The Permian and Triassic Chronostratigraphic Scales—Framework for Ordering Events

Chapter 2

The Permian and Triassic Chronostratigraphic Scales—Framework for Ordering Events S.G. Lucas New Mexico Museum of Natural History and Science, Albuquerque, NM, United States

Chapter Outline 1 Introduction 2 Building a Chronostratigraphy 3 Permian Chronostratigraphic Scale 3.1 Some History 3.2 Subdivisions of the Permian 3.3 Regional Permian Chronostratigraphic Scales

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4 Triassic Chronostratigraphic Scale 4.1 Some History 4.2 Subdivisions of the Triassic 4.3 Other Triassic Chronostratigraphic Scales 5 Integrated Timescales Acknowledgments References

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1 INTRODUCTION The Paleozoic Era ended with the approximately 47-million-year-long Permian Period, a major juncture in Earth history when the vast Pangean supercontinent continued its assembly (Fig. 1), and the global biota faced its greatest diversity crisis, the end-Permian mass extinction, the most extensive biotic decimation of the Phanerozoic. The Mesozoic Era followed with the approximately 50-million-year-long Triassic Period, another major juncture in Earth history when the vast Pangean supercontinent completed its assembly and began its fragmentation (Chapter 3), and the global biota diversified and modernized after the end-Permian mass extinction. Across Permo-Triassic Pangea, paleoenvironments ranged from deserts to epeiric seas, and some of the latter were important loci of salt accumulation (Chapter 1). The temporal ordering of geological and biotic events during Permian and Triassic time thus is critical to the interpretation of a unique and pivotal time in Earth history. This temporal ordering is mostly based on the Permian and Triassic chronostratigraphic scales—relative geologic time scales of series, stages, and substages that have been developed and refined for nearly two centuries. My goals here are to briefly review how the Permian and Triassic chronostratigraphic scales have been constructed, summarize the state of the art of those chronostratigraphic scales and indicate the path to future chronostratigraphic refinement.

2  BUILDING A CHRONOSTRATIGRAPHY The geological timescale has developed over more than two centuries, primarily by constructing a relative timescale of chronostratigraphic units. These units are nested in a well-known hierarchy of systems (such as Permian), series (such as Cisuralian), and stages (such as Asselian). This began, mostly in the early 1800s, with the proposal of terms that correspond to what were judged to be distinctive stratigraphic successions, often with characteristic fossils. By the end of the 1800s, there was nearly universal agreement on most of the geological system and series terms. Beginning about in the 1960s, an effort was made to standardize stage terminology, which was diverse and complex for most systems, with many competing alternatives for stage subdivisions and their names. This led to current stratigraphic

Permo-Triassic Salt Provinces of Europe, North Africa and the Atlantic Margins. http://dx.doi.org/10.1016/B978-0-12-809417-4.00002-1 © 2017 Elsevier Inc. All rights reserved.

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FIG. 1  Map of Permian-Triassic Pangea showing locations of ratified GSSPs (Global Stratotype Section and Point) of some of the Permian and Triassic stages. (Map modified from Lucas, S. G., Schneider, J. W., & Cassinis, G. (2006). Non-marine Permian biostratigraphy and biochronology: An introduction. In S. G. Lucas, G. Cassinis, & J. W. Schneider (Eds.), Non-marine Permian biostratigraphy and biochronology (Vol. 265, pp. 1–14). London: Geological Society (Special Publications).)

practice, which seeks to recognize a single global stage for each interval of geological time, and each series and system base corresponds to the base of a stage (this is the standard global chronostratigraphic scale). Furthermore, the definition of stages is now based on the method that defines the bases of stages by a global stratotype section and point (GSSP) correlatable by one or more “signals,” usually a biostratigraphic datum. A heavy emphasis is also placed on integrated stratigraphy, which applies multiple data sets (different fossil groups, radioisotopic ages, magnetostratigraphy, and chemostratigraphy) to the definition of chronostratigraphic units (e.g., Salvador, 1994; Remane et al., 1996; Walsh, Gradstein, & Ogg, 2004). This work of defining a chronostratigraphic scale by GSSPs is largely that of the International Commission on Stratigraphy (ICS), a body politic of the International Union of Geological Sciences. Under ICS oversight, each geological system has a subcommission devoted to defining and refining the timescale of that system. Thus, both the Permian and the Triassic systems have ICS subcommissions who have been working on their timescales for decades.

