2. Paläontologisches Institut und Museum, Karl-Schmid-Strasse 4, Zürich 8006, Switzerland;
3. Department für Geo-und Umweltwissenschaften, Paläontologie und Geobiologie, Ludwig-Maximilians-Universität and Bayerische Staatssammlung für Paläontologie und Geobiologie, Richard-Wagner-Straße 10, München 80333, Germany
The Lopingian (Upper Permian) to Lower Triassic interval is marked globally by important changes in faunas and floras, including the famous Permian-Triassic extinction and subsequent recovery (e.g., Cascales-Miñana and Cleal, 2014; Chen and Benton, 2012; Benton et al., 2004; Benton and Twitchett, 2003). The four distinct Late Permian phytoprovinces (Gondwana, Angara, Cathaysia, and Euramerica) were replaced by only two (Gondwana and Laurasia) in the Early Triassic (Utting and Piasecki, 1995; Dobruskina, 1987). In contrast, provincialism of animals increased (Sidor et al., 2013). In Lopingian successions, macrofossils of land plants are common, contrasted by comparatively rare and mostly impoverished floras dominated by pleuromeiacean lycopsids in Lower Triassic sediments (e.g., Cascales-Miñana and Cleal, 2014; Grauvogel-Stamm and Ash, 2005; Rees, 2002). Despite the reduced abundance of preserved macrofossils, there is no shortage of spores and pollen grains from the latter interval suggesting that there might be a considerable taphonomic bias for the macroremains during this period.
The International Stratigraphic Commission recognizes the Wuchiapingian and Changhsingian stages as subdivisions of the Lopingian, and the Induan and Olenekian stages for the Lower Triassic (Cohen et al., 2013), but informal regional stages and substages are still in use. The bases of the Olenekian and Anisian stages are not yet defined. According to the current working definition, the FAD (first appearance datum) of the conodont Neospathodus waageni marks the base of the Olenekian (Krystyn et al., 2007a). A potential biostratigraphic marker for the base of the overlying Anisian Stage is the FAD of the conodont Chiosella timorensis (Hounslow et al., 2007). Conodonts are also chosen as markers for the bases of the Wuchiapingian (Clarkina postbitteri postbitteri; Jin et al., 2006a), Changhsingian (Clarkina wangi; Jin et al., 2006b), and Induan (Hindeodus parvus; Yin et al., 2001) stages. Ammonites, bivalves, and foraminifers are often used for biostratigraphic correlation of marine sediments in these intervals as well, while conchostracans and tetrapods are employed in terrestrial settings (Ogg et al., 2016). Plant macrofossils are used exceptionally for dating, but provide only low time resolution (Cleal, 2016). Detailed biozonation schemes based on palynomorphs are available for many regions (Fig. 1), but inter-regional correlation is often problematic due to floral provincialism. Palynomorphs used for biostratigraphy in this context are mostly spores, pollen, and acritarchs, but also Reduviasporonites. The latter is a controversial taxon that may represent fungi or algae and appears in mass occurrences close to the Permian-Triassic boundary (PTB) (e.g., Rampino and Eshet, 2017; Hochuli, 2016; Pereira et al., 2016; Visscher et al., 2011, 1996; Steiner et al., 2003; Afonin et al., 2001; Ouyang and Utting, 1990; Góczán et al., 1986). Previous revisions of palynostratigraphic schemes are available for the Permian (Stephenson, 2016) and the Late Triassic (Kustatscher et al., 2018; Cirilli, 2010). Here we review the available biozonations (Figs. 2–8) based on palynomorphs and land plant macroremains from the Lopingian up to and including the Middle Triassic, with an emphasis on correlation to the current stage boundaries (or their working definitions).1 PALYNOSTRATIGRAPHY 1.1 Gondwana 1.1.1 India
The Permian–Triassic palynostratigraphy of India (Figs. 2, 8) is well documented, but most studies have focused on the description and correlation of specific assemblages without defining biozones. Hart (1971) defined five "florizones" for the Permian of India, of which the uppermost Florizone 5 corresponds to the Lopingian. Vijaya and Tiwari (1987) described the Densipollenites/Crescentipollenites and Klausipollenites [=Krempipollenites]/Lunatisporites assemblage zones presumably representing the uppermost Permian and lowermost Triassic in the Raniganj Coalfield. A comprehensive zonation was published by Tiwari and Tripathi (1992), who defined assemblage zones based on an extensive compilation of species occurrences from the Lower Permian to the Cretaceous. The Permian-Triassic data in this work derived from the Damodar and Rajmahal basins. A revision of this initial zonation provided the inclusion of the Krishna-Godavari Basin and the correlation of the palynozones to the international stages (Tiwari and Kumar, 2002; Tiwari, 1999a, b). Zonations for individual basins were presented by Tripathi (1996). Limited age-control is provided through tetrapods and conchostracans (Tiwari, 1999a, b). Several of these assemblage zones have been correlated with the Australian palynofloral zones of Helby et al. (1987) and Backhouse (1993) (Vijaya et al., 2012; Tripathi et al., 2005). Permian palynofloras from India are also closely related to southern African ones (Jha, 2006) and show similarities with Antarctic ones (e.g., Awatar et al., 2014).1.1.2 Pakistan
A palynozonation for the Mianwali (Early Triassic) and Tredian (Anisian) formations in the Salt and Surghar ranges of Pakistan was formalized by Hermann et al. (2012; Fig. 2) and dated by ammonoids (Brühwiler et al., 2011, 2010; Guex, 1978), conodonts (Wardlaw and Pogue, 1995), plant remains (Balme, 1970; Kummel and Teichert, 1970), and carbon isotope stratigraphy (Hermann et al., 2011). The assemblage zones and two informally described associations from the uppermost Chhidru (Lopingian) Formation are correlated with assemblages from Australia, India, and Madagascar. Zia-ul-Rehman et al. (2015) proposed a complementary set of four assemblage zones for the Chhidru Formation in the Salt Range. Unfortunately, independent dating is not provided in this case.1.1.3 Australia
