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Posts tagged with "mass extinctions"

Why Ammonites Died Out

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Show me your teeth, and I’ll tell you what you eat. Living mammals easily show their teeth, but ammonites that have been extinct for 65 million year, is a different challenge. Although their shells make good fossils, other ammonite structures such as their feeding apparatuses are rarely discerned. In a new study published in Science Kruta et al. have used a new method (synchrotron-based x-ray microtomography) to visualize and reconstruct the mouthparts of three ammonite specimens (Baculites). Based on the feeding apparatus and the (food remains from the) last meal found between the jaws they concluded that Baculites lived in the water column and fed on small zooplankton.

Ammonites of all sorts were teeming in the oceans for about 340 million years, from the Early Devonian to the end of the Cretaceous. As dinosaurs they became extinct at the end of the Cretaceous (65.5 million years ago). This timing is coincident with an abrupt decline in several groups of plankton. The collapse of the marine food web at that time apparently contributed to the extinction of Baculites and similar ammonites, which depended on plankton as a food source. Other ammonites, however, also became extinct simultaneously. A plausible hypothesis for the terminal extinction of ammonites is related to their early life history. Newly hatched Mesozoic (from about 250 million years ago to about 65 million years ago, also referred to as the Age of Dinosaurs) ammonites had shells that ranged in size from 0.5 to 1.8 mm, which suggests that ammonites laid a large number of small eggs and that newly hatched juveniles ate small plankton. The abrupt decline of plankton at the end of the Cretaceous would thus have greatly affected the survival prospects of newly hatched ammonites.

Another aspect is that the radiation of certain forms of ammonites might be associated with the radiation of plankton during the Early Jurassic.


Unlike the probably better known “coiled” ammonites, Baculites ("walking stick rock") had a nearly straight shell.

Reference:
Kruta, et al.
The Role of Ammonites in the Mesozoic Marine Food Web Revealed by Jaw Preservation
Science 331, 70 (2011)
DOI: 10.1126/science.1198793





Academics

Microbial Mounds at Guilin, China

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The Guilin area in South China boasts some of the most spectacular tropical karst topography in the world, in my view only surpassed by Halong Bay in North Vietnam. When I visited Guilin some years ago and made a boat trip on the Li river, it was unfortunately raining (it often does I have been told). I have found a picture from Wikipedia that strongly reminds me of my visit to the area.



Guilin is surrounded by Devonian limestone representing a shallow marine carbonate platform extending nearly 40 km from east to west and nearly 80 km from south to north, and it is through this (karstified) carbonate platform that the river Li has has carved its river bed. Below is a generalised cross-section of a typical carbonate platform also taken from Wikipedia.



The sediment succession at Guilin also covers the Late Devonian mass extinction event at the Frasnian-Famennian boundary - this event (or possibly 2 closely spaced events) is also known as the Kellwasser event.

Reefs studied at the margin of the carbonate platform a few km west of Guilin show a transition from reefs built up by multi-cellular organisms during the Givetian to dominating microbial reefs and mounds around the Frasnian-Famennian boundary. During the Givetian massive corals were common as reef-builders, but at he Frasnian-Famennian boundary the reefs/mounds became dominated by microbes, other unicellular organisms and algae. Calcimicrobes were the dominating reef-builders throughout the Famennian, but declined near the Devonian-carboniferous boundary.

Calcimicrobes are calcareous colonial microfossils, which include many morphologically dissimilar organisms, whose effect in massive aggregations, in association with shelly multi-cellular organisms, was to lay down the earliest recognizable reef systems. The term was first applied by James & Gravestock in 1990.

The late Devonian shift from skeletal to microbial reef-builders is seen as a major event in Phanaerozoic reef evolution, generally associated with the Frasnian-Famennian extinction event. The Phanaerozoic covers roughly speaking the latest 542 million years.

I would also like to mention that the extinction only seems to have affected marine life. The causes of these extinctions are unclear. Leading theories include changes in sea level and ocean anoxia, possibly triggered by global cooling or oceanic volcanism. The impact of a comet or another extraterrestrial body has also been suggested. Some statistical analysis suggests that the decrease in diversity was caused more by a decrease in speciation than by an increase in extinctions. Please notice that the occurrence of microbial reef-building communities is not solely a function of the extinction of larger skeletal organisms.

Definitions?

