Evolution Through Punctuated Equilibrium: History Of An Idea

Palaeontologist Niles Eldredge explains how one of the most famous papers on paleontology and evolution came to be published: 

Steve was determined to be a part of Tom’s plan to do a GSA symposium and publish a book of essays on this new-fangled concept of “paleobiology.” Tom had a list of topics and was shopping around for speakers to be assigned to each one. When Steve saw the list, he told me that he had first wanted “morphology”—but that was already assigned to Dave Raup. So he opted instead for “phylogeny”—but that had been grabbed up by Mike Ghiselin. That left only “speciation,” the last of the evolutionarily imbued topics on Tom’s list, as yet unassigned. Steve called me up, explained the situation, and said he had settled for speciation—but could not think of anything much to say about it beyond the manuscript I had written and recently submitted to Evolution—there of course being no Paleobiology as yet. “The Allopatric Model and Phylogeny in Paleozoic Invertebrates”—a distinctly un-Gouldian, plodding, if accurate, title (Eldredge 1971). Without Ralph Gordon Johnson in the editorial chair of Evolution at that time, I doubt that that early paper would have been accepted. As it was, it was likely to have gone relatively unnoticed—had not Tom come along, Steve grabbing “Speciation”—and Steve asking if we could coauthor the paper along the basic lines of my first effort. He was stuck with “speciation,” and couldn’t think of anything much to say beyond what I had said in the Allopatric Model manuscript.

This passage is from an article by Niles Eldredge titled Reflections On Punctuated Equilibria, published in a recent issue of Paleobiology. The 1972 paper he refers to, coauthored with Stephen Jay Gould, was, Punctuated equilibria: an alternative to phyletic gradualism. It marked the beginnings of a long debate on how to interpret the patterns of morphological change observed in the fossil record. Do species remain in stasis, showing little morphological change through much of their existence as Eldredge and Gould argued? Do periods of rapid morphological change coincide with the origin of new species (speciation)? Supporters hailed it as a revolutionary work. Critics called it 'evolution by jerks', a jibe aimed not just at the patterns of change.

Dr. Eldredge provides a very insightful look at the history of this idea including some fascinating snippets on Darwin's thinking about divergence and species origins. For Darwin, change accumulates incrementally over long passages of time. Divergence via natural selection can give rise to descendant varieties even without geographic isolation of a population. Later thinking has given more importance to exogenous factors like climate change in causing habitat fragmentation and reproductive isolation. Populations gets geographically isolated first, and then diverge from the ancestral species either through natural selection or random genetic drift. Eldredge and Gould applied this idea to the fossil record and emphasized that the sudden appearance of new fossil species is a manifestation of long periods of stability interrupted by episodes of isolation and geologically rapid shifts in morphology (allopatric speciation).

There is a lot to take in and think about the long term patterns of change preserved in the fossil record. But it is enriching reading. The article is open access.

Patterns Of Angiosperms And Insect Evolution

Charles Darwin famously called it an 'ábominable mystery'. He was referring to the sudden appearance and diversification of flowering plants in the Cretaceous fossil record. He noticed that these early fossils resembled modern flowering plants. 'Primitive' or ancestral stages were missing. Today, biologists categorize these as crown and stem representatives of a group. 

The first fossil evidence of flowering plants is from 140-130 million year old sediments. These are early types of pollen grains with one aperture (uniaperturate). Triaperturate pollen is found in slightly younger 125 million year old rocks. Towards the end of the early Cretaceous, by around 100 million years ago, flowers, leaves, and other organs appear from several continents representing all the major groups of angiosperms.

The picture below is of an early Cretaceous (~100 million year old) flowering plant from the lotus family. The location is northeast Brazil. There is a remarkable preservation of the whole plant, with connected roots, rhizome, leaves, and aggregate fruit. 

Source: William Vieira Gobo et.al. Nature Scientific Reports 2023- A new remarkable Early Cretaceous nelumbonaceous fossil bridges the gap between herbaceous aquatic and woody protealeans.

Taking a long view of their evolutionary pattern, angiosperm diversification is structured in three phases. The first phase was a steady expansion through early to late Cretaceous. There was more rapid diversification in late Cretaceous by around 70 million years ago. Enumeration of floral species through the Cretaceous indicate that angiosperms made up about 5% of species in early Cretaceous, increasing to 80% by Maastrichtian times (late Cretaceous). Despite this increase in species numbers, in terms of biomass, angiosperms were still a small component of Cretaceous floras. Their domination of floral communities, including the origin of modern wet tropical forests, began in the Paleogene (65-24 million years ago) after the end Cretaceous mass extinction. Michael J. Benton, Peter Wilf, and Herve Sauquet have provided a good overview in New Phytologist of this pivotal phase of ecosystem change.

These evolutionary changes did not occur in isolation. Throughout the Cretaceous, significant changes were occurring to terrestrial ecosystems, with the origination of many plant and animal groups. This extended phase of ecosystem reorganization is known as the Cretaceous Terrestrial Revolution. Angiosperm diversification is thought to have played a key role in this transformation of land biodiversity, so much so, that the phase from about 100 million years to 50 million years ago is known as the Angiosperm Terrestrial Revolution.

The Cretaceous -Paleogene mass extinction hit angiosperms hard, as well as altering the trajectory of their evolution. For example, there was a 40% loss of diversity of flowering plants in Colombia following the mass extinction. But certain attributes of angiosperms, such as their partnerships with other organisms, their ability to efficiently capture energy and enhance photosynthetic rates, and an underlying genetic propensity to speciate, resulted in them expanding rapidly in the post extinction landscape. Angiosperm evolution opened up opportunities for a variety of land creatures including insects, spiders, lizards, birds, and mammals,  eventually driving up terrestrial biodiversity to 10 times more as marine biodiversity.

Paleobiologists are interested in understanding the interaction and impact angiosperm diversification could have had on other groups of plants and animals. Of particular interest is the diversification of insects in the Cretaceous and Paleogene.

Modern insect lineages began diversifying by 245 million years ago, long before angiosperms evolved. Gymnosperm and insect communities preserved in amber and sediments show that insects had an intricate relationship with host gymnosperms like cycads, conifers and ginkgoaleans.  Insect pollination of gymnosperms predated the origin of angiosperms by at least 100 million years and their fossil record show phases of diversification even when angiosperms were rare. 