Numerous palynological zonations have been established for the Permian and Triassic in Australia (e.g., Mantle et al., 2010; Mory and Backhouse, 1997; Price, 1997; Backhouse, 1993, 1991, 1990; Helby et al., 1987; Foster, 1982; Dolby and Balme, 1976; Helby, 1973; Evans, 1966; Figs. 3, 8). They are applicable to single or several basins. In general, Western and Eastern Australian palynofloras are treated separately, despite a clear differentiation between the two realms only appearing in the Middle and Upper Triassic with the development of the 'Ipswich' (Eastern Australia) and 'Onslow' (Western Australia) microfloras (Dolby and Balme, 1976). Helby et al. (1987) correlated palynozones from Eastern and Western Australia and provided biostratigraphic tie-points (bivalves, ammonites, and conodonts). In addition to sporomorphs, Helby et al.'s Staurosaccites quadrifidus Zone also includes dinoflagellate cysts which are assigned to the Sahulodinium ottii range zone (Late Anisian to Early Ladinian), the (isolated) lowermost zone of the Shublikodinium Superzone. A compilation of Australian biozones by Mantle et al. (2010) produced a continuous Devonian to Quaternary palynozonation correlated against the Geological Timescale. In recent years, U-Pb zircon dating has indicated younger ages than previously assigned for the Permian zones (Laurie et al., 2016; Smith and Mantle, 2013).1.1.4 Antarctica
Kyle (1977) described three assemblages/zones from Victoria Land, Antarctica (Fig. 2). The Alisporites Zone, the youngest of the three, is Triassic in age, and divided into four subzones (A–D). These subzones are compared with Middle to Upper Triassic palynozones from Australia. The Antarctic assemblages are generally very similar to the Australian and Indian ones. Awatar et al. (2014) specifically identified the Densipollenites magnicorpus assemblage zone of India and the Aratrisporites parvispinosus Zone of Australia in the Lashley and Weller formations of the Central Transantarctic Mountains. Other workers have preferably used Australian palynozones for correlation (e.g., McLoughlin et al., 1997; Lindström, 1995).1.1.5 Southern Africa
Hart (1970) proposed the Striatiti Florizone as a biostratigraphical unit for the Tatarian (~Upper Permian) of the Karoo Basin in South Africa. Anderson (1977) described seven Permian palynozones from the Northern Karoo Basin, of which the last two (zones 6 and 7) are Lopingian in age, corresponding to the Cistecephalus and Daptocephalus terapod zones, respectively. Three zones, presumably documenting the Permian-Triassic boundary interval, were described by Steiner et al. (2003) from the Carlton Heights Section in the Karoo Basin (Fig. 2). These are associated with the tetrapods Dicynodon and Lystrosaurus. In the Mid-Zambezi Basin (Zambia and Zimbabwe), Falcon (1975) described three assemblage zones from the Madumabisa mudstone formation (Tatarian=?Guadalupian– Lopingian; dated indirectly by tetrapods in neighbouring localities with similar assemblages). Nyambe and Utting (1997) also described––but left unnamed––one zone for the Madumabisa mudstone formation, and one for the interbedded sandstone and mudstone formation. The age of the latter was given as "Late Scythian" (Lower Triassic) or Anisian based on comparison of the palynoflora with Western Australia.1.1.6 Madagascar
Goubin (1965) proposed a palynological zonation for the Permian–Triassic Sakamena Group of the Morondava Basin (Fig. 2). Notably, this zonation included important differences between the southern and northern parts of the basin. The Sakamena Group was later restudied palynostratigraphically (Hankel, 1993; Wright and Askin, 1987), but without providing a new zonation. Wescott and Diggens(1998, 1997) presented a succession of numbered palynological zones without further details. The Madagascan assemblages show a close relationship with those from Pakistan (Hermann et al., 2012; Hankel, 1993; Wright and Askin, 1987).1.1.7 South America
In South America, the Lopingian may be represented in the Tornopollenites toreutos Zone (Middle and Upper Andira Formation; Late Permian) in Brazil (Playford and Dino, 2000), and the Tornopollenites toreutos-Reduviasporonites chalastus Zone in the Tunas Formation of Argentina (Balarino, 2014). Lower or Middle Triassic palynozones are currently not available for Argentina.1.2 Germanic Basin
In the Germanic Basin (Northwestern and Central Europe; Figs. 5, 8), the Wuchiapingian corresponds to the Upper Rotliegendes and Lower Zechstein, the Changhsingian is mostly represented by the main part of the Zechstein, and at the top by the lowermost part of the Buntsandstein. The Germanic Triassic has traditionally been divided into "Buntsandstein", "Muschelkalk", and "Keuper". This triad was eponymnous for the Triassic, but does not conform to the current international series or stages. The Induan more or less corresponds to the Lower Buntsandstein, except for its basal part. The Olenekian covers most of the Middle Buntsandstein, while the Upper Buntsandstein ("Röt") is Aegean (Early Anisian) in age (Deutsche Stratigraphische Kommission, 2016; Ogg et al., 2014). The Muschelkalk corresponds to most of the Anisian and the Lower Ladinian, while the Keuper encompasses the rest of the Triassic (Deutsche Stratigraphische Kommission, 2016). Age-control for palynoassemblages in the Germanic Basin is sporadic, but more or less provided by conchostracans, magnetostratigraphy, and cyclostratigraphy and locally, such as for example in the Röt and the Jena formations and in the Zechstein and Muschelkalk, also by ammonoids, conodonts, holothurians and ostracods (Scholze et al., 2017; Kozur and Bachmann, 2008, 2005; Bachmann and Kozur, 2004).
Visscher (1971) defined a succession of eight Lopingian to Lower Triassic zones (four main zones) based on quantitative changes between forms ("norms") of the Lueckisporites "palynodemes" (Lueckisporites, Guttulapollenites, Stellapollenites) in the Kingscourt Outlier, Ireland (Fig. 4). Fijałkowska (1994) used a similar approach for the five subdivisions of her Lueckisporites virkkiae Zone. Mainly based on assemblages from the Germanic Basin, Brugman (1983) defined four palynological phases for the Lower Triassic (LT-1 to LT-4) with an inter-regional (European) to worldwide scope, but this was not pursued further. Three Ladinian palynofloral phases, together with numerous, not time-specific ecophases as subdivisions, were described by Brugman et al. (1994) from a well in Southern Germany. These phases were dated by comparing the palyno-floras to similar assemblages with better age-control in the Germanic Basin and the Tethys Realm (Southern Alps and Transdanubian Mountains). Heunisch (1999) carried out a very detailed study, dividing the Triassic into 20 numbered assemblage zones, eleven of which (GTr1–11) for the Lower and Middle Triassic. In this case, age-control is provided by lithostratigraphy.