I wish could give you a good definition of reef, but unfortunately the experts do not agree. Rest to say that reefs are not necessarily coral reefs, but e.g. often build up by other calcium carbonate precipitating organisms. The differences between reefs and mounds are also rather vague. Generally speaking however reefs are considered to be held up by a macroscopic skeletal framework, whereas mounds lack such a macroscopic skeletal framework - mounds are built by microorganisms or by organisms that don't grow a skeletal framework. Many reefs are a mixture of the two at various proportions. I suppose that what used to be called mud mounds in most cases actually are microbial mounds, and that microbial carbonate production has so far been underestimated. Anyway the microbial production of carbonate is now considered the most important contribution to the formation of Paleozoic mud mounds (Paleozoic Era: 542 - 251 million years ago).


Blue G marker = Guilin. Red H marker = Halong Bay





Academics

Accretionary Wedge - Geo-Image

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The current (May 2010) edition of the geoblogging carnival, The Accretionary Wedge, is held at Highly Allochthonous and the theme is geo-images. I am early out as I don’t expect to post much the rest of this month for various reasons.

I have chosen a picture I took some years ago in winter time (with snow). It is a picture of Stevns Klint in Denmark.



Why this choice? A. o. because it is as well an important K/T boundary site as the type locality of the Danian Stage.

On 8 January 2010 the site was submitted to the tentative list of UNESCO World Heritage Sites. Let me quote some of the reasons according to the site description:

“The coastal cliff site, Stevns Klint is arguably the most famous, scenic and best exposed K/T (Cretaceous-Tertiary) boundary section in the world with the exceptional K/T boundary layer easily recognisable immediately beneath a pronounced topographic overhang separating the underlying soft Cretaceous chalk from the overlying, harder Tertiary limestone.

The K/T boundary coincides with one of the most pronounced faunal mass extinction known in the geological record, a turnover that affected both terrestrial and marine faunas 65.5 million years ago. The recovery after the extinction event lead to the life we know on Earth today. At the same time the boundary represents the only mass extinction and change of the global ecosystems which has been related to an extraterrestrial impact.

Stevns Klint is a 14.5 km long coastal cliff located about 45 km south of Danish capital, Copenhagen on the east coast of the Danish island of Sjælland. The exposed succession is about 45 m thick and shows the stratigraphic evolution from the latest Cretaceous, across the K/T boundary and into the early Tertiary.

Stevns Klint is a classical K/T boundary site and is one of the three discovery localities of the famous iridium anomaly, which formed the basis for the asteroid impact hypothesis of Alvarez et al. (1980) proposed to explain the end-Cretaceous mass extinction. Stevns Klint is therefore a key locality in the ongoing debate about mass extinction in general and the K/T boundary in particular. In addition Stevns Klint is type locality of the Danian Stage. The Danian is represented by bryozoan limestone mounds outlined by thick black flint bands which illustrate the geometry, dimensions and architecture of one of the finest, ancient cool-water carbonate mound complexes in the world.

...”



The picture shows one of the bryozoan limestone mounds quite clearly (middle of image).

You may notice that the whole succession is chalk or limestone, also above the K/T boundary. Primarily for this reason (I suppose) the Danian (at least in Denmark and a few other countries) was considered part of the Cretacious (Cretacious is Latin for "chalky") - when I was young. The situation in today’s stratigraphy: Cretacious about 100 to 65 million years ago with Maastrichtian as last stage and Danian (65.5 ± 0.3 to 61.7 ± 0.2 million years ago) first stage of Tertiary.

Personally I find that Stevns Klint certainly deserve to be a UNESCO World Heritage Site.

PS:
See my next post for a few notes on "Bryozoan Mounds".





Academics

Dinosaurs died out, but what about Insects?

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The Cretaceous–Paleogene extinction event, which occurred approximately 65 million years ago, was a large-scale mass extinction of animal and plant species in a geologically short period of time. The most famous victims were the dinosaurs. More than half of the species that lived in the sea died out at this time - including ammonites and rudists.

What happened to the insects?

Gunnar Ries at Amphibol commented in a post of 28 October 2009 some of the different causes used to explain the extinction event. His post is in German. This post drew my attention (thank you!) to a publication by a team of biologists form Bonn in the Proceedings of the Royal Society.

