!

Genomes Reveal Humanity’s Journey into the Americas.

3) And next, onwards to a bit of Indian history. A very interesting conversation between Live History India editor Mini Menon and author Yashaswini Chandra on Ms. Chandra's new book, The Tale of the Horse: A History of India on Horseback. Fascinating story of the horse trade from Central Asia into India and its assimilation as a war animal and into Indian society. 

The Tale of the Horse (video). 

Niche: Two Examples From Deep Time

The term 'niche' can very simply mean an ecologic space which a particular type of organism exploits. Scientists are a pedantic lot though. They need more rigorous definitions to work with. This has spawned many different ideas about what a niche means and how it can best be described and measured. There is the environmental niche concept which focuses on the physical and chemical attributes of an available space that may or may not be filled by organisms. In this idea, there may be vacant niches, which opportunistic organisms may come to exploit. On the other hand, there is the population niche concept where the niche itself is an attribute of the population. Organism-specific use of its resources, uniquely shaped by the organism's physiology, community structure, and behavior, defines the niche. 

Similar organisms may co-exist in a particular space. This may result in niche overlap and niche partitioning as different species vie for the available resources. Ideas of competitive exclusion (competition theory) derive from such co-habitation of space.

I am just giving a flavor of the arguments here and not diving into a discussion of the many niche concepts. For the purpose of this short post I will use the term niche to mean the actual utilization of a space and of available resources by a species/population.

Palaios is of my of my favorite science journals. It published papers on themes intersecting palaeontology, ecology, sedimentology and stratigraphy. Unfortunately, most of the papers are behind a paywall, so I have to make do with reading the abstracts and finding the occasional open access paper on Research Gate and Academia. Browsing through it last week, I came across two interesting examples of very specific fossil niches, one from the Cretaceous and another from the earliest Triassic. 

In the March 2020 issue, Alison J. Rowe and colleagues ( Late Cretaceous Methane Seeps As Habitats For Newly Hatched Ammonites) describe a community of ammonites (a type of mollusc) living in the vicinity of cold methane seeps. These fossils are preserved in the sedimentary rocks of the Late Cretaceous Western Interior Seaway, a time when sea levels were high and the mid North American continent was under the sea. Methane seeps often occurred along faults which provided the pathways for the gas to rise from the subsurface and be released on the sea floor. Ancient methane seeps are recognized by the presence of typical communities of clams, fossilized worm tubes, ammonites and bacterial microstructures. There is often the development of cone shaped sediment mounds known as Teppe Buttes, made up of calcium carbonate mud and skeletal remains of organisms. The carbon in the calcium carbonate is enriched in the lighter isotope C12, suggesting its derivation from a hydrocarbon source (the hydrocarbon itself is transformed organic matter which is richer in C12). 

Anaerobic oxidation of methane (bacteria stripped electrons and protons from the hydrogen in the methane to drive respiration) provided the energy to build a food web that formed the base of this chemosynthetic ecosystem. These ammonites were small, implying that they were born in this habitat, and their geochemistry indicated that they were incorporating the carbon released from methane to build their shells. What an utterly fascinating mode of life!

The second example is from the Latest Permian-Earliest Triassic (Dwelling In The Dead Zone- Vertebrate Burrows Immediately Succeeding The End-Permian Extinction Event in Australia), preserved in strata deposited just after the biggest mass extinction in earth history. Stephen McLoughlin and colleagues find and describe burrow structures made by small tetrapods. The sediments which contain these burrows are poor in other organic traces suggesting a terrestrial ecosystem which has been stripped bare due to prolonged debilitating environmental conditions.  Paleogeographic reconstruction indicates that at this time the Sydney Basin  occupied a high paleo-latitude. A burrowing lifestyle, coupled with relatively cooler climate of higher paleo-latitudes may have provided these tetrapods protection from the otherwise harsh post extinction conditions. A nice example of the ecology of post mass extinction survivor fauna.

If you want to explore the history of ideas on the niche concept I can recommend two essays. Niche: Historical Perspectives by James R. Griesmer, and, Niche: A Bifurcation in the Conceptual Lineage of the Term by Robert K. Colwell. Both have been published in the book Keywords in Evolutionary Biology, edited by Evelyn Fox Keller and Elisabeth A. Lloyd.

We have so much more to learn about life.

Magmas And Mass Extinction: Late Triassic

A new study on the synchronicity of igneous activity and the Late Triassic mass extinction which occurred around 201.5 million years ago.

Large-scale sill emplacement in Brazil as a trigger for the end-Triassic crisis- Thea H. Heimdal, Henrik. H. Svensen, Jahandar Ramezani, Karthik Iyer, Egberto Pereira, René Rodrigues, Morgan T. Jones & Sara Callegaro. The article is open access.

Magma intruded a thick pile of sediments in Brazil. The thermal reactions in the sediment would have resulted in the release of 88 trillion tons of CO2 from the degassing of sediments!