Did angiosperm evolution also drive a rise in insect diversity? Pollinator insects particularly would seem to benefit from an abundance in flowering plants, and if so, what co-evolutionary patterns are apparent from the fossil record?

David Perise and Fabien Condamine have tackled this question in a new study in Nature Communications. I will share this beautifully compiled infographic from the paper that conveys so clearly the patterns of angiosperm and insect diversification through the Cretaceous and Cenozoic.

Digging into published databases, the researchers compiled data on the origination and extinction times of angiosperm and insect families. They then statistically analyzed whether angiosperm and insect origination and extinction times, and pulses of their diversification coincide. Their analysis showed that angiosperms seemed to have played a dual role in insect evolution. They mitigated insect extinction through the Cretaceous and spurred on the origination of new insect groups in the Cenozoic. Besides a broad analysis of insects, they also found that pollinator insects like bees and long proboscid butterflies show a pronounced diversification alongside angiosperm lineages. 

The success of angiosperms in the late Cretaceous and Cenozoic coincided with the decline in gymnosperms. Intrinsic mechanisms of genomic rearrangements in angiosperms resulted in repeated evolution of novel traits and specializations. They competitively displaced gymnosperms. The impact on gymnosperm dependent insects was variable. Generalist insect pollinators such as several beetle lineages transitioned to angiosperms. Much of the co-diversification of angiosperm and insects can be explained by this shift of gymnosperm pollinators to angiosperm hosts.  Gymnosperm specialized insect groups did not fare that well. For example, gymnosperms like Cheirolepidiaceae and Bennettitales went extinct by the latest Cretaceous. This was followed by the extinction of insect groups that were dependent on these plants such as some specialized long-proboscid flies, scorpionflies and lacewings.

Insect diversification did not depend only on angiosperms. Analysis also shows that warmer climate phases negatively impacted insect diversity and coincided with higher insect extinction rates. There seems also to be a relationship with other plant types. Spore plant and gymnosperm diversity had a positive impact on origination rates of insects. Ecosystem relationships and dependencies are multifarious and complex as this analysis between angiosperm and insect co-evolution shows.

Darwin's anxiety over flowering plants reflected his insistence that evolution is gradual. Nature does not make leaps, he stressed. He explained abruptness in the fossil record by invoking missing strata due to non deposition and erosion. Regarding flowering plants, he suggested that fossils were perhaps preceded by a period of cryptic evolution of that lineage that took place in a remote area or a lost continent, although he conceded that this was a poor explanation. However, this latter view, that a substantial lag time or a long fuse precedes the bang, continues to resonate among many biologists. Molecular methods that compares accumulated genetic difference to calculate the time of divergence of groups indicate a fairly long gap between the genetic branching of lineages and their first fossil appearance. 

Most familiar is the example of the origin of animals. Molecular data indicate that animals originated by 750 million years ago, yet unequivocal animal fossils appear by 570 million years ago, close to 200 million years later. Similarly, some molecular estimates put angiosperm origins to pre-Cretaceous times, stretching back 240-200 million years ago to Triassic-Jurassic, a good 100 million to 60 million years before first appearance of fossils.

This idea of a phylogenetic fuse has been recently criticized. Published in Systematic Biology, Graham E. Budd and Richard P. Mann have undertaken a critical examination of molecular clock methods. Their analysis indicate that popular methods used to assign probabilities to maximum age of lineages are biased against rapid lineage radiations being true evolutionary events. In their view, the mismatch between molecular dates of lineage origin and the timing of the first appearance of their fossils is an artifact. They point out that the coincident appearance of fossils from widespread localities in a particular sequence and across different modes of preservation faithfully records evolution. The time gap between the origin and later diversification of lineages is not that deep.

The 'abominable mystery' of the sudden appearance of fossil groups may in fact be a real biological motif in earth history, signalling the rapid radiation of lineages filling ecologic spaces following an environmental crises and evolutionary innovation.

Agglutinated Foraminifera From The Deep Sea

No matter how many David Attenborough specials you may watch, nature always throws more surprises at you.

Foraminifera are protists that build a skeleton or a test made up of calcium carbonate. The calcium carbonate is precipitated out of sea water. There is a sub-group of foraminifera which construct a shell not by capturing the calcium carbonate via chemical precipitation, but by assembling sedimentary grains and then cementing them together like a brick and mortar structure. This group of forams are known as agglutinated foraminifers. 

All this is well known. Many different species of forams use a variety of grains as bricks. They are mostly different shell fragments, but even mineral grains like ilmenite (iron titanium oxide) , rutile (titanium dioxide), or garnet are used. 

Now there is a new report of a deep sea living agglutinated foraminifera which constructs a tube made up of  planktonic foraminifera shell fragments of a single species. 

Read that again. A benthic (bottom living) foram shell made up of bits of another foram which lived floating in the upper water column!  

These shells are selected to the exclusion of all other types of available sedimentary grains. The specimens were recovered in a core drilled off shore northwest Australia by the International Ocean Discovery Program. The paper by Paul N. Pearson and IODP 363 Shipboard Scientific Party is published in the Journal of Micropalaeontology.

Here is the entire abstract. It is mind boggling. 

Agglutinated foraminifera are marine protists that show apparently complex behaviour in constructing their shells, involving selecting suitable sedimentary grains from their environment, manipulating them in three dimensions, and cementing them precisely into position. Here we illustrate a striking and previously undescribed example of complex organisation in fragments of a tube-like foraminifer (questionably assigned to Rhabdammina) from 1466 m water depth on the northwest Australian margin. The tube is constructed from well-cemented siliciclastic grains which form a matrix into which hundreds of planktonic foraminifer shells are regularly spaced in apparently helical bands. These shells are of a single species, Turborotalita clarkei, which has been selected to the exclusion of all other bioclasts. The majority of shells are set horizontally in the matrix with the umbilical side upward. This mode of construction, as is the case with other agglutinated tests, seems to require either an extraordinarily selective trial-and-error process at the site of cementation or an active sensory and decision-making system within the cell.

 

The photographs from the paper shows the tube of the agglutinated foraminifera made up of planktonic foraminifera shells of a single species.

