McEwain 2007 Cambio de Clima Triasico-jurasico

8/11/2019 McEwain 2007 Cambio de Clima Triasico-jurasico

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2007 The Paleonto logical Soci ety. Al l rights reserved. 0094-8373/07/3304-0004/$1.00

Paleobiology, 33(4), 2007, pp. 547573

Macroecological responses of terrestrial vegetation to climaticand atmospheric change across the Triassic/Jurassic boundary inEast Greenland

Jennifer C. McElwain, Mihai E. Popa, Stephen P. Hesselbo, Matthew Haworth,

and Finn Surlyk

Abstrac t.The magnitude and pace of terrestrial plant extinction and macroecological change as-sociated with the Triassic/Jurassic (Tr/J) mass extinction boundary have not been quantified usingpaleoecological data. However, tracking the diversity and ecology of primary producers providesan ideal surrogate with w hich to explore patterns of ecosystem stability, collapse, and recovery andto explicitly test for gradual versus catastrophic causal mechanisms of extinction.

We present an analysis of the vegetation dynamics in the Jameson Land Basin, East Greenland,spanning the Tr/J extinction event, from a census collected paleoecological data set of 4303 fossilleaf specimens, in an attempt to better constrain our understanding of the causes and consequencesof the fourth greatest extinction event in earth history. Our analyses reveal (1) regional turnoverof ecological dominants between Triassic and Jurassic plant communities, (2) marked structuralchanges in the vegetation as reflected by potential loss of a mid-canopy habit, and (3) decline in

generic-level richness and evenness and change in ecological composition prior to the Tr/J bound-ary; all of these findings argue against a single catastrophic causal mechanism, such as a meteoriteimpact for Tr/J extinctions. We identify various key ecological and biological traits that increasedextinction risk at the Tr/J boundary and corroborate predictions of meta-population theory or plantecophysiological models. These include ecological rarity, complex reproductive biology, and largeleaf size.

Recovery in terms of generic-level richness was quite rapid following Tr/J extinctions; however,species-level turnover in earliest Jurassic plant communities remained an order of magnitude high-er than observed for the Triassic. We hypothesize, on the basis of evidence for geographically ex-tensive macrofossil and palynological turnover across the entire Jameson Land Basin, that the na-ture and magnitude of paleoecological changes recorded in this study reflect wider vegetationchange across the whole region. How exactly these changes in dominance patterns of plant primaryproduction affected the entire ecosystem remains an important avenue of future research.

Jennifer C. McE lwain.* Depar tment of Geology, T he Fie ld Museu m, 1400 South Lake Shore D rive, Chicago,Illinois 60605. E-mail: [email protected]

Mihai E. Popa. Universit y of Buchare st, Faculty of Geology and Geophysic s, Depart ment of Geology and

Palaeontology, 1, Nicolae Balcescu Avenue, 010041, Bucharest. E-mail: [email protected] P. Hesselbo. Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR, United King-

dom. E-mail: [email protected] Haworth. UCD School of Biolog y and Environ mental Science, Universit y College Dublin, Na-

tional University of Ireland, Belfield, Dublin 4, Ireland. E-mail: [email protected] Surlyk. Institute of Geography and Geology, University of Copenhagen, ster Voldgade 5-7, DK-1350

Copenhagen, Denmark. E-mail: [email protected]*Present address:UCD School of Biology and Environmental Science, University College Dublin, National

University of Ireland, Belfield, Dublin 4, Ireland

Accepted: 13 June 2007

Introduction

The nature, causes, and consequences of the

Triassic/Jurassic (Tr/J) mass extinction event(200 Myr ago) have received increasing atten-

tion over the past decade. Sepkoskis (1981)original global compendium of marine faunalextinction rates classified the Tr/J boundary

as one of the big five extinction events inearth history. Estimates of the magnitude of

diversity loss across the boundary vary from

group to group and are dependent on the

scale of study. Stage-level compilations for

marine families and genera (Sepkoski 1981,1993) indicate losses of 23% and 50% respec-

tively. At the regional or single locality level,

extinction magnitudes of species are extreme-

ly high: 80% of terrestrial plant species in

Greenland and Sweden (Harris 1937; Mc-

Elwain et al. 1999) and 42% of terrestrial ver-

tebrates families in North America (Olsen et


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548 JENNIFER C. MCELWAIN ET AL.

al. 1987), as well as widespread extinction of

ammonites (Newell 1963; Tozer 1979), bi-valves (McRoberts and Newton 1995), radio-larians (Tipper et al. 1994; Carter and Hori

2005), and coral reefs (Kiessling 2001, 2005).

In contrast, other authors have questionedwhether the boundary can be characterizedunequivocally as a mass extinction. Hallam

(2002) has argued that the tempo of extinctionwas gradual rather than catastrophically rap-id, whereas others (Tanner et al. 2004) suggest

that most of the apparent biodiversity lossesacross the Tr/J boundary are due to biases or

artifacts of sampling or poor stratigraphiccontrol. Reanalysis of Sepkoskis global ma-

rine faunal data has also demonstrated thatlow origination rates were more responsible

than high extinction rates for Tr/J biodiversityloss (Bambach et al. 2004).

Uncertainties regarding the nature and tem-

po of the Tr/J boundary extinctions are fur-ther complicated by the fact that most biotic

records spanning the RhaetianHettangianinterval are based on presence-absence data,

which can artificially indicate a catastrophicevent if taxonomic groups are oversplit orbias interpretation in favor of a gradual event

owing to taphonomic control of last occur-rences (Signor III and Lipps 1982). Presence-

absence data sets also limit our understanding

of the ecological and physiological selectivityof extinctions. For instance, is there evidencefor gradual ecosystem decline or instabilityprior to the extinction event at the Tr/J

boundary? Does the extinction event repre-sent a biotic threshold response to long-term

gradual forcing or was it truly catastrophic?Were the taxa that suffered extinction ecolog-

ically important dominants within ecosys-tems or rare? In the absence of paleoecological

data these critical questions about the re-sponses of terrestrial plant communitiesacross the Tr/J boundary remain unan-

swered. Without paleoecological data it is alsodifficult to decipher potential forcing mecha-

nisms of biotic turnover, many of which makeexplicit predictions about the nature and tem-

po of macroecological response. To addressthese uncertainties we have undertaken a de-tailed paleoecological study of terrestrial

plant communities through Rhaetian and Het-

tangian strata of the Kap Stewart Group in Ja-

meson Land, East Greenland. From a databaseof 4304 census-collected macrofossil plant

specimens we have investigated the tempo ofmacroecological change across the Tr/J

boundary and tracked stability, collapse, andrecovery of plant communities, the primaryproduction and therefore foundation of terres-

trial ecosystems in this region. We define cen-sus-collected as follows: where every fossil

specimen discovered within a specific fossilplant bed was collected within a standardized

time frame for all plant beds.We have conducted this paleoecological

analysis in the context of a dramatically

changing global environment throughout theTriassicJurassic interval, with peak environ-

mental changes coinciding with the Tr/Jboundary, as reflected by a negative carbon

isotopic excursion in both organic and inor-ganic carbon (Palfy et al. 2001; Ward et al.2001; Hesselbo et al. 2002; Guex et al. 2004;

Galli et al. 2005). We have used relative abun-dance data and two measures of biodiversity

(evenness and richness) collected f rom ninefossil plant beds to assess whether there was

a gradual decline in ecosystem functioningprior to the Tr/J boundary, or whether an ob-served 80% species-level extinction at the

boundary (Harris 1937; McElwain et al. 1999)

represents a geologically instantaneous event.We have also investigated compositionalchanges (including the degree of heterogene-ity) in terrestrial vegetation and the ecological

and physiological selectivity of the Tr/J eventin an attempt to understand better the causal

mechanisms of Tr/J boundary floral turnover.

