Global coal gap between Permian–Triassic extinction and Middle Triassic recovery of peat-forming plants

Gregory J. Retallack Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403-1272 John J. Veevers School of Earth Sciences, Macquarie University, New South Wales 2109, Australia Ric Morante }

ABSTRACT A number of possible explanations for the coal or permineralized peat. Veevers et al. coal gap have been advanced. Land masses (1994a) introduced the term ‘‘coal gap’’ for Early Triassic coals are unknown, and of the world may have been riding too high a sharp break between thick and widespread Middle Triassic coals are rare and thin. The with respect to sea level for the accumu- coal up to the Permian-Triassic boundary, Early Triassic coal gap began with extinc- lation of peat (Daragan-Sushchov, 1989; lack of coal in the Early Triassic, followed by tion of peat-forming plants at the end of the Faure et al., 1995). Naturally acidic swamps thin and uncommon coals in the Middle Permian (ca. 250 Ma), with no coal known may have been overwhelmed by additional Triassic and thick and widespread coals in

anywhere until Middle Triassic (243 Ma). acid, such as sulfuric acid from SO2 of mas- the Late Triassic (Fig. 1). The coal gap thus Permian levels of plant diversity and peat sive eruptions of the Siberian Traps (Mc- includes an absolute gap in the Early Tri-

thickness were not recovered until Late Cartney et al., 1990), or nitric acid from NOx assic (Scythian) and recovery extending over Triassic (230 Ma). Tectonic and climatic ex- generated by impact of a large extraterres- the Middle Triassic (Anisian and Ladinian). planations for the coal gap fail because de- trial bolide at the Permian-Triassic bound- The rock closest to coal within the Early posits of fluctuating sea levels and sedimen- ary (Zahnle, 1990). Global climate may have Triassic coal gap is 10 cm of carbonaceous tary facies and paleosols commonly found changed from an icehouse with wet and cool claystone bearing the lycopsid plant Pleu- in coal-bearing sequences are present also climatic zones to a greenhouse that was too romeia in the Newport Formation of south- in Early Triassic rocks. Nor do we favor ex- hot and dry for peat (Worsley et al., 1994). eastern Australia (Retallack, 1975). Reports planations involving evolutionary advances Microbes such as fungi may have become of coal in Early Triassic rocks of northern in the effectiveness of fungal decomposers, more effective or herbivory by insects and Tibet (Deng et al., 1980; cited by Ziegler et insects or tetrapod herbivores, which be- tetrapods more destructive in Early Triassic al., 1993) are not reliable, because they are came cosmopolitan and much reduced in wetlands (Moore and Worsley, 1994). Fi- based on arbitrary placement of the Per- diversity across the Permian-Triassic nally, plants may have re-evolved adapta- mian-Triassic boundary. There is no coal boundary. Instead, we favor explanations tions to peat swamps well after the end-Per- within the stratigraphic sequence between involving extinction of peat-forming plants mian extinctions (Ziegler et al., 1993). Some localities for Early Triassic ammonites and at the Permian-Triassic boundary, followed of these possible explanations invoke events clams (Wen et al., 1981), but the Late Per- by a hiatus of some 10 m.y. until newly at the Permian-Triassic boundary, others mian and Late Triassic of northern Tibet evolved peat-forming plants developed tol- postulate long-term hostile conditions. A va- include coal (Li and Wu, 1981). We know of erance to the acidic dysaerobic conditions of riety of paleontological and geological lines no Early Triassic coal anywhere in the wetlands. This view is compatible not only of evidence are considered here on both world. with the paleobotanical record of extinction short and long time scales. Middle Triassic coals are uncommon and of swamp plants, but also with indications A marine parallel to the coal gap is Flu¨- thin (recovery of Fig. 1), like coals of the of a terminal Permian productivity crash gel’s (1994) Early Triassic ‘‘reef gap.’’ Com- Devonian (Ergolskaya, 1936; Chi and Hills, 13 from ␦ Corg and total organic carbon of mon features of tropical reefs and peat 1976). Middle Triassic coals include the both nonmarine and shallow marine shales. swamps provide additional clues to the rea- Cloughers Creek and Nymboida Coal Mea- sons for their Early Triassic disappearance. sures of New South Wales (Retallack et al., INTRODUCTION 1977, 1993), the Tank Gully and Long Gully EVIDENCE FROM STRATIGRAPHY Coal Measures of New Zealand (Retallack, It is a curious fact that no coal seam of 1979; Retallack and Ryburn, 1982), and the Early Triassic age has yet been discovered, The oldest known permineralized peats Lettenkohle of western Europe (Mader, and those of Middle Triassic age are rare and cuticle coals are Early Devonian (Kid- 1990; Aigner and Bachmann, 1992). A brec- and thin. In this paper we demonstrate this ston and Lang, 1921; Krassilov, 1981), and cia of permineralized peat blocks in the up- striking lacuna in coal formation. We also the oldest woody coal is Late Devonian in per part of the Fremouw Formation of the attempt to understand why there is such a age (Gillespie et al., 1981). Ever since then, central Transantarctic Mountains is also of gap, using evidence from stratigraphy, coal there has been a fossil record of peat-form- Middle Triassic (probably Ladinian) age geology, paleosols, and fossil fungi, plants, ing wetland ecosystems somewhere in the (E. L. Taylor et al., 1989; Farabee et al., and animals. world. Early Triassic rocks, however, lack 1989).

GSA Bulletin; February 1996; v. 108; no. 2; p. 195–207; 3 figures; 3 tables.

