Tuesday, July 21, 2015

Simon Conway Morris On The Burgess Shale

Don't miss listening to Prof. Simon Conway Morris on the Burgess Shale fauna on Paleocast hosted by Dave Marshall. The Burgess Shale is an important Middle Cambrian deposit in the British Columbia Rocky Mountains. It is a Lagerstatte, i.e. it contains exceptionally well preserved fossils and therefore gives us rich details about the animal life and biodiversity of the early Paleozoic oceans and some insights into the geologically rapid diversification of early metazoans. (the Cambrian "explosion").

Why is preservation so exquisite in the Burgess Shale? Reconstructions of the sedimentary basin indicate that the mud that became the Burgess Shale was deposited at the base of a high relief limestone reef which essentially formed a sort of an underwater sea cliff.  Periodic turbidity currents swept in fauna living in shallower  areas and buried them rapidly in the deeper  water at the base of the cliff. These currents form deposits a few cm thick, encasing animal remains a few mm in dimensions. The waters were oxygen starved, thus there was less aerobic bacterial degradation of soft tissue. Add to that were some peculiar geochemical conditions of the Cambrian ocean. One was a paucity of sulphate which retarded degradation by sulphate reducing bacteria. The other, as some recent work by Robert Gaines and colleagues suggest, was the high calcium carbonate saturation levels of the ocean, which lead to rapid cementation of the sea floor in between episodes of turbidity flows.

The image on the left shows CaCO3 cement rich layers in the Burgess shale (source: Gaines et al 2012). These cemented crusts on the sea floor formed an impermeable barrier and reduced the influx of sulphate and oxygen bearing sea water in to the sediment, further slowing down microbial activity. How do we know there was less activity of sulphate reducing bacteria? The researchers analyzed the patterns of sulphur isotopes in the fossil rich turbidity layers and the background sediment.  Sulphate reducing bacteria preferentially take up the lighter isotope of sulphur from sea water. Thus, background deposits with normal or enhanced microbial activity have a lighter isotope signature relative to the Cambrian sea water standard. On the other  hand, less microbial  activity means  less fractionation of the lighter isotope into bacteria and ultimately into the sediment matrix. In Burgess Shale type deposits the fossil rich turbidity layers capped by CaCO3 cements show an enriched or heavier sulphur isotope signal indicating less microbial activity. Finally, since the bottom  waters were anoxic, there was little benthic fauna living there. This meant that the cement crusts were not disturbed and broken by bioturbation and remained effective seals throughout the crucial first few weeks of burial when degradation is at its peak. Soft tissue does break down due to slowed microbial activity and fermentation and methanogenesis. The three dimensional carcass collapses into a nearly two dimensional carbon rich film. The final result is that recalcitrant extracellular organic material like cuticles, chaetae, and jaws are preserved as compressed thin carbonaceous films often just a few microns thick, the soft fine grained mud encasing the carcass helping preserve fine morphological details. This preservation style also meant that some animals lacking recalcitrant tissues like flatworms, mesozoans, nemerteans and unshelled molluscs are less well represented in the Burgess Shale style deposits ( Butterfield 2003). Peculiar preservational styles by their very exceptional and localized nature impose a bias on the fossil record that palaeontologists must recognize to understand true evolutionary patterns.

The examples on the left shows Burgess Shale style preservation of Arthropod (B), Polychaetae worm (C) and Arthropod (D) [source: Gaines 2014]. This style of preservation actually appears first in the early Neo-Proterozoic  and then disappears for about 150 million years until the earliest Cambrian. It again declines by late Cambrian with the earliest Ordovician being the last recorded example of this taphonomic style. A unique combination of geological conditions and early diagenesis of sediment prevailing in the latest Proteozoic and earliest Cambrian resulted in these fossil deposits. This time period also has other forms of detailed preservation of soft tissues, the two most important being the Edicaran style preservation wherein the remains of macroscopic plants and animals deposited in sandy and silty sediment were draped by microbial mats and compressed to form impressions (death masks) on the sediment surface. The other important style is the Doushantuo style preservation (named after the Doushantuo fossil beds of late NeoProterozoic age, China, containing preserved algae and putative embryos and larval stages of early animals ) where phosphate minerals are attracted to and precipitate around organic tissue preserving delicate cell outlines and internal organs. Very occasionally, the same fossil will show two different preservational styles, for example, the extracellular tissue preserved in the Burgess Shale style while  internal organs preserved in the Doushantuo style. These taphonomic "windows", as they are referred to, appear and disappear through the Neo-Proterozoic to Cambrian period. For example, the Edicaran style preservation first appears in the late NeoProterozic around 580 million years ago or so. Considering that microbial mats which play an important role in this style of  preservation are pervasive through the late Archaean and the Proterozoic, the first appearance of the Edicaran remains is then likely an evolutionary signal of the first appearance of  macroscopic multicellular  eucaryotes on earth. The disappearance of Edicaran style by the earliest Cambrian also suggests a biological feedback. The evolution of macroscopic benthic animals burrowing and grazing on bacterial mats may have destroyed the cover protecting the faunal remains. Preservational styles are controlled not just by geological conditions but due to contemporaneous evolutionary innovations too.