3  PERMIAN CHRONOSTRATIGRAPHIC SCALE 3.1  Some History Recognition of a distinctive interval in Earth history (originally identified as a distinct succession of stratified rocks) that corresponds to the current concept of Permian began in Germany more than 200 years ago. The history of the development of the Permian chronostratigraphic scale can be divided into five distinct phases: (1) initial studies of the Permian strata of Germany during the 1700s; (2) in 1841, Murchison coins the term Permian to refer to a succession of rocks in the Ural Mountains region of European Russia; (3) a century-long process that ended with recognition of the Permian as a distinct system of the geological timescale; (4) development of diverse Permian stage nomenclatures and alternative Permian chronostratigraphic scales between about 1874 and 1976; and (5) beginning in 1975, the work of the ICS Subcommission on Permian Stratigraphy (SPS) to develop the current Permian chronostratigraphic scale (Lucas & Shen, 2017). Continental European geologists of the late 1700s united the German Rotliegend and Zechstein into one “group” or “system.” These units comprised an economically important stratigraphic interval because they included important sources of copper and salt. Inclusion of the Rotliegend and Zechstein in a single system thus had long precedence in German geological research (Zittel, 1901) and without doubt facilitated later acceptance of the Permian System, which united them. About half a century later, British geologist Roderick Murchison (1792–1871) named the Permian as a result of fieldwork he undertook in Russia. This fieldwork, in 1840 and 1841, is well documented by Murchison, de Verneuil, and von

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Keyserling (1845) and Collie and Diener (2004), who published an edited and annotated version of Murchison’s previously unpublished narrative description of his work in Russia. To summarize, while in Paris in 1840, Murchison became aware of an extensive, flat-lying (little deformed) and fossiliferous Paleozoic section in the Baltic region of European Russia. That summer he went to Russia to examine the older Paleozoic strata and to confirm their succession and in particular to establish further the validity of the Devonian System. Murchison returned to Russia the next summer for 5 months in 1841, but this time with the financial support of the Russian Czar, ostensibly to evaluate coal resources. An incidental byproduct of the second trip was the naming of the Permian, first proposed in a letter Murchison wrote in the Fall of 1841 to the Russian government official Johann Gotthelf Fischer von Waldheim in St. Petersburg. Fischer von Waldheim (1841) published this letter in Russian (it was originally written in French), and identified Murchison as its author. Later, in 1841, Murchison published an article (essentially an English translation of the letter) in the Philosophical Magazine establishing the Permian Period for a succession of marls, limestones, sandstones and conglomerates on the western flank of the Urals. According to Murchison, in Russia the Permian System overlies Carboniferous rocks (including the “grits of Artinsk”) that he correlated to the British Millstone Grit. Murchison judged fossil fishes and amphibians from the Russian Permian similar to those of the German Zechstein, which supported correlation of the Russian Permian to the British Magnesian Limestone. Murchison also considered the Permian fossil plants to be intermediate between those of Carboniferous and Triassic ages. He thus equated part of the Permian to the British “lower New Red Sandstone,” which supported correlation to the German Rotliegend. However, it has long been clear that Murchison’s type Permian is not the entire Permian of most later usage, and even included some Lower Triassic strata (Fig.  2). Murchison regarded as Carboniferous the underlying strata that are now considered the majority of the lower Permian (Cisuralian) Series. This means that the original base of the Permian sensu Murchison was much younger than the current base of the Permian. Murchison’s (1841) Permian soon gained wide use. By the 1880s, Permian rocks and fossils were recognized well beyond Europe, in North America, India, South Africa, and China. Indeed, the idea of “Gondwána-land” published by Austrian geologist Eduard Suess (1831–1914) in his classic book Das Antlitz der Erde (1885) was based in part on his recognition of Permian rocks in Australia, India, and Africa. Accepting the Permian as a separate system proved most difficult among North American stratigraphers, who generally combined it with the Carboniferous till about the time of the Second World War. Thus, the US Geological Survey long recognized a Carboniferous System divided into three series—Mississippian, Pennsylvanian, and Permian (Wilmarth, 1925). However, in 1941, the US Geological Survey finally recognized the Permian as a separate system (Cohee, 1960). With that late recognition by American geologists, the Permian came into universal use as a distinct system/period of the geological timescale. Subdivision of the Permian into series and stages began with Karpinsky (1874), who used the term Artinskian to refer to the ammonoid-rich clastic succession of strata immediately below Murchison’s type Permian (the “grits of Artinsk” of Murchison et al., 1845). Almost all other Permian stage terminology developed by about 1976. This development took place primarily in three separate countries, at least in terms of stage names that have attained global usage. Thus, Russian regional stages of the type Permian—Kungurian, Ufimian, Kazanian, Tatarian—were coined.