Abstract:

The end-Triassic is characterized by one of the largest mass extinctions in the Phanerozoic, coinciding with major carbon cycle perturbations and global warming. It has been suggested that the environmental crisis is linked to widespread sill intrusions during magmatism associated with the Central Atlantic Magmatic Province (CAMP). Sub-volcanic sills are abundant in two of the largest onshore sedimentary basins in Brazil, the Amazonas and Solimões basins, where they comprise up to 20% of the stratigraphy. These basins contain extensive deposits of carbonate and evaporite, in addition to organic-rich shales and major hydrocarbon reservoirs. Here we show that large scale volatile generation followed sill emplacement in these lithologies. Thermal modeling demonstrates that contact metamorphism in the two basins could have generated 88,000 Gt CO2. In order to constrain the timing of gas generation, zircon from two sills has been dated by the U-Pb CA-ID-TIMS method, resulting in 206Pb/238U dates of 201.477 ± 0.062 Ma and 201.470 ± 0.089 Ma. Our findings demonstrate synchronicity between the intrusive phase and the end-Triassic mass extinction, and provide a quantified degassing scenario for one of the most dramatic time periods in the history of Earth.

This prolonged phase of igneous activity resulted in the formation of the Central Atlantic Magmatic Province. Its connection to the mass extinction was hard to pin down due to a lack of accurate dates of the oldest igneous activity. This and some other work now show that phases of this magmatic episode were synchronous with the mass extinction.

A similar problem of lack of accurate dating of events had limited our understanding of the role of Deccan Volcanism in the mass extinction that took place at 66.04 million year ago. New geochronology work (summarized by Kale et. al. 2019)  is showing that volcanism spanned this mass extinction. Significant amount of lava eruptions took place before the mass extinction and would have played a role in the deterioration of environmental conditions. And volcanism continued well after the mass extinction delaying biotic recovery for hundreds of thousand of years.

Large injections of magma as laterally extensive intrusions (sills) into sediment has also been thought to have been the trigger for the end-Permian mass extinction that took place around 252  million years ago. Interestingly, like the end-Triassic, it was not emissions of carbon dioxide and methane directly from lava eruptions that is thought to be the driver of environmental change. Rather, it was the subsurface emplacement of sills and the thermal reaction (contact metamorphism) in buried sediment in contact with this hot magma that resulted in volumetric degassing from sediments. Limestones when heated this way would have released carbon dioxide upon breakdown of the mineral calcite. And organic matter would have released methane.

The long trajectory of evolution on earth has been disrupted and reoriented many times from deep within.

Links: Kimberlites, Ecosystem Recovery, Early Atmosphere, Carbonates

Some readings on assorted subjects.

Enigmatic origin of diamond-bearing rocks revealed

These are volcanic rocks which are the primary source of diamonds. Kimberlite magmas originate from deep in the earth's mantle. A recent geochemical survey has provided insights into the nature of that source. In India, the famous Panna diamonds are derived from the Majghawan Kimberlite which erupted about 1073 million years ago in the Proterozoic Vindhyan Basin.

Diversity decoupled from ecosystem function and resilience during mass extinction recovery

The mass extinction that took place 66 million years ago devastated both marine and terrestrial ecosystems. How long does post-extinction recovery take and exactly how do community structure and ecosystem functions reboot? A study using a 13 million year record of nannoplankton (unicellular protists) spanning the mass extinction has yielded some insights.

The study suggests that essential ecosystem functions such as geochemical cycling of nutrients was established by few hardy species very soon in the extinction aftermath. This recovery preceded by million of  years the reestablishment of species richness.

Did Bacterial Enzymes Cap the Oxygen in Early Earth’s Atmosphere?

Photosynthetic cyanobacteria that expelled oxygen evolved by 2.4 billion years ago. But oceanic and atmospheric oxygen levels remained quite low, about 10% of current levels, until about 400 million years ago.

The Pre-Salt Hydrocarbon Reservoirs of the South Atlantic

A superb example of how an understanding of the environments in which sediments are deposited helps petroleum exploration strategy.  Focus is on the unusual alkaline lake carbonate deposits of Brazil, formed during the Cretaceous when South America and Africa started splitting away from each other.

Conversations: Tempo Of Deccan Traps Eruptions

My friend V.V Robin who is an evolutionary biologist and researches the genetics of bird species along the Western Ghats also takes a keen interest in geology. He pointed me to this new work on the tempo of Deccan Volcanism;

Tempo of the Deccan Traps eruptions in relation to events at the Cretaceous-Paleogene boundary - Renne, Paul et al 2016; Presented at the meeting of the EGU General Assembly 2016, held 17-22 April, 2016 in Vienna Austria

Robin wrote- I found this quite cool - that all the volcanism was possibly within one million time frame! Must have also had a huge impact on the organisms living then.