Coming back to the talk! Prof Simon Conway Morris describes the history of research on the Burgess Shale,  how he got into researching it, details of some of the animals found in it including the famous Pikaia. This has been interpreted as an early representative of  the chordates from which the vertebrates evolved. Overall, Conway Morris gives a masterly authoritative talk.

I would have loved to hear him talk a little more about the broader questions that arise from this deposit. Are the origins of the Burgess animals to be found in the earlier Edicaran fauna? Does the Cambrian have greater morphological disparity than later periods in earth history? Has life followed a contingent unique pathway or does examples of convergence tell us something deeper about the general principles of evolution? ..or an intelligence which frames the ultimate laws and guides evolutionary processes. Simon Conway Morris has indicated elsewhere his thoughts that the Universe is the product of a rational mind and that evolution is but a search engine and I wish Dave Marshall had pressed him on his theist beliefs. But I guess the topic was the fauna of the Burgess Shale and in particular that iconic quarry in British Colombia.

He did mention in passing something which I think  is an important aspect of this story. Just as he had finished his Master's degree from Bristol University in 1972, a project headed by Prof. Harry Whittington on the Burgess Shale was starting at Cambridge. Simon Conway Morris saw this as a good opportunity. At around the same time, the Chicago school of palaeontologists lead by David Raup (who died last week), Jack Sepkoski and Tom Schopf had started a program to broaden the scope of palaeontology to include rigorous quantitative methods on large sample sets to understand biodiversity and patterns of evolution, bringing the field of palaeontology, as John Maynard Smith famously said, to the "high table of evolutionary theory". These events underscore the important point that for all your brilliance in something, circumstances and timing matter. Simon Conway Morris was present at the right time at the right place. And he did the Burgess Shale fauna justice.

Friday, July 10, 2015

Darwin: Ancestry In Lyell's Pebbles And Herschel's Words

A lovely passage from the 19th century astronomer John Herschel's  letter to geologist Charles Lyell quoted in Darwin: The Life Of A Tormented Evolutionist:

Words are to the Anthropologist what rolled pebbles are to the Geologist- Battered relics of past ages often containing within them indelible records capable of intelligible interpretation- and  when we see what amounts of change 2000 years has been able to produce  in the languages of Greece & Italy or 1000 in those of Germany, France & Spain we naturally begin to ask how long a period must have lapsed since the Chinese, the Hebrew, the Delaware & the Maleass [from Madagascar] had a point in common with the German & Italian & each other. - Time! Time! Time! - we must not impugn Scripture Chronology,but we must interpret it in accordance with whatever shall appear on fair enquiry to be the truth for there cannot be two truths. And really there  is scope enough: for the lives of the Patriarchs may as reasonably be  extended to 5000 or 50000 years apiece as the days of Creation to as many thousand millions of years.

Besides a great age for earth, a notion which was becoming more accepted as geological observations poured in from Britain and other parts of the world,  in the passage are ideas on ancestry and divergence from a common stock. Herschel's musings on language change influenced Darwin. Were these applicable to life too? Darwin on his return from the Beagle voyage in 1836 was engrossed in geological thinking,  hoping to write a book on South American geology. Charles Lyell encouraged him,  and unlike the cantankerous zoology community, he found the geology fraternity more  approachable and gentlemanly. His paper to the Geological Society on the uplift of the Chilean coast was very well received.

But his zoological samples too were being processed by a variety of experts. Richard Owen found out that the large animal bones Darwin had collected were ancient relatives of living South American sloths and armadillos and not related to European and African mammals. There was, Darwin realized, a genealogical succession of fauna which jarred with the commonly held ideas about separate creation of species.  The ornithologist John Gould pointed out that the seemingly disparate collecting of Galapagos Islands ground dwelling birds with distinctive beaks that Darwin had presented to him and thought of as distinct species of finches, wrens and warblers were in fact a closely related group of finches, with more distant relatives found on the South American mainland. And his collection of mockingbirds which unlike  the finches were correctly labelled according to the islands,  turned out to be 3 related species, each distinct to an island... Ancestry and divergence from a common stock..

John Herschel's "mystery of all mysteries" began tempting him. His long association with dissenting intellectuals and radical thinkers who claimed that life should be explainable by natural laws and his own observations of  nature's bounty and variability had made him  receptive to the idea of species transforming into new species. In this, he departed from his geology mentor Charles Lyell who was a scientist molded in the conservative Anglican tradition. Lyell believed that the dynamics of landscapes and the succession of fauna ultimately revealed the hand of a creator. That humans could have originated via transmutation from an ape was anathema to him. For Darwin, such thoughts held no terror.