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Figure 1. Distribution of Permian and Triassic coal and carbonaceous sediment on the Permian and Triassic Pangean platform and in the Chinese blocks related to global events. Time scale is from Palmer (1983), modified by the Permian-Triassic boundary calibrated at ca. 250 Ma (as in Veevers et al., 1994a). Sea level is measured against Euramerican platform; Permian (280–250 Ma) from Ross and Ross (1988, their Figs. 2–4), Triassic–Jurassic from Haq et al. (1988, their Figs. 16 and 17). Percent sedimentary cover of platform in marine, paralic, and nonmarine facies from Holser and Magaritz (1987, their Fig. 5); ␦34 S in marine sulfate and ␦13 C in marine carbonate from Holser et al. (1988); 87 Sr/86 Sr in marine carbonate from Koepnick et al. (1990) and Denison et al. (1994). Reef development from Flu¨gel (1994), including a Scythian–earliest Anisian (250–238 Ma) ‘‘reef gap’’ followed by very rare later Anisian reefs and common Ladinian–Rhaetian reefs. An additional, tentative reef gap spanned the earliest Jurassic. Diversity (number of families) of land plants from Knoll (1984) and of tetrapods from Benton (1987). Glacigenic sediment, coal measures, carbonaceous sediment, and red beds along the Panthalassan margin of Gondwanaland—southern South America, southern Africa, Transantarctic Mountains (TAM), New Zealand, and eastern Australia into central and western Australia—are from Veevers et al. (1994b, their Fig. 3), with East Antarctic data from Webb 13 and Fielding (1993) and Foster et al. (1994). Australian ␦ Corg and total organic carbon (TOC) from Morante et al. (1994) and Morante Kiama Sandstone. Indian data are from Veevers and Tewari (1995); data from North America, Europe, and China ؍ unpubl. data); K) are from Veevers et al. (1994a); additional European occurrences from Aigner and Bachmann (1992) and Janoschek and Matura (1980); data from the former Soviet Union (F.S.U.) are from Nalivkin (1973). EXTINCTION AND RECOVERY OF PEAT-FORMING PLANTS

The coal gap and recovery are empha- Plant extinction as an explanation for the sulfur isotopic record show anything unusu- sized by thick and productive coal measures coal gap was supported by Morante et al. al at the onset of the coal gap, although the of latest Permian age in the Sydney Basin of (1994), who showed that the boundary be- late Early Triassic sulfate sulfur isotopic Australia (Diessel, 1992; Morante et al., tween coaly and coal-barren facies in east- spike may reflect a threshold in oceanic sul- 1994) and in the Kuznetsk Basin of Siberia ern Australia coincided with the great ex- fate reduction resulting from rising levels of (Gorelova et al., 1973; Betekhina et al., tinction at the end of eastern Australian organic matter from increasing oceanic pro- 1984) on one side of the gap, and on the palynological zone 5 associated with the ductivity (Holser and Magaritz, 1987) at a other, the Late Triassic (Carnian) Ipswich Glossopteris megaflora and the succeeding time of recovery from the coal gap. Coal Measures of Queensland (Mengel and Protohaploxypinus microcropus palynozone The onset of the coal gap at the Permian- Carr, 1976) and Productive Coal Measures representing the Dicroidium megaflora. In Triassic boundary was a time of exception- Member of the Tuckahoe Formation of the addition, the abrupt lightening of carbon ally low sea level (Fig. 1) as determined by eastern United States (Cornet and Olsen, isotopes in kerogen and carbonate carbon both sequence stratigraphy and the percent- 1990). across the Permian-Triassic boundary may age of marine sedimentary cover (Holser The beginning of the coal gap has been be due to lower levels of primary production and Magaritz, 1987; Ross and Ross, 1988; dated in eastern Australia as ca. 250 Ma by along with oxidation of large amounts of Haq et al., 1988). The curves show (1) low radiometric dating of biotite in tuff at the buried carbon including methane clathrates, sea level for much of the latest Permian, top of the coal measures (Veevers et al., releasing carbon dioxide into the atmo- when thick peats accumulated in Siberia, 1994a). This is also the age of the Permian- sphere as a greenhouse gas (Holser and Ma- Australia, India and South Africa, and (2) Triassic boundary (250.0 Ϯ 0.3 Ma) deter- garitz, 1987; Baud et al., 1989; Hsu¨ and marked transgression during the Early Tri- mined radiometrically in Chinese sections of McKenzie, 1990; Erwin, 1993; Wang et al., assic without evidence of peats. The coal gap high stratigraphic resolution (Claoue´-Long 1994). This hypothesis is supported by the and relative sea level are thus not related in 13 et al., 1991; Renne et al., 1995). synchronous drop in ␦ Corg and total or- a simple manner. Nor would one expect The demise of peat formation also can be ganic carbon in shale, from 1%–4% by them to be, considering the many thick coal dated by chemostratigraphy using carbon weight in the latest Permian to 0%–1% in measures that have accumulated in inter- 13 isotopes of carbonate (␦ Ccarb) and kero- the earliest Triassic of Australia (Fig. 1). montane basins remote from the sea (Men- 13 gen (␦ Corg). The Permian-Triassic bound- Other environmental events also coincide gel and Carr, 1976; Casshyap and Tewari, ary in marine sections in Eurasia, particu- with the onset of the coal gap. The drop in 1984; Betekhina et al., 1984; Olsen, 1989; 13 larly along the margins of the former Tethys ␦ Corg in Australian sections, both marine Taylor et al., 1990; Stricker, 1991; Wu et al., Ocean, is characterized by a sharp drop in and nonmarine, is ϳ4 per mil (Fig. 1; –28 to 1992). The exceptionally low sea level at the 13 ␦ Ccarb of 3 per mil (Fig. 1; from ϩ3to0 –24 per mil). The atmosphere is the only Permian-Triassic boundary has been ex- per mil; Holser and Magaritz, 1987; Baud et common reservoir for such widely scattered plained by assembly of Pangea (Faure et al., 13 al., 1989) and a similar drop in ␦ Corg of ϳ3 records of isotopic shifts on land and sea 1995), but most of the supercontinent was per mil (from mean values of –24.6 to –27.4 (Fig. 2), thus the shift probably coincided already in place by ca. 320 Ma (Veevers,