But it was not clear from this where the epicentre was - "Poladpur or uppermost Bushe Fm., near the base of the laterally extensive Wai Subgroup"

Right and this is very important... this refers to when the style of magmatism changed in relation to the K-Pg boundary event. Within the enormous pile of lava, in the upper parts of the section subdivided based of geochemical signatures into the Bushe (older), Poladpur and Ambenali (youngest) formations lies the K-Pg boundary. See figure to the left (Source: Earth Magazine; credit K. Cantner AGI) which shows the chemical stratigraphy of the Deccan Basalts as defined primarily by sampling the Western Ghat sections. The change in magmatism style is somewhere near the top of the Bushe Formation. Lava of the entire Wai Subgroup may have erupted in just a few hundred thousand years.  It is key to know at what stratigraphic level the K-Pg boundary is. If the boundary lies say near the base of Bushe, then this would strengthen the hypothesis that the Chicxulub impact triggered a more effusive phase of Deccan volcanism. In this scenario, since it would have taken hundreds to thousands of years before volcanic rates increased, the more effusive phase of volcanism may not have caused the mass extinction, but played a role in stifling recovery of ecosystems for hundreds of thousands of years after. On the other hand if the boundary lies higher up, say somewhere in upper part of Poladpur or Ambenali, then the implication would be that the more effusive phase of volcansim started before the impact and that would mean a more direct causal link between volcanism and mass extinction.

Robin though had read the abstract a little differently and he answered back -

So, they dont really say much about WHERE the epicentre was, as suggested in the initial part of the abstract. Maybe that's ongoing research. Very exciting stuff! This would certainly be interesting to folks studying older taxa - centipedes, frogs etc

He wanted to know where the volcanic epicentre was geographically. He was referring to these sentences from the abstract -

 40Ar/39Ar geochronologic study is providing the first indications of variable time-averaged eruption rates in the important Western Ghats region, in addition to providing the first precise location of the KPB within the Deccan pile

" radioisotope geochronology has failed to clarify the tempo of the eruptions or to delineate where the KPB age-equivalent horizon occurs within the eruptive sequence"

 Well, in the language of stratigraphy the authors are not referring to a geographic place but where in the stratigraphic sequence.  They are referring to its temporal position .

Geographically, the K-PG age equivalent horizon, meaning a lava flow which erupted 66.04 million years ago (coincident with the Chixculub impact) could be preserved anywhere. Near Lonvala, near Mahabaleshwar, in southern Konkan or Goa or all of these places. We haven't found it yet, but it could be a matter of time, with better radiometric resolution now allowing finer and finer sampling.

Off course it is a perfectly reasonable question to ask if there was an geographic epicenter of Deccan volcanism. Here is a cross section of the Western Ghat with formation boundaries and structural features.

Source: M Widdowson  and K.G Cox 1996

Note that there is a large vertical exaggeration (40X) to the profile which is about 650 km in length. The long wavelength fold and dips are not apparent at outcrop scale and have been revealed through accurate measurements of topographic levels of formation boundaries. What you do see is that the older formations (Kalsubai Subgroup)  are to the north and the younger to the south (Wai Subgroup). Is this due to a southerly migration of volcanism or is this due to deeper erosion of the lava pile to the north which has stripped away younger lavas and exposed the older section of the lava pile?
 
Likely there was not one epicentre of volcanism in the Deccan volcanic province. Rather, there would have been many many eruptive centers spread out along the N-S oriented western rifted margin and the E-W oriented Narmada Tapi rift zone and more in the central parts of the plateau too. Within the Western Ghat section there have been some suggestions that the lava thickness and laterite geochemistry might be consistent with the idea of a south moving focus of volcanism during the eruption of the Wai Subgroup. But on a larger scale, taking into account the entirety of the province it is hard to make a case of any one epicentre.

Take a look at this map with plotted ages of Deccan basalts. Younger and older basalt ages are scattered all over.

Source: Hethu Sheth - The Deccan Beyond The Plume Hypothesis

The Western Ghats have been the most intensely studied and sampled region of the province and hence its age distribution is now better constrained. But the volcanic province is spread over large areas in Madhya Pradesh, Gujarat, eastern Maharashtra and importantly a large area under the Arabian sea covered by hundreds of feet of Cenozoic sediment.

Here is a map which shows the distribution of selected Deccan lava sections beyond the Western Ghats. Many sections to the north, northeast and the east contain the Bushe, Poladpur and Ambenali Formation lavas which are thought to span the K-Pg boundary. Geochemical work has identified dykes feeding these lavas, and they are distributed over wide regions. A few have been identified in the Narmada rift region which may have erupted upper formation lavas present in the north and northeast of the province. There is also a cluster between Nasik and Pune and between the coast to Sangamner on the plateau to the east of the Western Ghats also. Some of these feeder dykes likely gave rise to the bulk of the Western Ghat upper formations. So, eruptive centers of lavas spanning the mass extinction was occurring in regions well away from the Western Ghat region too, suggesting there wasn't any one epicenter of volcanism during this time.

 Source: The Feeder System of the Deccan Traps (India): Insights from Dike Geochemistry

..to be continued as more questions come in!

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.

Quote: Stephen Jay Gould On Paleontology

This study of periodicity of mass extinction was published last month-

Periodic impact cratering and extinction events over the last 260 million years - Michael Rampino and Ken Caldiera.
 