Charles Darwin knew of agglutinated foraminifera from reports he had read and was astonished....“almost the most wonderful fact I ever heard of. One cannot believe that they have mental power enough to do so, and how any structure or kind of viscidity can lead to this result passes all understanding”.  This was a letter he wrote to W.B Carpenter who had described them in 1873.

I'll leave you to ponder upon this most exquisite of natural wonders. The paper is open access: A deep-sea agglutinated foraminifer tube constructed with planktonic foraminifer shells of a single species.

Holiday Readings: Ancient Amputations, First Americans, Fossil Molluscs

Wishing my readers a very Happy New Year! I hope these readings will be to your liking.

1) Can ancient amputations tell us about the care systems of our ancestors? Paleoanthropologist John Hawks surveys the fossil record of ancient humans for signs of severed limbs due to trauma or disease. He also presents cases of limb loss in other primates and offers a perspective on what all this can tell us about past social systems. 

"Both humans and nonhuman primates show us that survival and life after extreme injuries happen under varied circumstances. Bioarchaeologists tend to highlight severe injuries, which stand out from the more subtle patterns of osteological signs of disease that can be understood only across large samples of skeletons. But such individual stories rarely yield unambiguous interpretations".

2) Finding the First Americans. Anthropologist Jennifer Raff brings together often conflicting genetic and archaeological data on this ever vexing and complicated question of how the Americas were populated. 

3) Finding Molluscs. This podcast (with transcript) is part of an excellent continuing series of earth science and paleontology podcasts by Mongabay India. In this episode, host Sahana Ghosh talks with paleoecologist Devapriya Chattopadhyay on her research on fossil molluscs. Dr. Chattopadhyay uses these creatures to track ancient environmental conditions and ecology. She also speaks on the urgent need for India to create a national fossil repository and museum which will help preserve our deep history for future generations.

Links: Tonga Volcano, Old Carbon, Indian Palaeontology

 Sharing these readings.

1) My first impression of the massive Volcanic eruption in Tonga was the mushrooming ash plume seen in a satellite imagery. Over the days more sensors have captured additional information about this event. When it is safe, geologists will travel to the site to sample the volcanic debris and subject it to detailed textural and geochemical analysis to piece together the journey of magma from its source to its explosive entry on the surface.

Scientific American has a good summary - Why the Tonga Eruption Was So Violent, And What to expect next. 

2) Debate is an integral part of scientific progress. One common platform to engage in a critique and discussion with your colleagues is the Comment and Reply section in scientific journals. You can submit a note explaining the issues you have about a paper, and it is published along with the author’s response. I am across a good example of this in the journal Science. The topic was the recent announcement of roughly 22,000 year old human footprints from Lake Otero, New Mexico. These dates suggest that humans  were present in North America during the Last Glacial Maximum, a few thousand years earlier than what other data has indicated.

The debate revolves around the accuracy of dating these footprints. Of particular interest here is the problems one can encounter with carbon dating a sample. Living beings have amounts of the radioactive isotope carbon 14 in them in equilibrium with the atmosphere. After death, the amount of this isotope in organic tissue starts decreasing due to natural radioactive decay of carbon 14. Knowing the decay rate and measuring its proportion in the organic material gives us estimates of how old the sample is. But what if the source of carbon is old? For example,  it is from very old groundwater or from the bottom of a lake which is not exchanging gases with the atmosphere? The carbon 14 values in such a réservoir will be very low due to ongoing decay and no replenishment of newly formed carbon 14 from the atmosphere. If organisms consume carbon from such a reservoir (which contains carbon 12 and carbon 13 too) and then are sampled , they will be estimated to be older than they really are. 

The Comment and Reply focuses on this problematic aspect of recognising and correcting for the ‘reservoir age’ of carbon 14. Of pointed importance too is the context and location of collected samples. 

A very informative debate on the nuances of sampling and assigning ages. 

Comment- Évidence of humans in North America during the last glacial maximum. 

Reply- Evidence of humans in North America during the last glacial maximum. 

3) I have posted about this topic before. Thé challenges and triumphs of Indian palaeontology very well described by Kamala Thiagarajan in this recent article. 

Why India’s Fossil Wealth Has Remained Hidden. 

Previously, Sreelatha Menon had written about the lack of importance palaeontology is accorded in the earth sciences and the devastation this neglect is inflicting to palaeontology education, awareness, and research. Her essay is worth reading too; What do you do when palaeontology is itself endangered in India?

Reverend William Buckland's Pie Crust

I came across this delightful passage in Elsa Panciroli's book, Beasts Before Us: The Untold Story of Mammal Origins and Evolution.

Naturalists exploring southern Scotland in the early nineteenth century came across some intriguing looking footprints impressed in red sandstone in a quarry at Corncockle Muir. The geologist Reverend Henry Duncan described these footprints and send some casts to the Reverend William Buckland who was making a name for himself in the emerging field of geology. The thinking was that these tracks were most likely made by crocodiles and turtles. Reverend Buckland came up with a clever way to test this idea. 

From Elsa Panciroli's book-

'Ist I made a crocodile walk over soft pye-crust [sic]' he wrote in a letter to Duncan, 'and took impressions of his feet...[second] I made tortoises, of three distinct species, travel over pye-crust, and wet sand and soft-clay...' Buckland's wife supplied the pie-crust and Buckland supplied the tortoises. Where the crocodiles came from is unclear, but as Buckland had a penchant for eating them he probably also had access to  live ones. The results : the marks matched the tortoises. He concluded, 'though I cannot identify them with any of the living species.... the form of the footsteps of a modern tortoise corresponds sufficiently well.... so I conceive your wild tortoises of the red sandstone age would move with more activity and speed... than my dull torpid prisoners.'

It was understood much later that these footprints in sand from Permian times (299 -252 million years ago) were made by early Therapsids from which arose the mammalian lineage. 

I am really enjoying this book! 

India Fossil Outcrops, Horse Domestication, Mars Landscapes

 From the past few days:

1) India is rapidly losing fossil rich outcrops to urbanization, expanding agriculture, mining, and unregulated fossil collection.

On International Fossil Day, October 23, 2021, the Paleontological Society of India, Pune Mumbai Student Chapter, organized a very informative online symposium on this topic. I have linked to part of the talks held that day. Paleontologist Dr. Rajani Panchang was the moderator. Several young researchers describe their field work in Kutch, Tamil Nadu, and Spiti Valley. Over the past several years, changes in land use and unchecked fossil removal has resulted in outcrop degradation and impoverishment.