per mil) in shale and marl interbedded with with a rise in atmospheric levels of CO2 and 1989), well before the onset of the coal gap carbonate in Austria (Magaritz et al., 1992). global warming (Morante et al., 1994). Such at ca. 250 Ma. The resumption of thick coal Along the former margin of the Panthalas- global warming inferred from carbon iso- measure accumulation at ca. 230 Ma coin- san Ocean in Canada, in deep-water marine topic data has been difficult to substantiate cided with rifting during an early phase in 13 sediment, the drop in ␦ Corg across the Per- by other stratigraphic lines of evidence. It the breakup of Pangea (Fig. 2B; Veevers, mian-Triassic boundary is again ϳ3 per mil may be indicated by the wider paleolatitu- 1989, 1990a). (from –29 to –32.6 per mil; Wang et al., dinal spread of evaporite deposits in Triassic Recovery from the coal gap can be dated 13 1994). The drop in ␦ Corg also can be compared with Permian rocks; however, the in Australia and New Zealand, where thin traced from marine (Tethyan) sections in role of warming in promoting the coal gap is coals reappear in the early part of the Mid- northwestern Australia to alluvial-paralic weakened by similar evidence for continued dle Triassic (Anisian) near the first appear- (Panthalassan) sections in southeastern global warming and drying into Jurassic time ance of the biostratigraphically important Australia (Morante, 1993; Morante et al., (Parrish et al., 1986). seed fern Dicroidium odontopteroides and 1994), and into the wholly nonmarine sec- Other sharp isotopic events, such as the the halobiid bivalve Aparimella apteryx (for- tion of the Cooper Basin in the Australian Late (but not latest) Permian minimum merly Daonella; Retallack et al., 1977; Re- continental interior (Morante, unpubl. value of 87Sr/86Sr in shallow marine carbon- tallack, 1979; Retallack and Ryburn, 1982; data). There is also a precipitous drop in ate (Denison et al., 1994), and the Early (but Campbell, 1994). These taxa appear in New 13 34 ␦ Corg at the Permian-Triassic boundary in not earliest) Triassic ␦ Ssulfate peak, the Zealand before 242.8 Ϯ 0.6 Ma (Retallack easternAustraliabetweenPermiancoalmea- Ro¨t Event (Holser, 1984), occurred some et al., 1993). Thick and productive coal mea- sures below and coal-barren Narrabeen and millions of years before and after the Per- sures of Late Triassic (Carnian) age in Rewan Groups of the Sydney and Bowen mian–Triassic extinction (Fig. 1). They were Queensland appear with the biostratigraph- Basins, respectively (Morante et al., 1994), not so directly involved with the extinctions ically important gymnosperm Yabeiella (Re- 13 thus revising the previous view from pa- as the synchronous ␦ Corg shift. Strontium tallack, 1977b), an event that is less well lynology that the Permian-Triassic boundary isotopic data give no support for theories of dated, but probably ca. 230 Ϯ 5 Ma (Retal- lay some hundred meters above the last coal environmental acidification due to volcan- lack, 1977b; Veevers, 1989). Similarly thin below the base of the Narrabeen Group and ism or bolide impact comparable to those coals appear in the Middle Triassic (Ladin- correlative strata. Thus the coal gap began proposed for the Cretaceous-Tertiary ian) Lettenkohle of Europe (Mader, 1990; at the Permian-Triassic boundary. boundary (MacDougall, 1988). Nor does the Aigner and Bachmann, 1992) and thick

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Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/2/195/3382412/i0016-7606-108-2-195.pdf by guest on 28 September 2021 Figure 2. Pangea before and after the coal gap. Base map and paleolatitude drawn by computer from the Terra Mobilis program (Scotese and Denham, 1988). The center of the projection (Lambert equal-area) is given beneath the age and was chosen to concentrate the continents in the middle of the diagram. (A) At the Permian-Triassic boundary (250 Ma), modified from Veevers et al. (1994a). Places with the change from Permian coal measures to Triassic red beds are marked RB/CM. The shallow sea is shown by light stipple. Paleogeography is from Ziegler et al. (1979, their Figs. 38–43, for Kazanian), with addition of the Siberian traps (ST) marked by v’s. (B) In the Carnian (230 Ma) or Late Triassic, modified from Veevers (1994). Deposition of coal (C) resumed in newly created rift basins in the eastern United States (Newark Group or NG; Olsen, 1989) and Australia (Ipswich Coal Measures or I, Peera Peera Formation or PP, Leigh Creek Coal Measures or L, and coal measures of Tasmania or T; Veevers, 1990b). Coal measures of the Molteno Formation (M) of South Africa (Dingle et al., 1983) followed the terminal (Gondwanide) deformation of the Cape Fold Belt. Carbonaceous sediment was deposited conformably on Early and Middle Triassic noncarbonaceous sediment in the central Transantarctic Mountains (CT; Collinson et al., 1994), ?Early–Middle Triassic red beds of the Jetty Member of the Flagstone Bench Formation in the Amery area of East Antarctica (Webb and Fielding, 1993; Foster et al., 1994), and in the Damodar River area of India (DA). In the Chinese cratons and Tibet, coal succeeds red beds (Yang et al., 1986). The coal and carbonaceous sediment were deposited during the start of continental rifting that prefigured rift oceans of the Atlantic and Indian Oceans (Veevers, 1989), and immediately after widespread deformation of the Gond- wanides, Cimmerides, Indosinian Movement, and Malayan orogeny, and less intense folding and faulting in the Damodar River area (DA) of India and the Canning Basin (CB) of northwest Australia. Paleogeography is from Ziegler et al. (1983, their Fig. 2, Early Jurassic), but the (Late Triassic) Keuper of Europe is from Ziegler (1988, his Plate 10).