The claims of periodicity in impact cratering and biological extinction events are controversial. A newly revised record of dated impact craters has been analyzed for periodicity, and compared with the record of extinctions over the past 260 Myr. A digital circular spectral analysis of 37 crater ages (ranging in age from 15 to 254 Myr ago) yielded evidence for a significant 25.8 ± 0.6 Myr cycle. Using the same method, we found a significant 27.0 ± 0.7 Myr cycle in the dates of the eight recognized marine extinction events over the same period. The cycles detected in impacts and extinctions have a similar phase. The impact crater dataset shows 11 apparent peaks in the last 260 Myr, at least 5 of which correlate closely with significant extinction peaks. These results suggest that the hypothesis of periodic impacts and extinction events is still viable.

This idea is not new and I remembered Stephen Jay Gould's essay "The Cosmic Dance of Siva" (in The Flamingos's Smile) which describes one of the early such hypothesis followed by his trademark meditations of the role of paleontology in understanding the nature of evolution  and the history of life. Gould was always irked by the opinions held by many that paleontology is a dusty sort of a science, where people spend their careers fighting over species names, and that they have nothing important to say about the theoretical aspects of evolution. A large part of his popular science writing effort was devoted to demolishing  this notion. He was very successful in it with essay after essay beautifully demonstrating the utmost importance of paleontology in highlighting life's little oddities as well as its grand patterns.

Paleontology in the early 1970's underwent something of a change in attitude. Decades before "big  data" became a buzzword, researchers led by David Raup, Jack Sepkoski,  Leigh Van Valen and Tom Schopf, to name a few,  began amassing enormous data sets on fossil characteristics and species distributions and subjected them to rigorous statistical analysis in an effort to elucidate distribution of biodiversity and macro-evolutionary trends. Gould's own work was substantial. Ideas such as "punctuated equilibrium" (with Niles Eldridge) which relies on the fossil record to tease out patterns in the mode and tempo of evolution, were met with admiration as well as fierce criticism as was his thinking on the role of contingency and chance and the limits of natural selection and adaptive evolution as explanations for life's historical trajectories. His theoretical forays did make paleontologists think more broadly about their data and what it tells us about evolution but my take is that it didn't cause a revolution in evolutionary theory as was made out by the media and by Gould's  rhetoric. But he did elevate the status of paleontology and that can only be good for science.

He writes:

Most hot  ideas turn out to be  wrong. I can only hope that I will not be remembered as the man who campaigned with a name for the nonexistent (surely worse than a moon for the misbegotten). Some chances are certainly worth taking. If Thalia smiles and Siva exists, think what it all will mean for my beloved science of paleontology. We have labored so long under the onus of boredom and dullness. We are guardians of life's history, but we are often depicted as mindless philatelists of stone; specialists in tiny corners of space, time, and taxonomy; purveyors of such arcane names  as Pharkidonotus percarinatus in extended orgies of irrelevant detail. The editors of Britain's leading scientific journal wrote of us in 1969: "Scientists in general might be excused for assuming that most geologists are paleontologists and most paleontologists have staked out a square mile as their life's work." 

That impression has changed since to include paleontology as an important contributor to evolutionary theory. For that, the field owes him a debt.

Open Access- History Of Life Articles In Current Biology

Update: November 6 2015- The articles are no longer open access.

This is a treat!

A special section in Current Biology on the History of Life with short essays and longer reviews on a variety of evolutionary history topics. So far I have read four:

The Cambrian Explosion - Derek E.G. Briggs
The Neoproterozoic- Nicholas J Butterfield
Novelty and Innovation in The History Of Life- Douglas H Irwin
Life in the Aftermath of Mass Extinctions- Pincelli Hull


These are densely written and immensely informative articles about various aspects of the evolution of the earth and its biosphere.

Couple of passages-

..from Novelty and Innovation in The History Of Life on the different styles of diversification seen in the fossil record-

Several challenges have arisen to claims that adaptive radiations are responsible for most evolutionary diversifications. For one, many events have been identified among both living and fossil clades that cannot be explained as the outcome of diversification from a single species. Examples range from the Cambrian explosion of animals, which involved many major clades but relatively few species, to the diversification of grasses. I have already discussed cascading radiations where increased diversity was driven by ecological interactions between clades. Other diversifications, for example the spread of a genus across a continent, may be largely non-adaptive. The most striking observation, however, is the absence of evolutionary novelty associated with classic adaptive radiations. Indeed, by their nature, adaptive radiations concern the adaptive exploitation of ecological opportunities via variation on existing adaptive themes, but not the formation of the themes themselves. While the fossil record documents adaptive radiations that encompass greater morphological diversity than Darwin’s finches, mockingbirds or Anolis lizards, including the spread of insects and angiosperms, and the Mesozoic radiation of mammals, the origins of morphological novelties often seem to involve a different process.

..from The Aftermath of Mass Extinctions-

Macroevolution is shaped as much by those who survive as those who did not [3,121]; it is shaped as much by extinction, as by innovation and speciation [3,122]. More than 99% of all the species that have ever lived are now extinct [3], and the losses have often been distinctly non-random [7,8]. The largest biotic crises eliminate entire branches of the tree of life [1], drive the decline of once diverse clades [123], and lead to the radiation of new species and ecosystems [13,124,125]. In the prolonged aftermath, ecosystem change across the globe exerts an evolutionary influence distinct from the extinction itself, with a timing characteristic of the earth system (i.e., earth system succession). As such, mass extinctions should not be considered as macroevolutionary point events, but rather as prolonged intervals of varying selection spanning the mass death and subsequent radiation of taxa.