Previous studies of insect-damaged fossil leaves in the US Western Interior showed major plant and insect herbivore (plant feeding) extinction at the Cretaceous–Palaeogene boundary. The Bonn team studied leaf fossils from the middle Palaeocene Menat site, France, which has the oldest well-preserved leaf assemblage from the Palaeocene of Europe, to test the generality of the observed Palaeocene US pattern. Apparently the insects were harder hit in the US than in Europe, and where it took about 10 million years for the insect fauna in the US to recover, Europe did in half that time, namely only 5 million years.

The diversity and complexity of plant–insect interactions at Menat suggest that the net effects of the Cretaceous–Paleogene extinction were less at this greater distance from the Chicxulub, Mexico, impact site. Along with the available data from other regions, the study seems to show that the end-of-Cretaceous event did not cause a uniform, long-lasting depression of global terrestrial ecosystems. Rather, it gave rise to varying regional patterns of ecological collapse and recovery that appear to have been strongly influenced by distance from the Chicxulub structure.

This does not end the discussion, but seems to back up the Chicxulub hypothesis. Who makes the next goal?

Reference:
Wappler et al.
No post-Cretaceous ecosystem depression in European forests? Rich insect-feeding damage on diverse middle Palaeocene plants, Menat, France
Published online before print September 23, 2009
doi: 10.1098/rspb.2009.1255

Unfortunately NOT open access !

The Palaeocene Epoch is a geologic epoch that lasted from around 65 to around 56 million years ago.
The Palaeogene Period (that began around 65 and ended around 23 million years ago) on the other hand comprises the Palaeocene, Eocene and Oligocene





Academics

Algae Key To Mass Extinctions?

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There are lots of hypotheses about what causes mass extinctions, one more fanciful than the other. Two ideas taken more seriously are volcanoes and cosmic impacts. Now a new hypothesis suggests algae may be the killer behind the world's great fauna annihilations.

The idea was introduced in "Hypothesis for the role of toxin-producing algae in Phanerozoic mass extinctions based on evidence from the geologic record and modern environments" , a paper published in the March 2009 issue of the peer-reviewed journal Environmental Geosciences, and also presented in a talk on 19 October 2009, at the annual meeting of the Geological Society of American in Portland, Oregon, USA. (The full paper by Castle and Rodgers from March can be downloaded from this Clemson University site).

Castle and Rodgers have spent two years analyzing data from ancient stromatolite structures finding evidence that blue-green algae, which produce poisons and deplete oxygen, were present in sufficient quantities to kill off untold numbers of plants and animals living on land or in the sea. The geologic record demonstrates a pronounced increase in abundance and environmental range of algae, including stromatolitic cyanobacterial mats, coincident with the first four of the five major Phanerozoic (the last 545 million years) mass extinctions. During these past events of algal expansion, population decline of animals could have been caused by effects of algal blooms, including algal-produced toxins, at a scale sufficient to generate a fossil record of mass extinction.


Mass extinctions have often been attributed to climate changes, sea level, volcanic activity, and asteroids. Castle and Rodgers claim that these causes are contributors, but algae were the mass killer.

Environmental changes such as climatic warming, sea level fluctuation, and increased nutrient supply may have promoted algal blooms over vast expanses of marine to freshwater environments. and indeed, in my humble opinion toxic algae may just be another contributor together with other factors leading to (too) harsh environmental conditions.


Schematic profiles illustrating the influence of climate-induced sea level change on algal growth. (A) Sea level is low, shelves are narrow, and water temperatures are less favorable for algal growth during periods of cool global climate. (B) During periods of warm global climate, sea level is high resulting in extensive areas of shallow marine and coastal environments favourable for algal growth. Warm water temperatures promote the growth of algal blooms, domal and columnal stromatolites, and stromatolitic mats, which increases the potential for toxin production and release.





Academics

Shiva Crater

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At the 2009 Portland GSA Annual Meeting (18-21 October 2009) Sankar Chatterjee will present a paper on the Shiva Crater (18 October 2009, 3:45-4:00 p.m.). The massive Shiva basin is a submerged depression west of India that is intensely mined for its oil and gas resources. Some complex craters are among the most productive hydrocarbon sites on the planet.

If Chatterjee is right this could be the largest, multi-ringed impact crater the world has ever seen, with a diameter of ~500 km. The diameter of the so far known largest impact crater, the Vredefort Crater in South Africa has a diameter of about 300 km. Furthermore the Shiva crater has an age that makes it a suspect for the killing of the dinosaurs ca. 65 million years ago.