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Figure 3. Long-term changes in coal paleolatitude, seam thickness, and coal-producing floras. Coal thicknesses include maximum values of unusually thick seams (to right) and (to left) averages of at least 10 coals within a single stratigraphic sequence (or 3 consecutive coals for Early Carboniferous and Devonian only). Paleolatitudes of coal occurrences were compiled from Habicht (1979). Coal thickness and floristic data are summarized in Tables 1–3, with floristic data supplemented by paleoecological studies of compression fossils in coal measures (Gorelova et al., 1973; Krassilov, 1981; Gillespie et al., 1981; Retallack and Dilcher, 1981; Meyen, 1982; Scheckler, 1986; DiMichele et al., 1987; Cornet and Olsen, 1990; Schneider, 1990; DiMichele and Phillips, 1994; Blackburn and Sluiter, 1994; Retallack, 1994).

coals in the Late Triassic (Carnian) Produc- The earliest coals of Devonian age were coal balls (Stubblefield and Taylor, 1978), tive Coal Measures Member of the Tucka- mainly tropical and subtropical (Fig. 3). This but there is no comparable indication of hoe Formation of Virginia, in the United remained the case during Carboniferous changing effectiveness of decomposers be- States (Olsen and Cornet, 1990). Thus peat time when coal formed also at high lati- fore or after the Early Triassic, and this fal- and coal formation ceased entirely for some tudes. By Permian time most coals formed sifies the idea that decomposers are respon- 10 m.y., and coal seams as thick as those of in temperate climatic belts of high latitudes, sible for the coal gap (Moore and Worsley, the Late Permian were attained only after and this situation persisted into Triassic and 1994). an additional 10 m.y. Jurassic times. It was not until Tertiary time Further information on peat-forming eco- that coals once again became widespread in systems may be gained from the average EVIDENCE FROM COAL GEOLOGY the tropics (Habicht, 1979). The marked pa- thickness, range of thickness, and maximum leolatitudinal shift of coals from tropical to thickness of coal seams through time The coal gap interrupted long-term temperate latitudes in the Late Carbonifer- (Fig. 3). Our compilation of maximum changes in the paleolatitudinal distribution ous has been attributed to the evolution of average thickness was culled from 141 pub- of coals, in the thickness of coal seams, and tropical decomposers such as fungi and bac- lished sequences with at least 10 consecutive in their petrographic composition, which teria (Briden and Irving, 1964). Such an in- seams, although this criterion was relaxed to could give clues to possible biological causes terpretation is debatable considering the only 3 consecutive seams for rare Early Car- such as the coevolution of plants and fungi. abundance of fossil fungi in Carboniferous boniferous and Devonian coals (Table 1).

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Many of these same sequences include the those abruptly curtailed at the Permian- rich coals remain common through every thickest individual coal seams for their time, Triassic boundary. The Middle Triassic re- subsequent geological period, but Permian but some exceptionally thick coals are in se- covery of coal thickness is similar to the Late and younger coals have a greater proportion quences of Ͻ10 consecutive seams (Ta- Devonian–Early Carboniferous evolution of of macerals other than vitrinite (Diessel, ble 2). Both sets of data reveal an increase in thick coal seams. 1992). Many Permian coals, for example, are seam thickness from Early Devonian to Late The petrographic composition of coals very rich in inertinite macerals such as fusin- Carboniferous, with the Late Carboniferous has changed through time, and this could ite, which is thought to be charcoal from the plateau sustained until the last coals of the reflect changes in the ability of swamp plants burning of dry peat. Jurassic and younger Permian. As already discussed, no Early to resist physical disturbance and biological coals are sometimes rich in exinite, such as Triassic coals are known, and Middle Tri- decay (Robinson, 1990). Carboniferous resinite produced by conifers (Gould and assic coals are few and thin (Retallack, 1979; coals are rich in vitrinite, a maceral derived Shibaoka, 1980). There has not yet been Retallack et al., 1977, 1993; Retallack and from cordaite and lycopsid wood and bark found a significant change in the petro- Ryburn, 1982). Our data showing thin coal (DiMichele and Phillips, 1994). Vitrinite- graphic composition of coal shortly before seams of the Middle Triassic (Fig. 3) can be interpreted as a recovery phase from the coal gap. By Late Triassic and into the Ju- rassic, coal thickness reached values compa- rable with that of the Late Carboniferous– Late Permian. There is considerable variation in these data as a result of local paleoclimatic and tectonic settings of the coals, but the coal gap and its recovery re- main prominent. The increased thickness of Devonian–Carboniferous coals can be at- tributed to evolutionary improvements in lignin or phenolic production, biomass, or productivity, which increased effectiveness of the peat-forming flora in persisting against disturbances such as storms, floods, decay, or herbivory (Diessel, 1992), or to a less-perturbed landscape (Shearer et al., 1994). Similarly, Middle Triassic peat swamps may have been re-evolving commu- nities that could cope with disturbance or were in less-perturbed landscapes. By Late Triassic time, levels of disturbance resis- tance were attained that were comparable to