Geology was to remain an important part of his work life, but in July 1837 he opened and began scribbling ideas in what is one of the most famous scratchpads in the history of science- Notebook B which he titled Zoonomia (after his grandfather Erasmus Darwin's book), marking a decisive shift in emphasis in his thinking from geology to evolution.

Monday, July 6, 2015

Monsoon Trekking Season Is Here

I hereby declare the Deccan Volcanics Western Ghat 2015 monsoon trekking season officially open!



That picture is of the steep slopes of Fort Rajgad about an hour's drive from Pune, taken a couple of years ago.

Last year the geology highlight of my treks into the Deccan Basalt countryside was this dyke with horizontal columnar joints near the summit of Fort Ghangad.



Columnar joints are cooling cracks which develop perpendicular to the cooling surface. In lava flows, the cooling surface is mostly horizontal and so the result is vertically oriented, often hexagonal shaped columnar  joints.  In this case, the cooling surface was the near vertical contact between the intruding hot magma and the colder older lava flow. The cooling cracks therefore are horizontal.

I will be posting more pictures from this year's trekking season. The Western Ghats in the monsoon and the winter are a sight to savor!

Sunday, June 21, 2015

Cartosat 1 DEM- Two Strike Slip Faults

My two favorite strikes slip faults in India as rendered by Cartosat 1 Digital Elevation Model-

1) Yamuna Left Lateral Fault-



Source: Cartosat 1 DEM National Remote Sensing Center, ISRO

You can see that the Siwalik hills are breached by the Yamuna and dislocated in a left lateral sense, i.e. one would have to turn left to follow the narrow marker rock bed across the fault.

From a previous post on this fault-

Miocene onwards a thick wedge of fluvial sediments filled up a foreland basin that formed in front of rising thrust sheets uplifted along the active Main Boundary Thrust (MBT). That phase ended about 0.5 to 1 mya.

This fluvial wedge over the last half a million years has been deformed into the Siwalik mountains. These mountains form broad synclines and tight anticlines cut by north dipping thrust faults, a result of the continuing compression of the sediment wedge. The southernmost of these thrusts which brings into tectonic contact the anticlinal Frontal Range of the Siwaliks over the alluvial plains in called the Himalayan Frontal Thrust (HFT).

The HFT is broken into segments and the amount of displacement along these segments or thrust blocks is unequal. For example the blocks west of the Yamuna and east of the Ganga have moved southwards with an opposite sense of movement relative to the central block known as the Dun block. To view this, turn on labels and pan southeastwards in embeddable map below until the town of Haridwar where the Ganga enters the plains.

Thus the Yamuna fault has a left lateral sense of movement while the Ganga fault has a right lateral sense of movement. These faults can be thought of as lateral ramps of the HFT accommodating the displacement caused by the southwards movement of the HFT blocks. 

Structural considerations indicate that during the last 0.5 my there has been about 8 km of displacement along the Yamuna and Ganga faults, a slip rate of approx. 16 mm year. - See more at: http://suvratk.blogspot.in/2010/12/remotely-india-3-left-lateral-yamuna.html#sthash.H9K4tfrI.dpuf
 Miocene onwards a thick wedge of fluvial sediments filled up a foreland basin that formed in front of rising thrust sheets uplifted along the active Main Boundary Thrust (MBT). That phase ended about 0.5 to 1 mya.

This fluvial wedge over the last half a million years has been deformed into the Siwalik mountains. These mountains form broad synclines and tight anticlines cut by north dipping thrust faults, a result of the continuing compression of the sediment wedge. The southernmost of these thrusts which brings into tectonic contact the anticlinal Frontal Range of the Siwaliks over the alluvial plains in called the Himalayan Frontal Thrust (HFT).

The HFT is broken into segments and the amount of displacement along these segments or thrust blocks is unequal. For example the blocks west of the Yamuna and east of the Ganga have moved southwards with an opposite sense of movement relative to the central block known as the Dun block. To view this, turn on labels and pan southeastwards in embeddable map below until the town of Haridwar where the Ganga enters the plains.

Thus the Yamuna fault has a left lateral sense of movement while the Ganga fault has a right lateral sense of movement. These faults can be thought of as lateral ramps of the HFT accommodating the displacement caused by the southwards movement of the HFT blocks.

Structural considerations indicate that during the last 0.5 my there has been about 8 km of displacement along the Yamuna and Ganga faults, a slip rate of approx. 16 mm year.