FIG. 2  Woodcut diagram showing the inferred European equivalents of the Russian type Permian. Note that the “Rothe-todte-liegende” is actually older than the Russian type Permian, and that most of the “Lower Bunter” is Triassic. (From Murchison, R. I., de Verneuil, E., & von Keyserling, A. (1845). The geology of Russia in Europe and the Ural Mountains. Volume 1. Geology (p. 204). London: John Murray.)

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The early Permian stages Asselian and Sakmarian were mostly developed by the Soviet expert on late Paleozoic ammonoids, V.E. Ruzhentsev (1899–1978) in the 1930s–1950s. Chinese Permian stage names began with Huang (1932). The North American series terms (often treated as stages) were introduced by Adams et al. (1939). Also, Tethyan stage names were originally based on work in Tajikistan by Miklucho-Maklay (1958). About half a century ago, Glenister and Furnish (1961), Furnish (1973), and Waterhouse (1976, 1978) provided useful reviews of Permian stage terminology. They presented three Permian chronostratigraphic scales similar to each other but different in details. It is fair to say that these scales reflect the state of Permian chronostratigraphy when the SPS began its work. The SPS began its work in the early 1970s. It was authorized by the ICS to develop a standardized and globally applicable Permian timescale. After more than 40 years of work, the current SPS chronostratigraphic scale (Fig. 3) is nearly finished, recognizing three series and nine stages, six of which have ratified formal definitions of their basal boundaries. Much of the ongoing work of the SPS is focused on resolving the definitions of the bases of the three remaining, undefined stages, the Sakmarian, Artinskian, and Kungurian (Fig. 3).

3.2  Subdivisions of the Permian Division of the Permian into two series, lower and upper, corresponding in some sense to the European Rotliegend and Zechstein, had a tremendous amount of precedence to as far back as the 1700s. But, by the 1990s, it became clear to many students of the Permian timescale that dividing the Permian into three series would better represent geologic and biotic

FIG. 3  The SPS (Subcommission on Permian Stratigraphy) Permian chronostratigraphic scale showing a possible set of Permian substages. Note that no substages of the Guadalupian have been proposed.

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events during the period. Thus, they advocated and ultimately agreed on lower, middle, and upper Permian series, the Cisuralian, Guadalupian, and Lopingian (Fig. 3). Major changes in fusulinid and ammonoid evolution, in particular, mark the boundaries of the Guadalupian, as does the middle Permian marine transgression and global highstand so evident in Tethys and western North America. Furthermore, the Guadalupian ends with a major marine regression and a substantial extinction. Waterhouse (1982) introduced the term Cisuralian Series to encompass the Asselian, Sakmarian, and Artinskian stages. Jin, Wardlaw, Glenister, and Kotlyar (1997) added the Kungurian to the Cisuralian. Cisuralian is now the official term for the lower Permian series (Fig. 3). Based on a now classic Permian marine section in the Guadalupe Mountains of West Texas, Girty (1902) coined the term Guadalupian period. Adams et al. (1939) used Guadalupian as a series, but some other workers (e.g., Schenck et al., 1941; Glenister & Furnish, 1961) used it as a stage. Glenister et al. (1992) reviewed use of the term Guadalupian and formally proposed it as the middle Permian series to include the Roadian, Wordian, and Capitanian stages. This proposal was ratified by ICS in early 2001 (Henderson, Davydov, & Wardlaw, 2012) (Fig. 3). The Guadalupian was the time of the most extensive marine transgression and the warmest climates of the Permian. Among ammonoids, the cyclolobids and ceratatidans first appeared during the Guadalupian. The advanced fusulinids (Verbeekinidae, Pseudodoliniacea) diversified, and the Neoschwagerinida, Polydexodinidae, and Tangchienidae appeared. The goniatitids and most of the fusulinids disappeared at or by the end of the Guadalupian at a substantial mass extinction event. Grabau (1923) had early used the term Loping horizon for a limestone stratigraphically beneath the Changhsing Limestone in southern China. Huang (1932) used the Loping as a series to refer to all Chinese Permian strata above the Maokou Formation. Huang’s (1932) Lopingian thus is the oldest name for an upper Permian series based on a relatively complete marine succession, in preference to Ochoan (Adams et al., 1939), Djulfian (also spelled Dzhulfian) (Schenck et al., 1941), and Yichangian or Transcaucasian (Waterhouse, 1982). Lopingian is now the official upper Permian series name and encompasses the Wuchiapingian and Changhsingian stages (Fig. 3). The Lopingian is bracketed by two severe biotic crises, the end-Guadalupian (or pre-Lopingian) and end-Permian extinctions. The base of the Lopingian is also a major global regression of sea level that marks the boundary between the middle and upper Absaroka megasequences.