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or after the Early Triassic that would impli- mon in a humid nonseasonal climate, such blages of Early Triassic age in South Africa, cate changing plant woodiness or decay re- as paleosols of the Bald Hill Claystone of India, Antarctica, China, and Russia (King, sistance as a cause of the coal gap (Robin- Australia (Retallack, 1976, 1977a). These 1990). Cynognathus and Rechnisaurus are son, 1990). paleosols could be regarded as evidence of additional Early–early Middle Triassic environmental acidification, but this was (Scythian–Anisian) therapsid reptiles with EVIDENCE FROM PALEOSOLS clearly not universal, considering calcareous wide geographic distribution (Parrish et al., Early Triassic paleosols found in other parts 1986; de Fauw, 1993). During Late Triassic Many Early and Middle Triassic nonma- of the world. time vertebrate faunas were again diverse rine sequences contain red beds (Fig. 1), In addition to red, formerly well-drained and provincial with the rise of rhynchosaurs which have been interpreted as indicating soils that formed in environments unsuit- and dinosaurs (Benton, 1987; Sereno et al., onset of desert to monsoonal climates at the able for peat accumulation, there also are 1993). Both on land and in the sea, the coal beginning of the Triassic (Wang, 1993). The Early Triassic suites of paleosols that gap was initiated during a period of biolog- aridity and seasonality would have been very formed in waterlogged environments. Pa- ical extinction and followed by recovery severe indeed if they eradicated peat forma- leosols of the Early Triassic Newport, Garie, from a protracted time of low biological pro- tion globally, considering the peats now and Wombarra Formations of southeastern ductivity. Neither vertebrates nor insects an- forming in climates as dry and seasonal as Australia include ganisters, siderite nodules, ticipated or profited from the coal gap. the ‘‘Sudd’’ of Sudan and Okavango Delta of and sphaerosideritic fireclays similar to pa- Botswana (Lottes and Ziegler, 1994). In any leosols (including ‘‘underclays’’ and ‘‘fire- EVIDENCE FROM PALEOBOTANY case, evidence from the distribution of red clays’’) of Carboniferous coal measures of beds and the detailed study of paleosols in Europe and North America (Retallack, Plant extinction and evolution through red beds contradict the idea of globally dry 1976). Kaolinitic and podzolized paleosols the coal gap also can be examined from pa- Early Triassic paleoclimates. in these formations are also evidence for a leobotanical data. Although plant fossils can Red beds are not restricted to the Early humid paleoclimate (Retallack, 1977a). Yet be abundant and diverse in coal measures, Triassic, but are widespread in the Carbon- the closest thing to a coal in the Newport not all were necessarily peat formers. For- iferous (Besly and Turner, 1983; Loope, Formation is thin carbonaceous claystone tunately there are rare permineralized peats 1988), Permian (Steel, 1974; Bull and Cas, (Retallack, 1975). Thus humid wetland en- in which peat-forming vegetation is pre- 1989; Kent, 1980; Wang, 1993; Veevers and vironments were present locally during the served and can be compared with modern Tewari, 1995), Middle–Late Triassic (Hu- Early Triassic, but they did not accumulate vegetation of mires (Table 3). Trunks of per- bert, 1977; Kraus and Middleton, 1987; Ser- peat. This line of evidence falsifies theories mineralized wood and fossil forests were not eno et al., 1993), and Jurassic (Kitching, that the coal gap was due entirely to arid included in our compilation unless shown to 1979; Dodson et al., 1980). The unique fea- paleoclimate (Worsley et al., 1994) or low be from a laterally continuous permineral- ture of Early Triassic nonmarine strata is base level (Daragan-Sushchov, 1989; Faure ized peat. Our compilation also includes not their red color, but their lack of coal. et al., 1995). only fertile organs such as sporangia and Red color of beds is caused by local drain- seeds, when these are present with vegeta- age and has little paleoclimatic significance EVIDENCE FROM PALEOZOOLOGY tive remains named using form genera. in itself. Studies of paleosols in red beds in- These data can be used to reconstruct flo- dicate that the principal reason for their red The Permian-Triassic boundary has long ristic diversity through time (Fig. 3). color is oxidation in well-drained soils, as been known as a major crisis for animal life Calcareous coal balls of Late Carbonifer- indicated by abundant burrows of non- in the sea, with extinction of Ͼ90% of ma- ous (Namurian–Westphalian) age from the aquatic animals, deeply penetrating root rine species (Raup, 1979), including all fusu- United States are mainly of tree lycopsids or traces, abundant illuvial clay skins, and thor- line foraminifera, trilobites, and tabulate cordaites (Phillips et al., 1974), with the lat- ough chemical weathering (McPherson, and rugose corals. This profound break in ter common in marine-influenced peats 1980; Besly and Turner, 1983; Loope, 1988; the evolution of life has attracted a variety of (Raymond and Phillips, 1983). Following Retallack, 1990, 1991). Enhanced reddening environmental explanations, including im- Middle–Late Carboniferous (Stephanian) is due to dehydration of ferric hydroxides to pact of large extraterrestrial bolides, mas- extinctions, the dominant trees in swamps oxides during burial, but the primary oxida- sive eruption of Siberian flood basalts, and were marattialean tree ferns of the genus tion in most cases was during exposure to air unusually low sea level. Erwin (1994) has Psaronius. Permian permineralized peats in a soil (Retallack, 1991). reviewed and proposed a combination of from Queensland (Gould and Delevoryas, Red beds include paleosols representing a these—a ‘‘world went to hell’’ hypothesis. 1977) and Antarctica (E. L. Taylor and Tay- wide range of paleoclimates: wet and dry, Recent reassessment of the fossil record lor, 1990) are dominated by wood and leaves monsoonal, and nonseasonal climates. of insects (Labandeira and Sepkoski, 1993) of the seed fern Glossopteris. Permian coal Some Early Triassic red beds are sequences and vertebrates (Benton, 1987) has demon- balls in Siberia are dominated by wood of of paleosols with calcareous nodules of the strated that the Permian-Triassic boundary endemic Ruflorian and Voynovskyan cor- kind expected (pedocals) in monsoonal was a time of crisis for animals on land as daites, which are also abundant in associ- semiarid climates—for example, paleosols well (Fig. 1). Early Triassic vertebrate fau- ated shales (Meyen, 1982; personal obser- of the Buntsandstein of Germany (Ortlam, nas are distinguished by both lower diversity vations of Retallack, 1984). Permian coal 1971; Mader, 1990; Weber, 1993). Other and more cosmopolitan distribution than balls of China are dominated by cordaites Early Triassic sequences of red paleosols are faunas of Late Permian or Late Triassic age and lepidodendralean lycopsids (DiMichele noncalcareous, and in some cases deeply (Parrish et al., 1986). The therapsid reptile et al., 1987), and permineralized peats of weathered to bauxitic (pedalfers), as is com- Lystrosaurus dominates vertebrate assem- this age from France are dominated by