.Video Permanent Link - India Fossil Outcrops .

Even though the Geological Survey of India and some local agencies have identified locations of geological importance, at present India does not have a law for the preservation of geoheritage sites. D.M. Banerjee writes about the struggle to get the Indian government to take up this issue seriously in his article Fate of Indian Geoheritage and Geopark Bill, published in the July 2021 issue of Current Science.

2) The origin of domestic horses has been a tough case to crack. It was long held using archeological evidence that horses were domesticated by the Botai Culture in Central Asia around 3500 B.C. But ancient DNA studies indicated that these early domesticated lines are not the ancestors of the modern domestic horse. Instead, the origin of the modern domestic horses have been tracked to the Volga-Don region in the Western Eurasian steppes between 2500 and 2000 B.C. The abstract of the paper is worth reading through- 

Domestication of horses fundamentally transformed long-range mobility and warfare. However, modern domesticated breeds do not descend from the earliest domestic horse lineage associated with archaeological evidence of bridling, milking and corralling at Botai, Central Asia around 3500 bc. Other longstanding candidate regions for horse domestication, such as Iberia and Anatolia, have also recently been challenged. Thus, the genetic, geographic and temporal origins of modern domestic horses have remained unknown. Here we pinpoint the Western Eurasian steppes, especially the lower Volga-Don region, as the homeland of modern domestic horses. Furthermore, we map the population changes accompanying domestication from 273 ancient horse genomes. This reveals that modern domestic horses ultimately replaced almost all other local populations as they expanded rapidly across Eurasia from about 2000 bc, synchronously with equestrian material culture, including Sintashta spoke-wheeled chariots. We find that equestrianism involved strong selection for critical locomotor and behavioural adaptations at the GSDMC and ZFPM1 genes. Our results reject the commonly held association between horseback riding and the massive expansion of Yamnaya steppe pastoralists into Europe around 3000 bc driving the spread of Indo-European languages. This contrasts with the scenario in Asia where Indo-Iranian languages, chariots and horses spread together, following the early second millennium bc Sintashta culture.

The paper is open access. And there is an easier to understand article in Nature as well. 

3) The remarkable range of technologies brought to bear on understanding the geology of Mars is giving some spectacular payoffs. Two studies caught my eye:

a) Mars' surface shaped by fast and furious floods from overflowing craters: Lake breach floods produced fast flowing streams that cut deep drainage valleys, reshaping the Mars landscape. Catastrophism has played a large role in the history of Martian surface evolution.

b) The Perseverance Rover rocks on!! The stunning images it has taken of rock outcrops on Mars is enabling geologists to reconstruct details of ancient sedimentary environments. N. Mangold and colleagues describe a delta lake system and flood deposits at Jezero Crater. 

Take a look at the details available to geologists for interpreting sedimentary processes and the rock history.

Source: N. Mangold et.al. 2021:  Perseverance rover reveals an ancient delta-lake system and flood deposits at Jezero crater, Mars.

On Mars, large crater lakes were sites of sediment deposition. Rivers meeting such craters dumped their sediment on the crater floor in lobes that expanded lakewards forming a delta. The architecture of the sedimentary layers within this delta environment has been vividly captured and described in this study. In the image, the bottomset strata are fine grained sediment deposited in waters ahead of the delta. The foreset strata represent deposition on the inclined growing delta front. And the foreset strata are deposits of rivers associated with the delta. The paper is quite detailed and a treat for sedimentologists. But the images can be enjoyed by all. Open Access too!  

Palaeontology Musings

It struck me a couple of days back that the field of paleontology and evolution has come up with some very evocative terms to describe phenomenon and name theories.

Take for instance the Red Queen Hypothesis. The term was coined by University of Chicago evolutionary biologist Leigh Van Valen in 1973. It is an explanation for his observations on patterns of extinction which came to be known as Van Valen's Law of Constant Extinction (itself a cool name). Van Valen did a broad survey of genus and family level extinction patterns of several different marine invertebrate groups and found out that the probability that a group could go extinct was independent of their age. The expectation might be that longer lived groups may have evolved more efficient adaptations and thus the likelihood that they could go extinct might decrease for older groups. 

Van Valen's finding was counterintuitive. A small clarification. The finding here is not that the rate of extinction is constant over time. It is not, obviously we just have to look at times of mass extinctions when rates of extinction increase enormously. What Van Valen found was that longer lived taxa were no better at avoiding extinction than newly appeared groups.

Why?...Enter the Red Queen. The inspiration for the name comes from Lewis Carroll's Through The Looking Glass. In it the Red Queen says to Alice; "Now, here, you see, it takes all the running you can do, to keep in the same place".

Van Valen reasoned that organisms are in a perpetual competition over resources. If one evolves a more efficient way of extracting resources, a cohabiting species will do so too. A sort of a metaphorical "arms race" results leaving both species at the same level of efficiency relative to each other. Besides, since evolution is changes in response to immediate challenges, already acquired adaptations cannot guarantee a fit to future environmental change. Longer existing taxa thus are likely to perish just as easily as newly emerged ones.

The Red Queen invited a lot of interest from evolutionary biologists and ecologists and has spawned rich directions of research since.

The other name I stumbled upon recently is Dead Clade Walking. This too concerns patterns of extinction and recovery. Paleontologist David Jablonski, also from the Chicago school of thought, in 2002, invented the term to describe his finding that many marine groups experience sudden drops in diversity spanning mass extinctions. Many don't go extinct but never quite recover fully either. It is not well understood why certain groups survive such global extinction events but then cannot rediversify. Some further work has shown that such drops in diversity without recovery need not be associated with mass extinctions but occur even during the background extinction that is going on. Understanding these patterns is another active area of research in paleontology. 

The type of research I've described readily invites a comment on how the field of paleontology has itself evolved. Besides the two scientists I mentioned, I will add David Raup, Jack Sepkoski, Elisabeth Vrba, Niles Eldridge and Stephen Jay Gould to name a few more. Beginning in the early 1970's these paleontologists collated large data sets of fossil groups, combing through literature and museum archives. They devised more expansive sampling strategies and subjected morphological measurements and life history attributes to rigorous statistical analysis. They used the emerging patterns to reconstruct broad histories of diversification and extinction and to test various evolutionary principles.  Their work reinvigorated paleontology from what was thought of as a descriptive field to one that began making significant contributions to evolutionary theory. This big picture approach inspired biologist John Maynard Smith to acknowledge that paleontology is ready to join the "high table of evolutionary theory". 

