Journal of Earth Science  2018, Vol. 29 Issue (4): 778-793   PDF    
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Permian-Triassic Charophytes:Distribution, Biostratigraphy and Biotic Events
Spencer G Lucas    
New Mexico Museum of Natural History, 1801 Mountain Road N. W., Albuquerque, New Mexico 87104, USA
ABSTRACT: Permian charophytes are known from the Ukraine, Russia, Kazakhstan, Germany, Saudi Arabia, China, the USA, Brazil, Paraguay and India. Most of these records are of Middle-Late Permian Age and are the basis of local biostratigraphic zonation in southern Russia and China. Development of a robust Permian charophyte biostratigraphy will require a more extensive record. Triassic charophytes are known from Germany, Sweden, Poland, Slovenia, Bulgaria, the Ukraine, Russia, Morocco, Congo, the USA, Argentina, Kazakhstan and China. This encompasses records from all Triassic stages and has been the basis of detailed biostratigraphic zonation in southern Russia-Kazakhstan-eastern Europe. Permian and Triassic charophyte biostratigraphy at the level of genus does not provide detailed correlations beyond local or regional schemes. Nevertheless, it does identify some important evolutionary datums that constrain the timing of important biotic events in the Permian-Triassic evolutionary history of the Charophyta, including:(1) Early Permian extinction of the Palaeocharaceae; (2) Late Permian extinction of the "Trochiliscales" (Moellerinales); (3) Carboniferous origin of the paraphyletic Porocharaceae, soon followed during the Permian by the origin of the multicellular basal plate; and (4) an important generic turnover of charophytes across the Triassic-Jurassic boundary, though there are insufficient data to identify this as a mass extinc-tion.
KEY WORDS: Permian    Triassic    charophytes    biostratigraphy    biotic events    

0 INTRODUCTION

Charophytes are fresh and brackish water green algae with a fossil record that extends back to the Silurian. Calcification occurred in most charophytes, and the group is particularly well known from fossils of the calcified female fructification, the gyrogonite (Fig. 1). Indeed, the taxonomy of fossil charophytes is based almost entirely on features of the gyrogonite, as have interpretations of the phylogeny and evolution of the group.

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Figure 1. Selected Permian and Triassic charophytes (drawings not to scale, after Sommer, Saidakovsky and Kiselevsky).

Permian and Triassic charophyte fossils have a broad geographic distribution across Laurussia, but are less well known from the Gondwana continents. Biostratigraphic schemes have been developed using Permian charophytes in Russia and China and using Triassic charophytes in Kazakhstan and eastern Europe (Russia, the Ukraine, Poland and Germany). Here, I review the distribution and evaluate the biostratigraphy of Permian–Triassic charophytes, with the ultimate goal of assigning ages to the major biotic events during the Permian–Triassic evolution of the charophytes.

1 TAXONOMY

A useful biostratigraphy relies on the unambiguous recognition of operational taxonomic units (OTU) to establish correlation. Identical OTU are assumed to indicate identical ages, a statement that I have termed the "index-fossil hypothesis" (Lucas, 2013). Taxonomy that is sound and replicatable thus is critical to producing a viable biostratigraphy. Like most workers, I treat the charophyte taxa based on gyrogonites as biological entitites, though, as Horn af Rantzien (1959) stressed, they are, technically, organ genera and species. Here, the OTU is the genus because most Permian and Triassic charophyte species are unique to one locality, which suggests to me that they are endemic ecophenotypes not useful in biostratigraphy. Furthermore, taphonomic differences between charophyte assemblages and oversplitting of species-level taxonomy has also contributed to the apparent endemism of many Permian and Triassic charophyte species.

The genus-level taxonomy used here follows Feist et al. (2005), though not all agree on the taxonomy compiled by those workers (for varied views of charophyte genus-level taxonomy see, for example, Lu, 1997; Saidakovsky, 1993; Feist and Grambast-Fessard, 1991; Bilan, 1988; Breuer, 1988; Schudack, 1986). Furthermore, the possibility that some of the genera here recognized as valid are not monophyletic taxa needs to be considered. Clearly, a comprehensive review of the taxonomy of Permian and Triassic charophytes, based on more data than that were available to Feist et al. (2005) is needed, but beyond the scope of this article.

For the Permian and Triassic, I thus recognize the following 11 valid charophyte genera and their synonyms used for some Permian and/or Triassic species: (1) Auerbachichara Kiselevsky, 1967 (=Shaikiniella Kiselevsky, 1993a); (2) Clavatorites Horn af Rantzien, 1954 (=Cuneatochara Saidakovsky, 1962); (3) Gemmichara Wang, 1984; (4) Latochara Mädler, 1955; (5) Leonardosia Sommer, 1954 (=Leonidiella Kiselevsky, 1993c; =Luichara Kiselevsky, 1993c; =Paracuneatochara Wang, 1984); (6) Palaeochara Bell, 1922; (7) Porochara Mädler, 1955 (=Aclistochara Peck, 1937, in part; =Euaclistochara Wang, Wang et al., 1976); (8) Stellatochara Horn af Rantzien, 1954 (=Maslovichara Saidakovsky, 1962); (9) Stenochara Grambast, 1962 (=Praechara Horn af Rantzein 1954); (10) Stomochara Grambast, 1961 (=Catillochara Peck and Eyer, 1963; =Horniella Shaïkin, 1966; =Altochara Saidakovsky, 1968); and (11) Vladimiriella (=Porosphaera Wang and Huang, 1978; =Triassic uses of Sphaerochara and Tolypella) (Fig. 1).

2 PERMIAN CHAROPHYTE DISTRIBUTION

Permian charophytes are known primarily from the Ukraine, Russia, western Kazakhstan and China, and most records are of Middle–Late Permian Age (Fig. 2).

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Figure 2. Permian charophyte distribution. Localities are: 1. Kansas-Oklahoma, USA; 2. Ohio, USA; 3. São Paulo State, Brazil; 4. Paraná State, Brazil; 5. Paraguay; 6. Saudi Arabia; 7. India; 8. Ukraine and Germany; 9. Russia; 10. western Kazakhstan; 11. Xinjiang, China; 12. northern China; 13. southern China.
2.1 Ukraine, Russia, Kazakhstan

Saidakovsky (1966a) described Permian charophytes from Tatarian strata in the Donetz Basin of the Ukraine that he assigned to Stellatochara (as "Maslovichara") and Porochara. Saidakovsky (1989) described new species of Stomochara (some as Horniella) from the Permian of Russia. These are primarily from boreholes in the Vyatka-Kama Basin near Kazan. Saidakovsky (1989) also reassigned Stomochara gracilis Esaulova and Saidakovsky, 1985 from Kazanian strata in the Kama River region to Leonardosia.