Palynozones associated with the lithostratigraphical units of the Germanic Basin have been specifically described in Poland by Orłowska-zwolińska(1988, 1985, 1984, 1977), who divided the Early–Middle Triassic in seven zones and partially subzones. The palynostratigraphy of Poland and Germany (e.g., Orłowska-zwolińska, 1988, 1985, 1984, 1977; Reitz, 1985; ) was then combined to a unified palynozonation for the Germanic Basin (Kürschner and Herngreen, 2010; Herngreen, 2005; Reitz, 1988). Biozonations based on megaspore assemblages correlated with the microfloral zones also exist for Poland and Germany (Marcinkiewicz et al., 2014, and references therein), although they are stratigraphically very restricted.1.3 Tethys Realm
Figure 6 shows palynozonation schemes from the Transdanubian Mountains (or Transdanubian Mid-Mountains, Transdanubian Central Range, etc.) in Hungary and the Southern Alps in Northern Italy. In the Transdanubian Mountains, Góczán et al. (1986) defined 19 assemblage/dominance zones covering the uppermost Permian to Anisian. Dating was possible based on palynomorphs, bivalves, and foraminifers. Van der Eem (1983) defined a sequence of six palynomorph phases for the Illyrian to Carnian in the Western Dolomites (Southern Alps). These were correlated to regional ammonoid zones of Mojsisovics (Mojsisovics et al., 1895) from the same lithostratigraphical units. Four phases serve as subdivisions for the Ladinian and the Fassanian and Longobardian substages. The biostratigraphic scale proposed by Van der Eem was extended downwards by Brugman (1986) with material from the Vicentinian Alps (Northern Italy) and the Transdanubian Mountains, spanning the Olenekian to Fassanian. Brugman (1986) re-defined the thiergartii-vicentinense phase of Van der Eem (1983) reassigning the sample from the thiergartii-vicentinense phase sensu Van der Eem (1983) to his vicentinense-scheuringii phase. He also doubted the validity of the plurianulatus-novimundanus phase sensu Van der Eem (1983), because it was based on only one samples. An assemblage that is comparable with this plurianulatus-novimundanus phase has been reported from a borehole in Gröden/Val Gardena (Hochuli and Roghi, 2002). In his unpublished thesis, Roghi (1995) revised the sequence of phases for the Anisian to lower Carnian of the Southern Alps, based on different stratigraphic successions dated with the help of ammonoids in combination with the two aforementioned works (Brugman, 1986; Van der Eem, 1983). The resulting six phases were divided into 22 zones, which are defined by FOs (local First Occurrences). Kustatscher et al. (2006) and Kustatscher and Roghi (2006) described rich palynological assemblages in the Lower Pelsonian to Illyrian Dont Formation of the Dolomites (mostly Kühwiesenkopf/Monte Prà della Vacca), which were assigned to the thiergartii-vicentinense phase (both sensu Brugman, 1986 and sensu Roghi, 1995). The individual assemblages were compared to Roghi's four zones for this interval and due to earlier FOs for Jerseyiaspora punctispinosa and Cristianisporites triangulatus, the zones were emended and reduced to only three. Hochuli et al. (2015) defined a succession of Middle–Upper Triassic palynozones (TrS-A to TrS-F) from the Seceda drill core. The correlation with Van der Eem's (1983) material evidenced a partial overlap of his zones. Dal Corso et al. (2015) designated two palyozones (TrSM-A and TrSM-B) correlated to the ammonoid zones of Mietto and Manfrin (1995) from the Dolomites and Valsugana, both of Illyrian age. Megaspores from the Ladinian to Carnian in the Southern Alps have been assigned to the Horstisporites selaginelloides megaspore assemblage zone (Marcinkiewicz et al., 2014; Wierer, 1997). A tentative composite zonation for the uppermost Permian to Middle Triassic in the Tethys Realm based on the biozones of Góczán et al. (1986) and the phases of Brugman (1986) and Roghi (1995) is shown in Fig. 8.1.4 China
Assemblages and biozones from China are presented in Fig. 7. A number of studies report palynological assemblages from South China, but to our knowledge, no formal biozones have been defined so far. Triassic palynological assemblages from North and South China were grouped into "assemblage (zones)" and palynofloras by Sun et al. (1995), but these do not have defined boundaries. Notable are the assemblages representing the latest Permian and earliest Triassic at the Meishan Section, where the GSSP (Global Stratotype Section and Point) for the base of the Triassic is located at the boundary between the Leiosphaeridia changxingensis-Micrhystridium stellatum assemblage zone and the Vittatina-Protohaploxypinus assemblage zone (Yin et al., 2001; Ouyang and Utting, 1990). As the Meishan Section documents the PTB in a marine environment, Peng et al.(2006, 2005) studied the palynofloral succession in the terrestrial Xuanwei (Late Permian to earliest Triassic) and Kayitou (Early Triassic) formations at Chahe and Zhejue (Weining, Guizhou). They described three assemblages, assigned to the Late Permian, the "Permian-Triassic boundary stratigraphic set", and the Early Triassic. However, the quality of these sections has recently been questioned (Bourquin et al., in press, 2018; comp. Zhang et al., in press). The same formations are found in Eastern Yunnan Province, where Ouyang (1982) similarly described a succession of three assemblages; Torispora gigantea-Patellisporites meishanensis assemblage (Lungtan Formation=Lower Xuanwei Fm.), the Yunnanospora radiata-Gardenasporites spp. assemblage (Changhsing Formation equivalent=Upper Xuanwei Fm.), and the Lundbladispora-Aratrisporites-Pteruchipollenites assemblage (Kayitou Fm.).
For North China, Ouyang and Norris (1988) presented a succession of six Changhsingian to Middle Triassic assemblage zones (based on earlier works and partially emended), which are connected to formations and associated with plant macrofossils and vertebrates. Further assemblage successions related to formations from the Permian and Triassic of North China were presented by Zhang et al. (2003; and references therein). In addition, Liu et al. (2015) described the Patellisporites meishanensis Biozone from the Baode Section in Shanxi Province, which was dated indirectly from known occurrences of the eponymous species as Capitanian–Wuchiapingian. Megaspores from the same section, corresponding to the upper part of the P. meishanensis Biozone, have been assigned to the Biharisporites cf. foskettensis assemblage zone (Liu et al., 2011).
Informal assemblages described from the Olenekian to Ladinian successions of the Tarim Basin were recently reviewed by Peng et al. (2017) and correlated to six zone-like intervals (TT1–TT4 and two unnamed intervals). The assemblages reported from the Permian in the Junggar and Tarim basins were summarized and correlated by Zhu et al. (2005). In the Tarim Basin, the Lopingian is represented by the Lueckisporites virkkiae-Scutasporites xinjiangensis assemblage from the Upper Pusige Formation and the Lueckisporites virkkiae-Klausipollenites schaubergeri assemblage from the Duwa Formation (Wuchiapingian). In the Junggar Basin, the Kraeuselisporites spinulosus-Potonieisporite turpanensis assemblage from the Wutonggou Formation and the Lueckisporites virkkiae-Klausipollenites schaubergeri assemblage from the Lower–Middle Guodikeng Formation are assigned to the Lopingian. In addition, the Lundbladispora-Lunatisporites-Aratrisporites assemblage has been described from the earliest Triassic of the Junggar Basin (Ouyang and Norris, 1999).