Do you know of a cool name for an earth science phenomenon or theory? Drop in a comment.

Articles: Trace Fossils, Supercontinents, Harappan Hydrology

 Some interesting geology rich readings from the past few weeks:

1) Ichnology is a branch of palaeontology that studies the traces made by organisms in soft sediment. These could be tracks and trails as animals move around on a substrate, or burrows constructed as escape structures or as dwellings, or bite marks on shells and bones. All these are indicative of behavior, which otherwise would be hard to discern from just the fossilized remains of body parts. Science writer Jeanne Timmons has written this lovely article on Ichnofossils and what they tell us about past ecology and animal behavior.

Trace fossils, the most inconspicuous bite-sized window into ancient worlds.

2) The earth has seen over its long geological history episodes of continents coming together to form a supercontinent, then breaking up and drifting apart forwhat seems an eternity, but eventually coalescing to form another giant landmass. When did this supercontinent cycle begin on earth. What are the forces that initiated and subsequently has maintained this mode of surface reconfiguration, and what are its consequences on tectonics, and the physical and chemical evolution of earth. A great review article by Ross N. Mitchell and colleagues.

The Supercontinent Cycle.

3) The rivers that sustained the Bronze Age Harappan Civilization have been the subject of lively research in recent years. Ajit Singh and colleagues have worked on the Markanda river catchment in the Sub-Himalaya dun region. Markanda joins the Ghaggar-Hakra river flowing through present day Harayana, Punjab and Rajasthan. They find that during the Mature Harappan Period (2600 B.C. to 1900 B.C.), large floods in the Himalaya foothill rivers sustained flow in downstream reaches, making  agricultural viable, even as northwestern parts of India experienced a reduction in summer monsoon strength.

Larger floods of Himalayan foothill rivers sustained flows in the Ghaggar–Hakra channel during Harappan age (behind paywall).

Cretaceous Cauvery Basin Stratigraphy

In the second year of my bachelor's degree course, a few of us friends had gone fossil hunting near the town of Ariyalur in Tamil Nadu. Ariyalur sits on Cretaceous age sediments deposited in a basin that formed as India broke away from Antarctica and Australia. The basin got filled slowly over time, by sediments brought in by rivers, as well as in the marine realm, as the sea episodically kept encroaching on to the continent interior. 

Before leaving for the trip we had approached Dr. V.D.Borkar, a research scientist with the Agarkar Research Institute in Pune, to help us plan the fossil collection. He very generously lent us maps and gave us a detailed idea of the villages to travel to and nearby field locations. 

All in all it was a fun field trip. We roamed the countryside around Ariyalur and collected plenty of fossils. In our collection were plant impressions on clay, ammonoids, belemnites, echinoids, coral fragments, and a variety of bivalves. The non geology highlight was the absolutely delicious vegetarian thali meal served in the canteen next to the town bus station! We used to gorge on it everyday, twice a day.

At that time I didn't have a good understanding of stratigraphy and even sedimentary geology. As it happened I did not grasp the broader implications of the distribution of particular fossils and the arrangement of strata that I was observing in the field. 

Its never too late to update yourself! The past month I have been reading three papers on the Cretaceous outcrops around Ariyalur which focus on basin development and stratigraphic evolution. In simpler language, stratigraphic evolution means the patterns by which basins fill up. A closer look reveals that basins are not made up of uniform continuous layers (layer cake stratigraphy) of one sediment type succeeding another, but rather there is lateral interfingering of different types of sediment, controlled by sediment distribution patterns, water energy, and basin topography.  

There are exogenous influences too. A long term drop in sea level will result in a particular arrangement of strata known as 'progradation', formed for example when deltas build out in to the sea. This may be followed by a long term sea level rise forming an overlay of a different sedimentary pattern, called  'retrogradation'. In this case as the sea encroaches on land, coarser sediments that are deposited closer to the shore get buried under deeper water fine grained sediments  A sedimentary section from base to the top (older to younger) reveals in its sediment characteristics these changing environmental conditions.

Documenting these patterns in not as esoteric an exercise as it may seem to some. Such analysis is very keenly taken up during petroleum exploration.  One may find during outcrop mapping that coarse sand deposits (potential petroleum reservoirs) occur at repeated intervals and are juxtaposed against finer organic rich mud rocks (potential hydrocarbon source rocks). This then may become a guide for optimizing detailed exploration strategy in areas of the basin where strata are buried and can't be observed directly. Just such a situation occurs in the Cretaceous Cauvery Basin. The sediments around Ariyalur is one of the main accessible outcrops. But further to the east, these sedimentary layers continue under the sea bed of the Bay of Bengal. A well documented and well understood outcrop provides an analogue for the unseen portions of the basin.

These three papers clarified to me much of the Cretaceous stratigraphy that I had failed to understand in my college days.

Here are the links:

1) Cretaceous tectonostratigraphy and the development of the Cauvery Basin, southeast India: Matthew P. Watkinson, Malcolm B. Hart and Archana Joshi

A broad study of basin formation by continental rifting and the resulting patterns of basin infilling interpreted in the context of tectonic events, major sea level fluctuation and depositional episodes.

2) Sea level changes in the upper Aptian-lower/middle(?) Turonian sequence of Cauvery Basin, India  An ichnological perspective: Amruta R. Paranjape, Kantimati G. Kulkarni, Anand S. Kale.

Ichnology is the study of trace fossils. These are tracks, trails and burrows made by the movement of  creatures living on the basin floor. Traces differ depending upon the nature of sediment substrate and environmental conditions and can be used along with other sedimentological and fossil data to interpret patterns of sea level change.,

3) Siliciclastic-carbonate mixing modes in the river-mouth bar palaeogeography of the Upper Cretaceous Garudamangalam Sandstone (Ariyalur, India): Subir Sarkar, Nivedita Chakraborty, Anudeb Mandal, Santanu Banerjee, Pradip K. Bose.