Kiselevsky (1993c) described species of Leonardosia (as Acutochara, and the new genera Leonidiella and Luichara), Clavatorites (as Cuneatochara) and Stomochara (some as Horniella) from Kazanian–Tatarian strata of western Kazakhstan and the Kirov and Orenburg regions of Russia. Kiselevsky (1998) reviewed earlier work (Esaulova and Kiselevsky, 1993; Kiselevsky, 1993a, b, c, d, 1980; Esaulova and Saidakovsky, 1985) on the Middle–Late Permian (Ufimian–Kazanian–Tatarian) charophyte assemblages of the Kazan region and plotted the stratigraphic ranges of the genera Clavatorites ("Cuneatochara"), Leonardosia ("Luichara"), Stellatochara and Stomochara (including "Horniella") (Fig. 3). Note that Arefiev et al. (2015) recently listed several records of Permian charophytes from the East European platform, but without taxonomic identifications.

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Figure 3. Biostratigraphy of Permian charophyte species in the Vyatka-Kama basin near Kazan, Russia (after Kiselevsky, 1998). Note that Cuneatochara is here considered as a synonym of Clavatorites; Horniella is a synonmy of Stomochara; and Luichara is a synonym of Leonardosia.
2.2 Germany

Stomochara is very common in Upper Pennsylvanian (Gzhelian) dm-thick limestones of the wet red-bed facies in the German Saale Basin (Schneider et al., 2005; Gebhardt and Schneider, 1985). From Permian deposits in Germany (originally called Carboniferous), Palaeochara has been reported as mass occurrences in lacustrine shales that straddle the Gzhelian/ Asselian boundary (Kozur et al., 1982). Though charophyte gyrogonites and vegetative parts (thalli) are known from thin sections of micritic limestones (algal-gastropod-ostracod limestones) found in several Early Permian German basins, the charophytes remain to be studied and identified (J. Schneider, personal com., 2018).

2.3 Saudi Arabia

Hill and El-Khayal (1983) reported Palaeonitella (a form genus for extinct vegetative remains of charophytes) from the Midhnab Member of the Khuff Formation of Central Saudi Arabia, which they regarded as of Late Permian Age. Vaslet et al. (2005) assigned a Changhsingian Age to this record based on foraminiferal biostratigraphy.

2.4 India

De (2003) reported two species of Leonardosia (as Paracuneatochara) from the Talchir Basin near Mumbay in India. These are from the upper part of the Middle Barakar Formation, assigned to a Permian Age, but with no more specificity. Goswani and Singh (2013) assigned a late Early Permian Age to this record based on macrofossil plant biostratigraphy and palynostratigraphy.

2.5 China

Wang (1984) named two new genera of charophytes, Gemmichara and Paracuneatochara (= Leonardosia) from the Late Permian Sunlan Formation in Gansu and the Hongla Formation in Liaoning. Gemmichara is particularly significant, as it is the stratigraphically highest genus of the "Trochiliscales" (see later discussion).

Lu and Luo (1984) reported Leonardosia (as "Paracuneatochara") from the Upper Permian Xiaolongkou Formation in Xinjiang. They noted the presence of similar charophytes in the Hongla Formation of Liaoning and the Sunan Formation of Gansu.

Wang and Wang (1986) reported Gemmichara and Leonardosia (as "Paracuneatochara") from Early–Middle Permian strata in eastern China (Henan, Anhui and Jiangsu provinces). The Gemmichara is from the Upper Shihotze Formation, whereas the Leonardosia is from the Lower and the Upper Shihotze Formation. Wang et al. (2003), based on floral and magnetostratigraphic data, considered the Lower Shihotze Formation to be Makouan (Early–Middle Guadalupian), and the Upper Shihotze Formation to be Lengwuan (Late Guadalupian).

Luo (1987) reported Stomochara and Porochara from Late Permian strata in the western Tarim Basin of China. Lu and Luo (1990) reported a single gyrogonite of Leonardosia from the Kangkelin Formation in the Tarim Basin of Xinjiang, which they regarded as an earliest Permian (Zisongian) record. Lu and Zhang (1990) reported Leonardosia from the Late Permian lower part of the Guodikeng Formation in Xinjiang.

2.6 USA

Peck and Eyer (1963) noted occurrences of charophytes in Early Permian strata (Council Grove Group) of Kansas (also see Peck, 1934; Lane, 1958) and in the Lower Permian Dunkard Group of Ohio. They assigned all of these records to Catillochara moreyi (Peck), which has been reassigned to the genus Stomochara (Feist et al., 2005).

Lucas and Johnson (2016) described an assemblage of Palaeochara from the Kungurian (Leonardian) Wellington Formation of Oklahoma. Note that both Glukhovskaya (1975) and Tappan (1980) considered Palaeochara to be based on anomalous ("pathological") specimens, because of its unique morphology of six, sinistrally spiral cells. At the time of their suggestions, little was known beyond the small original sample of Palaeochara described by Bell (1922). However, subsequent discoveries in the Carboniferous/Permian of Germany and Carboniferous of China (Gao et al. 2002; Lu and Luo, 1990; Kozur et al., 1982) and the large sample described by Lucas and Johnson (2016) from a single Permian locality validate the unusual morphology of Palaeochara.

2.7 Brazil

Sommer (1954) named the common Permian charophyte genus Leonardosia for material from the Permian Teresina Formation in Paraná State (also see Herbst, 1981). Faria et al. (2013; also see Ricardi-Branco et al., 2016; Christiana de Souza et al., 2012) described abundant Leonardosia from the Teresina Formation. Additional material of Leonardosia has been reported from the Permian of São Paulo State (Corumbatá Formation) by Ragonha and Soares (1974), Zampirolli et al. (1997) and Faria and Ricardi-Branco (2009). These workers assign the Brazilian records of Leonardosia a tentative Early Guadalupian Age.

2.8 Paraguay

Herbst (1981) reported Leonardosia from Permian strata in Paraguay considered to be correlative to the Brazilian Teresina Formation.

