A Review on the Global Stratotype Section and Point of the Permian-Triassic Boundary

This paper provides a synthetic review of researches on Meishan Section D, the Global Stratotype Section and Point of the Permian-Triassic Boundary （PTB）. The history of research, geographic and geological setting of the section are briefly introduced. Changhsingian to ＂Griesbachian＂ conodont and ammonoid zonations, the most perfect Permo-Triassic zonations over the world, are presented, with a short discussion on the age of Otoceras, The Changhsingian to ＂Griesbachian＂ strata are subdivided into three 3rd order sequences, and the sedimentary structure of each sequence is indicated. The paper presents a correlation of the Changhsingian to ＂Griesbachian＂ magneetostratigraphy of the Meishan section with the general scale, and informed the negation of a short magnetic reversal at the PTB suggested by Zhu and Liu （1999）. Recent developments of chemostratigraphy of δ^13C and δ^34S are introduced, especially the discovery of more than one negative δ^13C excursions across the PTB. Two important molecular researches reveal the profound changes at the base of eco-system, the microbial catastrophy, and the euxinic conditions in the photic zone during the Permian-Triassic superanoxic event. The paper reports the changes in dating the age of PTB since 2001. According to recent achievements, the age of volcanogenic claybed 25, 14 cm below the PTB, is now set at 252.4±0.3 Ma. A discussion on the eventostratigraphy concludes that, although there is a major event episode at beds 25-26, the events across the PTB at Meishan are multi-episodic and commenced prior to the Event Beds 25-26, thus the possibility that the mass extinction was mainly induced by an exterrestrial impact is largely excluded.

Volcanism at the Permian-Triassic Boundary in South China and Its Effects on Mass Extinction

This paper discusses the clayrocks widespread at the Permian-Triassic boundary, which are mostly of volcanic origin. Volcanogenetic textures, structures and minerals such as high-temperature quartz are found in clayrocks at the Permian-Triassic boundary in many places. Thousands of microspherules have been collected from the Boundary clayrocks, many of which exhibit the typical features of the process from melting to cooling and solidification. indicating that they were formed by volcanic eruption or extraterrestrial impact. Volcanic effects on the Permian-Triassic mass extinction may be reflected in conodonts, algae and ammonoids. The Boundary clayrocks are found in many Permian-Triassic sections along the coast of Tethys. Their orighin remains to be studied.

Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary

Aromatic compounds extracted from sedimentary rocks can reflect environmental conditions, organic sources and maturity. The aromatics, identified in association with mass extinction in particular, would provide a signature assisting our understanding of the causes of the biotic crisis. Aromatic hydrocarbons were fractionated from the total lipid extracts of 37 samples taken from the PermianTriassic boundary （beds 23 to 34） of section B at Meishan （煤山）, Zhejiang （浙江） Province in South China. These aromatics were analyzed by using gas chromatography-mass spectrometry （GC-MS）. Main compounds identified include naphthalene, phenanthrenes, fluorene, dibenzothiophene, dibenzofuran, fluoranthene, pyrene and some of their methyl homologues. The indices of methyl phenanthrene distribution fraction indicate the comparable maturity （within the oil window, 0.7%-1.0% of the mean vitfinite reflectance） of the organic matter throughout the whole profile analyzed. The ratio of dibenzothiophene to phenanthrene （DBT/PHN） varies generally at a comparable pace with lithology. Significantly, a gradual decrease of this ratio was observed within bed 24 limestone, which is probably due to the variation of sedimentary environment. This change is in line with the drop in the carbon isotope composition of carbonate, the loss of the Changhsingian reef ecosystem, and the decrease of cyanobacteria abundance within the bacteria population. The coincidence of these records suggests a close relation between the biotic crisis and marine environmental conditions, and these records clearly show the onset of the biotic crisis prior to event bed 25.