The last line adds some perspective.... that although mass extinctions are defined by death, there is also evolution, often at elevated rates, going on during this interval. Groups of organisms which are able to adjust and evolve their way through environmental crises will find and occupy emptied ecologic niches and so to speak inherit a new earth to settle and diversify.

As you would have guessed from the articles I first dived into, my interest lies primarily on the broad patterns of evolution as revealed from the fossil record. But there are plenty more biology oriented articles to capture your interest. 

Current Biology

Do Marine Animal Lineages Evolve Toward Larger Body Size Over Time

aka Cope's Rule-

I like these big questions about the history of life and I am fascinated and very impressed when palaeontologists take up such questions. It is incredibly laborious and time consuming work, to go through archival data on fossils and often generate new data from museum specimens and older compilations describing fossil taxa.

A recent study in Science Magazine:

Cope’s rule in the evolution of marine animals - Noel A. Heim, Matthew L. Knope1, Ellen K. Schaal, Steve C. Wang, Jonathan L. Payne

Cope’s rule proposes that animal lineages evolve toward larger body size over time. To test this hypothesis across all marine animals, we compiled a data set of body sizes for 17,208 genera of marine animals spanning the past 542 million years. Mean biovolume across genera has increased by a factor of 150 since the Cambrian, whereas minimum biovolume has decreased by less than a factor of 10, and maximum biovolume has increased by more than a factor of 100,000. Neutral drift from a small initial value cannot explain this pattern. Instead, most of the size increase reflects differential diversification across classes, indicating that the pattern does not reflect a simple scaling-up of widespread and persistent selection for larger size within populations.

What that means is that the size increase is not due to a uniform increase across all animal groups. Rather, groups that were larger very early in animal evolution have diversified disproportionally more than smaller sized groups. Why should that happen? The authors suggest that there may be advantages to being larger, such as, ability to move faster, to capture larger prey and to burrow deeper for protection and exploiting additional food resources.

That would seem to make larger animals more resilient to background extinction and make larger sized lineages longer lived. But why would that make larger animals more speciose? i.e. why would larger sized animals species split into more new species than smaller sized animal species? ..because that is what is the claim, that throughout the history of animal evolution larger sized species gave rise to more new species than smaller sized ones (differential diversification). In fact, one could make arguments favoring higher rates of speciation in smaller sized organisms, such as, their ability to disperse over greater geographic area resulting in greater chances of populations getting reproductively isolated resulting in new species, their ability to survive better during environmental crises (survivor fauna after mass extinctions tend to be smaller bodied, mass extinctions seems to kill of larger bodied species disproportionately). If mass extinctions differentially kill off larger bodied species, then is the observed trend really a series of trends, each reset at the aftermath of the crises, resulting in small pioneer /survivor fauna evolving towards larger size. There could be a physical limit to how small one could become and the only direction for size to vary (either through drift or natural selection) would be towards a larger size. I am just speculating without even reading the paper, the authors do mention that drift from a small initial size does not explain their findings, but it would interesting to know what role mass extinctions might be playing in disruption or amplifying trends.

So although a trend is apparent, the answers are not all clear cut. It would also be interesting to group the trends according to life habits, i.e. planktonic versus benthic, sessile versus mobile  and see if any of these life styles particularly favors evolution towards larger size.

Eurekalert has a summary of the study. 

Post Permian Mass Extinction Size Reduction In Glossopteris Plant Lineages

Mass extinctions prune away branches of the tree of life. The late Permian-Triassic mass extinction event affected marine and terrestrial animal life severely, perhaps more than it affected terrestrial plant life. One common observation is that survivor species tend to be smaller bodied representatives of groups. This is known as the Lilliput effect.

A lot of attention has been paid to evolutionary trends in size during and post mass extinction in various animal groups. An interesting paper in Current Science (open access)  documents the Lilliput effect in one of the iconic terrestrial plant groups  of Late Permian -Early Triassic times; the seed fern Glossopteris.

From the paper:

Modern day high-resolution regional palaeoecological studies have proved the myth wrong, which stated that the mass extinction event had little macroecological or evolutionary consequence for terrestrial plants The early Triassic (Panchet Formation) in India witnessed more arid or semi-arid climate in comparison to the Permian. The early Triassic experienced greenhouse conditions with a warmer phase (Figure 6) due to global rise of temperature coupled with episodes of intense volcanism. The reduced size of the lamina is one of the strategies adapted by the plants during the adverse condition in early Triassic when there was indeed a shortage of essential nutrients in the soil, in addition to seasonal dry climate, irregular rainfall and widespread aridity. The leaves possessed cuticles with sunken stomata since the atmosphere during the early Triassic had higher levels of CO2 and lower O2levels (Figure 7). The Glossopteris flora of late Permian was adapted to temperate, cool and moist environments. The phenomenon of dwarfism as evidenced in Glossopteris has been observed only up to th e early Triassic, which is represented by the Panchet Formation. During the middle to late Triassic, typical Dicroidium flora associated with Lepidopteris takes over the preceding Glossopteris flora.