3 PERMIAN CHAROPHYTE BIOSTRATIGRAPHY

Kiselevsky (1998) presented a biostratigraphy based primarily on borehole records of charophytes from the Kazan region of southern Russia (Fig. 3). This biostratigraphy can distinguish most of the horizons of the Russian Middle–Late Permian regional stages (Ufimian, Kazanian, Tatarian) by presence/absence of one or more charophyte species (Fig. 3). However, this is a very local biostratigraphy (almost all of the species are endemic) of no proven value to correlation outside of southern Russia.

In China, Lu et al. (2000; also see Wang et al., 2003) recognized a succession of five Permian charophyte assemblages (the two youngest of which are approximately coeval) in China: (1) Leonardosia sp. assemblage from the Early Permian Kangkelin Formation of Xinjiang; (2) Leonardosia yongchengensis assemblage from the middle and upper part of the Lower Shihotze Formation; (3) Gemmichara pingdingshanensis-Leonardosia? sp. assemblage from the Upper Shihotze Formation, to which they assigned a Wuchiapinguian Age; (4) Leonardosia jinxiensis-L. jimsarensis-Gemmichara sinensis assemblage from the lower Guodikeng Formation in Xinjiang, lower part of the Hongla Formation in Liaoning and the Sunan Formation in Gansu, to which they assigned a Changhsingian Age; and the coeval (5) Stomochara kunlunshanensis-Porochara moyuensis assemblage from the Duwa Formation in Xinjiang, also assigned a Changhsingian Age. The Leonardosia jinxiensis-L. jimsarensis-Gemmichara sinensis assemblage provides some basis for correlation across northern China, from Xinjiang to Liaoning. However, the biostratigraphic utility of most of this succession remains to be proven.

If we examine the stratigraphic distribution of Permian charophyte genera at the series level (Fig. 4), a few possible biostratigraphic datums can be identified: (1) Leonardosia is characteristic of Permian strata, though it has one Early Triassic record; (2) the HO (highest occurrence) of Gemmichara is Late Permian and the HO of Palaeochara is Early Permian; and (3) the LO (lowest occurrence) of Auerbachichara is Early Permian, the LO of Clavatorites is Middle Permian, and the LOs of Stellatochara and Porochara are Upper Permian. However, note that there is no standard Permian succession of charophytes that can be tied directly to the standard global chronostratigraphic scale (SGCS, the "marine timescale"). Instead, the ages assigned to the Permian charophytes listed earlier are based on macrofossil plant biostratigraphy, palynostratigraphy or other criteria that, themselves, need to be tied to the SGCS, and the correlation of which to the SGCS is often imprecise.

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Figure 4. Stratigraphic distribution of Permian and Triassic charophyte genera by series.

Clearly, Permian charophyte biostratigraphy is in a very early stage of development. Wang et al. (2003, p. 199) well observed that "their [Permian charophytes'] sporadic occurrence renders an attempt to establish precisely successive biozonation scarcely feasible, and it is insufficient for precise determination and correlation in [on a] global scale". Nevertheless, the Permian charophyte record does establish some important datums for charophyte biotic events discussed below.

4 TRIASSIC CHAROPHYTE DISTRIBUTION

Triassic charophytes are best known from Sweden, Poland, Germany, the Ukraine Russia and Kazakhstan and less well known from other localities that give them a broad distribution across Triassic Pangea (Fig. 5).

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Figure 5. Triassic charophyte distribution. Localities are: 1. New Mexico-Arizona, USA; 2. Argentina; 3. Congo?; 4. Morocco; 5. Sweden; 6. Germany and Poland; 7. Bulgaria and Slovenia; 8. Russia; 9. Ukraine; 10. western Kazakhstan; 11. Taimyr, Russia; 12. Tunguska, Russia; 13. northern China; 14. southern China.
4.1 Germany

Early reports of Triassic charophytes from Germany were little more than records without the kind of information that might allow taxonomic assignments (e. g., Horn af Rantzien, 1953; Krause, 1939; Wicher, 1939; Pia, 1924). Reinhardt (1963) reported two species of Stellatochara from the lower Keuper in boreholes in Thuringia. Kozur and Reinhardt (1969) reported charophytes from the Muschelkalk and lower Keuper assigned to Porochara, Stellatochara and Stenochara. Kozur(1975, 1974) proposed a succession of charophyte zones for much of the Triassic Section in the Germanic Basin (Fig. 6). Breuer (1988) reported Clavatorites (as Cuneatochara) from the Lower Keuper of southwestern Germany. Schudack (2013) recently presented a brief review of the German record of Triassic charophytes, noting its local/regional value in biostratigraphy.

4.2 Sweden

Horn af Rantzien (1954; also see Brotzen, 1950) described charophytes from Middle Triassic strata in a borehole at Höllviken in southern Sweden (Scania). Most significantly, he named the genus Stellatochara, which is the most common and widespread Triassic charophyte genus. Charophytes from the Middle Triassic in Scania were assigned to Clavatorites, Porochara (as Aclistochara), Stenochara (as Praechara), Stellatochara and Vladimiriella (as Sphaerochara).

Qvanström and Niedźwiedzki (2018) reported an Late Triassic charophyte assemblage from southern Sweden consisting of Auerbachichara, Porochara, Stellatochara and Stenochara. They correlated these charophytes to the Late Triassic Auerbachichara rhaetica range zone of Bilan(1991, 1988).

4.3 Poland

Bilan (1969) described species of Stellatochara (including Maslovichara), Stenochara and Porochara from the Keuper strata at Kolbark near Krakov in southern Poland. Bilan (1974) named two new species of Auerbachichara from the Polish "Rhaetian" and one new species of Stellatochara from the Polish Upper Keuper.

Bilan (1988; also see Bilan, 2005, 1991) published a monographic study of Triassic charophytes based on charophyte assemblages from borehole samples at diverse localities in Poland. From these data he constructed a charophyte biostratigraphy of five partial range zones that encompass most of Triassic time (Fig. 6). Significantly, Bilan (1988) correlated the Polish charophyte biostratigraphy to those developed by Saidakovsky and Kiselevsky in the Ukraine, Russia and Kazakhstan, establishing the most comprehensive Triassic charophyte biostratigraphy available.

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Figure 6. Triassic charophyte biostratigraphy in Russia, Ukraine, Kazakhstan and Eastern Europe (modified from Bilan, 2005, 1988).