Abiotic Context for Tr/J Extinctions

Causal mechanisms for Tr/J extinctions re-main unresolved. Suggested forcing factors

for Tr/J biodiversity loss include climatic andenvironmental disturbance due to a cata-

strophic meteorite impact (CAMP) (Olsen etal. 2002), gradual climatic and environmental

change associated with emplacement of theCentral Atlantic Magmatic Province (Marzoliet al. 1999, 2004; McElwain et al. 1999; Hes-

selbo et al. 2002), catastrophic release of meth-ane due to methane hydrate destabilization

resulting in global warming (Palfy et al. 2001;


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549TRIASSIC/JURASSIC VEGETATION DYNAMICS

FIGURE 1. Stable carbon isotopic (13C) record from fossil wood (Hesselbo et al. 2002) (A), cuticle (McElwain et al.1999) (B), and a paleoatmospheric carbon dioxide record (McElwain et al. 1999) (C), plotted against height of thesection at Astarteklft (cf. Fig. 3) across the Triassic (Rhaetian)/Jurassic(Hettangian) boundary.The carbonisotopicdata in A are all derived from fossil wood collected at Astarteklft. The cuticle material in B is derived from As-tarteklft and other sections throughout the Jameson Land Basin (see McElwain et al. 1999 for details).

Beerling and Berner 2002), and sea-level

change involving a fall followed rapidly by a

rise (Hallam 1997). Global cooling has also

been invoked as a potential extinction mech-

anism (Hubbard and Boulter 2000) although

supporting evidence for such a mechanism isopen to serious challenge.

Irrespective of the exact causal mechanism,

stable carbon isotopic records have identified

a pronounced double negative excursion of

23 coincident with the boundary in East

Greenland (McElwain et al. 1999; Hesselbo et

al. 2002), Hungary (Palfy et al. 2001), England

(Hesselbo et al. 2002), the United States (Guex

et al. 2004; Ward et al. 2006), Italy (Galli et al.

2005, 2006), and Canada (Williford et al. 2006).

These records clearly document a major per-

turbation of the carbon cycle, which was glob-al in nature, and they suggest major environ-

mental upheaval in the latest Rhaetian with

maximum environmental change coinciding

with the Tr/J boundary. Hesselbo et al. (2002)

interpreted the single carbon isotope excur-

sion apparent in the Jameson Land succession

as an amalgamation of an initial and a

main isotope excursion observed in ex-

panded marine sections.

In this paper we explore in detail the ter-

restrial plant communities in the region of As-

tarteklft, East Greenland, in the context of

the stable carbon isotopic composition fromfossil wood in the same section (McElwain et

al. 1999; Hesselbo et al. 2002) (Fig. 1A,B). Al-

though it is difficult to infer directly from iso-

topic records how the climate or atmospheric

composition was changing we interpret the

isotopic profile to provide an integrated re-

cord of environmental upheaval across the in-

terval. We therefore interpret any significant

global deviation in 13C (2 from average

background levels) as a perturbation in the

carbon cycle associated with a change in en-

vironmental conditions. We also investigatevegetation dynamics in the context of a likely

fourfold increase in paleoatmospheric CO2across the Tr/J boundary from levels that

were approximately three times higher than

present before the boundary, derived using

the stomatal proxy approach (McElwain et al.

1999), and a 16C regional climatic warming,


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FIGURE 2. Map of the Jameson Land Basin showing lo-cations of the fossil plant localities and position of pa-leolake.

inferred from a coupled ocean atmosphere

general circulation model forced with this CO2reconstruction (Huynh and Poulsen 2005).However, an estimated regional warming of

16C is remarkably high and likely to be over-

estimated, as their model does not take intoaccount the possible effect of higher SO2levelsdue to CAMP volcanism (which would cool

climates) on global or regional climate warm-ing. Furthermore, stomatal density-based CO2records are required from additional genera

to Ginkgoales, following recent proceduresfor other time intervals (i.e., McElwain et al.

2005) to test the fidelity of the existingGinkgo-based record. Paleo-CO2 records based on

multiple independently calibrated generawith temporally overlapping stratigraphicranges are more robust, as they minimize cal-

ibration uncertainty that can be introduceddue to genetic variability in stomatal frequen-

cy within and between genera, such as thatdemonstrated by (Cantor et al. 2006).

Material

Kap Stewart Flora. In a seminal series of

monographic papers published between 1926and 1937 Tom Harris documented a Rhaetian

Hettangian aged fossil flora of over 200 spe-cies from approximately 13 localities in Ja-

meson Land, East Greenland (Fig. 2). This fos-

sil flora is represented by a rich assemblage ofbryophytes, pteridophytes and gymnosperms(Harris 1926, 1931, 1932a,b, 1935, 1937). Twodistinct plant macrofossil biozones are rec-

ognized within the Kap Stewart Group: aRhaetian assemblage zone characterized by

the presence ofLepidopteris ottonis and a Het-tangianSinemurian assemblage zone with

Thaumatopteris brauniana (Harris 1937). A

80% species-level turnover of plant macro-fossils occurs between the highest occurrences

ofL. ottonis zone taxa and the lowest occur-rences ofT. brauniana zone taxa (Harris 1937).

This floral transitional zone was used byHarris to define the Tr/J boundary. Subse-

quent fieldwork (20002004) has shown thatthe first appearance ofThaumatopteris brauni-ana zone elements occur contemporaneously

with the last appearances ofLepidopteris zonetaxa in Bed 5 of our current study locality, As-

tarteklft. This level coincides with the most

negative carbon isotopic values in the stable

carbon isotopic record from the same section(Fig. 3) (Hesselbo et al. 2002). Palynologicalstudies of the Kap Stewart Group have de-

fined two microfloral zones that parallel themacrofloral biozones of Harris and correlate

with established Rhaetian aged Rhaetipollis-

Limbosporites zone and Hettangian aged Pi-nuspollenites-Trachysporites zones of Europe,Canada, and Svalbard (Pedersen and Lund1980). For these reasons, and in the absence of

a global boundary stratotype section andpoint (GSSP) for the Tr/J boundary, we inter-

pret the top of Bed 5 as representing theTr/J boundary at Astarteklft.

Geological and Depositional Setting. TheRhaetianHettangian aged Kap StewartGroup in Jameson Land was deposited in en-

vironments that ranged from fluvial to lacus-trine (Dam and Surlyk 1993b) (Table 1). The

group comprises three formations (Surlyk2003): the predominantly conglomeratic and

sandy Innakajik Formation at the base, depos-ited in coarse-grained alluvial plain environ-ments; a mixed sandy and shaley Primulaelv

Formation in the middle, deposited in a delta-plain setting; and a mixed sandy and shaley

Rhaetelv Formation at the top, deposited in a


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lacustrine setting. The plant fossils collected

for this study come exclusively from the Pri-mulaelv Formation at Astarteklft (Figs. 2, 3).The succession at Astarteklft comprises

mainly sandstone (7580%), of fine to coarse

grade and commonly texturally and compo-sitionally immature (Dam and Surlyk 1993b).Shaley units, which constitute 2025% of the

succession, are made up of pale blocky mud-stones ranging in thickness up to a maximumof4 m, and dark-gray leaf-rich shale in beds

1 m. Poorly developed black coal and coa-ly mudstone occurs about the middle of the

formation. Rootlets are present at the base ofthe coal, and at several other horizons in the

mudstone facies. Thin beds of siderite occur inthe more organic-rich shaley layers. Most ofthe mudstone units are traceable across the ex-

posure at individual localities and evidentlyhave lateral extents on the order of tens to

hundreds of meters. Mudstone beds also oc-cur within channelized sandstone bodies.