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horsetails (Calamites) and ferns (Psaronius; Triassic peat plants are provincial, as if dif- maceae, Peltaspermales), while others have Galtier and Phillips, 1985). ferent evolutionary lineages were reoccupy- mainly cycadophytes (Antarcticycas) and When the fossil record of permineralized ing wetlands in different parts of the world. ferns (Farabee et al., 1989; E. L. Taylor and peat picks up again with thin, local coals in Some Middle Triassic permineralized peats Taylor, 1990; T. N. Taylor et al., 1993). The the Middle Triassic, it is with different peat- from Antarctica are dominated by remains most common plant in carbonaceous shale forming plants. In addition, these Middle of the seed fern Dicroidium (Corystosper- partings within Middle Triassic coals of New

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Zealand and Chile is an enigmatic plant, boundary is immediately above the last glos- succulence, inrolled leaves, small leaves, Linguifolium, perhaps a seed fern or cycado- sopterid coal seam in the Bowen, Cooper, needle leaves, and heterophylly (Mader, phyte (Retallack, 1979; Retallack and Ry- and Sydney Basins of Australia. New radio- 1990). Gone were the latest Permian Tata- burn, 1982). Different plants again have metric dating also places Siberian flood ba- rina flora of northern Pangea, the Glossop- been found in shales associated with thin salts at the Permian-Triassic boundary teris flora of the southern Pangea, and the Middle Triassic coals of Europe (Mader, (Renne and Basu, 1991; Campbell et al., Gigantonoclea flora of tropical China. It was 1990). Late Triassic–Early Cretaceous 1992; Renne et al., 1995). The distinctive not until Middle Triassic time that floral swamps were dominated by osmundalean fern-dominated Korvunchanian flora in the provincialism and diversity was reestab- tree ferns, dipteridaceous ferns, conifers, Siberian traps can be correlated with similar lished with the Scytophyllum flora of north- cycadeoids, gnetaleans, and pentoxylaleans, floras in coal-barren sediments overlying the ern Pangea and the Dicroidium flora of as indicated by permineralized peats from last cordaite coals of the Kuznetsk Basin of southern Pangea (Retallack, 1977b, 1980; Arizona (Pigg et al., 1993), Sarawak Siberia (Gorelova et al., 1973; Betekhina et Meyen, 1987). The oligotrophic depauper- (Gastony, 1969), Tasmania (Tidwell et al., al., 1984; Meyen, 1987). ate interregnum of conifers and lycopsids, 1987; White, 1991), and India (Sahni, 1932, In a continuous sequence of high tempo- which appears abruptly at the Permian-Tri- 1948). ral resolution across the Permian-Triassic assic boundary, corresponds in time with the Permineralized peats include wetland boundary within the northeastern Sydney worldwide spread of Lystrosaurus faunas, fungi, which have also been invoked to ex- Basin (Herbert, 1993), palynological studies and with the coal gap. plain the coal gap (Moore and Worsley, have shown abrupt extinction of the Glos- Some paleobotanical support for the idea 1994). Both Permian and Triassic permin- sopteris flora, with a dramatic transient in- of a warm Early Triassic climate comes from eralized peats from Antarctica include a va- crease of fern spores and acritarchs before Late Permian (E. L. Taylor et al., 1990) and riety of endosymbiotic and saprophytic fungi establishment of a new and lower diversity Middle Triassic (E. L. Taylor and Taylor, (E. L. Taylor and Taylor, 1990; T. N. Taylor flora (Grebe, 1970; Helby et al., 1987). A 1993) permineralized forests in Antarctica, et al., 1993). The Triassic fungi in some similar floral reorganization with a fern at paleolatitudes too cold for vascular plants cases show exceptional preservation, such as spike was found also in palynological studies today (Parrish et al., 1986). The presence of arbuscles of mycorrhizae, but all the fungi of the Permian-Triassic boundary in Green- mosses in Permian (but not Triassic) peat, belong to groups with an older Paleozoic land (Hsu¨ and McKenzie, 1990). This fern and of cycads in Triassic (but not Permian) record (Stubblefield and Taylor, 1988). dominance may indicate ecological succes- peat may be an indication of a warmer cli- Plants of waterlogged environments were sion following catastrophic deforestation, as mate in the Triassic (E. L. Taylor et al., not protected from fungal decay, and there has been argued for the fern spike at the 1989; E. L. Taylor and Taylor, 1993). On the is no clear indication of change in fungal Cretaceous-Tertiary boundary (Nichols and other hand, depauperate conifer-lycopsid assemblages from Late Permian to Middle Fleming, 1990). Also in the Sydney Basin, as floras of the Early Triassic are like floras of Triassic. well as in Europe and the Middle East, is a cool climates today (Retallack, 1980), and Paleobotanical data also is useful in re- transient abundance of fungal spores and the small fossil cycads of the Triassic did not fining the extent of the coal gap and asso- hyphae, which may reflect a brief episode of have the large terminal meristem of frost- ciated Permian–Triassic life crisis (Vak- rotting of catastrophically destroyed vegeta- intolerant modern cycads (Spicer and Chap- hrameev et al., 1978; Anderson and tion (Visscher and Brugman, 1988; Eshet, man, 1990). Furthermore, Early Triassic Anderson, 1983; Mader, 1990; Retallack, 1992; Retallack, 1995). Transient acritarch vegetation can be regarded as a product of 1975). Compression fossil floras and pa- swarms at the Permian-Triassic boundary in a life crisis at the Permian-Triassic boundary lynology offer greater temporal resolution Greenland, Pakistan, China, and southeast- (Retallack, 1995), in which case modern than permineralized peats on changes in ern and Western Australia are evidence for comparisons are weak. Permian–Triassic ex- vegetation from Late Permian to Middle widespread marine incursions into predom- tinctions are a more attractive explanation Triassic time. Although several assessments inantly nonmarine basins (Balme, 1989). for the coal gap considering the taxonomic of fossil floras have stressed the gradual na- These also indicate catastrophic changes at differences between Late Permian and Mid- ture of floral change across the Permian- the boundary, and falsify the idea that the dle Triassic peat floras, together with cata- Triassic boundary (Knoll, 1984; Meyen, coal gap was the result of low sea level. strophic extinctions of plants at the Per- 1987), there are now indications of abrupt A catastrophic change in vegetation at the mian-Triassic boundary associated with and widespread extinctions of plants at the Permian-Triassic boundary is also apparent transient abundances of fungi, acritarchs, boundary (Retallack, 1995). Abrupt extinc- from the distinctive nature of Early Triassic and ferns. tions of glossopterids, gigantopterids, voy- plants, which are low in diversity and cos- If the coal gap was initiated in a sudden novskyaleans, and ruflorialeans at the Per- mopolitan compared with Late Permian catastrophe, there remains the problem of mian-Triassic boundary were unrelated to plants (Meyen, 1987). Early Triassic vege- its 10 m.y. duration. One explanation could long-term diachronous change in domi- tation is similar worldwide, with low-diver- be that Permian–Triassic extinctions cut nance of gymnosperms over pteridophytes, sity assemblages dominated by voltziacean deeply into ecosystem components such as which has been termed the Paleophytic- conifers (Voltzia, Voltziopsis) and herba- microbial nutrient procurement systems. Mesophytic transition (Fredericksen, 1972; ceous to weakly lignified lycopsids (Anna- This was the most profound life crisis in the Gray, 1993). New radiometric dating (Vee- lepis, Cylomeia, Tomiostrobus, Pleuromeia; history of life (Erwin, 1994), with severe mi- vers et al., 1994a) and international corre- Vakrameev et al., 1978; Mader, 1990). crobial disruption indicated by transient lation of a marked carbon-isotopic shift in These Early Triassic plants also show indi- abundances of acritarchs and fungi (Vis- kerogen (Morante, 1993; Morante et al., cations of adaptation to low nutrients or sea- scher and Brugman, 1988; Balme, 1989; Es- 1994) indicate that the Permian-Triassic sonal to sparse water supply in the form of het, 1992). A microbial crisis is indicated