Zatoń and Piecholta (2003) described charophytes from the Late Triassic Krasiejów bonebed in southwestern Poland, assigning them to Porochara, Stellatochara, Stenochara, and Stomochara. The age of the Krasiejów bonebed is either Late Carnian (Lucas, 2015) or Norian (Szulc et al., 2015).

4.4 Bulgaria

Saidakovsky (1968) reported (but did not fully document) various charophytes from Bulgaria, mostly from boreholes in strata of reported Anisian–Rhaetian Age, including Auerbachichara (Rhaetian), Clavatorites (as Cuneatochara) (Ladinian–Norian), Porochara (Anisian–Ladinian), Stellatochara (including Maslovichara) (Anisian–Carnian), Stomochara (including Altochara) (Anisian–Carnian, Rhaetian) and Vladimiriella? (as Sphaerochara?) (Carnian–Norian).

4.5 Russia, Ukraine

Apparently the first published reference to a Triassic charophyte was by Schimper (1869, p. 217), for material supposedly found near Moscow, though the stratigraphic provenance of these charophytes is uncertain (Horn af Raintzien, 1954). Soon after, Auerbach (1871) described Early Triassic charophytes that he named Chara bogdoana from Bolshoi Bogdo Mountain near the Caspian Sea.

Working in southern Russia, Demin (1956) described two species he assigned to Chara from Lower Triassic strata. These species are Chara donbassica, later assigned to Stellatochara, and C. karpinskyia, later assigned to?Vladimiriella (Kiselevsky, 1969d).

Saidakovsky's (1960) first publication on Triassic charophytes from the Dnepr-Donyets basin in Ukraine soon followed, although he noted that several earlier workers made mention of the presence of charophytes in these strata. Saidakovsky (1960) presented new species of Porochara ("Aclistochara") Stellatochara and Vladimiriella ("Tolypella"). In the Early Triassic, four charophyte zones were delineated.

Saidakovsky (1962) named the new genera Cuneatochara and Maslovichara. He presented a brief summary of seven Triassic biostratigraphic zones based on charophytes, primarily from the Russian platform (Fig. 6).

Saidakovsky (1966b) published a monographic study of the Triassic charophytes of the Bolshovo Donbass (greater Donbass) Basin on the Russian platform. This included new species of and taxonomic revisions of Clavatorites (as Cuneatochara), Latochara, Porochara, Stellatochara (including Maslovichara), Stenochara and Vladimiriella ("Sphaeochara"). The monograph organized the Russian charophyte assemblages into seven zones that were considered to be of Early and Late Triassic ages (Fig. 6).

Kiselevsky (1967) named new Triassic species of Porochara and Stenochara from the pre-Caspian basin. The new genus Auerbachichara Kiselevsky and Saidakovsky was named for material from Bolshoi Bogdo Mountain.

Saidakovsky (1968) described Middle and Upper Triassic charophytes from the Caspian basin and southern Russian platform. He assigned them to Auerbachichara, Clavatorites ("Cuneatochara"), Porochara, Stellatochara, Stenochara, Stomochara (including Altochara) and Vladimiriella ("Sphaerochara"). Saidakovsky (1968) also reassigned his Lower Triassic charophyte zones Ⅳ and Ⅴ to the Middle Triassic (Fig. 6).

Kiselevsky (1969a) reviewed Triassic charophyte biostratigraphy in the northwestern part of the pre-Caspian basin, recognizing six zones of Early, Middle and Late Triassic Age (Fig. 6). He correlated these zones to the charophyte zonations of the southern European part of the USSR as well as to Sweden and the Germanic basin. Kiselevsky (1969b) described new Triassic species of Porochara and Vladimiriella ("Sphaerochara"), and Kiselevsky (1969c) described Triassic charophytes in thin section. Kiselevsky (1969d) also described charophytes assigned to Porochara and Vladimiriella ("Sphaerochara") from the Early Triassic strata at Bolshoi Bogdo Mountain, assigning them to a single biostratigraphic zone equivalent to his zone Ⅱ in the pre-Caspian basin.

Saidakovsky (1971), in a summary of his dissertation, reviewed his already published charophyte biostratigraphy, recognizing seven zones of Early, Middle and Late Triassic ages. Saidakovsky and Kiselevsky (1985) presented a synopsis of their Triassic charophyte biostratigraphy of the East European platform. Kiselevsky (1991) synonymized Altochara and Horniella. He reprised the four Triassic charophyte zones recognized in western Kazakhstan by Saidakovsky and Kiselevsky (1985).

Saidakovsky (cf. Mogutcheva and Krugovykh, 2009) reported charophytes indicative of the Vladimiriella karpinskyi zone (Lower Induan) from the Keshin Formation in Taimyr, Siberia. Mogutcheva and Krugovykh (2009) reported charophytes from Early Triassic strata of the Tunguska Basin in Siberia. Saidakovsky identified these charophytes, assigning them to his Vladimiriella karpinskyi zone of Induan Age and to his Porochara triassica zone of Olenekian Age.

4.6 Slovenia

MartíMartín-Closas et al. (2009) documented Stenochara from Late Ladinian/Early Carnian marine strata in Slovenia.

4.7 Morocco

Medina et al. (2001) reported Stellatochara and Stellatochara ("Maslovichara") from the base of the Aglegal Member of the Timesgadouine (Timezgadiwine) Formation in the Argana Basin of Morocco. They advocated a Middle Triassic age based on these charophytes.

4.8 Congo

Peck (1953, p. 219) referred to a supposed Triassic charophyte record from the "Belgian Congo" listed by Groves (1933), but nothing more is known of this report.

4.9 Kazakhstan

Kiselevsky (1991) reviewed the Triassic charophytes of western Kazakhstan (Caspian depression, Ustyurt and Mangyshlak), concluding that they could be used to identify four biostratigraphic zones (also see Saidakovsky and Kiselevsky, 1985): (1) Vladimirella wetlugensus-Horniella continua zone of Induan Age; (2) Porochara triassica-Auerbachichara baskuntschakensis zone of Olenekian Age; (3) Stellatochara dnjeproviformis-Stenochara donetziana zone of Anisian Age; and (4) Stellatochara hoellvicensis-Stenochara pseudoovata zone of Ladinian Age (though note that Saidakovsky and Kiselevsky [1985] considered this last zone to range into the Late Triassic). Kiselevsky (1991) correlated these zones to other parts of the eastern European platform, Bulgaria, Poland, East Germany and China.