Palynozones have been described for the Middle and Late Triassic of Tulong, Xizang, Tibet (Peng et al., 2018). The two lowermost zones were dated as Anisian and Late Anisian to Early Norian, based on ammonoids, conodonts, bivalves, and brachiopods. The composition of the assemblages was compared to those from Australia, but there are differences in the ranges of taxa. Therefore and because of conceptual differences, the Staurosaccites quadrifidus taxon-range zone of Tibet is not equal to the Staurosaccites quadrifidus (Oppel) Zone (Mantle et al., 2010; Helby et al., 1987; Dolby and Balme, 1976) of Australia.1.5 Boreal Realm 1.5.1 Russia
Gomankov et al. (1998) synthesized four Palynozones for the Tatarian (Capitanian–Changhsingian) on the Russian (or East European) Platform (Fig. 4) from earlier results. In the Moscow Syneclise (or the Moscow Basin, a part of the Russian Platform), four assemblages have been described from the terrestrial Vokhma Formation by Yaroshenko and Lozovsky (2004). This succession was originally dated as Induan, based mainly on the presence of the tetrapod Tupilakosaurus and correlation by palynomorphs with assemblages from Eastern Greenland and the Barents Sea. The stratigraphy of this region was later revised, with the Nedubrovo Member being re-dated as uppermost Permian (Lozovsky et al., 2016). Kirichkova and Kulikova (2005) described Early to Late Triassic assemblages from the Timan-Pechora Basin with age-control by vertebrates, which they correlated with the palynostratigraphic units of Heunisch (1999) for the Germanic Basin, and with assemblages from the Eastern Urals and Siberia (Kirichkova and Kulikova, 2002). The Tatarian zones and Triassic assemblages from the Moscow and Timan-Pechora basins are combined to a nearly continuous scheme in Fig. 8. The four Olenekian to Ladinian assemblages described from the Eastern Urals and Siberia (Chelyabinsk and Anokhino grabens) are geographically close to and show a similar taxon distribution as the assemblages from the Timan-Pechora region (Kirichkova and Kulikova, 2005, 2002).1.5.2 Baltic region
Vigran et al. (2014) assigned a common zonation to the palynological findings from many outcrops and wells on Svalbard and in the Barents Sea area, with 15 composite assemblage zones covering the uppermost Permian and the entire Triassic (Figs. 4, 8). These zones partially relate to previously described assemblages (Mørk et al., 1999, 1990; Vigran et al., 1998; Hochuli et al., 1989). They are dated mainly by ammonoids, magnetostratigraphy, and organic carbon isotope stratigraphy. It should be noted that the Pechorosporites disertus composite assemblage zone of the Baltic Sea region in this scheme is not equivalent to its namesake from the Pechora-Timan region. The latter is defined by the FO of the eponymous species in the Induan, the former by its consistent appearance and the FO of Cordaitina gunyalensis in the Lower Olenekian.1.5.3 Canada
The Sverdrup Basin of the Canadian Arctic islands was located adjacent to Svalbard during Permian and Triassic times. Utting (1994) described the Tympanicysta stoschiana-Striatoabieites richteri Zone from the Blind Fiord Formation of this basin, and renamed it later on. The new Chordecystia chalasta-Striatoabieites richteri Zone is also recognized in the Montney, Toad and Grayling formations of western Canada (Alberta, British Columbia, Yukon; Utting et al., 2005). Both Tympanicysta stoschiana and Chordecysta chalasta are considered junior synonyms of Reduviasporonites chalastus (Foster et al., 2002). This assemblage zone is partially dominated by Uvaesporites imperialis. It is therefore comparable to the Uvaesporites imperialis and Reduviasporonites chalastus zones of the Barents Sea. The Canadian assemblages were dated as Griesbachian (based on ammonoids) to Dienerian (based on conodonts) (Utting et al., 2005; Utting, 1994). However, the current definition of the base of the Induan and the correlation with the Norwegian zonation indicates that the base of this zone is Changhsingian in age. Older assemblages from these areas are assigned to a Guadalupian or even older ages (Utting et al., 2005).1.5.4 Greenland
Palynological assemblages from the Permian and Triassic in eastern Greenland have been reported from Kap Stosch, Jameson Land, and Scoresby Land (Schneebeli-Hermann et al., 2017; Piasecki, 1984; Balme, 1980). Formal biozones have not been defined, but these assemblages have often been referenced for correlation (e.g., Vigran et al., 2014; Hermann et al., 2012; Ouyang and Utting, 1990; Orłowska-zwolińska, 1985). Balme (1980) described a Protohaploxypinus Association from beds yielding the ammonoids Otoceras boreale and Glyptophiceras triviale (Teichert and Kummel, 1972), pointing to a latest Permian–earliest Triassic Age. This is followed by a Taeniaesporites (=Lunatisporites) Association, which was dated as Griesbachian by megafauna and palynomorphs. Piasecki (1984) found similar assemblages.1.6 Middle East 1.6.1 Israel
Permian and Triassic palynofloras from wells in Israel were first ordered by Horowitz (1974) into the Klausipollenites schaubergeri and "Taeniaesporites" (=Lunatisporites) kraeuseli zones, which were dated as Thuringian (Late Permian) and Middle to Late Triassic, respectively. They were subdivided into three subzones each. Based on material from the same and additional boreholes, a different set of assemblage zones was proposed by Eshet and Cousminer (1986) (Figs. 4, 8). Eshet (1990) compared the succession across the PTB and suggested that it coincides with the boundary between the Lueckispories virkkiae Zone (=Protohaploxypinus spp.-Lueckisporites virkkiae assemblage zone in Eshet and Cousminer, 1986) and the Endosporites papillatus Zone (=Endosporites papillatus-Kraeuselisporites spp. assemblage zone in Eshet and Cousminer, 1986). Ages were derived from comparison with assemblages of other regions.1.6.2 Iraq
Nader et al. (1992) described three assemblage zones from borehole Mityaha-1 in Iraq. Assemblage zone A was dated as Tatarian (Late Permian) and assemblage zones B and C (only one sample) as Early Scythian (Induan). The ages were derived from comparable microfloras across the world.1.6.3 Abu Dhabi
From boreholes in Abu Dhabi, Loutfi and Abdel Sattar (1987) recognized the Potonieisporites microdens partial range zone from the Upper Khuff and Lower Sudair formations, the Aratrisporites paenulatus partial range zone from the Upper Sudair Formation, and the Taeniaesporites krauseli (=Lunatisporites kraeuseli) partial range zone from the Gulailah Formation. These were dated as Late Permian, Early Triassic, and Middle Triassic, respectively. A later calibration of the lithostratigraphy on the Arabian Platform based on sequence stratigraphy would place the Upper Khuff Formation in the Induan, the Sudair Formation in the Olenekian, and the Gulailah Formation in the Ladinian (Haq and Al-Qahtani, 2005; Sharland et al., 2001).1.7 Libya
Brugman and Visscher (1988) described five assemblages from wells in Libya (Fig. 4), dated respectively as Sakmarian-(?)Ufimian, latest Scythian (=Olenekian), Early Anisian, Late Ladinian–Early Carnian, and Rhaetian–Liassic. These ages are determined only indirectly by comparison with palynological phases from Hungary (heteromorphus-conmilvinus and crassa-thiergarti phases; Brugman, 1986) and with assemblages from Russia, Australia, and Kazakhstan.2 MACROFLORA
Macroremains are rarely used for biostratigraphy in Permian-Triassic successions, due to their reduced abundance and sporadic appearance in the fossil record during these periods, the still insufficiently documented stratigraphic distribution of specific taxa and their limited geographical distribution. Most attempts at macrofloral biostratigraphy have been carried out on the Southern Hemisphere. Retallack (1995), for example, divided the uppermost Permian and Lower Triassic in the Sydney Basin (Eastern Australia) into nine heteropic and diachronous zones. These were later recalibrated and reduced by Metcalfe et al. (2015) to five (mostly) diachronous macroplant zones (Glossopteris communis, Dicroidium calliptroides, Voltziopsis wolganensis, Dicroidium zuberi and Dicroidium odontopteroides zones) dated partially radiometrically, and correlated lithostratigraphically and to local palynomorph zones (e.g., Helby, 1973). Wang et al. (2005) divided the Lower and Upper Permian of China into four phytogeographical zones but identified also within the Upper Shihhotse Formation at least three biostratigraphical zones (Gigantonoclea lagrelii p., Gigantonoclea hallei and Gigantonoclea taiyuanensis lineage zones, and Lobatannularia multifolia-Psygmophyllum multipartitum assemblage zone), assigned by the authors to the Late Permian. Early to Middle Triassic plant assemblages from China were attributed to a Northern and a Southern China Floristic Subregion of the Eurasian Floristic Region (the European-Sinian area of the Laurasia Kingdom sensu Dobruskina, 1987) by Sun et al. (1995). They distinguished an Early Triassic Pleuromeia Flora and a Middle Triassic Annalepis-Scytophyllum Flora in North China and a poorly documented, unnamed Early Triassic Flora followed again by a Middle Triassic Annalepis-Scytophyllum Flora in South China.