Ammonoid Succession of Setorym River (Verkhoyansk Area) and Problem of Permian-Triassic Boundary in Boreal Realm

The presence of a single Otoceras species ( O. boreale ), morphologically very variable, at the base of the Nekuchan Formation in Verkhoyansk, we believe, is to be obvious. Some morphological evidence leaves no doubt that two described morphs of O. boreale are a strictly corresponding sexual dimorphic pair. It is very likely that Kummel's idea that Canadian O. concavum Tozer is an invalid species is truthful, considering the range of variability seen in larger Siberian and Himalayan Otoceras fauna. Just above the upper Tatarian Imtachan Formation, the six stages of ammonoid succession can be recognized within the lower part of the Nekuchan Formation in the Setorym River Section: (a) Otoceras boreale ; (b) Otoceras boreale-Tompophiceras pascoei ; (c) Otoceras boreale-Tompophiceras pascoei- Aldanoceras ; (d) Tompophiceras pascoei-Otoceras boreale-Aldanoceras ; (e) Tompophiceras morpheous-T. pascoei-Aldanoceras ; (f) Tompophiceras morpheous-T. pascoei-Wordieoceras domokhotovi-Ophiceras transitorium; (g) Tompophiceras morpheous-T. pascoei, corresponding to the Otoceras boreale and Tompophiceras morpheous zones. In spite of the domination of Otocerataceae or Xenodiscaceae in both of these zones and the presence of some Permian type conodonts in the lower part of the Otoceras boreale Zone, they seem to be early Induan in age on the basis of the following arguments: (1) in contrast to the underlying regressive type sediments of the Upper Tatarian Imtachan Formation, both the Otoceras boreale and the Tompophiceras morpheous zones of the lowermost part of the Nekuchan Formation correspond to the single transgressive cycle; (2) typical early Induan ammonoids ( Ophiceras and Wordieoceras ) have been recognized in the Tompophiceras morpheous zone; (3) all described ammonoid succession stages (a-g) are characterized by very gradual changes and therefore correspond to the different parts of the single zone or to the different zones of the same stage, but not to the different systems (Permian and Triassic); (4) elsewhere in the Boreal realm (Arctic Canada), the conodont index species for the base of the Triassic, Hindeodus parvus , has been reported from the Otoceras boreale Zone. A new scheme of the phylogeny for the Otocerataceae and its Induan Olenekian offspring (Araxceratidae- Otoceratidae- Vavilovitidae n.fam.-Proptychitidae-Arctoceratidae) and Xenodiscaceae is offered.

Research on Genesis of Pyrite near the Permian-Triassic Boundary in Meishan,Zhejiang,China

The content and crystal forms of pyrite and sulfur isotope composition of pyrite sulfur as well as its vertical distribution near the Permian-Triassic （P/T） boundary in the Meishan section, Changxing county, Zhejiang province, China were studied using geological, petrological, mineralogical and geochemical methods （techniques）. The result showed that the genesis of abundant pyrites in bed 24e2 at the uppermost part of the Changxing Formation in the Me- ishan section may be related to volcanic activity. In bed 24e2 of the Meishan section, pyrite has its highest content of 1.84% and the sulfur isotope composition has the highest 834S value at ＋2.2%0 which is very similar to that of the average value of volcanic gas, There are some volcanic products such as β-quartz, siliceous cylinders and siliceous spherules which coexisted with pyrites in beds 24e2 and 24f. It can be concluded that a large quantity of volcanic ash fell into the South China Sea and was incorporated into marine sediments during the formation of limestone at the uppermost part of the Changxing Formation. The volcanic eruption with massive amounts of H2S and SO2 gas at the end of the Permian period resulted in the enrichment of HES in the South China Sea areas, The reaction of H2S with reactive iron minerals formed the mass of abundant pyrites.

Determination of Fullerenes (C60/C70) from the Permian-Triassic Boundary in the Meishan Section of South China

Fullerenes (C 60 /C 70 ), clays and rocks near the Permian-Triassic (P/T) boundary in the Meishan section of South China are explored by means of comprehensive analytical techniques, including ultrasonic extraction with column purification, high-performance liquid chromatography (HPLC) and matrix assisted laser desorption/ionization time-of- flight mass spectrometry (MALDI TOF MS). The study confirms the existence of fullerenes toward the P/T event boundary and their absence in clays and limestones beyond the boundary. In particular, the white clay, known as the event boundary, contains fullerenes of 0.33 ppb, while the red material, as the first lamina fill of goethite and gypsum on the base of the white clay, contains fullerenes of 1.23 ppb, and the last lamina of 2.50 ppb. Significantly, distinct enrichment of fullerenes is coincident with the disappearance of fossil records of marine species (94%) just at the base of the white clay, implying that geological fullerenes would be one of temporal remnants led by the P/T catastrophic event. This work strongly supports that fullerenes would be one of significant records of the P/T catastrophic event but their origin remains to be studied further.