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also by the local abundance of cyanobacte- late in swamps if allowed to dry or become amounts of old continental strontium were rial stromatolites in nonmarine (Mader, aerated enough for decay by microbial de- released to the ocean by weathering. A 1990) as well as marine strata of Early Tri- composers. On the other hand, subsidence much broader perturbation of the strontium assic age (Schubert and Bottjer, 1992). The beyond the rate at which trees can maintain isotopic balance in the oceans extends from Permian-Triassic boundary crisis may have some degree of aeration of their roots kills the Late (but not latest) Permian into the been severe enough to reset life on earth swamp trees and creates a lake rather than Early (but not earliest) Triassic, perhaps re- back to earlier simpler ecosystems without a swamp (Falini, 1965). Although this line of lated to increased hydrothermal activity at the mycorrhizae, nitrogen fixers, and other argument could implicate changes of sea mid-ocean ridges. Strong perturbations at organisms required for the maintenance of level in their demise, reefs should still have the Permian-Triassic boundary in ␦13Cof vascular plants. been able to survive on evenly sloping vol- marine carbonate and kerogen and of non- An additional biological explanation for canic islands and peats in intermontane marine kerogen reflect a redox and produc- the 10 m.y. delay of the coal gap is that ad- basins. tivity crisis, which could plausibly have in- aptations to the acidic, oxygen-poor, and nu- A second common feature of reefs and volved also acid rain. These are short-term trient-starved conditions of swamps are peat swamps is oligotrophy, or nutrient star- effects compared with the 10 m.y. duration rare. A striking feature of fossil plants in vation. Reefs grow best in clear waters dis- of the coal gap. In addition there is evidence permineralized peats is their low diversity tant from mouths of large rivers that deliver from calcareous paleosols of Early Triassic and relictual character, compared with co- sediment and nutrients such as phosphorus age in Germany, South Africa, and else- existing plants of associated sedimentary (Hallock, 1987; Wood, 1993). Similarly, the where that protracted acidification was not rocks (DiMichele et al., 1987). Carbonifer- waters of peat swamps are commonly too universal on land. ous peat-forming plants of North America acidic for phosphorus and other cationic nu- belong to long-ranging, slowly evolving trients to be available to plants (Moore and Paleoclimatic Change clades that were replaced only after long in- Bellamy, 1974). By this reasoning, coral tervals at mass extinctions (Phillips et al., reefs and peat swamps should have survived Permineralized peats of Late Permian 1974). This may be an evolutionary conse- better than other ecosystems during Permi- and Middle Triassic age in Antarctica are quence of the ecological observation that an–Triassic productivity collapse, but this surprisingly rich in trees for such high pa- peat environments are difficult for plant nu- was not the case. Coincidence in time of leolatitudes, and the poles may have been trition and low in plant competition (Di- both the beginning and end of the reef and free of large ice caps at this time, but both Michele et al., 1987). In such difficult envi- coal gaps implies a cause that affected both Permian and Triassic peats contain fossil ronments, the coevolution of plants and land and sea, such as the Permian–Triassic plant assemblages of temperate woodlands. microbes may have taken longer than else- life crisis (Retallack, 1995). The oxidation of carbon indicated by where on land. marked isotopic shift at the Permian-Tri- A SUMMARY OF CAUSES OF THE assic boundary and later rise of cycado- COMPARISON WITH THE REEF GAP COAL GAP phytes could signal the advent of a carbon dioxide greenhouse. Although a simple pic- The absence of fossil coral reefs, corals, The various lines of evidence reviewed ture is often presented of a dry Triassic with and bryozoans in Early Triassic rocks has above are here summarized for possible ex- red beds following a wet Permian with coal long been a puzzle (Erwin, 1994). The planations for the Early Triassic coal gap. measures, recent interpretation of paleosols enigma is emphasized by impressive fossil and evaporites presents a different and more reefs of sponges and brachiopods of Late High Continental Freeboard detailed picture. Calcareous red paleosols Permian age in West Texas and of sponges indicate semiarid to subhumid climates dur- and bivalves of Middle and Late Triassic age Although continents were riding high dur- ing both the Permian and Early Triassic in in the southern calcareous Alps of Europe. ing the Late Permian and Early Triassic, this the United States, Britain, and Europe. Middle Triassic reefs have similar overall fa- would not have destroyed wetlands in inter- Noncalcareous kaolinitic paleosols indicate cies and structure to Late Permian reefs, but montane basins or oceanic islands. Global humid climates during both Permian and many new organisms. These included scler- sea level curves and locally abundant marine Early Triassic in Siberia, Antarctica, and actinian corals and cheilostome bryozoans acritarchs in nonmarine sequences of Early Australia. There were extensive regions cli- very different from Paleozoic clades of cor- Triassic age indicate marine transgression matically suitable for peat formation during als and bryozoans, as well as persistent cor- shortly after the Permian-Triassic bound- the Early Triassic. alline algae and sclerosponges (Stanley, ary, at the start of the coal gap. In addi- 1992; Flu¨gel, 1994). tion, paleosols in some places, such as the Rise of Decomposers and Herbivores Tropical reefs and peat swamps are very Early Triassic Newport Formation of Aus- different kinds of environments, but they do tralia, indicate humid bottomlands that at The meager fossil record of fungi in Late have common features. One of these is a other times would have encouraged peat Permian and Middle Triassic peats from narrow tolerance for changes in base level. accumulation. Antarctica shows no obvious differences in Few reef organisms can withstand exposure fungal decomposers over this interval. Evo- to air for longer than an intertidal cycle. Environmental Acidification lution of wetland fungi has also been impli- Corals with symbiotic zooxanthellae and red cated in the lack of tropical peat accumula- algae that are primary reef framework build- Acid rain at the Cretaceous-Tertiary tion after the Carboniferous, but the shift of ers are restricted to the photic zone (Stan- boundary produced a transient 87Sr/86Sr peat accumulation to high latitudes was al- ley, 1992). Similarly, peat will not accumu- spike in marine carbonate, as unusual ready completed by Late Permian and was