Thin sand laminae and wave-ripple structureshave not been observed in the mudstone facies

at Astarteklft.The thick sandstones were laid down from

river channels that were commonly several

meters deep (Sykes 1974; Dam and Surlyk1993a,b). The textural and mineralogical im-

maturity of the sand indicates a local source

terrain that was undergoing strong physicalerosion; in the case of the Hurry Inlet (east Ja-meson Land) localities this was likely to havebeen the Caledonian basement that is current-

ly exposed in Liverpool Land to the east (Fig.2). Neither discrete erosional channel margins

nor mud-plugged abandoned channels arecommonly observed, indicating rapid and

continuous lateral migration of channels dur-ing deposition. The thin coarsening-upwardsandstone sequences that form wings to ad-

jacent channel fills represent sheet splay orcrevasse splay deposits formed during river

flood episodes (Dam and Surlyk 1993b). Theseare the primary facies in which plant fossils

occur at Astarteklft (sheet splay in Table 1, SSin Fig. 3). A secondary setting for the plant

fossils is in sediments deposited at upwardtransitions from channel sand facies intofloodplain facies (abandoned channel in Table

1, AC in Fig. 3). These are interpreted to have

been shallow pools developed in semi-aban-

doned sand-filled channels.The predominant gray color of the mud-

stone units, combined with moderate lateralextent and sporadically rooted nature, indi-

cates deposition on a permanently or semi-permanently waterlogged floodplain or in aninterdistributary lacustrine bay. The differ-

ence between floodplain and interdistributarylacustrine bay environments is one of degree

of connection to open lake conditions. In ourstudy, in the Hurry Inlet area, interpretation ofthese facies as floodplain is preferred because

sedimentary structures indicative of open wa-ters (e.g., wave ripples) are absent, palynofa-

cies are exclusively of terrestrial origin (Kop-pelhus 1997), and there is no observed inter-

digitation of alluvial or delta plain with la-custrine facies. The true coal (coal swamp

[CS]), a third category of plant bed in JamesonLand Basin, possibly only occurs at one strati-graphic level (equivalent with the level of Bed

6 at Astarteklft; Table 1, Fig. 3) but is not ful-ly developed at Astarteklft. Only rarely did

the floodplain areas dry out enough for oxi-dizing conditions to become established, as

shown by the volumetrically subordinate pur-ple-gray mudstone. The abundance of green-clay cementation of the tops of sandstone beds

indicates the common development of anaer-

obic early diagenetic conditions beneath thewaterlogged floodplain.The floras preserved in these three plant-

bed settings are likely to be of somewhat dif-ferent origins. Floodplain deposits and coalswamps are likely to represent mainly autoch-

thonous assemblages, containing mostly insitu representatives of the floodplain and

swamp communities, respectively (Gastaldo1989). Sheet or crevasse splays contain autoch-

thonous assemblages from local wetland andfloodplain communities, combined with

allochthonous plant material derived from

channel levees and possibly from plant com-munities further upstream (Gastaldo 1989;

Spicer 1989). These types of deposits arethought to provide some of the most accurate

representations of the overall regional sourcevegetation despite taphonomic biases that

tend to overestimate upper delta or alluvialplain communities and underrepresent those


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TABLE 1. Fossil plant collection statistics from 2002 field season.

Bed AgeDepositionalenvironment

No. rock slabscollected

No. fossilleaves recorded

No.reproductive

specimensrecorded

Bed 8 Jurassic (Hettangian) Abandoned channel 150 221 0Bed 7 Jurassic (Hettangian) Abandoned channel 170 864 10Bed 6 Jurassic (Hettangian) Coal swamp 80 128 1Bed 5 Tr/J boundary Sheet splay 332 1023 (320)* 3 (2)*Bed 4 Triassic (Rhaetian) Sheet splay 412 876 103Bed 3 Triassic (Rhaetian) Sheet splay 127 525 0Bed 2 Triassic (Rhaetian) Sheet splay 157 258 4Bed 1.5 Triassic (Rhaetian) Sheet splay 43 62 0Bed 1 Triassic (Rhaetian) Sheet splay 87 224 1

* Numbers in brackets refer to number of specimens collected census style from Bed 5. The higher number in Bed 5 refers to the total number ofspecimens collected using census techniques, collected directly from the bed and from fallen blocks directly under the bed.

FIGURE 3. Measured section from Astarteklft, East Greenland from Hesselbo et al. (2002), updated based on fur-ther fieldwork in 2002 and 2004. Biostratigraphy and lithostratigraphy from Harris (1937), Pedersen and Lund(1980), Dam and Surlyk (1993a,b) and Surlyk (2003). The fossil plant beds located in the present study are shownin boldface; Beds 0, 5.5, and 9 and the moss bed have not yet been investigated in detail. Harriss (1937) plantbed names are also shown, together with his barome ter-b ased e stim ated dist ance below t he Jamesoni Horizon (Har-ris scale). We re-located Harriss beds on the basis of position in the section and on the contained macrofloras.Interpreted depositional environments are indicated in italics. SS, sheet splay; CS, coal swamp; AC, abandonedchannel.

of the local floodplain (Ferguson 1985; Gas-

taldo 1989; Spicer 1989). The abandoned chan-nels, which according to sedimentological

analysis were formed by avulsion, would beexpected to preserve predominantly parauto-

chthonous plant communities growing inclose proximity to the channel, with only veryrare occurrences of allochthonous compo-

nents transported from upstream (Behrens-meyer et al. 2000). Fossil plant assemblages in

these deposits would therefore most likelyprovide insights only into plant communities

in the immediate region of Astarteklft.

Taphonomic Considerations

Taphonomic studies have shown that un-transported and undecayed litter from tem-

perate and relatively low diversity subtropicalfloodplain forests provides an accurate indi-

cation of both the richness and the dominance-diversity relationship of the source forest

(Burnham 1989; Burnham et al. 1992). It isnoteworthy that the majority of these live-dead studies have been carried out on angio-

sperm dominated plant communities, withonly one detailed study to date on a forest

where a gymnosperm was co-dominant (Tax-

odium-Acer swamp [Burnham et al. 1992]).

Nonetheless, observations in different ecolog-ical settings and hemispheres have revealed

that representation of live species in leaf litterfrom both angiosperm- and gymnosperm-

dominated (Taxodium) forests is predominant-ly controlled by the same factors (e.g., relative

abundance, canopy height, leaf area, distancefrom tree) (Burnham et al. 1992; Steart et al.2005). In the absence of a live-dead study of

broad-leaved conifer dominated vegetation,we have therefore assumed that the same pri-

mary factors controlled preservation of leaflitter in the Triassic and Jurassic. General bi-

ases that should be noted include slight over-

representation of taxa with a woody vine (li-ana) habit (Burnham et al. 1989), and better

representation of larger canopy trees thansmaller sub-canopy taxa (Gastaldo 2001). Bi-

ases due to decay and transport of leaf littercan also be considerable (Ferguson 1985; Spi-

cer 1989).Despite these caveats, the macrofossil as-

semblages from Astarteklft are exquisitely

preserved and abundant, primarily autoch-thonous or parautochthonous and to a lesser

extent allochthonous, and show little evidence


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for decay or long-distance transport. Further-

more all fossil plant beds were census-collect-ed to avoid collector bias, and lateral samplingof each bed was as wide as the nature of ex-

posure would allow, so as to minimize biasing

of the rank-abundance data by very localizedinput of leaves from individual plants. Everybed was therefore laterally sampled across a

minimum of eight to a maximum of 20 me-ters. Six out of a total of nine fossil plant bedsare isotaphonomic, including all five of the

Triassic beds and the Tr/J bed (Table 1, Fig.3). Isotaphonomic assemblages afford the

least-biased means of tracking biodiversitytrends through time (Behrensmeyer et al.