Sulfur Isotopes of Framboidal Pyrite in the Permian-Triassic Boundary Clay at Meishan Section

Pyrite framboids were found in the Permian-Triassic boundary at Meishan Section,while their sulfur isotopes were determined.The majority of framboids is less than 5μm in diameter,with some large-sized framboids.Also,euhedral gypsums were observed in the boundary clay.The authors suppose that most of the pyrite framboids formed just below the redox boundary and stopped growing after entering the lower water column.The result indicates that it was probably lower dysoxia condition in the temporal ocean.Moreover,the authors also presume that some pyrite was oxidated to sulfates accompanying the fluctuation of redox condition,which would probably be the origin of the negative sulfur isotopes of gypsum and CAS reported before.In addition,sulfur isotope of framboidal pyrite suggests that sulfur is originated from bacterial sulfate reduction in anoxic condition.Therefore, this study confirms that the ocean was widely anoxic during the Permian-Triassic transitional period. However,the redox condition in temporal ocean was probably not stable,with short-term fluctuations.

Some Evidence for a Possible Extraterrestrial Event at/near Permian-Triassic Boundary

A selection of evidence, including a carbon isotopic excursion, iridium anomaly, fullerenes (C 60 and C 70 ) with trapped noble gases, microspherules and shocked quartz, is discussed in this paper. All the evidence in hand favors the hypothesis that the PTB event was probably related to an extraterrestrial cause, and the impact would lead to great physical change, including large volcanic eruptions on the earth's surface. The ET markers for the CTB event could be considered only as an example, and cannot be taken as a unique standard of an ET event.

Occurrence of Organic Matter in Calcimicrobialites across Permian-Triassic Boundary in Huayingshan Region,Sichuan,South China

Calcimicrobialites across the Permian-Triassic boundary in Huayingshan （华蓥山） region were investigated using the fluorescence microscopic measurements to understand the occurrence of organic matter. The microbialites are composed of micrite matrix and coarse spar cement. Abundant rhombic or magnetic needle-like carbonate minerals were observed adrift within the cement. The fluorescence microscopic measurement indicates the micrite matrix in microbialites shows the most abundant organic matter, with the rhombic or magnetic needle-like carbonate minerals and coarse spar cement coming to the 2nd and the 3rd, respectively. Organic matter is mainly preserved in the space between the grains of the micrite minerals but almost evenly distributed in the rhombic or magnetic needle-like carbonate minerals. As one of the common diagenesis types, dolomitization is observed to occur in the microbialites in Huayingshan. However, the carbonate cement in microbialites still has high content of element Sr as shown by the microprobe analysis, reflecting that the dolomitization might have happened in a restricted environment. Observation under the fluorescence microscope shows that dolomitization just led to the redistribution of organic matter in the grain space of dolomite minerals, inferring that the diagenesis has a slight effect on the preservation, and thus on the content of organic matter in the microbialites.

Alkane Biomarkers in Permian-Triassic Boundary Strata at Meishan Section, Changxing, Zhejiang Province

Meishan Section D in Changxing County, Zhejiang Province, China has been selected as the global stratotype of the Permian Triassic boundary and various studies have been done at the boundary, but the gas chromatographic mass spectrographic analysis of alkane biomarkers has not been investigated. This paper presents the results of a study of the biomarkers analyzed in a series of samples across the Permian Triassic boundary at both Meishan Section A and Section D. The results show that the overall concentration of alkane biomarkers in the Permian Triassic boundary strata is high in Bed 26 while it is low in Bed 27. A variety of biomarker parameters demonstrate that the main sources of organic matter in the sediment are algae and bacteria and that the depositional environment varied from weakly oxidizing to reducing during the studied interval.

A Proposed Area for Study of Accessory Section and Point of Terrestrial Permian-Triassic Boundary

After the establishment of the global stratotype section and point (GSSP) of the Permian Triassic boundary (PTB), the definition of the accessory section and point (ASP) of the terrestrial Permian Triassic boundary (TPTB) is now on the agenda. However, all good TPTB sections so far known have the following shortcomings: (1) the exact TPTB horizon is difficult to define paleontologically with high resolution, and (2) accurate correlation between marine and terrestrial PTBs is hard to attain. In order to enhance the understanding of the nature of the global life crisis in both the marine and terrestrial environments across the Paleozoic Mesozoic transition, these shortcomings need to be addressed. In western Guizhou and eastern Yunnan, Southwest China, some fossiliferous PTB sections which include marine, paralic and terrestrial are well developed, allowing bed to bed correlation of the PTB sequences. Fortunately, the marine PTB sequence in this area is almost the same as found at the Meishan Section, where the GSSP of the PTB is located, which may provide a reliable auxiliary marker for high resolution demarcation of the TPTB. These features found in western Guizhou and eastern Yunnan make this area a good place to study the ASP of the TPTB, so we propose to study the ASP of the TPTB in this area.