Plant macroremains are used in South America as biostratigraphic proxies for Triassic sediments, dividing them into five association biozones, namely the Dictyophyllum castellanosii-Johnstonia stelzneriana-Saportaea dichotoma (CSD; Anisian), Yabeiella marayesiaca-Scytophyllum bonettiae-Protophyllocladoxylon corderitaensis (MBC; lower Ladinian), Yabeiella brackebuschianaScytophyllum neuburgianum-Rhexoxylon piatnitzkyi (BNP; Upper Ladinian), Dicroidium odonotopteroides-Dicroidium lancifolium (OL; middle Upper Triassic) and Dictyophyllum tenuiserratum-Linguifolium arctum-Protocircoporoxylon marianaensis (DLM; upper Upper Triassic). Two or three markers characterize each zone (Artabe et al., 2003, 2001; Morel et al., 2003; Spalletti et al., 1999). There was a tentative correlation also with the other biozonations, such as the only Eurasian one, proposed by Dobruskina (1994). According to the author, the Late Permian was characterized in the Siberian area by the Cordaites Flora, in Europe by the Zechstein Flora and in the Asian area by the Gigantopteris Flora, locally represented in the latest Permian also by the Tatarina Flora. The Early to Middle Triassic were characterized by the diachronous Korvunchana (Siberia), Voltzia (Europa-Asia), and Pleuromeia Flora (in both areas). The Ladinian and Carnian ages were represented by the Scytophyllum Flora throughout the entire Laurasian Kingdom. The most remarkable suggestion in this scheme is, however, that the boundary between the Palaeophytic and the Mesophytic does not correspond to the boundary between the Palaeozoic and the Mesozoic (i.e., the PTB) but corresponds to the Anisian-Ladinian boundary or, in some areas, even to the boundary between the Ladinian and the Carnian (Dobruskina, 1994, p. 259). This incongruence between the palaeozoological and palaeobotanical boundary in the scheme of Dobruskina (1994) is due to the fact that the Palaeozoic-Mesozoic boundary was based on the mass extinction among animals at the end of the Permian, while Dobruskina considered the appearance of new plant families in the Middle Triassic as the beginning of the Mesophytic, even though "the greatest change [in floras] took place at the very beginning of the Early Triassic [..]" (Dobruskina, 1987, p. 84).
The Induan is depleted in plant macrofossils, making a qualitative comparison with the earliest recovery floras of the Olenekian very difficult. This is especially true for Euramerican successions, which so far did not yield any Induan plant assemblages, although the Olenekian ones are locally sometimes surprisingly diverse (Grauvogel-Stamm and Kustatscher, in press; Nowak et al., 2017; Kustatscher et al., 2014), suggesting that the dearth of Induan plant remains might also be due to a taphonomic bias. Likewise, very few plant assemblages are known from the Southern Hemisphere, where the lowermost Triassic floras are generally poorly diversified, whereas the upper Lower Triassic successions are generally dominated by seed ferns (mostly Corystospermales and Peltaspermales; e.g., Silvestro et al., 2015). Plant assemblages from China and Russia, on the other hand, are more diverse and in the Induan generally dominated by sphenophytes, lycophytes, ferns and Gigantopteridales (Xiong and Wang, 2011; Yu et al., 2010a, b; Dobruskina, 1994), although the credibility of age correlations of the terrestrial Chinese successions at the PTB has been questioned lately (Bourquin et al., in press, 2018; comp. Zhang et al., in press). Olenekian floras are more diverse with abundant lycophytes, sphenophytes, ferns, seed ferns and conifers (Xiong and Wang, 2011; Dobruskina, 1994). According to some of the above-cited papers, several of the taxa that survived the end-Permian mass extinction event, also survived the Induan-Olenekian boundary (IOB). The taxa that may have survived the end-Permian mass extinction but went extinct at the end of the Induan are typical Paleozoic forms (e.g., Annularia, Lepidodendron, Gigantopteris). On the other hand, typical Triassic elements such as e.g., Albertia, Anomopteris, Neuropteridium, and Bjuvia seem to first appear after the IOB (Nowak et al., 2017). The lack of a clear transition between the late extinction of Paleozoic taxa and the appearance of new Mesozoic forms in addition to the sporomorph data indicating a diverse flora suggest that the taphonomic and sampling bias concerning plant macroremains is still too high to determine a reliable stratigraphic biozonation around the PTB.3 STAGE AND SYSTEM BOUNDARIES 3.1 Capitanian-Wuchiapingian Boundary
The base of the Wuchiapingian Stage (and the Lopingian Series) is defined by the FAD of the conodont Clarkina postbitteri postbitteri in Bed 6k near the top of the Maokou Formation at the Penglaitan Section in Guangxi Province, South China (Jin et al., 2006a). This was intended to approximately coincide with the end-Guadalupian extinction event (Wignall et al., 2009; Jin et al., 2006a), but the extinction may have occurred earlier, in the Middle Capitanian (Bond et al., 2015). Palynomorphs are not known from this section. In the zonation of Liu et al. (2015) for the Baode Section in North China, the boundary is crossed by the long-ranging Patellisporites meishanensis Zone. It furthermore falls within the Lueckisporites virkkiae Zone in Israel (Eshet, 1990), the Dulhuntyispora ericianus Zone (APP42) in Australia (according to radiometrical datings by Laurie et al., 2016). It might correspond approximately to the boundary between zones Ⅱ and Ⅲ on the Russian Platform of Gomankov et al. (1998) and to the boundary between the Gondisporites raniganjensis and Guttulapollenites gondwanensis assemblage zones in India (Tiwari and Kumar, 2002), but age-control is lacking in these cases.3.2 Wuchiapingian-Changhsingian Boundary
The GSSP for the base of the Changhsingian was placed near the base of the Changxing Formation in Bed 4 at Meishan Section D, Changxing County, Zhejiang Province, South China, where the conodont Clarkina wangi first appears (Jin et al., 2006b). Palynomorphs are not documented from this part of the section. The boundary is generally not well documented in terms of palynology. It falls within the Lueckisporites virkkiae Zone in Israel (Eshet, 1990) and the Germanic Basin (Fijałkowska, 1994), the Dulhuntyispora parvithola Zone (APP5) in Australia (Laurie et al., 2016), and Zone Ⅲ on the Russian Platform (Gomankov et al., 1998).3.3 Permian-Triassic Boundary