3.3  Regional Permian Chronostratigraphic Scales Ideally, the Permian chronostratigraphic scale consists of one set of stages applied globally. However, the provinciality of fossil taxa compounded by limitations of facies distributions (rarely is any taxon or facies global in extent) have hindered universal recognition and use of a single chronostratigraphic terminology. This is why regional Permian stages, which are specific to paleoprovinces and/or facies, are of great utility in regional correlations (Fig. 4). Indeed, there is great value in regional stages, which Cope (1996) has aptly called the “secondary standard” in stratigraphy. The Permian has a variety of secondary standards, and these provided a rich source for the nine stages ultimately chosen to define the standard chronostratigraphy for the system (e.g., Menning et al., 2006). In the literature of Permian salt mining, the old German’s miners’ terms are still being applied. Rotliegend is now treated as a lithostratigraphic group, and has a very long temporal range from latest Carboniferous (Gzhelian) through earliest Lopingian (early Wuchiapingian) (e.g., Lucas, Schneider, & Cassinis, 2006; Schneider, Rossler, Werneburg, Scholze, & Voigt, 2014). The term Weissliegendes actually pertains to facies equivalent to the lower Zechstein. The Zechstein ranges in age from early Wuchiapingian through Changhsingian (e.g., Schneider et al., 2014).

4  TRIASSIC CHRONOSTRATIGRAPHIC SCALE 4.1  Some History As was the case with the Permian, recognition of a distinctive succession of stratified rocks that corresponds to the current concept of Triassic began in Germany more than 200 years ago. This early work was culminated by von Alberti’s (1834) monograph in which he coined the term Trias (Fig. 5). The 200-year-long history of the development of a Triassic chronostratigraphic timescale can be divided into five distinct phases: (1) the initial studies of the Triassic strata of Germany, culminated by von Alberti’s (1834) recognition of the Trias “formation”; (2) extension of the term Trias to marine rocks and fossils outside of its type section in Germany, particularly southward into the Alps of Austria-Italy; (3) recognition of subdivisions of Triassic time based on ammonoids, primarily in the Alps and in what is now Pakistan; (4) the New World

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FIG. 4  The most widely used Permian regional chronostratigraphic scales. Note that this is not intended to be a precise correlation chart of the different scales to each other. (After Henderson, C. M., Davydov, V. I., & Wardlaw, B. R. (2012). The Permian Period. In F. M. Gradstein, J. G. Ogg, M. D. Schmitz, & G. M. Ogg (Eds.), The geologic time scale 2012 (Vol. 2, pp. 653–679). Amsterdam: Elsevier.)

Triassic timescale of Canadian paleontologist E. Timothy Tozer, first published in the 1960s; and (5) the timescale still being developed by the Subcommission on Triassic Stratigraphy (STS) (Lucas, 2010). Rocks and fossils in central Europe now considered to be Triassic in age have been studied since the late 1700s and were particularly well studied in southwestern Germany. Thus, the German geologists of the late 1700s and early 1800s recognized that a thick succession of strata lay between the Zechstein and the marine strata that came to be called Lias or Jura. They saw the succession as tripartite—lower, sandstone-dominated interval (“Bunter”), middle, carbonate-dominated interval (“Muschelkalk”), and claystone-dominated upper interval (“Keuper”). One of these geologists was Friedrich August von Alberti (1795–1878), a salt mining engineer and mine manager who had a diverse knowledge of the geology of southwestern Germany. The rocks he came to call Trias in 1834 are extensively exposed in southwestern Germany.

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FIG. 5  Title page of von Alberti’s (1834) monograph in which the name Trias is proposed.