Paleoenvironments and geochemistry across a continuous Permian-Triassic boundary section at Bukk Mountains,Hungary

The Bálvány North Permian-Triassic boundary sediments were deposited on a carbonate platform in the tropical part of the western Paleo Tethys ocean.The overall elemental geochemistry of the detailed two-metre-thick section across the boundary that we studied shows that the clastic content of the sediments came from dominantly silica-rich continental sources though with some more silica-poor inputs in the uppermost Permian and lowest Triassic limestones as shown by Ni/Al and Nb/Ta ratios.These inputs bracket,but do not coincide with,the main extinctions and associated C,O and S changes.Increased aridity at the Permian-Triassic boundary with increased wind abrasion of suitable Ti-bearing heavy minerals accounts for both the high Ti/Al and Ti/Zr ratios.Various geochemical redox proxies suggest mainly oxic depositional conditions,with episodes of anoxia,but with little systematic variation across the Permian-Triassic extinction boundary.The lack of consistent element geochemical changes across the Permian-Triassic boundary occur not only in adjacent shallower-water marine sections,and in other marine sections along the SW Tethys margin such as the Salt Range sections in Pakistan,but also in deeper shelf and oceanic sections,and in non-marine African and European continental sediments.In the absence of significant changes in physical environments,chemical changes in the atmosphere and oceans,reflected in various isotopic changes,drove the Permian-Triassic extinctions.

Ostracodes(Crustacea) Witnesses of Paleoenvironmental Changes During Permian-Triassic Boundary Events in South China and Turkey

The transition between Permian and Triassic(252.4 Ma) is marked by the most devastating extinction the biosphere ever went through.Although its cause stays unclear,it is admitted that this mass extinction originated in the conjunction of several global events: tectonic(Pangaea closing),volcanic(Siberian traps), magnetic(reversal of superchron),climatic(end of Permian glaciation) and eustatic(tecto-and ther-

Analyses of Sequence Stratigraphy and Environments across Permian-Triassic Boundary in Liaotian, Northwestern Jiangxi Province

Based on the study of lithology, sedimentology and paleontology at the Permian Triassic boundary in Liaotian, Northwestern Jiangxi Province, the sequence stratigraphy and depositional environments across the boundary are reconstructed. The top part of the Upper Permian Changxing Formation is composed of very thick bedded light colored dolomitic limestone formed in high deposition rate on carbonate ramp, which indacates a transgression systems tract (TST). The Lower Triassic Qinglong Formation shows continuous deposition with the underlying Upper Permian. The lower member of Qinglong Formation consists of calcareous shale, shelly limestome and dolomitic limestone with abundant bivalves ( Claraia sp.)and trace fossils ( Chondrites ). The calcareous shale at the bottom of Lower Triassic indicates a calm deep water environment to form the condensed section (CS). The shelly limestome and dolomitic limestone with shell fossils, intraclast, algal ooide show clean but turbulent environment of carbonate ramp, which produce the deposition of highstand systems tract (TST).

Analysis of platinum-group elements in drill core samples from the Meishan Permian-Triassic boundary section, China

There is a long-standing controversy of what triggered the extinction at the Permian-Triassic boundary, the most severe mass extinction in the geologic record, including flood basaltic volcanism and/or bolide impact hypothesis. In order to clarify various pieces of evidence for the mass extinction event at the Permian-Triassic boundary, some researchers from some laboratories throughout the world have made a comprehensive study on a group of samples from the Meishan area of China. Some fresh core samples from the Permian-Triassic boundary in the Meishan area were analyzed in this study. The results showed that there is no Ir anomaly. Moreover, the PGEs patterns of those samples show obvious differentiation characteristics, that is different from the case encountered in meteorites. So no evidence supports the hypothesis of extraterrestrial impact. In contrast, the PGEs patterns are similar to those of Siberian and Emeishan basalts, which indicates that those PGEs are derived mainly from the basalts, lending a support to the correlation between mass extinction at the Permian-Triassic boundary and flood basaltic volcanism. This study has also confirmed the results for samples from section C prior to the analysis of the samples.