2000; Gastaldo 2001). The large database offossil leaf counts from macrofossil assemblag-

es at Astarteklft therefore provides an idealstudy system with which to investigate paleo-ecological changes in Triassic and Jurassic

vegetation. In the case of the six isotaphon-omic plant beds, we can explicitly examine

paleoecological changes that occurred leadingup to, and coincident with, the Tr/J extinction

boundary bed, and test gradual versus cata-strophic mechanisms of floral extinction.However, because the depositional setting of

the Jurassic plant beds at Astarteklft (twoabandoned channels and one swamp) are dif-

ferent from those of the Triassic (all sheet

splay), we use preliminary fossil plant occur-rence data from a Jurassic abandoned channelassemblage at a second Jameson Land field lo-cality (South Tancrediaclft [McElwain, I.

Glasspool, Popa, Hesselbo, D. Sunderlin, andSurlyk unpublished data]) to distinguish the

potential effects of taphonomy from larger-scale paleoecological patterns across the Tr/J

boundary.

Collection Methods

Macrofossil specimens were collected in2002 using census style techniques from a to-

tal of nine fossil plant beds at Astarteklft, Ja-meson Land, East Greenland (Table 1, Fig. 3).

Most these fossil plant beds were originallydiscovered by Harris and were relocated by

measuring distance in meters below a pecten-rich carbonate-cemented sandstone (JamesoniHorizon) of the Neill Klinter Group (Fig. 3).

Harris used this as his marker bed throughout

Jameson Land localities to correlate macroflo-

ras across the basin (Harris 1937: p. 71). Col-lection procedures were standardized for all

nine fossil plant beds. Each bed was collectedfor a total of 48 hours by excavating four small

quarries, spaced two to five meters apart lat-erally, as constrained by the nature of the ex-posure. Roughly the same amount of bulk

sediment was excavated per bed with the ex-ception of Beds 1.5 and 6, which were only

census-collected for a total of eight hours each.All macrofossil specimens excavated duringthe collection interval were collected irrespec-

tive of preservation state and are currentlystored in the paleobotanical collections at the

Field Museum, Chicago. Every identifiablemacrofossil specimen was identified to genus

in the laboratory; specimens of each genuswere counted and recorded in a fossil leaf oc-

currence matrix (Table 2). We use the termmacrofossil specimen to refer to any iden-tifiable vegetative or reproductive structure

preserved on a rock slab/specimen. Individ-ual slabs commonly preserved multiple fossil

leaves and reproductive structures from oneor many fossil plant taxa (Table 1). To estimate

the relative abundances of Triassic and Juras-sic genera at Astarteklft we used a samplingstrategy that represents a hybrid method be-

tween the widely used quadrat method of

counting (Pfefferkorn et al. 1975), which re-ports the occurrence of a taxon once per quad-rat irrespective of how many leaves there were

per quadrat, and those methods that count ev-ery individual leaf (Wing et al. 1993). Quadratmethods can inflate the relative abundances of

ecologically rare taxa and underestimate theabundances of dominants (see Wing and

DiMichele 1995 for a full discussion), whereasindividual counting methods are extremely

time consuming and it can be difficult to re-solve when, and if, leaf fragments should be

counted. We have counted every individual

leaf or leaf fragment per rock slab as one oc-currence in our abundance matrix (Table 2).

However, in cases where there were more thanfive leaves or five leaf fragments per rock slab

we recorded a count of five rather than count-ing every individual leaf. We recognize that

this counting strategy may underestimate therelative abundances of the ecological domi-


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TABLE 2. Generic abundance (number of occurrences) of macrofossil leaf and reproductive specimens at Astar-teklft, East Greenland.

Macrofossil taxon Bed 1 Bed 1.5 Bed 2 Bed 3 Bed 4 Bed 5 Bed 6 Bed 7 Bed 8

Anthrophyopsis 0 0 0 0 2 0 0 0 0Anomozamites 36 20 21 1 237 1 0 4 0

Baiera 14 0 0 7 0 0 0 0 0Cladophlebis 0 0 9 0 0 3 50 22 12Clathropteris 0 0 4 0 0 0 0 0 0Ctenis 0 0 0 0 0 4 0 0 0Ctenozamites 0 0 0 0 0 0 0 0 0Cycadocarpidium 0 0 0 0 0 1 0 0 0Cycadolepis 1 0 4 0 88 1 0 1 0Czekanowskia 5 0 0 0 0 0 0 442 27Dictyophyllum 2 19 1 2 144 0 11 0 17Doratophyllum 0 0 0 41 0 0 0 0 0Elatocladus 1 4 16 0 1 3 19 0 3Equisetites 24 0 0 0 10 5 2 0 0Ginkgoites 14 0 80 0 1 0 0 214 0Hausman nia 1 0 0 0 0 0 0 0 0Lepidopteris 1 2 12 0 27 0 0 0 0Macrotaeniopteri s 0 0 0 0 0 5 0 0 0Marattia 0 0 0 0 0 0 1 0 0

Mattonia 0 0 0 0 0 0 0 0 6Neocalamites 0 0 0 0 0 11 0 0 0Nilssonia 7 9 0 3 0 0 0 1 0Ontheodendron 0 0 0 0 0 3 0 0 0Pachypteris 0 0 0 0 1 0 0 0 0Pagiophyllum 0 0 0 0 0 0 1 0 0Phlebopteris 0 0 0 0 0 0 1 0 34Podozamites 62 1 28 453 13 519 0 0 44Pseudoctenis 0 3 0 0 1 28 0 0 0Pterophyllum 56 3 49 11 440 0 5 0 0Ptilozamites 0 0 1 0 0 9 1 0 0Rhinipteris 0 0 35 0 0 0 0 0 2Sagenopteris 0 0 0 0 0 0 2 0 0Sphenobaiera 0 0 0 6 0 0 0 181 76Spiropteris 0 0 0 0 4 0 1 0 0Stachyotaxus 1 1 2 0 0 416 35 0 0Taeniopteris 0 0 0 1 1 9 0 0 0

Thaumatopteris 0 0 0 0 0 4 0 0 0Vardekloeftia 0 0 0 0 0 1 0 0 0Weltrichia 0 0 0 0 4 0 0 0 0Wielandiella 0 0 0 0 5 0 0 0 0

nants; however, we feel that this is preferableto the loss of potentially important ecological

data caused by the inflation of abundances ofrare taxa. Furthermore, a sensitivity analysiscomparing our counting strategy for Bed 1 at

Astarteklft with a standard quadrat stylecounting approach has revealed only subtle

differences in the resultant relative and rank-order abundances of fossil leaf genera and in-

significant differences in standard measuresof biodiversity based on the relative occur-rence data (Appendix 1). Census collection of

the Kap Stewart Group at Astarteklft result-ed in a data set of 4303 recorded occurrences

of macrofossil leaf and reproductive speci-

mens, derived from a total of 40 Rhaetian andHettangian plant genera.

Paleoecological Analysis

Paleoecological analyses have all been car-

ried out at the genus level. An analysis of Har-riss original Tr/J macrofossil data (Harris

1937) indicates that 95% of generic occurrenc-es, from a total of 38 different beds, are mono-

specific. Those genera containing more thanone species occur predominantly in Triassic(9%) rather than Jurassic plant beds (0.5%)

and are typically represented by two (e.g.,

Ctenis, Equisetites) and rarely three to four spe-

cies (e.g.,Pterophyllum) in a single assemblage.


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From these data we infer that a generic-level

analysis is a fairly robust proxy for analysis atthe species level (which will be the subject offuture work) but that generic-level richness

may underestimate species richness in the Tri-

assic more than in the Jurassic. We note that ageneric-level paleoecological analysis mayalso mask interesting species dynamics be-

tween stratigraphic levels. However, thesecannot be resolved until we have a firmer un-derstanding of the real taxonomic value of leaf

surface micromorphological features (e.g., sto-matal density and distribution, trichome

abundance, presence and absence) on whichspecies determinations for the Kap Stewart

Group flora were largely based (Harris, 1926,1931, 1932a,b, 1937). It is now known than

many of these micromorphological traits arehighly plastic in response to changes in the cli-matic and atmospheric environment. A num-

ber of species as determined by Harris couldtherefore represent a continuum of ecophen-

otypes responding to environmental changesassociated with CAMP volcanic activity, rath-

er than biological species.