Kieselyevsky (1993a) reported charophytes from the Middle Triassic Inder horizon in the Caspian depression. He assigned them to various species of Porochara, Stellatochara (including Maslovichara), Stenochara and to a new genus, Shaikinella, now regarded as a synonym of Auerbachichara (Feist et al., 2005).

4.10 China

Wang and Huang (1978) reported charophytes from the Triassic Heshanggou, Ermaying and Wayaoba formations in Shaanxi Province. Vladimiriella (as Porosphaera) is present in the Heshanggou Formation, Stellatochara and Stenochara in the Ermaying Formation and Clavatorites (as Cuneatochara) in the Wayaoba Formation.Wang et al. (1976) reported Clavatorites (as Cuneatochara) from the Wayabao Formation in Shaanxi, and Li (1985) reported Stellatochara from the Tanzhuang Formation in Henan. Also, from Triassic strata in Henan, Zhao et al. (1980), Zhang (1981) and Ge (1995) reported Clavatorites (as Cuneatochara), Stellatochara, Stenochara and Vladimiriella.

Wang and Huang (1978) described "Porosphaera maxima" from the Lower Triassic Heshanggou Formation, and Wang and Wang (1986) reassigned that material to Leonardosia (as Paracuneatochara). This is the only Triassic report of Leonardosia, but I question the generic attribution. Thus, the type material of "Porosphaera maxima" illustrated by Wang and Huang (1978, pl. 1, Figs. 7–8 therein) lacks a distinct neck, so it is not Leonardosia, and does conform to their original generic assignment to "Porosphaera" (=Vladimiriella). The specimen attributed by Wang and Wang (1986, pl. 1, Figs. 9–11 therein) to "Porosphaera maxima" has a very short, thin and tubular neck. It is a different taxon from the type of "Porosphaera maxima" and likely a specimen of Stellatochara.

Wang (1981) reported Triassic charophytes from Anhui and Zhejiang provinces. These are Stellatochara from the Middle Triassic Tontoujian Formation, and Porochara (as Euaclistochara) from the Hongqian and Yushanjian formations. Huang (1983) documented Middle Triassic charophytes from the Huangmaqing Formation at Zhongshan, Nanjing. These are Porochara, Stellatochara, Stenochara and Vladimiriella (as Porosphaera), with the age assignment based primarily on the species of Stellatochara. Huang (1983) stressed that some of the species of Stellatochara from Nanjing are restricted to Middle Triassic strata in Sweden, Germany and Russia, and also to Middle Triassic strata in China (Ermaying Group, Shaanxi; Liaodeng Formation, Shandong; Badong Formation, Hubei) to assign the Huangmaqing Formation charophytes a Middle Triassic Age. However, Zhao (1985) noted that strata assigned to the Huangmaqing Formation at the Jiangsu-Anhui border yield what is more likely a Middle Jurassic assemblage of charophytes assigned to the genus Porochara (including Euaclistochara and Aclistochara).

Lu and Luo (1984) documented Triassic charophytes from the Turpan Basin of southern Xinjiang. From the Early Triassic Euhoubulake Formation, they reported Auerbachichara, ?Stellatochara, Stenochara and Vladimiriella. From the Middle Triassic Karamay Formation they reported Clavatorites (as Cuneatochara), Stellatochara? and Stenochara. Lu and Luo (1984, Table 1 therein) also plotted the global temporal ranges of Triassic charophyte genera.

Zhao and He (1988) reported Triassic charophytes from Fanxian in Henan Province. These are Stenochara and Vladimiriella from the Lower Triassic Heshanggou Formation, and Stellatochara and Stenochara from the Middle Triassic Liaochengtan Formation.

In an extensive monograph on the Carboniferous–Quaternary charophytes of the Tarim Basin in western China (Xinjiang), Lu and Luo (1990) described charophytes from Early–Middle Triassic strata in the northern part of the basin. On outcrop, in the Kuqa depression, they recovered charophytes from the Early Triassic Ehuobulake Formation (Porochara, Stenochara and Vladimiriella) (also see Yang, 1987) and the Middle Triassic Karamay Formation (Clavatorites [as Cuneatochara], Stellatochara? and Stenochara). From borehole data in the Tabei depression, they documented Auerbachichara, Porochara, Stellatochara? and Vladimiriella from the Ehuobulake Formation. They organized these charophytes into two biostratigraphic assemblages: Auerbachichara xinjiangensis-Vladimiriella karpinskyi-Porochara brotzeni assemblage of Early Triassic age and Stenochara ovata-Stellatochara wensuensis assemblage of Middle Triassic age. Significantly, they were not able to recognize and correlate these assemblages to other Chinese Triassic charophyte assemblages, as none of the species they documented from Xinjiang occur elsewhere in China.

Liu and Chen (1992) reported a charophyte assemblage dominated by Porochara (including Aclistochara) from the Late Triassic Xujiahe Formation in Sichuan. They reported a similar assemblage (but also with some Stenochara) from the overlying Lower Jurassic Yimen Formation. Liu and Chen (1992) assigned all Late Triassic charophyte assemblages from China known to them (from Shanxi, Henan and Sichuan) to a Cuneatochara tongchuanensis-Stellatochara jiyuanensis-Porochara dazuensis assemblage. They similarly identified an Early Jurassic Porochara xichangensis-Aclistochara sichuanensis-Stenochara xiangqiensis assemblage.

Jiang et al. (2001) reported charophytes from Early–Middle Triassic strata in a borehole drilled in the Lingqing depression along the Bohai Sea coast. They assigned these charophytes to Stellatochara and Stenochara and indicated the charophytes are similar to the charophyte assemblage from the Middle Triassic Ermaying Formation of eastern China.

4.11 USA

Peck (1934) reported charophytes from strata that he termed "upper Chugwater" in Wyoming and indicated they are likely of Triassic age, a record considered as Triassic by some later workers (e.g., Horn af Raintzien, 1954). However, these "upper Chugwater" strata are actually part of the Middle Jurassic Sundance Formation, so this is not a Triassic charophyte record (Peck, 1957). Indeed, when Peck (1957) monographed the North American Mesozoic charophytes, the oldest records he listed were Middle Jurassic.