The Garudamangalam Sandstone formed during a sea level highstand i.e. at the peak of a sea level change cycle, when the rate of sea level rise finally slows down and stops. Sediment transported by east flowing rivers began building a delta. The exposed Garudamangalam Sandstone is part of this delta complex. This is a very nice analysis of sedimentary processes and products. The various subenvironments in this delta complex are identified and the chemical changes in the sediment after their deposition are documented using various techniques like chemical staining and cathodoluminescence. I really enjoyed reading this one!

On a personal note, the Covid catastrophe unfolding in India is making reading and writing difficult. However, I did find that a few hours of geology time that I am managing to hold on to brings me some comfort. 

Fossil Dickinsonia in Bhimbetka Sandstones: Nature India Article

 My short piece published in Nature India on the surprising report of Ediacaran age fossil Dickinsonia in the Bhimbetka caves near Bhopal, Central India, and its geological and biological significance.

an excerpt:

"The biological affinity of Dickinsonia is controversial. Most scientists tend to accept it as an early animal. Some like Gregory Retallack, co-discoverer of this fossil, think of it to be a large algae or lichen. He argues that the mainstream view that Dickinsonia was a marine animal is based on weak evidence, while his own detailed work shows that Dickinsonia was a land creature, forming biogenic crusts on soils. Interestingly, the Bhimbetka rocks were deposited in a mostly terrestrial setting, more in alignment with Retallack's interpretation. Importantly for geology, its restricted time span, being found only in rocks between 555-550 million years old, makes it a diagnostic age indicator. So far, no animal fossils have been found in the Vindhyan rocks. This finding may inspire geologists to start searching contemporaneous Indian basins afresh for such subtle clues".

Fossil from dawn of animal life found in India’s famous caves.

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.

Interview: Palaeontologist Prof. Ashok Sahni

This is a rich conversation between paleontologist Prof. Ashok Sahni and Dr. Devapriya Chattopadhaya of the Indian Institute of Science Education and Research, Pune.

Email subscribers who can't see the embedded video can watch it at this link: Interview- Prof. Ashok Sahni.

It was so refreshing to hear Prof. Sahni talk candidly about the state of paleontology research in India,  preserving fossil sites, motivating students, the need for Indian scientists to proactively engage with the public about their work, and the importance of building bridges between research and societal needs.

Prof. Sahni comes from an illustrious line of scientists. His uncle was the paleobotanist Dr. Birbal Sahni after whom the Birbal Sahni Institute of Palaeosciences, Lucknow is named. And his father was also a palaeontologist. His mother was not keen on him taking up geology, admonishing him that there were already too many rocks in the house. However, he persisted. 

Definitely worth your time.

Infographic: Reefs Through Geologic History

Academia.edu is always sending me links to papers that I can download. It is a lure to get me to upgrade my membership. I am grateful for the riches that I have discovered on their site.

Last week they sent me a gem-  Palaeoecology: Past, Present and Future by David J. Bottjer. It is a full length book on the field of paleoecology summarizing its scope, its methods, and giving an overview of different ecological domains such as the shallow marine, the pelagic (deep marine), and the terrestrial environments and how they have changed through time. Dr. Bottjer is Professor of Earth Sciences, Biological Sciences and Environmental Studies with the University of Southern California Dornsife, Los Angeles.

One striking aspect of this book is the really informative diagrams that Prof. Bottjer has collated from various papers and books. They really add a lot of value, especially if you want to avoid reading every line of the text!

Here is one which shows the changing composition of reefs through time.

Source: James, N.P. and Wood, R. 2010. Reefs. In James, N.P.,  Dalrymple, R.W., Facies Models 4. Geological Association of Canada, pp. 421–447

They were not always built by corals.

I am going to learn a lot from this book.

Punctuated Equilibrium Is About Small Subtle Changes

The theory of punctuated equilibrium (PE) put forth by paleontologists Niles Eldredge and Stephen Jay Gould proposes that morphological evolution speeds up during lineage splitting events when new species form. In practice, a paleontologist sampling fossil species in a sedimentary section will find that a species over its lifetime does not show any trend in morphological changes. That species eventually goes extinct at a particular interval. Fossils of the inferred descendant species appear abruptly in the sedimentary bed above. A fossil population showing a mix of traits diagnostic of both species is not generally seen. 

Eldredge and Gould argued that this pattern of change observed in fossil species shows that the history of a species is really characterized by long periods of no change, eventually interrupted by rapid evolution to a new species.

A reassessment of one textbook example in bryozoans suggests no evidence of such a punctuated mode. 

A write up in Phys.org describes the new work. 

Revisiting a Landmark Study System: No Evidence for a Punctuated Mode of Evolution in Metrarabdotos - Kjetil Lysne Voje, Emanuela Di Martino, and Arthur Porto

I am not going into the work itself. There are well documented examples of the punctuated equilibrium mode of evolution while others as in this case may see revision from time to time. 

Instead, I wanted to comment on one of my pet peeves about this topic which is the common use of the phrase "major evolutionary change" to describe the actual evolutionary changes.

This idea which never seems to go way that PE involves big or major changes has caused a lot of misunderstanding about the theory.  As the authors of PE took pains to point out, the amount of morphological difference between ancestor and descendant species is subtle, often taking excruciating examination to recognize. 

Here is Niles Eldredge describing his work on Devonian trilobites.

I measured some 50 different lengths and widths- length of the head, distance between the eyes, height of the eyes, length of the tail and so on -on hundreds of specimens. This was tedious. Each specimen had to be cleaned to at least well enough to make all the anatomical landmarks visible. Each had to be mounted on a block of wood, stuck to a blob of plastilene, with the tops of the eyes (a flat surface) in a horizontal plane, perpendicular to my line of sight down the barrel of the microscope. There was a little scale inside the right eyepiece reticle from which to read off the various measurements. After a day of measuring, and until I got used to the microscope, I would see double from the bus window on my way home. 

 from Time Frames: The Evolution of Punctuated Equilibria.

Trilobites have compound eyes, made up of columns of lenses. All this detailed work led to the recognition that a new species had 17 columns of lenses, instead of 18 in the ancestral species found in slightly older strata!

This point about small changes was missed by many observers. Creationists conflated PE with a theory of macro-mutation or saltation, a sudden origin of big morphological change as for example seen in the transition from reptiles to mammals. They claimed that Darwin with his insistence on gradual accumulation of small changes had been wrong all along! 