And, Alberti recognized Triassic rocks in countries outside of Germany, including England, France, Austria, Italy, Poland, Russia, the United States, and India. This early recognition of the broad geographic distribution of the “Trias formation” served to insure the ready acceptance of the Triassic as part of the geological timescale. Alberti’s type Triassic in southwestern Germany is a sandwich of dominantly nonmarine red beds (Buntsandstein and Keuper) with a restricted marine middle portion (Muschelkalk). However, in Austria and northern Italy, the Alps contain a relatively complete section of Triassic marine strata, so extension of the Triassic into the Alpine marine strata became central to further subdivision and correlation of Triassic time. The subdivision of Triassic time owes more to the Austrian geologist Edmund von Mojsisovics (1839–1907) than to any other geologist. Mojsisovics 30-year career began in the 1860s. He worked for the Geological Survey (the Geologiches Reichsanstalt) of the Hapsburg monarchy, collecting and studying Triassic ammonoids throughout much of central and southern Europe, and publishing on ammonoids sent to him from locations as remote as the Olenek River in eastern Siberia. Recognition of subdivisions of Triassic time based on ammonoids by Mojsisovics and his collaborators, particularly Carl Diener (1862–1928) and Wilhelm Heinrich Waagen (1841–1900), produced most of the stage-level terminology of Triassic chronostratigraphy still used today. This work was culminated by von Mojsisovics, Waagen, and Diener (1895), perhaps the most important article written on Triassic chronostratigraphy. This article built largely on Mojsisovics’s and Diener’s work in Europe (primarily Austria, Italy, and Bosnia), and Waagen’s work in the Salt Range of what is now Pakistan. It coined the names of most of the marine stages and substages recognized today (Fig. 6). This timescale was refined subsequently, especially by the addition of von Bittner’s (1892) Ladinian, but remained the basic Triassic timescale until at least the 1960s. Beginning in the 1960s, Canadian paleontologist E. Timothy Tozer (1928–2010), in part collaborating with American geologist Norman J. Silberling (1928–2011), assembled a Triassic timescale based on North American ammonoid zones

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FIG. 6  The Triassic timescale of von Mojsisovics et al. (1895).

(e.g., Tozer, 1967, 1971, 1984, 1994; Silberling & Tozer, 1968). Key components of Tozer’s Triassic timescale were that it: (1) used most of the Middle and Upper Triassic stage names of von Mojsisovics et al. (1895), with the addition of Bittner’s Ladinian; (2) but, it defined Triassic stage boundaries based on North American ammonoid localities; (3) it rejected the Rhaetian as a distinctive stage; and (4) new stage names were coined to create a fourfold division of the Lower Triassic. For about two decades, Tozer’s Triassic chronostratigraphy, especially the fourfold subdivision of the Lower Triassic, found wide acceptance in the English language literature on the subdivision of Triassic time, though few abandoned the Rhaetian (e.g., Kummel, 1979; Harland et al., 1982, 1990). Conceived in 1968, and beginning its meetings in the 1970s (Tozer, 1985), the STS, as part of the ICS, was primarily created to establish a global Triassic timescale based on formal definitions of the bases of the Triassic stages (e.g., Gaetani, 1996). After initial acceptance in 1984 of most aspects of the Tozer timescale, in 1991, the STS agreed on a stage nomenclature of the Triassic in which key aspects of Tozer’s timescale were rejected (Fig. 7). Indeed, the STS is now close to a formal definition of the Rhaetian Stage. However, despite more than 40 years of work, few of the Triassic stage boundaries have been formally defined—only the bases of the Induan, Ladinian and Carnian have ratified GSSPbased definitions. Conodont-defined boundaries have been proposed for the bases of the Olenekian, Anisian, Norian, and

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FIG. 7  The current STS (Subcommission on Triassic Stratigraphy) Triassic chronostratigraphic scale. Note that the Griesbachian substage as originally defined extends downward into the Permian, and that no substages of the Rhaetian have been proposed.

Rhaetian but remain to be agreed on. The three Triassic series remain stable, as do the stages of the Middle and Upper Triassic. However, usage of two, three or four Lower Triassic stages reflects different approaches to that part of the Triassic chronostratigraphic scale (Fig. 8).

4.2  Subdivisions of the Triassic The Triassic has always been subdivided into three series: Lower, Middle, and Upper. No other series-level subdivisions of the Triassic have been proposed, other than Lucas (2013), who suggested a four-series Triassic. Some workers (e.g., Harland et al., 1982) have used the term Scythian to refer to the Lower Triassic series, but this has not been widely used. Thus, no formal terms are used for the three Triassic series—they are simply the Lower, Middle and Upper Triassic Series. The “type” Lower Triassic is the German Buntsandstein, a complex lithosome that consists mostly of continental redbed siliciclastic strata. Biostratigraphic and magnetostratigraphic data now indicate that the Buntsandstein ranges in age from latest Permian to earliest Middle Triassic (early Anisian) (e.g., Kozur & Bachmann, 2005). However, given that the type Lower Triassic is mostly nonmarine strata, it has not been used as a standard to subdivide Early Triassic time.