Peck and Eyer (1963) reported charophytes (Stellatochara) from two localities in the Triassic Moenkopi Group/Formation. One is in the Holbrook Member of the Moenkopi Formation near Cameron Arizona (this is a capitosaur amphibian bonebed) and is the type locality of Stellatochara prolata Peck and Eyer. The other locality is in Rio Blanco County, Colorado, between 0.6 and 4 m below the base of the overlying Late Triassic Chinle Group. The Holbrook Member locality is of Early Anisian Age (Lucas and Schoch, 2002), and the Colorado locality may also be that young, given its stratigraphically high position in the Moenkopi Section.

Kietzke (1987) reported Stellatochara and Stomochara (as Altochara) from the Bull Canyon Formation of the Upper Triassic Chinle Group in East-Central New Mexico. He also reported Stellatochara from the Sloan Canyon Formation of the Chinle Group in northeastern New Mexico. Kietzke(1989a, b, 1988) reported Porochara and Stomochara? (as Altochara?) from the Middle Triassic Anton Chico Member of the Moenkopi Formation in Central New Mexico. Lucas and Kietzke (1993) reported Porochara from the Late Triassic Painted Desert Member of the Petrified Forest Formation in eastern Arizona. Lucas and Kietzke (1996) briefly reviewed the North American Triassic record of charophytes to conclude that the paucity of charophyte-producing facies likely precludes their use in Triassic biostratigraphy on this continent.

4.12 Argentina

Benavente et al.(2014, 2012) reported charophytes from the Middle Triassic Cerro Puntudo Formation of San Juan, northwestern Argentina. These are poorly preserved forms that probably belong to Porochara and Stellatochara.

5 TRIASSIC CHAROPHYTE BIOSTRATIGRAPHY

With regard to Triassic charophyte biostratigraphy, both Riveline et al. (1996) and Feist et al. (2005) regarded the zonation of Bilan (1988) as standard, and Feist et al. (2005) presented an updated zonation provided by Bilan (2005) (Fig. 6). Saidakovsky(1973, 1962), Kiselevsky(1993d, 1969a) and Saidakovsky and Kiselevsky (1985) developed charophyte-based biostratigraphic schemes for the Donbass Basin (Ukraine), pre-Caspian depression (Kazakhstan), the European portion of the former Soviet Union and the eastern European platform. These biozonations show some regional consistency, although the ages assigned to some of these assemblages (derived from non-charophyte biostratigraphy) have varied (Fig. 6).

Lu and Luo (1990) identified two Triassic charophyte assemblages in the Tarim Basin of western China: Auerbachia xinjiangensis-Vladimiriella karpinskyi-Porochara brotzieni assemblage overlain by a Stenochara ovata-Stellatochara? wensuensis assemblage. The latter is also known from the Kelamayi (Karamay) Formation of Xinjiang, which produces fossil vertebrates of the Parakannemeyeria biochron, which Lucas (2010) considered to be of Middle Triassic (Anisian) Age. As Lu and Luo (1990) noted, their older assemblage has no counterpart in other Chinese localities, so its age is not readily determined. Indeed, the assemblage below it, the Stomochara hoitanensis-Porochara moyuensis assemblage, may also be Triassic, though Lu and Luo (1990) suggest that associated ostracods and palynomorphs indicate a Late Permian Age.

A critical review of the Triassic charophyte biozonation of Bilan(2005, 1988) indicates that it is of questionable utility. Bilan's zonation is largely composed of partial range zones not recognized by index taxa, but instead relies heavily on the absence of a taxon. As such, it is not an ideal biozonation, given that the absence of a taxon can be facies related or due to sampling and not necessarily reflect its actual stratigraphic range. A charophyte biostratigraphy based on partial range zones may prove to be of local/regional utility over an area where the factors that truncate taxon ranges (usually facies changes) are stratigraphically consistent. However, a more broadly applicable biostratigraphy needs to be based on the "complete" stratigraphic ranges of index taxa. Indeed, the inability to apply the Eocene charophyte biostratigraphy of Riveline et al. (1996) outside of western Europe (Zhamangara and Lucas, 1998) may in large part be due to the fact that this biostratigraphy relied heavily on partial range zones.

Indeed, Bilan's (1988, Table 9 therein) plot of the stratigraphic ranges of the charophyte taxa in the Polish section shows disjunct stratigraphic ranges of many taxa. Bilan's zonation appears to have been driven by a desire to create zones that are essentially the same as those proposed by Saidakovsky and Kiselevsky for the Triassic of the Russian platform, even though the Polish Triassic charophytes do not reprise those zones. Furthermore, correlation of the Polish Section to the classic Germanic Triassic strata was already clear before the work of Bilan, so the charophyte zonation of the German strata (e.g., Kozur, 1975, 1974) also heavily influenced Bilan's work. A critical review of Bilan's zonation identifies only three useful biostratigraphic constructs:

1. The oldest zone is the Vladimiriella globosa partial range zone, which in Poland is the interval in which V. globosa does not co-occur with Porochara triassica. V. wetlugensis and Porochara belorussica also occur in this zone, but have long stratigraphic ranges from Lower to Upper Triassic. V. globosa is restricted to the Early Triassic but has a range beyond that of its partial range zone. Therefore, a V. globosa range zone that approximates the Early Triassic could be defined based on Bilan's (1988) data. Also, note that the recognition of the V. globosa partial range zone relies on the absence of P. triassica.

2. Bilan considered the Porochara triassica partial range zone of Poland to be equivalent to the Porochara triassica/ Auerbachichara bakuntschakiensis Zone of Saidakovsky and Kiselevsky (1985). It occurs in Russia in strata of the Enoyatevka Formation that overlie strata that produce the Spathian ammonoid Tirolites. In Poland, it is in the upper part of the Middle Buntsandstein. Bilan's Porochara triassica partial range zone is the (small) portion of the stratigraphic range of P. triassica that does not overlap the stratigraphic range of Stellatochara dnjeproviformis. Significantly, the key taxa of the underlying zone—Vladimiriella globosa, V. wetlugensis and P. belorussica—are present in this zone. Thus, only the absence of S. dnjeproviformis defines this zone.

3. The Stellatochara dnjeproviformis partial range zone in Poland is the interval where this taxon does not overlap the range of S. hoellvicensis. In Poland, this encompasses the Röt to middle Muschelkalk (~Anisian). Only the very rare species Latochara acuta (known from seven gyrogonites from one locality) is restricted to this zone (only the lower part). Indeed, as Bilan (1988, p. 133) noted, the S. dnjeproviformis zone contains "numerous species known from both the Porochara triassica zone and Vladimiriella globosa zone". Note that Bilan's data support the idea of a Stellatochara acme zone equivalent to the Middle Triassic, though the genus in Poland ranges from low in the Early Triassic and extends into the lower part of the Middle Triassic.