Composition. Small changes in the ecolog-ical composition of plant communities can

greatly alter whole ecosystem processes, bi-otic interactions, and feedbacks with the cli-

matic system. Significant functional shifts in

ecosystem processes can result from changesin the relative dominance of different taxawithout any significant change in taxonomiccomposition. To track changes in generic-level

floral composition and dominance patternsacross the Tr/J boundary, we calculated rela-

tive generic abundances per bed for all 41 gen-era as a percentage of the total bed abundance

using the occurrence matrix (Table 2).Detrended correspondence analysis (DCA)

was carried out on the resultant relative abun-

dance matrix (Table 3) using PAST Version1.33 (Hammer et al. 2001). This eigenvector

technique was developed for detecting floris-tic gradients in modern botanical data sets

and displaying complex compositional differ-ences between sites in two dimensions (Hilland Gauch 1980). Using this technique, sites

or plant beds that are compositionally sim-ilar, plot close together in ordination space,

whereas those which are compositionally dis-

tinct plot far apart. In the case of this study we

display compositional change using the firsteigenvector (Axis 1) only. To minimize poten-tial biases introduced by disarticulated repro-

ductive structures of unknown phylogenetic

affinity, we restricted our investigations ofcompositional changes to vegetative macro-fossils only. DCA was also repeated with the

inclusion and exclusion of genera occurring inonly one bed to test for any distortion effects

that these may introduce in the analyses (Wilfand Johnson 2004).

Biodiversity. Biodiversity is variously de-

fined by different authors. However, we definebiodiversity here in terms of both richness (the

number of taxa) and evenness (the equality ofrelative abundances among taxa) of the pa-leovegetation. The more diverse or rich an eco-

system the more likely that it will containfunctional types that are highly productive

(Loreau et al. 2001). Although there are manyexceptions to the rule, richer ecosystems tend

to be more productive as long as climatic andedaphic variables are favorable. Also the richer

an ecosystem, the more likely it is to with-stand major abiotic changes, because there isa larger species pool from which new domi-

nant taxa or new productive functional typescan be recruited.

Evenness is a measure of niche partitioning

and facilitation within an ecosystem (Loreauet al. 2001). Ecosystems that have many co-dominant species all contributing to ecosys-tem function, such as in modern tropical low-

land forests, have a high degree of evennessand a higher diversity of phenotypic traits.

They are therefore more likely to be resilientto environmental changes or to exceeding crit-

ical thresholds (Loreau et al. 2001). These gen-eralized ecological relationships between bio-diversity and stability have been demonstrat-

ed for both short-term (Loreau et al. 2003) andgeological time scales (Kiessling 2005). For the

current study we use generic richness, even-ness, and composition of plant communities

as proxies for overall ecosystem persistencestability, which we define as the ability to

withstand abiotic perturbations as indicatedby changing isotopic composition.

Richness. Trends in generic richness across

the Tr/J boundary were investigated by stan-


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TABLE 3. Precentage representation of leaf genera at Astarteklft from generic abundance matrix (Table 2).

Taxon Bed 1 Bed 1.5 Bed 2 Bed 3 Bed 4 Bed 5 Bed 6 Bed 7 Bed 8

Anomozamites 16.07 32.26 8.14 0.19 27.05 0.10 0.00 0.46 0.00Baiera 6.25 0.00 0.00 1.33 0.00 0.00 0.00 0.00 0.00Cladophlebis 0.00 0.00 3.49 0.00 0.00 0.29 39.06 2.55 5.43

Clathropteris 0.00 0.00 1.55 0.00 0.00 0.00 0.00 0.00 0.00Ctenis 0.00 0.00 0.00 0.00 0.00 0.39 0.00 0.00 0.00Ctenozamites 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Czekanowskia 2.23 0.00 0.00 0.00 0.00 0.00 0.00 51.16 12.22Dictyophyllum 0.89 30.65 0.39 0.38 16.44 0.00 8.59 0.00 7.69Doratophyllum 0.00 0.00 0.00 7.81 0.00 0.00 0.00 0.00 0.00Elatocladus 0.45 6.45 6.20 0.00 0.11 0.29 14.84 0.00 1.36Equisetites 10.71 0.00 0.00 0.00 1.14 0.49 1.56 0.00 0.00Ginkgoites 6.25 0.00 31.01 0.00 0.11 0.00 0.00 24.77 0.00Hausman nia 0.45 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Lepidopteris 0.45 3.23 4.65 0.00 3.08 0.00 0.00 0.00 0.00Macrotaeniopteri s 0.00 0.00 0.00 0.00 0.00 0.49 0.00 0.00 0.00Marattia 0.00 0.00 0.00 0.00 0.00 0.00 0.78 0.00 0.00Matonia 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 2.71Neocalamites 0.00 0.00 0.00 0.00 0.00 1.08 0.00 0.00 0.00Nilssonia 3.13 14.52 0.00 0.57 0.00 0.00 0.00 0.12 0.00Ontheodendron 0.00 0.00 0.00 0.00 0.00 0.29 0.00 0.00 0.00

Pachypteris 0.00 0.00 0.00 0.00 0.11 0.00 0.00 0.00 0.00Pagiophyllum 0.00 0.00 0.00 0.00 0.00 0.00 0.78 0.00 0.00Phlebopteris 0.00 0.00 0.00 0.00 0.00 0.00 0.78 0.00 15.38Podozamites 27.68 1.61 10.85 86.29 1.48 50.88 0.00 0.00 19.91Pseudoctenis 0.00 4.84 0.00 0.00 0.11 2.75 0.00 0.00 0.00Pterophyllum 25.00 4.84 18.99 2.10 50.23 0.00 3.91 0.00 0.00Ptilozamites 0.00 0.00 0.39 0.00 0.00 0.88 0.78 0.00 0.00Rhinipteris 0.00 0.00 13.57 0.00 0.00 0.00 0.00 0.00 0.90Sagenopteris 0.00 0.00 0.00 0.00 0.00 0.00 1.56 0.00 0.00Schizoneura 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Sphenobaiera 0.00 0.00 0.00 1.14 0.00 0.00 0.00 20.95 34.39Stachyotaxus 0.45 1.61 0.78 0.00 0.00 40.78 27.34 0.00 0.00Taeniopteris 0.00 0.00 0.00 0.19 0.11 0.88 0.00 0.00 0.00Thaumatopteris 0.00 0.00 0.00 0.00 0.00 0.39 0.00 0.00 0.00

dardizing absolute generic richness for allnine fossil plant beds using rarefaction anal-ysis. Analytical Rarefaction 1.3 by S. M. Hol-

land was used to estimate rarefied leaf genericrichness and 95% confidence intervals for sev-

en of the nine fossil plant beds, all of whichhad more than 220 specimens (Table 1). In the

case of Beds 1.5 and 6, which had less than 220specimens (62 and 128, respectively), their re-spective richness values were forward-pro-

jected to 220 specimens by fitting the most sta-tistically appropriate curve to their respective

rarefaction curves: logarithmic in the case ofBed 1.5 (R2 0.99) and power in the case of

Bed 6 (R2 0.99). Generic richness reportedfor these two plant beds should therefore beconsidered preliminary. Further collecting

will be undertaken to test these preliminaryrichness estimates.