The boundary between the Changhsingian and Induan stages, which is also the boundary between the Permian and Triassic systems as well as the Palaeozoic and Mesozoic erathems, has received much attention since it is associated with the most severe mass extinction of the Phanerozoic (e.g., Benton and Twitchett, 2003). In the plant fossil record, the Induan is mostly characterized by a notable lack of fossils, which is an important change from the well-documented Lopingian palaeofloras, but this may in part be due to taphonomic and sampling bias. The GSSP defining the base of the Induan Stage is located in Bed 27c within the Yinkeng Formation at Meishan Section D, Changxing County, Zhejiang Province, South China, at the FAD of the conodont Hindeodus parvus (Yin et al., 2001). Historically, the ammonoid genus Otoceras was used as a marker for the Triassic and the Griesbachian was originally defined by the first appearance of O. concavum. Now, this datum has to be considered as Permian, with the Griesbachian in the old sense and Otoceras-bearing beds traversing the PTB. The base of the Griesbachian as a substage of the Induan Stage on the other hand coincides with the PTB. Otoceras boreale, O. woodwardi, and O. latilobatum can serve as markers for the basal Triassic. At the Meishan Section, the GSSP bed lies at the boundary between the acritarch-dominated Leiosphaeridia changxingensis-Micrhystridium stellatum and the pollen-dominated Vittatina-Protohaploxypinus assemblages sensu Ouyang and Utting (1990). The most important non-acritarch component in the lower assemblage is "Tympanicysta" (=Reduviasporonites). Mass occurrences of Reduviasporonites and similar microfossils of presumably fungal affinity ("fungal spike") have long been considered as typical for the PTB (e.g., Visscher et al., 1996). With the ratification of the GSSP, these would now be latest Permian. However, Reduviasporonites is not restricted to the boundary interval, and the validity and synchroneity of some of the observed instances of the "fungal spike" has been questioned (Hochuli, 2016). The megaspore Otynisporites eotriassicus appears in Central and Eastern Europe (Tesero Oolite, Southern Alps, Italy: Kozur, 1998; Lower Buntsandstein, Poland: Marcinkiewicz, 2014; Fuglewicz, 1980; Nedubrovo, Vologda Region, Russia: Foster and Afonin, 2005), East Greenland (Schuchert Dal Formation, Jameson Land: Looy et al., 2005), and North China (Guodikeng Formation, Junggar Basin: Liu, 1994) together with Lystrosaurus, Otoceras, Hindeodus praeparvus, and mass occurrences of Reduviasporonites (Foster and Afonin, 2006, 2005; Lozovsky et al., 2001; Krassilov et al., 1999). Its FAD is therefore considered as a useful marker for the latest Permian. Another possible marker might be Aratrisporites, but this genus has been recorded also from older sediments (Stephenson, 2016; Xiong and Wang, 2011).
The Lapposisporites-Kraeuselisporites assemblage zone from the Nádaskút Dolomite Member of the Arács Formation and from the lower part of the Alcsútdoboz Limestone Formation in Hungary was dated as basal Triassic in the presence of Claraia ex. gr. griesbachi and due to the "mixed" character of the assemblage with Triassic and Permian elements (Góczán et al., 1986). Claraia griesbachi appears in the Meishan section shortly above the GSSP, so the dating is valid (Yin et al., 2001). The underlying "Tympanicysta"[=Reduviasporonites]-Punctatisporites-Calamospora assemblage zone (at the base of the Alcsútdoboz limestone formation and the top of the Dinnyés dolomite formation; Góczán et al., 1986) also seems to correspond to other latest Permian assemblages with a notable abundance of Reduviasporonites. In the subsurface of Israel, the PTB was located at the boundary between the Lueckisporites virkkiae and Endosporites papillatus assemblage zones (Eshet, 1990; Protohaploxypinus spp.-Lueckisporites virkkiae and Endosporites papillatus-Kraeuselisporites spp. Assemblage zones in Eshet and Cousminer, 1986). E. papillatus was suggested as a marker for the Lower Triassic (Scythian), following Visscher and Brugman (1981), but this species has been reported from the Changhsingian as well (Vigran et al., 2014; Hermann et al., 2012). The uppermost Permian assemblages are again marked by abundant Reduviasporonites. Reduviasporonites is also a prominent constituent of the (?)uppermost Permian–Lower Triassic Chordecysta[=Reduviasporonites] stoschiana-Striatoabieites richteri assemblage zone in Canadia (Utting et al., 2005; Utting, 1994), and of the Reduviasporonites chalastus Zone (which is defined by the FO of Lundbladispora obsoleta and the consistent presence of R. chalastus) in the Baltic region (Vigran et al., 2014). The upper part of the R. chalastus Zone is correlated with the Otoceras boreale ammonoid zone, which indicates the basal Triassic (Yin et al., 2001). In India, the PTB is usually equated with the boundary between the Raniganj and Panchet formations of Damodar Valley and their equivalents. The main argument for this interpretation is a notable turnover in microfloras presumably coinciding with the mass extinction, but independent age-control is lacking (Tiwari and Vijaya, 1994). In terms of palynozones, this boundary lies between the Densipollenites magnicorpus and Krempipollenites indicus zones (Tiwari and Kumar, 2002; Tiwari, 1999a, b; Tiwari and Tripathi, 1992). The PTB lies within the lower part of the Lundbladispora obsoleta-Protohaploxypinus pantii composite assemblage zone (Kürschner and Herngreen, 2010), or the Otynisporites eotriassicus megaspore assemblage zone (Marcinkiewicz et al., 2014) in the German Basin. In Australia, it lies within the Lunatisporites pellucidus Zone or Unit APT101 (Metcalfe et al., 2015; Mantle et al., 2010). In Madagascar, it is located in Zone IIA (Goubin, 1965) or the Middle Sakamena assemblage (Wright and Askin, 1987).3.4 Induan-Olenekian Boundary (IOB)
Sections proposed as GSSP sites for the IOB are at Mud (Spiti Valley, India; Zakharov, 2010; Krystyn et al., 2007a, b), Chaohu (Anhui Province, China; Tong and Zhao, 2011; Chinese Triassic Working Group, 2007; Tong et al., 2004), and Nammal Gorge (Salt Range, Pakistan; Ware et al., 2010). Palynomorphs are not available from Mud (Ware et al., 2010), and only a few have been described from Chaohu (Tong et al., 2006), but assemblages from Nammal have been reported in detail (Hermann et al., 2012). Here, as in the Germanic Basin (Orłowska-zwolińska, 1984) and in the Transdanubian Mountains (Brugman, 1986; Góczán et al., 1986), an increase in the abundance of Densoisporites nejburgii is recorded from a level that coincides with the first occurrence of the ammonoid genus Flemingites and may be considered for the definitive boundary (base of assemblage zone PTr 2: Lundbladispora spp.-Densoisporites spp.). However, D. nejburgii does also appear less frequently in lower beds. In fact, no FAD of a widespread palynomorph taxon has been associated with the IOB so far. Furthermore, acmes of D. nejburgii are not necessarily coeval. In general, changes in palynofloras across the IOB are often inconspicuous or visible only in shifts of relative abundances. Consequently, palynomorphs are not suited for defining the IOB, but they can contribute to its correlation on regional scales. For inter-regional correlations, better age control is still needed in many cases.