Not so. The morphological differences observed involved small changes between the ancestor and descendant species.  It takes an expert to recognize this shift in form. 

The real significance of PE is not about the amount of change, but the pattern. PE proposes that very little morphological divergence takes place during the lifetime of a species. It is only when a sub-population of that species gets isolated that it may experience relatively rapid evolution. This genetic isolation results in the formation of a new species. The ancestral species may persist in the main parts of its range, while one of its peripheral populations has evolved into a new species. Biologists term this process of branching or lineage splitting as cladogenesis. 

PE is about explaining these rhythms in the history of lineages. Long periods of stasis (little or no change in form), punctuated by relatively rapid transition to a new form. This transition, though rapid in geologic time,  may take place over hundred of generations and thousand of years.

Why then isn't this transition to a new form preserved in the rock record as a fossil population with a mix of characters? The answer Eldredge and Gould prefer is that reproductive isolation takes place in outlier regions of a species range. Sediments deposited in these isolated basins will be archiving the incremental evolution of a new form, just as Darwin envisaged,  via a mixed or transitional population. However, these outliers containing transitional populations have less chance of getting preserved in the rock record.  The sedimentary sections that paleontologists examine are generally from the main part of the basin. This is not where the change to a new species has taken place.  The sudden appearance of an inferred descendant species in this strata is really recording the migration of the descendant from an isolated part of the species range where it evolved, into the central range of its extinct ancestor. 

Biologists have named this process of formation of a new species through reproductive isolation in a geographically distant area as allopatric speciation. Eldridge and Gould's theory of punctuated equilibrium invokes allopatric speciation and migration to explain the abrupt appearance of new species in the fossil record. In terms of the mechanisms of change, it is not an alternative to Darwinism as is often portrayed. Both the authors accept that even during the period of 'rapid' evolution, changes accumulate through natural selection or genetic drift incrementally across generations. But the vagaries of preservation means that sediments which record this transition are rarely available for study.

Palaeontology: Some Recent Spectacular Fossil Finds

Sharing some news on exciting fossil discoveries of the recent past:

1) Early animal evolution is a topic that continues to fascinate. A fossil rich sedimentary deposit from China dated to about 518 million years ago reveals exquisitely preserved soft bodied animals of the early Cambrian. This find, termed the Qingjiang biota, compliments the well known Burgess Shale of Canada and the Chenjiang site in China. It contains representatives of early cnidarians (related to corals), comb jellies, sponges, and many other creatures, and is helping paleontologists answer questions about the evolutionary relationships and timing of branching of animal groups.

Link: Spectacular new fossil bonanza captures explosion of early life.

2) Before the early Cambrian diversification of animals, is fossil evidence of the roots of some animal lineages, contained in the Ediacaran biota of late Neoproterozoic age ( 600-542 million years ago). At one site in S. Australia, a farmer is conserving a rich Ediacaran fossil site, turning it in to an outdoor research museum.

Link: This Australian farmer is saving fossils of some of the planet’s weirdest, most ancient creatures.

3) A 4 foot sedimentary layer in South Dakota contains a jumble of fossils of animals and plants. This 'event deposit' formed instantaneously from material gathered and dumped by a tsunami triggered by a large meteorite crashing into the Yucatan Peninsula, Mexico. Readers will recognize this! It happened 66 million years ago and resulted in the end Cretaceous mass extinction.

Link: Fossil Site Reveals Day That Meteor Hit Earth and, Maybe, Wiped Out Dinosaurs.

..and there is a longer article in the New Yorker on this fossil site and the hard work paleontologists have put in to tease out its secrets..  (thanks to Hollis for the reminder! ).

Link: The Day The Dinosaurs Died

Happy reading!

Interviews: Meteorite Researcher And A Palaeontologist

Came across these two interesting interviews with a meteorite researcher and a paleontologist.

Meenakshi Wadhwa grew up in Chandigarh, North India. She wanted to study architecture. She ended up being a meteorite researcher. Quanta Magazine highlights her path from college to Director of the Center for Meteorite Studies at Arizona State University.

I totally related to this!

Applying from India, at a time when there was no internet, I had the Barron’s guide to graduate schools in the U.S., which was outdated by like 10 years at that point. I didn’t care about geography or any of that. I didn’t care if it was East Coast or West Coast or the Midwest. It was all half a world away.

.. and this was pretty amazing-

We get something like 100 tons of stuff falling on the Earth every single day. Spread over the entire planet, it’s not all that much if you think about it. Most of that is sand-size particles — tiny, tiny particles. Things that are about the size of a car, or van-size bolides, they hit a few times a year. Something the size of the Chelyabinsk meteor [which exploded over Russia in 2013], that’s a few times a year.

It's a terrific interview.

Dr. Lisa White is a paleontologist. Her specialty is Diatoms. These are single celled algae. They have a lot to tell us about past ecology and climate.  African Americans are poorly represented in the geosciences, and Dr. White as the director of education and outreach at the University of California Museum of Paleontology is actively working to increase diversity in the geosciences.

An excerpt:

I work nationally on a number of boards and with working groups and communities that are constantly examining the diversity in geosciences. We know our numbers don’t compare to engineering and the biological sciences. African American students are more likely to know about those fields and see the direct link to jobs. So we do have a bit of an image problem.

[It can be] difficult for students to have access to information about geosciences careers. There aren’t often a lot of standalone courses in high school. But there are a lot of interdisciplinary connections between all the fields, especially geoscience engineering, chemistry, water science, even agriculture…soil science.

Black Enterprise has the full interview.

Its always fun to read about how people arrive at a particular career trajectory.  A casual conversation, a book read during a holiday, or a trip taken with friends or for some other work can lead someone down  a career path they never thought they would take.

Crisis In Indian Palaeontology

This is incredibly sad reading.

Two recent articles highlight the utter state of disarray Indian paleontology finds itself in.

Less offical importance, low budgets, low career prestige, no legal protection for fossil sites, no local fossil repositories to store collections and no national museum with an attached well funded research program.

From the article by Sanjay Kumar in Science:

With few legal protections, sites often fall victim to looting and development. And although funds are scarce for all science in India, the plight of paleontology is particularly acute. Little money is available for excavations and for acquiring and curating specimens, and the country lacks a national institution in which its natural heritage can be studied and preserved.