Conodont Biostratigraphy and Evolution across Permian-Triassic Boundary at Yangou Section,Leping,Jiangxi Province,South China

Series of large conodont samples with 20 species and 3 similar species in 3 genera were collected from the Permian-Triassic （P-T） boundary sequence in a shallow carbonate facies at Yangou （沿沟）, Leping （乐平） County, Jiangxi （江西） Province, South China. On the basis of the distributions of the identified species, seven conodont zones have been recognized in ascending order as follows, Neogondolella changxingensis zone, Neogondolella yini zone, Hindeodus changxingensis zone, Neogon- dolella taylorae zone, Hindeodus parvus zone, Isarcicella staeschei zone, and IsarciceUa isarcica zone. The successive occurrences of Hindeodus changxingensis, NeogondoleUa taylorae and Hindeodus parvus serve as proxies for defining the P-T biostratigraphy boundary at the base of Sub-bed 21-4 of Bed 21 in the Yangou Section. Correlations with the Meishan Section are also discussed in terms of conodont bio-stratigraphy. Three successive conodont faunal assemblages are grouped through the P-T transitional interval to examine the evolution of conodonts across the great transitional event.

Ages, Trace Elements and Hf-Isotopic Compositions of Zircons from Claystones around the Permian-Triassic Boundary in the Zunyi Section, South China: Implications for Nature and Tectonic Setting of the Volcanism

A growing body of evidence shows that volcanism near the Permian-Triassic boundary（PTB） may be crucial in triggering the Permian–Triassic（P–Tr） mass extinction. Thus, the ash beds near the PTB in South China may carry information on this event. Three volcanic ash layers, altered to clay, outcropped in the PTB beds in Zunyi Section, Guizhou Province, Southwest China. The U-Pb ages, trace elements, and Hf-isotope compositions of zircon grains from these three ash beds were analyzed using LA-ICPMS and LA-MC-ICPMS. The zircons are mainly magmatic in origin（241-279 Ma） except for two inherited/xenocrystic zircons（939 and 2 325 Ma）. The ages of these magmatic zircons indicate three episodes of magmatism which occurred around the MiddleLate Permian boundary（-261.5 Ma, MLPB）, the Wuchiapingian-Changhsingian boundary（-254.5 Ma, WCB）, and the PTB（-250.5 Ma）, respectively. The first two episodes of magmatism near the MLPB and WCB may be attributed to magmatic inheritance or re-deposition. All magmatic zircons share similar trace-element and Hf-isotope compositions. They have Y, Hf, Th and U contents and Nb/Ta ratios are typical of zircons from silicic calc-alkaline magmas. These zircons also exhibit enriched Hf-isotope compositions with _（εHf）（t） values of-11.4 to ＋0.2, which suggests that the three magmatic episodes involved melting of the continental crust. The more enriched Hf-isotope composition （_（εHf）（t）=-11.4--4.8） of Bed ZY13（-250.5 Ma） implies more input of ancient crustal material in the magma. Integration of the Hf-isotope and trace-element compositions of magmatic zircons suggest that these three episodes of magmatism may take place along the convergent continent margin in or near southwestern South China as a result of the closure of the Palaeo-Tethys Ocean.

Diagenesis of the microbialites in the Permian-Triassic boundary section at Laolongdong,Chongqing,South China

An interval of limestone,which generally has a dendroid appearance on outcrops,has been found in several Permian-Triassic boundary(PTB) sections in South China,and interpreted to be microbialites.Previous research has examined the paleontology and sedimentology of the unit,however,little attention has been devoted to its diagenesis.This paper discusses the diagenesis of the microbialites in the PTB section at Laolongdong,Chongqing,South China.Examination of multiple outcrops reveals that the structure in this kind of rock is not always dendroid;in many cases it is irregular in shape,and seldom shows the "up-branching" form.In addition,the speckled interval has also been found beneath the dendroid interval.In outcrop,both the speckled and dendroid rocks consist of the darkercolored areas and the lighter-colored areas.Examination of thin sections reveals that the darker-colored areas on outcrops are actually lighter-colored in thin sections,consist of larger crystals,and are more transparent.The lighter-colored areas in thin sections generally consist of large blocky calcite containing scattered small dirty calcite rhombs and irregular residual limestones,which are the same as the lime mudstones between the lighter-colored areas.It is inferred that the dendroid and speckled rocks have experienced the following diagenetic processes:(1) exposure of the carbonate sediments to the subaerial environment because of a sea level drop;(2) dolomitization caused by the downward migration of fluids formed the dendroid and speckled dolomitic patches;(3) dedolomitization of the dolomitic patches formed dendroid and speckled patches of calcite;(4) dissolution occurred in the interstices between relic dolomite crystals formed spongy pores;and(5) filling of the spongy pores by large blocky calcite.Therefore,even though microbes played a critical role in the formation of these microbialites,diagenesis contributed greatly to the formation of the speckled and dendroid pattern of the microbialites.