Taphonomic studies on modern leaf litter

demonstrate that multiple samples (four) oftemperate leaf litter capture on average 85% ofthe standing species richness of the source

vegetation, whereas subtropical and tropicalleaf litter typically underestimates standing

richness by four and eight times, respectively,owing to high spatial heterogeneity (Burnham

1993). We have addressed this important po-tential bias by estimating within-bed hetero-geneity following the methods of Burnham

(1993), which are interpreted to reflect the de-gree of ecological heterogeneity in the source

vegetation, using Sorensens Coefficient ofSimilarity (Sorensens Index, SI; 2C/[A B]),

where C is the number of species in commonbetween two samples and A and B are the to-tal number of species in each of the two sam-

ples (Burnham 1993). A mean Sorensens In-dex was calculated for Beds 1, 2, 3, 4, 5, 7, and

8 from six pairwise comparisons between dif-


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FIGURE4. Standing species richness (A), number of species extinctions expressed as a proportion of standing spe-cies richness (B), species originations (C), range-through species (D), and number of singletons (species occurringin one fossil plant bed only) expressed as a proportion of A (E). AE calculated from presence absence data of Harris(1937) for Astarteklft (Appendix 2). See text for more details on methods. Phases 1 to 4 represent distinct evolu-tionary/ecological phases in the vegetation of East Greenland which are described in detail in the text.

ferent collectors individual quarries, and for

Bed 1.5 from two pairwise comparisons. Rich-ness estimates derived from forest leaf litterwith Sorensens Indices of less than 50% canunderestimate that of source vegetation by as

much as three times, whereas leaf litter sam-ples with mean SIs greater than 50% under-

estimate source vegetation richness by only1.4 times (Burnham 1993).

For comparison we have also estimatedstanding species richness (Fig. 4A), speciesorigination/immigration (Fig. 4C), species ex-

tinction/emigration (Fig. 4B), and range-through species (Fig. 4D) from Harriss (1937)

original presence/absence data set for Astar-teklft following the methods of Foote (2000)

and elaborated on for application to paleobo-tanical specimens by Wilf and Johnson (2004).This analysis excludes singletons (species oc-

curring in one fossil plant bed only) and as-sumes the presence of taxa in a plant bed if it

occurs in the beds both immediately above

and below, i.e., with range-through occurrenc-

es (Appendix 2).Evenness. Changes in the patterns of dom-inance versus evenness of the vegetation span-ning the Tr/J boundary were assessed from

analyses of the relative abundance matrix ob-tained from the census-collected fossil plant

beds of Astarteklft using Shannons Diver-sity (H) divided by the logarithm of number

of taxa, using PAST Version 1.33. Specifical-ly, we were interested in addressing whetherthere was any evidence for ecosystem insta-

bility, as indicated by a decrease in evenness,prior to the Tr/J boundary. The advantage of

tracking changes in evenness in addition torichness is that evenness can be accurately es-

timated from the fossil record irrespective ofsample size (Peters 2004).

Results

Biodiversity Richness. There is an 85% de-

cline in standing species richness of plant


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FIGURE5. Rarefaction curves of summed Triassic (Beds1, 1.5, 2, 3, 4, 5) and Jurassic (Beds 6, 7, 8) leaf genericrichness versus number of macrofossil genera. Phases 1to 4 represent distinct evolutionary/ecological phasesinthe vegetation of East Greenland, which are describedin detail in the text.

FIGURE 6. Rarefaction curves of leaf generic richness

versus number of macrofossil specimens for each of thenine fossil plant beds at Astarteklft normalized for 220specimens per bed with the exception of Beds 1.5 and 6(see Fig. 4).

communities in the Astarteklft regionthroughout the Rhaetian with minimum levels

coinciding with the Tr/J boundary and themost negative carbon isotope values (Figs. 1,

4A). This marked decline in standing speciesrichness is attributable to a combination of el-

evated levels of species extinction at and im-mediately prior to the Tr/J boundary (70% inBed 4 and 80% in Bed 5) (Fig. 4B), depressed

levels of species originations (Fig. 4C), and asharp decline in the number of range-through

taxa (Fig. 4D), all initiated prior to the depo-

sition of Bed 3 and continuing through theRhaetian. At the species level, therefore, spe-cies turnover at this locality appears to haveincreased sharply at the onset of the negative

carbon isotope excursion and remained excep-tionally high through the latest Rhaetian with

peak proportional extinction coinciding withthe Tr/J boundary.

A comparison of lumped Rhaetian and Het-tangian rarefied leaf generic richness indicatesno significant differences (Fig. 5), suggesting

that, at the generic level at least, recovery ofJurassic plant communities following Tr/J

turnover must have occurred relatively rap-idly (within a few million years). When the

data are broken down into individual beds, aclearer pattern of changes in generic diversity

emerges (Fig. 6). Beds 1, 1.5, and 2, the oldestTriassic fossil plant beds, contain the richestmacrofossil assemblages at Astarteklft, with

on average 12 to 13 genera each. A marked

35% loss in generic richness occurs in Beds3 and 4 (with 7.2 and 8.3 leaf genera, respec-

tively). For Bed 3 in particular, the diversityloss coincides with the most positive stablecarbon isotope values recorded for the whole

succession. Bed 5, which marks the transitionzone between the Lepidopteriszone of the Tri-

assic and Thaumatopteris zone of the Jurassic,

is richer than Beds 3 and 4, with rarefactionresults indicating 10.9 leaf genera. However,Bed 5 groups with Beds 3 and 4 in demon-strating the lowest generic richness among all

nine fossil plant beds analyzed. A conserva-tive estimate suggests an average generic loss

of17% between the oldest and youngest (in-cluding the boundary) Triassic fossil plant

beds. Importantly, the loss occurs in both ge-nus and species richness, and prior to onset ofthe negative carbon isotope excursion but co-

incident with an initial 250 to 500 ppmV risein atmospheric CO2 concentration and pre-

sumed warming of global temperatures (Fig.1). This suggests that profound changes in

richness and stability of these plant commu-nities were in place before maximum environ-mental change occurred at the Tr/J boundary.

The lowest Jurassic assemblage (Bed 6),which represents the only coal, has an unex-

pectedly high rarefied generic richness of 14.5.


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FIGURE7. Stable carbon isotopic record from fossil wood (A) compared with changes in rarefied leaf generic rich-ness (B), evenness (which is opposite to ecological dominance) (C), and ecological composition (D), calculated fromrelative abundance data derived from a total of 4303 macrofossil leaf specimens from Astarteklft.

It must be noted that this value was extrapo-

lated from 120 to 220 individuals on the basisof the relationship between rarefied generic

richness and 120 individuals. However, even iffurther collection of fossil plant specimensfrom this bed demonstrates that the true ge-neric diversity has already been capturedwith 120 specimens, a raw diversity measureof 12 (for 120 individuals) is still higher thanfor the underlying three beds. These resultsindicate that plant generic richness may haverebounded rapidly in the Astarteklft region

of Jameson Land (i.e., within a few millionyears rather than tens of millions of years) fol-lowing Tr/J extinctions. As Bed 6 representsthe only coal within the sections at Astar-teklft, an alternative interpretation of theseresults would be that generic richness re-bounded rapidly among peat-forming plantcommunities or that they were unaffected byevents at the Tr/J boundary. We cannot ex-clude any of these interpretations until a Tri-assic assemblage with a similar depositionalenvironment has been identified, for compar-ison with Bed 6.

Considering richness in isolation from othermeasures of biodiversity, these data suggest thatecosystem instability, as indicated by loss inplant primary productivity via emigration andregional extinction, was in place before the Tr/J boundary. Although generic diversity mayhave rebounded rapidly in the Jurassic (Figs. 7,8B), the depleted levels of standing species di-

versity at Astarteklft (Fig. 4A) in both swamp

and non-waterlogged plant communities sug-gest that species turnover remained high in the

Jurassic. If the communities rebounded fromTr/J boundary environmental disturbances,their composition shows little temporal persis-tence. This pattern is supported by the obser-vation that Jurassic plant communities are char-acterized by an extremely high relative propor-tion of singletons (on average 40%)an orderof magnitude greater than observed amongthose in the Rhaetian (on average 4%)sug-

gesting exceptionally high species flux in thepost-boundary interval.