According to Chatterjee it is the remnant of a giant meteorite impact that left high-resolution stratigraphic signals in the sedimentary and volcanic rocks such as shocked quartz, iridium anomaly, nickel-rich spinels, sanidine spherules, magnetic nanoparticles, high pressure fullerenes, megatsunami deposits, and melt lavas. If the author and his team are right, this is the largest crater known on our planet. The bolide may have been perhaps 40 kilometres in diameter - as compared to the bolide of between 8 and 10 kilometres that hit Yucatan Peninsula, and is commonly thought to have killed the dinosaurs.

The impact was so powerful that it led to several geodynamic anomalies: it fragmented, sheared, and deformed the lithosphere mantle across the western Indian margin and contributed to major plate reorganization in the Indian Ocean. It initiated rifting between India and Seychelles in the west and created the Laxmi Ridge; it shattered the Indian plate easterly along the Narmada-Son Rift extending 1500 km across, dividing the Indian shield into a southern peninsular block and a northern foreland block. Because of topographic barrier of the Western Ghat Mountain range, the impact-triggered tsunami was restricted along the Narmada-Son Rift at the KT boundary.

The team hopes to go India later this year to examine rocks drill from the center of the putative crater for clues that would prove the strange basin was formed by a gigantic impact.

The rest of us are waiting to hear more.



In Norwegian:




PS of 21 October 2009:
The hypothesis met with sharp criticism. See
http://www.space.com/scienceastronomy/091018-dinosaur-crater.html

PS of 2 November 2009 - More about the Shiva Crater:
http://suvratk.blogspot.com/2009/11/end-cretaceous-how-many-impacts-how.html
and
http://books.google.co.in/books?id=3IORF1Ei3LIC&pg=PA35&lpg=PA35&dq=bombay+high+stratigraphy&source=bl&ots=ng1Gm3E1r-&sig=lYxnwzsvy96JmE821U2z40VDcKw&hl=en&ei=fLzmSvHfK5DOsQOf36TfBQ&sa=X&oi=book_result&ct=result&resnum=9&ved=0CB4Q6AEwCDgK#v=onepage&q=bombay%20high%20stratigraphy&f=false

Academics

Sixth Mass Extinction

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The classical "Big Five" mass extinctions are End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous (see note below). The Holocene extinction event is referred to as the Sixth Extinction, that is the extinction event that is taking place NOW. This means indeed that the earth's 6th great mass extinction is occurring while you read this post.

A study published in the international journal Conservation Biology is the first comprehensive review of more than 24,000 scientific publications related to conservation in the Oceanic region. Compiled by a team of 14 scientists, it reveals a sorry and worsening picture of habitat destruction and species loss. It also describes the deficiencies of and opportunities for governmental action to lessen this mounting regional and global problem. The review highlights destruction and degradation of ecosystems as the main threat. In Australia, agriculture has altered or destroyed half of all woodland and forests. Around 70% of the remaining forest has been damaged by logging. Loss of habitats is behind 80% of threatened species, the report claims.

According to the IUCN Red List of Threatened Species, 2008
  • Nearly 17,000 of the world's 45,000 assessed species are threatened with extinction (38 percent). Of these, 3,246 are in the highest category of threat, Critically Endangered, 4,770 are Endangered and 8,912 are Vulnerable to extinction.
  • Nearly 5,500 animal species are known to be threatened with extinction and at least 1,141 of the 5,487 known mammal species are threatened worldwide.
  • In 2008, nearly 450 mammals were listed as Endangered, including the Tasmanian Devil (Sarcophilus harrisii), after the global population declined by more than 60 percent in the last 10 years.
  • Scientists have catalogued relatively little about the rest of the world's fauna: only 5 percent of fish, 6 percent of reptiles, and 7 percent of amphibians have been evaluated. Of those studied, at least 750 fish species, 290 reptiles, and 150 amphibians are at risk.
  • The average extinction rate is now some 1,000 to 10,000 times faster than the rate that prevailed over the past 60 million years.