In the Germanic Basin, the IOB presumably occurs in the lower part of the Volpriehausen Formation (Middle Buntsandstein; Ogg et al., 2014), which falls into the Densoisporites nejburgii-acritarch acme subzone of the D. nejburgii zone (Kürschner and Herngreen, 2010). The FO of D. nejburgii is at the base of this zone, shortly below the boundary, but the species only first abundantly occurs in the upper part of the Volpriehausen Formation, above the prospective IOB. The pattern is similar in the Tethys Realm, where the IOB would be located in the lower part of the nejburgii-bisaccate Oppel Zone (Góczán et al., 1986) or the nejburgii-heteromorphus Phase (Brugman, 1986). In the zonation of Vigran et al. (2014), the IOB approximately falls together with the boundary between the Maculatasporites spp. composite assemblage zone and the Naumovaspora striata composite assemblage zone. The latter is defined by the first consistent occurrence (not the FO/FAD) of N. striata. The zonal ages are constrained by the ammonoids Vavilovites sverdrupi (Dienerian) and Euflemingites romunderi (Smithian), as well as by magnetostratigraphy (Hounslow and Muttoni, 2010). This boundary also coincides with formational boundaries on Svalbard and in the Barents Sea. In India, the Krempipollenites indicus assemblage zone (Tiwari and Kumar, 2002; corresponding to the Klausipollenites schaubergeri assemblage zone of Tiwari and Tripathi, 1992) has been assigned to the Induan and the Playfordiaspora cancellosa assemblage zone to the Olenekian, but without an evident age control for the boundary (Tiwari, 1999a). A tentative correlation with Australia would place the boundary within the K. indicus assemblage zone (Vijaya et al., 2012; Tripathi et al., 2005). In Australia, the IOB would be located within the long-ranging Protohaploxypinus samoilovichii Oppel Zone in Eastern and Western Australia (Metcalfe et al., 2015; Helby et al., 1987), unit APT102 (Price, 1997) of Eastern Australia, or the Kraeuselisporites septatus assemblage zone of Western Australia (Dolby and Balme, 1976). In North China, the "Lundbladispora" (=Densoisporites) nejburgii assemblage from the Upper Liujiaguo Formation and the Voltziaceaesporites heteromorpha assemblage from the Upper Heshangguo Formation were dated as Induan and Olenekian, respectively, based on plant remains and ammonoids (Ouyang Shu and Norris, 1988), but the IOB is not documented. Due to similarities, the Limatulasporites-Cycadopites-Tubermonocolpites-Micrhystridium assemblage and the Lundbladispora-Cycadopites-Veryhachium assemblage from Qinghai Province were also correlated to the Induan and Olenekian, respectively (Ji and Ouyang, 2005).3.5 Olenekian-Anisian Boundary (OAB)
Like the IOB, the Olenekian-Anisian boundary (OAB) is not yet defined by a GSSP. A candidate section is located at Deşli Caira Hill (Dobrogea Province, Romania). Biostratigraphic data from this section are available for ammonoids, nautiloids, orthoceratids, conodonts, foraminifers, and palynomorphs. A possible marker for the boundary is the FAD of the conodont Chiosella timorensis (Grǎdinaru et al., 2007). Alternatively, the OAB might be placed at the base of magnetozone MT1n (Hounslow et al., 2008, 2007). Based on the palynostratigraphy of Milne Edwardsfjellet (Svalbard), where this magnetozone is also recognized, Hounslow et al. (2008) suggested that the base of assemblage Svalis-5 (see Vigran et al., 1998) lies just below the OAB and that the consistent presence of Triadispora might serve as a secondary indicator for the boundary. Vigran et al. (2014) placed the OAB at the base of the Anapiculatisporites spiniger composite assemblage zone, which includes Svalis-5.
Stellapollenites thiergartii has its well-dated first occurrence at or near the OAB at least in the Germanic Basin (base of the S. thiergartii composite assemblage zone of Kürschner and Herngreen, 2010) and in the Tethys Realm (base of the conmilvinus-crassa phase of Brugman, 1986 or thiergartii-heteromorphus dominance zone of Góczán et al., 1986). This species has a wide geographic distribution. Thus, it might be a useful marker for other regions as well. In Israel, Eshet and Cousminer (1986) dated the Voltziaceaesporites heteromorphus- Alisporites grauvogeli A assemblage zone as Lower Anisian mainly because of the presence of A. grauvogelii, V. heteromorphus, and Triadispora falcata, but their FOs are above the base of the zone (which is defined by last appearances), thus the lower part might precede the OAB. V. heteromorphus and A. grauvogeli are also known from the Olenekian, whereas T. falcata is restricted to the Anisian. The prospective OAB coincides with the bases of the Aratrisporites parvispinosus Oppel Zone (Helby et al., 1987) or unit APT3 (Price, 1997) in Australia, and consequently by correlation with the base of subzone B of the Alisporites Zone in Antarctica (Kyle, 1977). The base of the Goubinispora morondavensis assemblage zone (Tiwari and Tripathi, 1992; corresponds to the Goubinispora indica assemblage zone of Tripathi et al., 2005) might also coincide.3.6 Anisian-Ladinian Boundary
The GSSP for the base of the Ladinian is located in the lower part of the Buchenstein Formation at Romanterro, Bagolino in Eastern Lombardy, Northern Italy (Brack et al., 2005). The commonly used informal Ladinian substages––Fassanian and Longobardian––are also originally based on the stratigraphy of this region. Consequently, Anisian-Ladinian palynozonations from the Southern Alps have a well-established correlation with the stage boundary and substages, and also with ammonoid zones. In terms of ammonoid zonation, the boundary as defined by the GSSP corresponds to the boundary between the Nevadites secedensis and Eoprotrachyceras curionii zones. Prior to the ratification of the GSSP, the base of the underlying Nevadites secedensis or Nevadites Zone has commonly been considered as the base of the Ladinian. Thus, compared to the old usage of these terms, the Fassanian substage of the Ladinian is shorter, while the Illyrian substage of the Anisian is longer.
Palynostratigraphic data are not available from the GSSP Section itself, but from the same formation at other localities in the region. The plurianulatus-secatus phase (Van der Eem, 1983) and the scheuringii-pseudoalatus phase (Roghi, 1995) traverse the boundary. Zone TrS-C of Hochuli et al. (2015) represents the basal Ladinian, as it is associated with E. curionii, but its lower boundary is unclear due to a barren interval.