Records of plant fossils substantiate the evidence that numerous physiological, reproductive and behavioural traits enabled the smaller sized plant species to persist in extreme climatic conditions.

I didn't know there was a myth that mass extinctions have little macro-ecological or evolutionary consequences for terrestrial plants. Although, I do get the impression that studies of plant evolution get less attention in the media than animal evolution.

Meteorite Impact May Have Triggered Largest Pulse Of Deccan Basalt Eruptions

What caused the mass extinction 65 million years ago?

a) It was a meteorite impact and the resulting environmental crises.
b) No,  it was the Deccan basalt eruptions and the resulting environmental crises.
c) It was both, the meteorite impact and the eruptions.

The two mechanisms were distinct. One, a calamity from space and the other a gigantic eruption whose cause was from deep within the earth.

Now, it seems they may be more intimately linked. The meteorite impact may have triggered the largest pulse of the Deccan volcanic eruptions. Be careful here. The impact did not initiate Deccan volcanism. That was caused by India rifting away from Madagascar (88 mya) and Seychelles (66 mya). The rifting and an unusually hot mantle underneath resulted in copious amounts of melt being generated in the mantle which found its way to the surface via the great tensional cracks formed when continents separate. The impact though may have resulted in increasing the permeability of the mantle underneath thus making it possible for larger amounts of magma to make its way to the surface.

Abstract- 2014 GSA Annual Meeting, Vancouver:

New constraints on the timing of the Cretaceous-Paleogene (K-Pg) mass extinction and the Chicxulub impact, together with a particularly voluminous and apparently brief eruptive pulse toward the end of the “main-stage” eruptions of the Deccan continental flood basalt province, suggest that these three events may have occurred within less than about a hundred thousand years of each other. Partial melting induced by the Chicxulub event does not provide an energetically-plausible explanation for this coincidence, and both geochronologic and magnetic-polarity data show that Deccan volcanism was underway well before Chicxulub/K-Pg time. However, historical data document that eruptions from existing volcanic systems can be triggered by earthquakes. Seismic modeling of the ground motion due to the Chicxulub impact suggests that the impact could have generated seismic energy densities of order 0.1-1.0 J/m3 throughout the upper ~200 km of the Earth’s mantle, sufficient to trigger volcanic eruptions worldwide based upon comparison with historical examples. Triggering may have been caused by a transient increase in the effective permeability of the existing deep magmatic system beneath the Deccan province, or mantle plume “head.” It is therefore reasonable to hypothesize that the Chicxulub impact might have triggered the enormous Poladpur, Ambenali, and Mahabaleshwar (Wai sub-group) lava flows that account for >70% of the Deccan Traps main-stage eruptions. This hypothesis is consistent with independent stratigraphic, geochronologic, geochemical, and tectonic constraints, which combine to indicate that at approximately Chicxulub/K-Pg time a huge pulse of mantle plume-derived magma passed through the crust with little interaction, and erupted to form the most extensive and voluminous lava flows known on Earth. High-precision radioisotopic dating of the main-phase Deccan flood basalt formations may be able either to confirm or reject this hypothesis, which in turn might help determine whether this singular outburst within the Deccan Traps (and possibly volcanic eruptions worldwide) contributed significantly to the K-Pg extinction.

Elsewhere in a GSA special issue on volcanism, impacts and mass extinctions more evidence that Deccan volcanism had a significant impact on the fauna and flora.

Chronology of the volcanic episodes is improving and pointing to a scenario wherein the volcanism coincided (or was causally connected) to within a hundred thousand years of the Chicxulub impact and overlapped the stratigraphic horizon defined as the Cretaceous -Paleogene boundary. Looks like everyone is going to be partially right on this one.

Multiple causes for the mass extinction.

Late Cretaceous Dinosaurs In India- Diversity, Habitat And Extinction

I attended a talk on Friday by Dr. Dhananjay Mohabey former Deputy Director General of the Geological Survey of India at the Agarkar Research Institute in Pune. The subject was Late Cretaceous dinosaurs along with a history of dinosaur research in India and some anecdotes that come with a long interesting career in the field.