Extinction certainly threatens amphibians [http://www.pnas.org/content/105/suppl.1/11466.abstract] — frogs, salamanders, and caecilians. A detailed worldwide assessment and subsequent updates show that one-third or more of the 6,300 species are threatened with extinction.

http://www.science.unsw.edu.au/news/extinction-crisis-oceania/
http://www.guardian.co.uk/environment/2009/jul/28/species-extinction-hotspots-australia
http://wwf.org.au/news/queensland-land-clearing/

In Danish:
http://politiken.dk/videnskab/article759526.ece

Note on 5 previous mass extinctions:

  • Cretaceous-Tertiary. 65 million years ago, the dinosaurs were wiped out in a mass extinction that killed nearly a fifth of land vertebrate families, 16% of marine families and nearly half of all marine animals.
  • End of Triassic. About 200 million years ago, lava floods erupting from the central Atlantic are thought to have created lethal global warming, killing off more than a fifth of all marine families and half of marine genera.
  • Permian-Triassic. The worst mass extinction took place 250 million years ago, killing 95% of all species.
  • Late Devonian. About 360 million years ago, a fifth of marine families were wiped out, alongside more than half of all marine genera.
  • Ordovician-Silurian. About 440 million years ago, a quarter of all marine families were wiped out.




AcademicsTop Blogs

Extinction Distinction

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The Triassic-Jurassic extinction approximately 200 million years ago is one of the five major extinctions in Earth's history. The cause has of course been widely discussed. An important factor in any discussion is the pace with which it took place. Was it catastrophically rapid - or did it occur gradually over a long period of time.

According to a study of Late Triassic biodiversity in East Greenland published in the journal Science of 19 June 2009 the decrease in plant species (in what is now East Greenland) was fairly abrupt and seemed to coincide with a period with increased atmospheric CO2 levels and global warming. As there is no current way of detecting changes in sulfur dioxide in the past, it is difficult to evaluate whether sulphur dioxide, in addition to a rise in carbon dioxide, influenced the extinction pattern.

The authors find that the abrupt plant diversity loss is consistent with expected plant responses to a catastrophically rapid rather than gradual environmental change and argues against the currently favored extinction mechanisms invoking gradual CO2-induced global warming due to slow release of CO2 from the mantle associated with extrusion of basalt over an area of more than 10 million km2, namely the Central Atlantic Magmatic Province in the eastern US, South America, and western Africa - during the breakup of Pangaea.

A rapid environmental change could however be related to sulphur dioxide aerosol released during volcanic eruptions in the Central Atlantic Magmatic Province. Several other mechanisms have been suggested over the years, including a meteorite impact, with the Manicougan impact as a possible candidate. Unfortunately for this hypothesis U-Pb zircon dating of the impact melt has proved that the crater has an age of 214 ± 1 million years. As this is 12 ± 2 million years before the end of the Triassic, the crater cannot be the cause of the Triassic-Jurassic extinction event.

As to increased atmospheric CO2 levels and global warming - If we compare with the situation today, it is expected that the level of carbon dioxide in the modern atmosphere may reach as high as two and a half times today's level by the year 2100. It is at exactly this level, namely 900 parts per million, that the ancient biodiversity crash was detected.

Reference:
Fossil Plant Relative Abundances Indicate Sudden Loss of Late Triassic Biodiversity in East Greenland
By McElwain et al.
Science 19 June 2009:
Vol. 324. no. 5934, pp. 1554 - 1556
DOI: 10.1126/science.1171706

http://www.sciencemag.org/cgi/content/abstract/324/5934/1554
http://www.eurekalert.org/pub_releases/2009-06/nsf-sci061809.php
http://my.opera.com/nielsol/blog/2008/02/11/manicouagan-impact-structure
http://www.geologytimes.com/research/Sudden_collapse_in_ancient_biodiversity_Was_global_warming_the_culprit.asp



AcademicsTop Blogs

Middle Permian Extinction

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A major global extinction in the Middle Permian 260 to 270 million years ago preceded the huge end-Permian extinction. This extinction is also known as the Guadalupian Mass Extinction. New research suggests it may have been caused by volcanic eruptions in what is now China.

Wignall et al. have studied the Emeishan flood basalt province in south-west China. The eruption in the Emeishan province unleashed around half a million km2 of lava, covering an area 5 times the size of Wales. The eruptions occurred in a shallow sea so that the lava appears today as a distinctive layer of igneous rock sandwiched between layers of sedimentary rock containing easily datable fossilised marine life. A close temporal link between the onset of eruptions and extinction suggests a cause-and-effect scenario. Cooling and acid rain (caused by sulphur dioxide (SO2) effusion and sulfate aerosol formation) and consequent environmental deterioration are candidates for this link.