4. The Stellatochara hoellvicensis partial range zone is the stratigraphic range of that species that does not overlap that of the younger species, S. thuringica. No species are restricted to this zone, and, as Bilan (1988, p. 135) noted, "there appear also numerous species known from earlier defined zones". S. hoellvicensis is a relatively widely known species in late Middle Triassic (~ Ladinian) strata, so this in part underlay using it as a zone name species, even though its stratigraphic range in Poland extends well into the Late Triassic. Again, Bilan's zone here seems to be created to match the biozonation of others, not as a useful biostratigraphic construct.

5. The Stellatochara thuringica partial range zone is that part of the range of this species below the LO of Auerbachichara rhaetica. Again, no species are restricted to the zone, and most of the species present are also found in underlying zones. The S. thuringiaca zone has no counterpart in the Soviet biozonatians, and Bilan (1988) viewed it as encompassing much of Late Triassic time.

6. The Auerbachichara rhaetica range zone is the only zone proposed by Bilan that corresponds to the actual stratigraphic range of a charophyte species. A. rhaetica is restricted to the zone, but no other species are restricted to the zone, originally considered Rhaetian (Bilan, 1988) but later considered to be Norian (Bilan, 2005).

What, then, can we conclude about charophyte biostratigraphy based on the Polish Triassic record? I think the answer is found in Bilan's (1988) discussion of charophyte assemblages associated with particular facies. This indicates that an abundance of small representatives of Porochara and Vladimiriella characterizes the Middle Buntsandstein (part of Early Triassic) in the Polish section. A more diverse assemblage dominated by Stellatochara and Stenochara characterizes the Upper Buntsandstein (Röt) and Muschelkalk (Middle Triassic). Late Triassic charophytes comprise more diverse assemblages, with the introduction also of Auerbachichara-dominated assemblages. Bilan's (1988, p. 151) conclusion that "the Lower and Middle Triassic assemblages of the Polish part of the Central European Basin and that of the East European Basin show a marked similarity in respect to the composition and succession of the predominant genera" is reasonable.

Riveline et al. (1996) and Feist et al. (2005, p. 84, Fig. 25), in a discussion of Triassic charophyte biozonation, simply refer to the scheme of Bilan (1988). They also list (table 9) the temporal ranges of charophyte genera in the Triassic, with all genera ranging throughout the entire Triassic (Auerbachichara, Latochara, Stellatochara, Stomochara, Clavatorites, Stenochara, Vladimirella and, possibly, Porochara and Feistiella, though I cannot verify the records of Feistiella? in the Permian and Triassic listed by Feist et al., 2005). Mädler (1957, text-fig. 2, table 2) shows the stratigraphic distribution of Triassic charophyte genera, and Tappan (1980, Figs. 11.16–11.17) also plotted the Triassic ranges of charophyte genera. Lu and Luo (1984, Table) show the temporal ranges of Triassic charophyte genera, indicating substantial increases in the abundance of porocharaceans during the time period.

The important conclusion about Triassic charophyte biostratigraphy is that the biostratigraphic schemes proposed for Russia, eastern Europe and western Kazakhstan show some consistency that allow correlations in that region only. The Germanic Basin charophyte biostratigraphy is closely correlated to the SGCS (e.g., Kozur and Bachmann, 2008), but the stage assignments of the other biostratigraphic successions in Fig. 6 are of varied precision and reliablility. This regional charophyte biostratigraphy of Russia-eastern Europe-western Kazakhstan merits further testing by developing more extensive Triassic charophyte records elsewhere, especially in China and the USA. However, at present, there is no global Triassic charophyte biostratigraphy, only a regional biostratigraphy of some utility in Russia, western Kazakhstan and eastern Europe.

6 BIOTIC EVENTS 6.1 Diversity

Early reviews stressed how little known and little studied were Permian and Triassic charophytes (e.g., Tappan, 1980; Grambast, 1974, 1963; Mädler, 1957; Peck, 1953). Indeed, charophyte diversity was low throughout the Paleozoic, Triassic and Early–Middle Jurassic, with about 4–8 genera known per series (Fig. 7). The diversification of the Clavatoracaea began in the Late Jurassic and culminated in the highest generic diversity (32–35 genera) of charophytes, during the Cretaceous and Paleogene, which truly was the "age of charophytes".

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Figure 7. Charophyte generic diversity through time (modified from data in Feist et al., 2005).

Shaikin(1991, 1988) showed a spike in charophyte diversity during the Early Triassic that he attributed to the change from cooler, glacial Permian climates to warmer Triassic climates. Earlier, Tappan and Loeblich (1973, Fig. 5 therein) plotted species diversity of charophytes from the Late Permian through the end of the Triassic. They inferred that the highest diversity during this time interval, during the Early–Middle Triassic, indicated low levels of freshwater productivity. Tappan (1980) amplified this idea, arguing that Late Paleozoic freshwaters were nutrient rich, and this eutrophication explained the rarity and low diversity of Carboniferous–Permian charophytes. She further argued that Triassic red beds indicate lower levels of organic matter, so the nutrient deficient waters of Triassic Pangea underlie the expansion of charophytes that began in the Triassic. However, the lack of hydrophytic competitors during the Triassic may have been a more important factor, insuring that charophytes were dominant elements of the Triassic macrophytes (Martín-Closas, 2003).

Low generic diversity of charophytes during the Silurian–Middle Jurassic may be at least in part a function of sampling and preservation (taphonomy). However, given the relative intensity and geographic breadth of study during some time intervals (particularly the Triassic), this low diversity is more likely to be real, as it is today. Indeed, there has been no appreciable change in Triassic charophyte generic diversity between the compilations of Bilan (1988, Fig. 7 therein) and Feist et al. (2005, Fig. 7 therein). It thus seems that Permian–Triassic charophyte generic diversity may have been characteristic of a real, relatively low diversity of charophyte genera during the Paleozoic and much of the Mesozoic.