The Digital  Elevation Model also brings out beautifully the Quaternary alluvial fans with tiers of river terraces deposited in the valley between the Siwaliks and the Lesser Himalayas and the intricate drainage on the southern slopes of the Siwalik frontal range.

2) Gani Kalava Fault-

This too has a left lateral movement and has a prolonged history of being reactived during different phases of sedimentation in the Proterozoic Cuddapah Basin of south India.  I  did my M.Sc dissertation on this area, concentrating on the asymmetric anticline with a gentler dipping southern limb and a nearly vertical dipping northern limb located south of Gani village.


Source: Cartosat 1 DEM, National  Remote Sensing Center, ISRO

From a previous post on this fault-

The feature is an ENE plunging asymmetric anticline with spectacular dip slopes of quartz arenites making up the southern limb of the fold and left-lateral movement along a regional fault steepening the northern limb of the fold. There is some copper mineralization along the fault. The Cuddapah basin is an intra-cratonic basin which was filled up in several depositional mega cycles. Sections of two of these mega cycles are exposed in this area. The older mega cycle comprising the Cuddapah subgroup is exposed in the core of the anticline. An angular unconformity separates the older cycle from the younger Kurnool cycle (sub group) which is exposed along the limbs.

Brown arrows shows the offset marker beds (grey arrows show sense of movement) of the oldest sedimentary formation of the Cuddapah basin, the early Proterozoic Gulcheru Quartzites and Vempalle limestones of the Papaghani Group. These lie unconformably on the Archean Peninsular  Gneiss which is the fawn colored peneplain in the left part of the DEM. The very distinct dip slopes of  the Gani anticline (south of Gani village) are made up of a much younger sedimentary unit, the Banganpalle  Quartzites of the Kurnool Group. They were deposited in energetic shallow seas, forming a vast sand shelf, wherein waves and currents winnowed out unstable minerals, leaving behind a nearly pure well sorted and rounded quartz sand. The quartz grains are polycyclic, meaning they show evidence of being derived by weathering of older sandstones, the most likely source being the Gulcheru Quartzites.

Remember this is a Digital Elevation Model,  same color means the same altitude range and not mineral composition.

Thursday, June 11, 2015

Chimpanzees Cooking

What do you have to say to this statement by psychologist Felix Warneken ?

“The logic is that if we see something in chimpanzees’ behavior, our common ancestor may have possessed these traits as well. If our closest evolutionary relative possesses these skills, it suggests that once early humans were able to use and control fire they could also use it for cooking.”

I would say that one has to be very careful in drawing general implications about ancestral abilities from the behavior of chimpanzees. They are not some frozen Pliocene ape. The human-chimpanzee split from our common ancestor may have occurred 7-8 million years ago. Since divergence,  the lineage that led to chimpanzees has been evolving for the same amount of time as our lineage. Many aspects of modern chimpanzee behavior may not reflect the ancestral condition but instead may have evolved later in their evolutionary history.

What the study that is described in the Guardian shows is that chimpanzees prefer cooked food to raw food and have abilities to defer instant gratification for later preferred reward. This though need not be restricted to cooked food. You could conceivably show that they for example behave the same way if given a choice between a raw and a ripe fruit. Or for that matter something not to do with food at all. There is no evidence that chimpanzees throw raw meat in natural forest fires and come back and eat the cooked meat. Awaiting for cooked food does not mean they posses "most of the intellectual abilities required for cooking"

And nor did our very early human ancestors. One implication drawn from these chimpanzee "cooking" experiments is that our ancestors may have developed a taste for grilled meats early on (possible) and the timeline for cooking may have to be shifted to an earlier date. I don't see the latter connection at all. The oldest confirmed evidence for cooking is about 1 million years ago, although some scientists like Richard Wrangham based on changes in physiology (larger brain, smaller molars)  seen in the fossil record push it back to around 2 million years ago.  Even assuming that the chimpanzees preference for cooked food reflects the ancestral state, one can't draw a connection between that and cooking appearing earlier in our lineage. It took 4-5 million years after our lineage split from the chimpanzee for us to evolve the ability to control fire and the addition of deliberately cooked food as part of our regular diets. That is hardly "early". It took a long time, in fact cooking has been absent for most of the time of our lineages existence, and it was contingent on a host of other unique changes in our cognition, social evolution and our abilities to use our limbs to manipulate objects.

There was nothing inevitable about it.. one just has to look at the chimpanzees. In fact, this precaution taken during the experiments gives the game away-

“Originally we thought of setting up a camping cooker in their sleeping area, but you could imagine them getting hold of a gas tank or burning themselves,” said Warneken. “This was not a viable option.”

It was not a viable option because chimpanzees don't even come close to possessing most of the required abilities to cook.

That is one of the problems of drawing sweeping evolutionary implications about behavior from contrived experimental situations.  They just don't bear any resemblance to behavior in the natural state.