The base of the Ladinian coincides with the base of the Heliosaccus dimorphus assemblage zone in the Germanic Basin (Kürschner and Herngreen, 2010) and the Echinitosporites iliacoides assemblage zone in the Norwegian Arctic (Vigran et al., 2014). It possibly also coincides with the base of the Dubrajisporites isolatus assemblage zone in India (Tiwari and Kumar, 2002) and (by correlation with the Germanic Basin) with the base of the Echinitosporites iliacoides-Podosporites amicus assemblage zone in Israel (Eshet and Cousminer, 1986). Echinitosporites iliacoides appears to be globally restricted to the Ladinian, and can therefore serve as a useful inter-regional marker. Heliosaccus dimorphus can be used as a marker within the Germanic Basin. In Australia, the boundary is crossed by the long-ranging Triplexisporites playfordii (Spathian to Fassanian in Western Australia; Mantle et al., 2010; Helby et al., 1987) and Aratrisporites parvispinosus (Aegean to Fassanian; Helby et al., 1987; Helby, 1973) zones, and best approximated by unit APT32 (Illyrian to Fassanian; Price et al., 1997).3.7 Ladinian-Carnian Boundary
The GSSP for the base of the Carnian was placed in Bed SW4 of the Prati di Stuores/Stuores Wiesen Section in Cordevole Valley, Northern Italy, in the San Cassiano/St. Cassian Formation (about 45 m above the base of the formation). The chosen biostratigraphic marker is the FAD of the ammonoid Daxatina canadensis (Mietto et al., 2012; Gaetani, 2009). In terms of ammonoid biozones, this marks the boundary between the regoledanus Subzone of the Protrachyceras Zone and the canadensis Subzone of the Trachyceras Zone. Trachyceras aon and the conodont Paragondolella polygnathiformis, which have been used as markers for the base of the Carnian in the past, appear well above the GSSP level. The palynofloral succession across the boundary at the Prati di Stuores/Stuores Wiesen Section shows, e.g., the FOs of Patinasporites densus, Vallasporites ignacii, Camerosporites secatus, Weylandites magmus, and Samaropollenites speciosus in the Lower Carnian canadensis Subzone (Mietto et al., 2012; Cirelli and Roghi in Broglio Loriga et al., 1999). However, Van der Eem (1983) reported Camerosporites secatus in the stratigraphically Lower Buchenstein Formation. He also studied a stratigraphically higher part of the succession, where he documented the boundary between the secatus-vigens and vigens-densus phases (as well as the Longobardian-Cordevolian boundary) in the Upper San Cassiano/St. Cassian Formation, based on the first appearance of Vallasporites ignacii, with P. densus only occurring sporadically. As noted by Cirilli and Roghi (Broglio Loriga et al., 1999), the appearance of P. densus and V. ignacii just above the GSSP indicates a lithostratigraphically lower base for the vigens-densus phase, in the lower canadensis Subzone. On the other hand, Van der Eem (1983) reported the presence of the typical Cordevolian ammonoid Trachyceras aon from the Bulla and Grohmann sections in beds assigned to the secatus-vigens phase. This might be a mistake (as it did not affect the dating) or indicate a significant diachroneity for this phase boundary. Similar issues have been pointed out for the lower phases of Van der Eem (Hochuli et al., 2015).
Yaroshenko (1980) suggested Camerosporites secatus as a palynological indicator for the Carnian or the Upper Triassic in general, but this species is also known from the Ladinian (e.g., Hochuli et al., 2015; Roghi, 1995; Brugmann, 1983; Van der Eem, 1983). Its FO is still the defining characteristic of the Camerosporites secatus Zone of Kürschner and Herngreen (2010) in the Germanic Basin, where it is placed at the base of the Carnian. Here, the base also coincides with the FO of Triadaspora verrucata, but as with C. secatus, this species occurs already in the Ladinian in the Southern Alps (Hochuli et al., 2015). C. secatus occurs in Israel first at the base of the Pityosporites ruttneri-Triadispora modesta assemblage zone (Eshet and Cousminer, 1986), which was therefore only tentatively dated as Early Carnian. The Aulisporites astigmosus composite assemblage zone (Vigran et al., 2014) in the Norwegian Arctic is dated as Early Carnian due to the presence of D. canadensis. In India, the Ladinian-Carnian boundary has been associated with the bases of both the Rimaesporites potoniei assemblage zone and the overlying Rajmahalispora rugulata assemblage zone (comp. Vijaya et al., 2012; Tripathi et al., 2005; Tiwari and Kumar, 2002; Tiwari, 1999a, b). In Australia, it is located at the base of the Samaropollenites speciosus Oppel Zone or within the Craterisporites rotundus Oppel Zone (Helby et al., 1987) and unit APT41 (Price, 1997).4 CONCLUSIONS
The compilation of globally distributed palynostratigraphic schemes reveals that not all chronostratigraphic stages are equally well aligned with these schemes. In many cases independent age control is still needed for calibration, and to our knowledge formal palynozones have yet to be defined in Greenland, South China, and in the Triassic of Russia. The bases of the Wuchiapingian and Changhsingian are indistinct with respect to palynology and the Permian-Triassic boundary by itself is not always recognizable, but the latest Permian is marked by important changes in palynofloras. Spores and pollen might be useful as (approximate) indicators of the Induan-Olenekian boundary in several regions. However, the comparability of palynozones between regions is limited due to a lack of marker taxa and dependence on relative abundances, which may reflect local ecology rather than global effects. Another limitation is that useful palynostratigraphic data are currently only available from one of the GSSP candidate sections (Pakistan).
In contrast to the Lopingian and Early Triassic stages, the bases of the Anisian, Ladinian, and Carnian are palynologically distinct. Echinitosporites iliacoides is a reliable marker for the Ladinian, and the FAD of Stellapollenites thiergartii might serve as an indicator for the base of the Anisian, depending on the decision concerning the position of the corresponding GSSP (Fig. 8). The applicability of palynostratigraphic biozonation for the identification of stage boundaries varies geographically and stratigraphically, but their usefulness increases with the accumulation of data.
Plant macroremains, on the other hand, appear not to be reliable as biostratigraphic markers for the Lopingian to Middle Triassic, at least not on supra-regional scales, whereas they may be useful for correlation of stratigraphic horizons within specific sedimentary basins. In this work, we have focused on formally described biozones, but much more biostratigraphical information is available in the literature. Ideally, we should leverage all the data to approximate the true geographical and stratigraphical ranges of relevant taxa, as well as the relationships (including diachronous first and last occurrences) between microfloras of different regions. This would allow for a more precise and secure dating using palynomorphs than what is normally possible at the moment.ACKNOWLEDGMENTS
This study was supported by the Euregio Science Fund (call 2014, IPN16: "The end-Permian mass extinction in the Southern and Eastern Alps: extinction rates vs taphonomic biases in different depositional environments") of the Europaregion/Euregio Tirol-Südtirol-Trentino/Tirolo-Alto Adige-Trentino and by SYNTHESYS (access call 4, 2016, GB-TAF-6751: "Diversity changes of spores and pollen during the Permian-Triassic mass extinction"). We thank the two anonymous reviewers, whose suggestions helped us to improve the article substantially. This work is a contribution to IGCP Project 630—"Permian-Triassic climatic and environmental extremes and biotic response". The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0790-8.
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