Biodiversity Evenness. The pattern of change in generic richness from the Triassic toJurassic was in part mirrored by observedshifts in evenness (Fig. 7C). Plant communitycomposition was extremely even in the oldestTriassic plant beds (1, 1.5, and 2) with no sin-gle taxon dominating. Forest canopy domi-nants of the levees include Podozamites, Ginkgo,and Baiera, whereas trees of the floodplainswamps were represented by Elatocladus,

Stachyotaxus, and to a much lesser extent Po-

dozamites. A rich and even understory of ben-nettites (Pterophyllum and Anomozamites) oc-curred in the better-drained areas, whereasdipteridaceous ferns (Dictyophyllum,Hausman-nia), cycads (Pseudoctenis and Nilssonia), andAnomozamiteswere prevalent in the floodplainsettings. A dramatic decrease in evenness isevident in Beds 3 and 4. The levee and


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FIGURE 8. A, Changes in heterogeneity of the sourcevegetation calculated using Sorensens Index (SI) on dif-ferent quarries from nine fossil plant beds at Astar-teklft. An SI of 70% indicates that fossil leaf specimenscollected from two different quarries from the same fos-sil plant bed (and assumed two different samples from

the source vegetation) share 70% of the same taxa. B, Es-timates of paleolatitude and inferred paleotemperaturebased on Reess morpho cat syst em (Rees et al. 2000). Alower mean DCA score indicates that the composition ofthe particular fossil plant bed in East Greenland pro-vides a signal that was typically associated with lowerpaleolatitudes throughout the entire Jurassic and there-fore from assumed higher paleotemperature (Rees et al.2000).

swampy floodplain environments becamecompletely dominated byPodozamites(86% ofrelative abundance) and Pterophyllum (50%relative abundance) respectively. This patternof increasing ecological dominance by a single

taxon and decreasing evenness continues tothe Tr/J boundary in Bed 5 where Stachyotaxusseptentrionalis became one of the dominanttaxa. Evenness did not rebound to pre-excur-sion levels until the youngest Hettangianplant bed (Bed 8, Fig. 7C).

Heterogeneity. Moving from the oldest tothe youngest fossil plant beds of the Triassic,a slight but insignificant increase in mean Sor-ensens Index (SI) is observed, indicating a de-crease in spatial heterogeneity of the vegeta-tion through the Triassic (Fig. 8). The most un-expected shift in ecological heterogeneity oc-

curs in Bed 5 at the Tr/J boundary, which hasa lower SI (47 3%) than all other Triassic andJurassic plant beds analyzed. If our paleoeco-logical data provide an accurate reflection ofthe ecological makeup of the standing vege-tation, these results suggest that despite de-creasing evenness the vegetation was slightlymore patchy at the Tr/J boundary than any

other time before or after the extinction event.The increased patchiness could reflect in-creased isolation of dry-ground communitiesto relatively higher and therefore drier topog-raphy as wet and swampy habitats favoring

wet-loving plant communities expanded. In-creased ecological importance of the wet-lov-ing communities in Bed 5 is indicated by in-creased relative abundances of Stachyotaxus(from 2% to 40%) and the presence ofNeo-calamitesandPtilozamites(Appendix 3). An al-ternative explanation is that patchy extinctionamong Late Triassic plant communities andsubsequent invasion by different Jurassicplant taxa resulted in increased spatial hetero-geneity in the boundary bed. The shift in het-erogeneity at the boundary is not, however,statistically significant and is unlikely to bias

our estimates of generic richness any morethan the other Triassic and Jurassic plant beds,as the SI is above 50% if standard errors aretaken into account.

Composition. Marked compositional differ-ences are evident between Triassic and Juras-sic plant communities in the region of Astar-teklft. In the Triassic, four genera,Podozami-tes, Pterophyllum, Anomozamites, and Dictyo-phyllum make up on average over 75% of therelative abundance, but collectively these gen-era constitute less than 10% of the relativeabundance in Jurassic communities. Similarly,

four taxa (Czekanowskia, Sphenobaiera, GinkgoandCladophlebis) that collectively dominate Ju-rassic plant communities (89% of relativeabundance) were only very minor compo-nents of Triassic ecosystems, each contribut-ing less than 5% of the relative abundance. Re-sults from detrended correspondence analysisindicate that the age of the fossil plant assem-blage exerts a stronger control on compositionthan does depositional environment (Fig. 9).These results suggest large-scale turnover ofthe ecological dominants after the Tr/Jboundary, that cannot be explained by taph-

onomic differences between Triassic (all sheetsplay) and Jurassic (swamp and abandonedchannel) fossil plant assemblages. For in-stance, the floral composition of Bed 1 fromSouth Tancrediaklft groups more closelywith the same-aged (Triassic) plant beds fromAstarteklft than with assemblages from thesame depositional environment (Fig. 9).


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FIGURE 9. Scatter plot results of detrended correspon-dence analysis (DCA) of fossil plant occurrence datafrom Astarteklft compared with preliminary occur-rence data collected from South Tancrediaklft (Tan.).Triassic fossil plants beds are in italics and Jurassic fos-

sil plant beds are underlined. Note that Bed 1 at Tancre-diaklft groups more closely with similarly aged fossilplant beds from Astarteklft (despite differences intheir depositional setting) than it does with Jurassicaged fossil plant beds at Astarteklft from the same de-positional setting (abandoned channels). Axis 1, eigen-value 0.4462; Axis 2, eigenvalue 0.3563.

The rate of compositional change at Astar-

teklft is reflected by a stepped change in thefirst eigenvector (axis 1 using DCA) throughout

the Rhaetian and a steady recovery after Tr/Jextinctions (Fig. 7D). Importantly, the exclusionof genera that occur in only one bed from our

DCA does not change the observed pattern ofcompositional change (data not shown). DCA

also reveals that the Tr/J boundary bed (Bed 5)is the most compositionally distinct of all nine

fossil plant beds analyzed (Fig. 7D). Relativeabundance data from Bed 5 indicate that vege-tation was co-dominated byPodozamites (50%)and by Sachyotaxus(40%), a rare component ofthe vegetation in the oldest Rhaetian beds, withan average relative abundance of 2%. Thefloodplain ground cover and perhaps subcan-opy, where present, were dominated almost ex-clusively by Pseudoctenis with rare occurrences

of Neocalamites, Ptilozamites, and Taeniopteris.Other paleoecological studies have shown thatsuch a floristic composition is usually associatedwith permanently or frequently flooded envi-ronments (Archangelsky et al. 1995; Howe andCantrill 2001).

These ecological changes parallel those ob-served for both richness and evenness, indi-

cating that marked changes in plant commu-nities were initiated before the Tr/J boundaryand 13C excursion. Together the results morestrongly support a gradual or pulsed trigger-ing mechanism for Tr/J boundary biodiver-

sity loss rather than a single catastrophic eventcoinciding with peak negative isotopic valuesused to define the boundary globally. Long-term environmental change appears to haveresulted in a major change in the dominance-diversity structure of plant communities priorto and coincident with the Tr/J boundary atAstarteklft.

Ecological Selectivity of Tr/J Extinction

Comparison of the relative abundance distri-butions of summed Rhaetian and Hettangianmacrofossil plant beds (Fig. 10) indicates that

several key ecological and biological traits areassociated with increased extinction risk at theTr/J boundary. These include ecological rarityand complex reproductive biology, two traitsthat are predicted to increase extinction riskalone or synergistically according to meta-pop-ulation theory (Gaston 1994; Lawton et al. 1994;McKinney 1997), and large leaf size, a trait pre-dicted to be disadvantageous by ecophysiolog-ical modeling under a Tr/J global warming sce-nario (McElwain et al. 1999). At the genericlevel, extinctions recorded at Astarteklft and inthe Jameson Land region occurred predomi-

nantly among (1) rare genera (defined here asgenera with relative abundances in the lastquartile of the rank abundances, following Gas-ton, 1994) including dipteridaceous fernsClath-ropteris andHausmannia, the bennettite flowerWeltrichia, and foliage generaAnthrophyopsis (in-certae sedis, ?Cycadales), Pachypteris(Corystos-permales), andMacrotaeniopteris(incertae sedis,?Cycadales); (2) genera with extremely largeleaves (Anthrophyopsis, Clathropteris,Hausmannia,Macrotaeniopteris); and (3) those genera, judgingfrom their reproductive anatomy, that likelywould have required specialist insect vectors for

pollination (WielandiellaandWeltrichia).Physiognomic Selectivity of Tr/J Extinctions.