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maintained when coals reappeared during helped with detailed reviews. Funded by cene) of British Columbia: Lythraceae: Canadian Journal of Botany, v. 63, p. 303–312. the Middle Triassic. National Science Foundation grant Cevallos-Ferriz, S. R. S., and Stockey, R. A., 1988b, Permineral- ized fruits and seeds from the Princeton chert (middle Eo- Plant-eating insects and reptiles had OPP9315228 to Retallack, an Australian cene) of British Columbia: Araceae: American Journal of evolved to considerable diversity by Late Research Council grant to Veevers, and a Botany, v. 75, p. 1099–1113. Cevallos-Ferriz, S. R. S., and Stockey, R. A., 1989, Permineralized Permian time, but were decimated by ex- Macquarie University postgraduate re- fruits and seeds from the Princeton chert (middle Eocene) of British Columbia: Nympheaceae: Botanical Gazette, tinctions at the Permian-Triassic boundary. search award to Morante. v. 150, p. 207–217. Cevallos-Ferriz, S. R. S., and Stockey, R. 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A., eds., Lower Permian coal measures of Son-Mahanadi and Koll- Systematic and taxonomic approaches in palaeobotany: Sys- Damodar Valley basins, India, in Rahmani, R. A., and tematics Association Special Volume 31, p. 201–222. Flores, R. M., eds., Sedimentology of coal and coal-bearing Ergolskaya, Z. V., 1936, Petrograficheskoe izuchenie barzasskikh We thank P. J. Conaghan and C. Herbert sequences: International Association for Sedimentology uglei [Petrographic study of the Barzass coal]: Trudy Zhen- for discussion. A. M. Ziegler, J. T. Parrish, Special Publication 7, p. 121–147. tralinoe Geologisheskoe Rasvestiya Instituta, v. 170, p. 5–58. Cevallos-Ferriz, S. R. S., and Stockey, R. A., 1988a, Permineral- Erwin, D. H., 1993, The great Paleozoic crisis: New York, Co- R. A. Gastaldo, and W. A. DiMichele ized fruits and seeds from the Princeton chert (middle Eo- lumbia University Press, 327 p.

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