The study is published in the 29 May 2009 edition of the journal Science:
Volcanism, Mass Extinction, and Carbon Isotope Fluctuations in the Middle Permian of China
Vol. 324. no. 5931, pp. 1179 - 1182
DOI: 10.1126/science.1171956
http://www.sciencemag.org/cgi/content/abstract/324/5931/1179

There is a relevant press release from the University of Leeds:
http://www.leeds.ac.uk/media/press_releases/current09/volcanic.htm

And a plenitude of media coverage - a.o.:
http://www.eurekalert.org/pub_releases/2009-05/uol-ave052709.php
http://www.physorg.com/news162738601.html
http://www.scientificblogging.com/news_articles/global_extinction_event_volcanoes_did_it_says_hypothesis
http://news.yahoo.com/s/ap/20090528/ap_on_sc/us_sci_volcano_extinction_2
http://www.scientificamerican.com/blog/60-second-science/post.cfm?id=new-evidence-for-volcanoes-as-sourc-2009-05-28
http://www.abc.net.au/science/articles/2009/05/29/2584220.htm
http://www.terradaily.com/reports/Ancient_eruption_killed_off_worlds_sea_life_scientists_999.html
http://www.nytimes.com/aponline/2009/05/30/science/30volcano-wire.html?_r=1&partner=rss&emc=rss
http://blogs.discovermagazine.com/80beats/2009/05/29/in-the-permian-period-erupting-super-volcanoes-may-have-killed-half-the-planet/



AcademicsTop Blogs

Magnetostratigraphy and the Permian Mass Extinction

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Magnetostratigraphy is a field within stratigraphy that studies the magnetic characteristics of rock bodies. If the magnetic properties of rocks have measurable differences stratigraphically, that is, from one strata to the next, those differences can be used to identify their relationships and identify varying stratigraphic units. Stratigraphic units are known collectively as magnetostratigraphic units (magnetozones). The most useful magnetic property for magnetostratigraphy results from a change in the direction of the magnetization of the rocks. Crystals in rocks are magnetically aligned with the Earth’s magnetic field. The Earth’s magnetic field has changed over the eons and those magnetic alignments are ‘recorded’ in the crystals of rocks because the rocks become magnetized in the direction of the Earth's magnetic field at the time of their formation. The change in the earth’s magnetic fields is caused by reversals in the polarity of the Earth's magnetic field, the Earth’s magnetic poles literally change locations. These reversals of Earth’s polarity have taken place many times during geologic history. (http://en.citizendium.org/wiki/Magnetostratigraphy)

The long procedure of sampling and analysis required to obtain a reversal stratigraphy for a succession of sedimentary rocks means that this technique is normally only used when other (biostratigraphic) methods cannot be used or a high-resolution stratigraphy is required. (Gary Nichols, Sedimentology & Stratigraphy, Blackwell, 1999)

Permian-Triassic gap in the fossil record?

Magnetostratigraphy has now been used to determine whether there was a Permian-Triassic gap in the fossil record of the Russian Ural Mountains.

The world’s single most severe mass extinction event which took place at the end of the Permian and start of the Triassic ages, some 250 million years ago. The extinction event, thought to be the result of runaway global warming, wiped out between 80-95 per cent of the planet’s species. Was this extinction event a real biological catastrophe or was it merely the result of gaps in the fossil record? So far it was thought that ten million years worth of rock from around that time was missing in Russia (we are talking about the continental uppermost Permian Russian stages, the Kazanian and Tatarian).

The scientists matched the magnetic record fossilised within the disputed Russian rocks with those from the rest of the World, demonstrating that the Russian rocks do indeed record the run-up to the event and the Permian/Triassic boundary and therefore the fossil losses in these rocks are part of the mass extinction.The sampled sections span the upper Guadalupian to Induan stages without any obvious break, so confirming the traditional view that the Tatarian is Late Permian in age.

Yet another piece matching in a larger jigsaw puzzle.

Reference:
Taylor et al.
Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus
Earth and Planetary Science Letters
Volume 281, Issues 1-2, 30 April 2009, Pages 36-47
doi:10.1016/j.epsl.2009.02.002
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-4VS3P43-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=26333ea71795423cab79750f8e75508d
http://www.bris.ac.uk/news/2009/6320.html



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