Here is the summary of the talk which was handed to us and which I am reproducing below:

Late Cretaceous Dinosaurs In India- Diversity, Habitat And Extinction - Dr. Dhananjay Mohabey

The first dinosaur from the Indian subcontinent was discovered in the year 1828 by Captain W. H. Sleeman of the Bengal Army from the Lameta Formation near Jabalpur. The bones collected were passed on to a series of learned amateur palaeontologists that included Spilsbury to James Princep (1832) to Thomas Oldham (1862) to Hugh Falconer who identified them as reptilian bones (1868). Richard Lydekker studied these bones along with the bones collected by H.B Medlicot (1877) from the overlying horizons at Jabalpur and established a type species Titanosarus indicus - the first dinosaur to be describe from India. During the period, a few more finds of dinosaurs were recorded that included collections of bones by W.T. Blanford from Lameta of Pisdura, later described as T. blanfordi and Laplatosaurus madagascariensis by Lydekker (1877). The majority of initial discoveries in India came from the Late Cretaceous sediments of the Central Province during the periods 1828-1879.  Between 1917 and 1933, Charles Matley carried out systematic excavations in two expeditions (1918-1919 and 1932-1933) in the Lameta sediments at Bara Simla and Chota Simla at Jabalpur and also the Lameta bed at Pisdura. He published his monumental work on systematics of Indian Late Cretaceous dinosaurs in 1933. Following years, 1960 onwards, witnessed excavations of thousands of dinosaur bones from the Late Cretaceous sediments, mostly Lameta of Jabalpur in Madhya Pradesh. Pisdura-Dongargaon in Maharshtra and Kheda in Gujarat. Discovery of dinosaur eggs in the Lameta sediments revived interest in research on India dinosaurs particularly with repect to their nesting behavior, habitat and environments. The discovery of plant bearing coprolites in the years 2000 provided a rare insight in to the dietary habit of the Indian sauropods. 

Of the vast collection of dinosaur bones since 1828, very few associated bones could be collected. Based mostly on the study by Charles Matley, at least twenty species of sauropod and theropod dinosaurs were described in India. However, current understanding based on the revised taxonomy recognises only two sauropod genera of Titanosauriforme dinosaurs viz Isisaurus colbeti and Jainosaurus septemtrionalis and four large-bodied abelisauridai theropods - Rajasaurus narmadensis, Rahiolisaurus gujaratensis, Indosuchus matleyi and Indosaurus raptorius and a small bodied theropod Laevishuchus indicus. 

Our study suggests that Late Cretaceous dinosaurs in India first appeared during the Maastrichtian in magnetochron C30n, ca. 500K years before the Cretaceous-Palaeogene boundary. Both titanosauriforme sauropods and abelisaurid theropods diversified and well established during C30n-C29r (Maastrichtian) with acme of their breeding and nesting. A change in biodiversity and abundance in dinosaur fauna from C30n to C29r is observed. The diversity and abundance of dinsoaurs of C30n -C29r declined rapidly with initiation of Deccan volcanism. Only a single or two titanosauriforme species with few individuals could survive the initial volcanic onslaught. The last stratigraphic level of the surviving dinosaurs in recorded in the C29r of Maastrichtian and 350k before the Cretaceous-Palaeogene boundary and they were all extinct before the K-Pg boundary. Continued work by the Geological Survey of India on the existing collection and new discoveries of dinosaur material from India, Pakistan and elsewhere in Gondwana have begun to resolve the composition and affinities of Indian dinosaurs.

Taking the Mesozoic as a whole the dinosaur record of India is quite poor. That does look like a preservation artifact. Jurassic rift basins of Western most India are mostly marine. Further in the Central and East parts, Early -Mid Mesozoic fluvial sedimentation in India took place in continental rift basins known as Gondwana basins since India at that time was a part of Gondwanaland.  These basins closed by around mid Jurassic except the Satpura and Godavari basins. There are some fluvial deposits of mid Jurassic and younger age from the Godavari basin especially (Kota Formation) that have yielded a few dinosaur fossils but the sample is too small to be able to say much about their diversity. Later in the Cretaceous E-W trending basin formed in West and Central India along the Narmada rift zone. Significant terrestrial sediments i..e sediments deposited in rivers and lakes accumulated in these basins.  These contain dinosaur remains preserved in the Maastrichtian Lameta Formation. So, much of the Jurassic to mid Cretaceous record is missing from Central and Eastern basins either due to erosion or non-deposition. There is only a tiny time slice of about 150 thousand years of the Maaschrictian with a record good enough to address in details questions about dinosaur diversity and evolution.

Overall it was an interesting talk. Some 400 crates of fossils of dinosaurs and other fossils were shipped out of India in the 1930' s by the British. The GSI is actively trying to trace if any of that collection still remains in British archives. There are archives in India too that have remain unstudied and so the picture of dinosaur diversity will certainly change as more archives are opened up and the fossils analyzed.

On the disheartening side we heard stories of dinosaur fossils being stolen from both the field site as well as museums. Field sites rich in dinosaur and other biota are being destroyed often to creeping urbanization around cities like Jabalpur and other towns in Gujarat. There were some really beautiful pictures of dinosaur nests with impressions of the clutch of eggs clearly seen. And one remarkable nest had preserved the remains of a snake, coiled and with jaws opened up in readiness to swallow an egg. And then there was a memorable picture of a large oblong dinosaur egg being used as a Shiva Lingam in a local temple near Dhar in Madhya Pradesh.

If the stratigraphic calibration that Dr. Mohabey presented is robust then it does seem clear that the Deccan volcanism wiped out dinosaurs in Central India at least. As Gerta Keller and colleagues have demonstrated, the volcanism had an impact on marine life in this part of the world as well and appears to have contributed to the mass extinction that took place 65 million years ago. This does not mean that the asteroid impact scenario is wrong. Just that 65 million years ago the earth experienced multiple cataclysmic events that reshaped ecology and life.