6.2 Originations

A variety of phylogenetic schemes have been proposed for Permian and Triassic charophytes (e.g., Kieselevsky, 1992; Feist and Grambast-Fessard, 1991; Martín-Closas and Schudack, 1991; Saidakovsky, 1989; Bilan, 1988; Grambast, 1974). Prior to the Permian, gyrogonites had as many as five helical cells, but during the Permian it became fixed at five enveloping cells that are twisted clockwise (sinistrally spiraled). Those forms with six clockwise-twisted cells and those with cells twisted counter clockwise (dextrally spiraled) became extinct by the end of the Permian (see below). Indeed, the sinistral spiral of five cells appeared during the Carboniferous and became dominant during the Permian, and is the number and orientation of helical cells in all Triassic-Recent charophytes (the Quniquespiralia of Martín-Closas and Schudack, 1991).

All Triassic charophytes are usually assigned to the paraphyletic Porocharaceae, which has a long stratigraphic range from Carboniferous to Paleocene (Carboniferous Stomochara is its earliest known representative). The Porocharaceae are well represented in the Permian–Triassic by genera with five, clockwise-coiled spiral cells and an apical pore. There are two charophyte lineages evident in the Triassic, one with unicellular (undivided, single) and the other with multicellular (divided, multipartite) basal plates (Martín-Closas, 2003; Martín-Closas and Schudack, 1991; Soulié-Märsche, 1989). The unicellular basal plate is well seen in Stomochara (e.g., Peck and Eyer, 1963), and its antiquity goes back to the Carboniferous. The multicellular plate, seen, for example, in Stellatochara, goes back to at least the Late Permian. Thus, the origin of the multicellular plate lineage was likely a Permian Event.

6.3 Extinctions

There are no charophyte records that closely bracket the Permo-Triassic boundary. Instead, there are Late Permian charophytes known from some localities in China and eastern Europe, and Early Triassic charophytes are known from these regions as well as Germany, but they only coarsely bracket the boundary (e.g., Fig. 6). The extinction of the Palaeocharaceae took place during the Early Permian, and the final extinction of the dextrally-coiled charophytes, with the Late Permian record of Gemmichara (which has 8–9 dextrally coiled cells), appears to have been a Late Permian Event (e.g., Soulié-Märsche, 2004, 1999; Liu et al, 1996; Wang, 1984). Indeed, Lu et al. (2000) noted the presence of Gemmichara (together with Leonardosia) in the lower Guodikeng Formation in Xijiang, China, which is a very Late Permian record, just below the Permo-Triassic boundary (Metcalfe et al., 2009). Shaikin (1991) showed high rates of origination for charophyte genera during the Early Triassic and relatively high extinction rates at both the Permo-Triassic and Triassic-Jurassic boundaries. However, few Permian charophytes were known at the time of his analysis.

Saidakovsky (1989) stated that the Permian-Triassic boundary can be readily located using charophytes. Thus, according to Saidakovsky (1989), Gemmichara, "Paracuneatochara", Leonardosia, Stomochara and "Horniella" have their HOs in the upper Permian, whereas Stellatochara, "Maslovichara", Auerbachichara, "Altochara", Vladimiriella, Latochara, Stenochara and "Cuneatochara" have their LOs in the Early Triassic. However, the compilation of generic diversity and temporal ranges presented here (Fig. 4) shows little generic turnover in charophytes across the Permo-Triassic boundary.

Liu and Chen (1992) described superposed Chinese charophyte assemblages of Late Triassic and Early Jurassic Age. These are dominated by Porochara and "Aclistochara, " with some species overlap, a change in genus dominance and the addition of Stenochara to the Lower Jurassic assemblage (though Stenochara has Triassic records elsewhere). Thus, there appears to be little change in charophyte assemblages across the Triassic-Jurassic boundary in China. On face value, the loss of most charophyte genera by the end of the Triassic (six out of eight genera extinct: Fig. 4) fits the concept of an End-Triassic mass extinction (e.g., Bilan, 1988), though there is no evidence of a mass extinction of land plants across the Triassic-Jurassic boundary (Lucas and Tanner, 2018). However, the HOs of most of the charophyte genera that disappear across the Triassic-Jurassic boundary are not precisely constrained, and it is not clear that many of them were present during the last stage of the Triassic, the Rhaetian (Fig. 6). It thus seems that no case can be made for mass extinctions of charophytes across the Permo-Triassic and the Triassic-Jurassic boundaries, though important evolutionary turnover took place among the charophytes across these boundaries.

6.4 Biotic Events

The following major biotic events can thus be identified in the Permian–Triassic evolution of charophytes.

1. Early Permian extinction of the Palaeocharaceae. The Palaeocharaceae were a low diversity experiment in a gyrogonite with six sinistral coils. They are the last such experiment in coiling before the fixing of all gyrogonites to five sinistral coils.

2. Late Permian extinction of the "Trochiliscales". Gemmichara from the Late Permian of China is the last record of the "Trochiliscales", an early group with dextrally spiralled gyrogonites.

3. Carboniferous origin of the paraphyletic Porocharaceae, soon followed during the Permian by the origin of the multicellular basal plate.

4. An important generic turnover of charophytes took place across the Triassic-Jurassic boundary, but there are insufficient data to identify this as a mass extinction.

7 CONCLUSIONS

The use of charophytes in biostratigraphy has been discussed and extolled by various authors. Tappan (1980, p. 932) is characteristic, noting that "many [charophytes] had a short existence and are good index fossils … for age determination of nonmarine beds at least to the period level". Kozur (1972) argued that charophytes will be very important biostratigraphically in the Triassic, noting the example of Stellatochara sellingii, a species characteristic of the Ladinian (Fassanian/Longobardian). Despite this, Permian and Triassic charophyte biostratigraphy at the taxonomic level of genus is highly problematic. Clearly, charophyte genera had very slow evolutionary turnover rates––most persist at minimum for a geological period, and many persisted for two or more periods (Fig. 4; Feist et al., 2005, Table 9 therein).

The fossil record of Permian and Triassic charophytes does not provide detailed biostratigraphy beyond local or regional schemes. Nevertheless, it does identify some important evolutionary datums that constrain the timing of important biotic events in the Permian–Triassic evolutionary history of the Charophyta. Hopefully, the development of a more extensive fossil record, particularly of Permian charophytes, will improve the utility of Permian and Triassic charophytes in biostratigraphy and provide even more precise temporal constraints on their evolutionary history.

ACKNOWLEDGMENTS

I thank M. Feist, the late N. Grambast-Fessard, I. Soulié-Märsche and A. Zhamangara for educating me about charophytes and for improving my understanding of their Permian-Triassic record. Comments on an earlier version of the manuscript by J. Schneider and an anonymous reviewer improved its content and clarity. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0786-4.


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