Sedimentary features of the Permian-Triassic boundary sequence of the Meishan section in Changxing County,Zhejiang Province

The Global Stratotype Section and Point(GSSP) for the Permian-Triassic boundary was placed in Bed 27 of Meishan section D where the conodont species Hindeodus parvus first occurs.Bed 27 was usually considered comprising continuouslydeposited,homogeneous silty limestone,with no depositional hiatus near the boundary.Detailed study on the boundary sequence revealed that a typical firmground characterized by Glossifungites ichnofacies developed about 2 cm below the Permian-Triassic boundary in Bed 27.Fossil content and lithology show apparent differences across the firmground crust.The abundance of the Permian bioclasts decreases significantly across the firmground,and is accompanied by a shift of dominating carbonate precipitation from calcite to dolomite.The firmground marked a rapid transgression at the very end of the Late Permian and significant shifts of sedimentary environment and paleoclimate.This transgressive submerging surface is also observed at the Huangzhishan section of the shallow-water carbonate platform facies in Zhejiang Province,the Jiangya section of the lower-slope to basinal-margin facies in Hunan Province,the Pingdingshan section of the basinal facies in Anhui Province of South China,as well as the Selong section in Tibet of the northern peri-Gondwana.The transgressive submerging surface marks the onset of a rapid global transgression at the latest Permian.

Palaeogeographic variation in the Permian-Triassic boundary microbialites:A discussion of microbial and ocean process esafter the end-Permian mass extinction

Shallow marine carbonate sediments that formed after the end-Permian mass extinction are rich in a thin(maximum ca.15 m) deposit of microbialites.Microbial communities that constructed the microbialites have geographic variability of composition,broadly divisible into two groups:1) eastern Tethys sites are calcimicrobe-dominated(appearing as thrombolites in the field),with rare occurrence of sedimentconstructed microbialites and uncommon cements either within microbial structure or as inorganic precipitates,2) other Tethys sites are sediment-dominated structures forming stromatolites and thrombolites,composed of micrites and cements,with some inorganic precipitates.These other Tethys locations include western and central Tethys sites but their palaeogeographic positions depend on the accuracy of continental reconstructions,of which there are several opinions.In contrast to geographic variation of microbialites,the conodont Hindeodus parvus,which appeared after the extinction and defines the base of the Triassic,is widespread,indicating easy lateral migration throughout Tethys.Conodont animals were active nekton,although being small animals were presumably at least partly carried by water currents,implying active Tethyan surface water circulation after the extinction event.Post-extinction ammonoid taxa,presumed active swimmers,show poor evidence of a wide distribution in the Griesbachian beds immediately after the extinction,but are more cosmopolitan higher up,in the Dienerian strata in Tethys.Other shelly fossils also have poorly defined distributions after the extinction,but ostracods show some wider distribution suggesting migration was possible after the extinction.Therefore there is a contrast between the geographic differences of microbialites and some shelly fossils.Determining the cause of geographic variation of post-extinction microbialites is problematic and may include one or more of the following possibilities:1) because calcifying microbial organisms that create calcimicrobes were benthic,they may have lacked planktonic stages that would have allowed migration,2)eastern Tethyan seas were possibly more saturated with respect to calcium carbonates and microbes,so microbes there were possibly more able to calcify,3) significant reduction of Tethyan ocean circulation,perhaps by large-scale upwelling disrupting ocean surface circulation,may have limited lateral migration of benthic microbial communities but did not prevent migration of other organisms,and 4) microbes may have been subject to local environmental controls,the mechanisms of which have not yet been recognized in the facies.The difficulty of distinguishing between these possibilities(and maybe others not identified) demonstrates that there is a lot still to learn about the post-extinction microbialites and their controls.