Multiple Events Caused K-Pg Mass Extinction?

Tobin et.al. have published work on a K-Pg section from Antarctica. From the latest issue of Palaeogeography, Palaeoclimatology, Palaeoecology:

Although abundant evidence now exists for a massive bolide impact coincident with the Cretaceous–Paleogene (K–Pg) mass extinction event (~ 65.5 Ma), the relative importance of this impact as an extinction mechanism is still the subject of debate. On Seymour Island, Antarctic Peninsula, the López de Bertodano Formation yields one of the most expanded K–Pg boundary sections known. Using a new chronology from magnetostratigraphy, and isotopic data from carbonate-secreting macrofauna, we present a high-resolution, high-latitude paleotemperature record spanning this time interval. We find two prominent warming events synchronous with the three main phases of Deccan Traps flood volcanism, and the onset of the second is contemporaneous with a local extinction that pre-dates the bolide impact. What has been termed the K–Pg extinction is potentially the sum of multiple, independent events, at least at high latitudes.

I don't have access to the paper so just some general thoughts.

Gerta Keller and colleagues have also challenged the theory that a single meteorite impact caused the mass extinction 65 mya. Except, their theory is that the bolide impact everyone is familiar with, which is the Chicxulub impact event in Yucatan Mexico took place about 300,000 thousand years before the peak Deccan volcanism. They base this on observations made on K-Pg boundary sections in Mexico and Texas and some work in India published in Journal of the Geological Society of India and Earth and Planetary Science Letters which support a link between Deccan volcanism and regional extinctions.

So, was the Chicxulub meteorite impact before the peak Deccan volcanism as Keller and colleagues say or was it 200,000- 300,000 thousand years after Deccan volcanism as this new paper by Tobin et. al. suggests. The famous iridium anomaly observed in many K-Pg boundary sections has been dated to after the main phase of Deccan volcanism indicating that there was a meteorite impact some time after peak volcanism.

Or were there two impacts with the Deccan volcanism sandwiched in between? It does seem that the single bullet theory for the K-Pg mass extinction is under serious threat.

Science Daily summary of Tobin et.al.' s paper. 

Would It Matter Where You Placed The Anthropocene?

I have been listening to an absolutely riveting series of talks hosted at Generation Anthropocene. This is a project dreamed up by young Stanford University researchers and it involves interviewing scientists working on the broad theme of human impact on ecology and environment and consequentially the question of whether there is now a case to be made for defining a new geologic epoch called the Anthropocene.

The answers to where one would place the Holocene-Anthropocene boundary usually converges to either the beginning of Agriculture or the Industrial Revolution because both involve big changes to various aspects of the earth's environment. While these two, especially the Industrial Revolution, came through as a strong candidate, the most thought provoking answer was given by ... well off course .. a geologist... Jon Payne.

He suggested we take a methodological approach and understand how geological boundaries are defined. There has to be a distinctively recognizable sedimentary section which contains evidence of a faunal break i.e. a change in the earth's biota marked by the extinction of some species or the appearance of some new species.  We subdivide geologic time by recognizing that there has been some change in the earth's biota. Looking way back in the earth's past, sedimentary sections containing these transitions cover time periods well beyond the human time scale i.e. the sedimentary layers may represent a time passage of thousands of years.

Sediments usually accumulate in layers quite slowly over time periods longer than the scale of historical events. So, taking a long view, choosing as the beginning of the Anthropocene either the Industrial Revolution or World War II  won't matter, since all these events will collapse as one instance of sediment deposition. A geologist looking back several million years from now won't be able to discriminate events that took place a few tens of years apart.

In my opinion that doesn't make the concept of Anthropocene a useless exercise. For example, the boundary between the Pleistocene and the Holocene is defined by the disappearance of two foraminifera species and is dated as 10,000 BP radiocarbon years. These events mark a passage of few tens of years. Similarly,  today there would be sediment accumulating in a lake or a quiet deep sea that contains very thin layers that mark a sudden rise in a geochemical indicator of the Industrial Revolution or some other human activity.

We may mark that as a beginning of Anthropocene today. These deposits may even get preserved into the geological future. It's just that millions of years from now absolute dating methods like carbon14 that may discriminate events on the order of few tens of years will be useless on these deposits and the error bar on the dates obtained for these rocks will be on the order of hundreds to thousands of years. As a physical entity, the boundary in deep future may be recognizable as a sedimentary layer marking the disappearance of a significant number of species, although correlating that boundary with a specific historical event may not be possible.

Other talks that I liked were by Jon Christensen who talked about the myth of the American Frontier, Rodolfo Dirzo who talked about tropical biodiversity and Doug Bird who spoke about the native Martu peoples of Australia and their history of landscape management.

One refreshing aspect of these talks are that the hosts are all graduate students. What an opportunity to have an extended chat with your intellectual mentors and find inspiration in their work.

And who knows... start thinking of a career in science journalism.

Highly highly recommended.