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FIG.  8  Various proposed subdivisions of the Lower Triassic. (Modified from Tozer, E. T. (1974). Definitions and limits of Triassic stages and substages: Suggestions prompted by comparisons between North America and the Alpine-Mediterranean region. Schriftenreihe Erdwissenschaftlichen Kommissionen Osterreichische Akademie der Wissenschaften, 2, 195–206.)

There has been more disagreement about the stage-level subdivisions of the Lower Triassic than there has been about subdividing the Middle and the Upper Triassic. Indeed, as Tozer (1978) noted, about 20 chronostratigraphic terms have been proposed to refer to all or part of the Lower Triassic (Fig. 8). The STS now divides the Lower Triassic into two stages, the Induan and the Olenekian. However, other schemes of subdividing the Lower Triassic exist, particularly in North America, where at least three Lower Triassic stages have been recognized (Fig. 8). The STS voted in 1984 and in 1991 to use two stages to divide the Middle Triassic—the Anisian and Ladinian (Fig. 7). The Anisian and Ladinian stages have also been subdivided into substages based on ammonoid biostratigraphy. At present, the STS recognizes three stages—Carnian, Norian, and Rhaetian—as the Upper Triassic. These stages also have substage divisions based on ammonoid biostratigraphy (Fig. 7).

4.3  Other Triassic Chronostratigraphic Scales Like the Permian, the Triassic has a variety of provincial stage names, and some of the disagreement about a single stage nomenclature of the Lower Triassic well reflects this (Fig. 8). Furthermore, there are also provincial Triassic chronostratigraphies, such as that for New Zealand (e.g., Carter, 1974). Here, I do not review these provincial scales, but note that their regional utility will guarantee their continued use.

5  INTEGRATED TIMESCALES Nearly two centuries of Permian and Triassic biostratigraphic research have produced remarkably detailed and reasonably precise correlations within a nearly complete, stage-level chronostratigraphic framework. Nevertheless, the biostratigraphy has its limitations, mostly due to facies restrictions of fossils, diachroneity often related to provinciality, variable evolutionary turnover rates, and continuous developing (improving?) taxonomy. The use of other chronological tools to calibrate and correlate Permian and Triassic rocks thus is critical to refinement of the Permian and Triassic timescales. This includes integrating the chronostratigraphic scale with radioisotopic ages, magnetostratigraphy, and chemostratigraphy, particular of carbon and strontium isotopes (Fig. 9). These data provide relatively new tools for correlation and timescale subdivision that are currently being studied and utilized.

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FIG. 9  Permian and Triassic chronostratigraphic scale with numerical ages added. (Lucas, S. G., Tanner, L. H., Kozur, H. W., Weems, R. E., & Heckert, A. B. (2012). The Late Triassic timescale: Age and correlation of the Carnian-Norian boundary. Earth-Science Reviews, 114, 1–18; Wotzlaw, J.-F., Guex, J., Bartolini, A., Gallet, Y., Krystyn, L., McRoberts, C. A., et al. (2014). Towards accurate numerical calibration of the Late Triassic: High-precision U-Pb geochronology constraints on the duration of the Rhaetian. Geology, 42, 571–574; Ogg, J. G., Huang, C., & Hinnov, L. (2014). Triassic timescale status: A brief overview. Albertiana, 41, 3–30; Lucas, S. G., & Shen, S. (2017). The Permian chronostratigraphic scale: History, status and prospectus. In Lucas, S. G., & S. Shen (Eds.), The Permian timescale (Vol. 450). London: Geological Society (Special Publications).)

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Formal definition of the Permian stage boundaries is nearly complete, and that of the Triassic is well underway. The frontier of Permian and Triassic chronostratigraphy is in definition and characterization of substages—this is the way forward based on biostratigraphy. As new and more reliable radioisotopic ages, magnetostratigraphy, and chemostratigraphy become available, their continued integration with the chronostratigraphic scale will only increase our ability to assign precise ages and to correlate accurately events of Permian and Triassic time.

ACKNOWLEDGMENTS I thank Juan Soto and the other editors of this volume for inviting me to contribute this chapter. Joerg Schneider generously provided information on the age of the German Permian section. The reviewer comments helped improve the content and clarity of this manuscript.

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