All of this discourages young people from entering the field. Cash-strapped universities are curtailing or axing paleontology courses, says Ashok Sahni, of Panjab University in Chandigarh, a leading figure in Indian paleontology. Sahni, best known for his finds of dinosaur nesting sites in Jabalpur and insects trapped in amber in Vastan, in Gujarat state, says he has watched waves of colleagues retire—with few young talents stepping in to replace them. "There is no critical mass of researchers left," he says. "Indian paleontology is dying."

..and Sreelatha Menon in The Wire writes about the problems in palaeontology, and more broadly, geology education:

“In well known centres of paleontological teaching and research, such as BHU, Lucknow University, Panjab University, Jadavpur University, etc., the number of palaeontologists has gone down drastically and new, prestigious educational institutions like the IISERs are not showing much interest in hiring palaeontologists,” Prasad said (IISERs: Indian Institutes of Science Education and Research). “So the country has very few palaeontologists working on large invertebrate fossils at present.”

Pratul Saraswati, a micropalaeontologist in the department of earth sciences, IIT Bombay, thinks it’s not about people not being interested in palaeontology. “If you ask me to name some micropalaeontologists other than myself, I won’t be able to give you more than five names. If you ask Prof Sahni to name some vertebrate palaeontologists, he won’t be able to name more than three or four.”

“The problem is with geology departments as a whole across the country. Except in the IITs and central universities, just one or two faculties teach all the subjects coming under geology – and that includes palaeontology,” Saraswati said. “There is no faculty for geology across India. So it is not just palaeontology but all the subjects coming under geology that are taking a beating.”

At the IITs, every subject is taught by a specialist – which is good because, according to Saraswati, “It is difficult for a non-specialist to teach palaeontology.” But in the other universities, “One or two teaching all the subjects in geology is fine till graduation. For post-graduation and research,” that will not be enough.

A couple of weeks ago I visited the Dept. of Geology at Sinhagad College of Science in Pune. One of the faculty there is working on the sequence stratigraphy of the Cretaceous deposits of the Cauvery Basin in Tamil Nadu, South India. She mentioned that many of the famous outcrops and fossil sites are being destroyed as farmlands and small villages and towns expand. This story is repeated elsewhere across India.

That really struck me hard. During my undergraduate years I had visited that area on a field trip. I saw and collected ammonoids, echnoids, molluscs, corals and plant fossils in the field and had come back with a finer appreciation of the stratigraphic and sedimentologic context in which fossils are entombed and preserved. In retrospect, we should not have collected so many fossils. But in those days we weren't taught, and neither did we introspect, about ethical issues regarding fossil collection and outcrop integrity.

India's natural history must be given more importance. It will be a real tragedy if these localities are lost to future generations.

Paper: Evaluating The Fossil Record Of Earliest Life

Understanding ancient life: how Martin Brasier changed the way we think about the fossil record - JONATHAN B. ANTCLIFFE, ALEXANDER G. LIU, LATHA R. MENON, DUNCAN MCILROY, NICOLA MCLOUGHLIN and DAVID WACEY

I really enjoyed reading this paper which came out in a special publication issue of the Geological Society, London, in 2017. It is a tribute to the work of Martin Brasier who made significant contributions to our understanding of early life and early animal evolution. In particular, Dr. Brasier argued for a more rigorous approach to analyzing fossils, or claimed fossils, of very early life. He developed detailed criteria for describing and interpreting enigmatic structures as either abiogenic or biogenic, and promoted the use of cutting edge imaging technology to better visualize 'fossil' structures in two and three dimensions.

An excerpt:

Crucial to our understanding of life on Earth is the ability to judge the validity of claims of very ancient fossils. Structures reported from the Apex chert (3.46 Ga) that were interpreted to occur in sedimentary rocks and to be biological in origin (Schopf & Packer 1987; Schopf 1992, 1993) were, for a decade or more, considered compelling candidates for the earliest fossils. Martin Brasier’s most important contribution to this debate was to characterize those structures in great detail and to develop a framework within which claims of the ‘oldest’ or ‘earliest’ life should be couched. In his lectures on this subject, Martin referred to the competitive tendency among palaeontologists working on early life as the MOFAOTYOF principle: My Oldest Fossils Are Older Than Your Oldest Fossils.

In particular, Brasier et al. (2002) made it clear that the burden of proof must fall on those making the claim of ancient life, not those refuting it: Ancient filamentous structures should not be accepted as being of biological origin until all possibilities of their non-biological origin have been exhausted. In particular, it is important to note that complex ‘septate’ carbonaceous structures can result from experimental hydrothermal processes. (Brasier et al. 2002, p. 80) In other words, we should assume that ancient structures resembling fossils, such as those in the Apex chert, are abiological until it can be shown beyond reasonable doubt that they are not, rather than the other way around. Brasier (2015) articulated this concept clearly:

This . . . allows palaeobiologists to set up a hypothesis which will prevail until proved false . . . Any newsworthy, and culturally challenging, interpretation must therefore be tested against a less exciting interpretation. This ‘null hypothesis’ is usually regarded as the ‘most boring explanation’. It is boring precisely because it is thought to have a higher probability of being correct. Brasier (2015, p. 9).

This could be thought of as Brasier’s razor: ‘the most boring answer is probably the correct one’.

This critical approach applies to the problem and controversies surrounding  the fossil record of the earliest animals too. A reassessment led Dr. Brasier  to retract his previous claim about the earliest sponge spicules from the Late Ediacaran ( ~ 560 million to 541 million years ago) age deposits of Mongolia.

Finally, his work on accurately characterizing the scratches, pits, holes, undulations, blobs and globules on and within sedimentary deposits has enormous implications for the search for potential fossils on other planets.

Dr. Martin Brasier died in a car accident in 2014. 

Open Access.

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

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

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

I played around a bit with India specific fossil locations.

Paleozoic versus Mesozoic Basins

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

Source: Paleobiology Navigator

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

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

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

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

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

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

Source: Paleobiology Navigator

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

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

Marine versus Continental Interior Basins in Mesozoic Central India

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

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

 Source: Paleobiology Navigator

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

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

Global and Indian Dinosaur Diversity Patterns

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

Source: Paleobiology Navigator

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

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

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

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