The selective regional extinction of rare large-leaved Triassic taxa such as Anthrophyopsis(20 100 cm), Clathropteris(30 40 cm),Hausmannia(20 cm in width), and Macro-taeniopteris (20 50 cm) is consistent withmodel expectations based on leaf physiologi-


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FIGURE10. Rank-order abundance plots for summed Triassic versus Jurassic fossil leaf and reproductive genera cal-culated from 4303 macrofossil specimens collected from Astarteklft, East Greenland. Dotted line indicatesthe four quar-tile (to the right) of the data, which are taxa considered ecologically rare using the definition of Gaston (1994).

cal responses to increased atmospheric CO2concentration and mean summer temperature.McElwain et al. (1999) modeled leaf temper-

ature responses to a fourfold increase in at-mospheric CO2 and a 34C global tempera-

ture rise across the Tr/J boundary. They pre-dicted that large-leaved taxa would becomemore dissected and/or reduced in size in or-

der to avoid exceeding lethal leaf temperaturelimits under a 5C global warming. Recent pa-

leoclimatic modeling shows that a fourfold in-crease in atmospheric CO2may have raised lo-

cal summer temperatures in the Jameson LandBasin by 16C, resulting in mean summertem-peratures of 36C (Huynh and Poulsen 2005).

Under such high summer temperatures, leaftemperatures with average undissected leaf

widths of 3 cm would readily exceed 50

55C, a temperature range considered lethal

for tropical/subtropical woody taxa (Gauslaa1984; Larcher 1994). The demonstration by

McElwain et al. (1999) of the replacement oflarge canopy leaves with progressively small-

er and more dissected leaves throughout theLate Triassic corroborates expectations basedon ecophysiological modeling of leaf temper-

ature. The results presented here suggest thatlethal leaf temperatures may also have ad-

versely affected large-leaved and presumedunderstory or ground cover taxa such as Clath-ropteris,Hausmannia, Anthrophyopsis, andMac-

rotaeniopteris.

Reproductive Selectivity of Tr/J Extinctions.Analysis of Harriss (1937) presence/absencedata set for the entire Jameson Land Basin re-

veals that 11 of 13 known bennettite taxa dis-


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covered in the Kap Stewart Group fossil plantbeds (Wielandiella and Weltrichia) are knownonly from the Lepidopteriszone (Triassic). Ourrelative abundance data set (Table 3) indicatesthatPterophyllumand Anomozamites, both ben-

nettite foliage taxa, experienced the secondand third most severe average losses in rela-tive abundances of all taxa recorded from theTriassic to Jurassic, respectively: Pterophyllumdeclines from 20% to 1.3 %, andAnomozamitesfrom 17% to 0.1% (Table 3). Together thesedata suggest that bennettites were severely re-duced, in terms of both their taxonomic rich-ness and ecological importance in Jurassiccompared with Triassic plant communitiesacross the entire Jameson Land Basin. Theprotection of ovules among interseminalscales and within flower-like reproductive

structures (Delevoryas 1963; Harris 1974) andthe presence of nectaries (Crane 1985; Crepet1974; Delevoryas 1963) are all indicative of aspecialized pollination syndrome, most likelyinvolving insects, among bennettites.

For cycads, only Doratophyllum experiencedregional extinction, and Ctenis, Pseudoctenis,and Nilssonia, all of which had low to rareabundances in the Triassic, are completely ab-sent from Jurassic plant beds of Astarteklft.The majority of modern cycad taxa displaymutualistic relationships with highly effectivespecialist insect vectors including beetles,

weevils, and thrips (Oberprieler 1995a,b;Mound and Terry 2001; Terry et al. 2005). Vol-atiles released from both male and femalecones attract insect vectors to visit, acquire,and transport pollen (up to 10,000 pollengrains per individual) between male and fe-male cones, thus pollinating ovules (Donald-son 1997). In return, insect vectors receive aready food source (pollen from the male andpollen droplet from the female), protection,warmth, and oviposition sites (Terry et al.2005). If these highly mutualistic relationshipsbetween insects and cycads were in place by

the Mesozoic, then extinction of one of thepartners in the relationship would result inthe relatively rapid demise of the other. Recentobservations of Triassic insect coprolites load-ed with pollen within a permineralized cycadcone suggest that such mutualistic relation-ships may have a very long evolutionary his-tory (Klavins et al. 2003, 2005). We hypothe-

size therefore that the higher levels of extinc-tion among entomophilous (including bennet-tites and cycads) than in anemophilous plantsat the Tr/J boundary were caused by a con-temporaneous extinction of insect taxa. Alter-

natively, extinction and ecological demise ofthese plants could have triggered extinction oftheir insect vectors. In the absence of insectbody fossils, or a comprehensive record of in-sect feeding damage from the Jameson LandBasin, this hypothesis remains to be tested.

The extinction of Lepidopteris at the Tr/Jboundary may also be due to reproductivespecialization; however, this hypothesis is notconclusive and requires further investigation.Peltaspermum, the female reproductive struc-tures ofLepidopteris, consist of relatively com-plex radially symmetrical peltate ovuliferous

disks from which hang the ovules (Harris1932a). The micropyle of the ovule in Lepidop-teris is extremely elongated and angled atabout 30, an anatomical character that mayindicate adaptation for insect rather than windpollination. Lepidopteris pollen does not pro-vide strong support for an entomophilous re-productive habit, as it is small, 23 to 40 m inlength (Townrow 1960), and unsculpturedwith a single sulcus. However, cycads, whichare entomophilous, also have pollen is thissize range. Additional, but admittedly moreindirect, evidence for relatively complex re-productive biology in Lepidopteris can begleaned from its likely habit, which we inter-pret as vinelike or liana-like (see Appendix 3).

Vegetation Recovery

Twenty-two percent (five) of fossil plantgenera observed in Jurassic aged plant bedswere not recorded in Triassic assemblages atAstarteklft, suggesting that they likely im-migrated from elsewhere. All of these newJurassic immigrants can be classified ecologi-cally as either rare or of low abundance interms of their relative abundance distributions

(Fig. 10). Phlebopteris and Matonia (matonia-ceous fern), Sagenopteris (Caytoniales), Marat-tia (marattiaceous fern), and Pagiophyllum, allnew genera to East Greenland in the Jurassic,have relative abundances of less than 2.5%.Recovery of the plant communities followingthe marked biodiversity loss before and coin-cident with the Tr/J boundary was therefore


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achieved by recruitment of new dominantsfrom within existing plant communities fromthe same region, rather than through origi-nation at the generic level or via significantlevels of immigration of exotic taxa. For in-

stance, there is evidence for ecological expan-sion ofCladophlebis,which increased in relativeabundance from 4% in the Triassic to 39%in the earliest Jurassic plant bed (Bed 6), andofStachyotaxus, which increased in abundancefrom 2% to 40% at the Tr/J boundary (Bed5) and to 27% in Bed 6.

Bed 6 at Astarteklft occurs 14 meters abovethe Tr/J boundary, but well within the negativeisotopic excursion (Figs. 1, 3) that characterizesthe boundary interval globally. It is noteworthyin containing the highest proportion of ther-mophilous elements, which likely represent im-

migrants from much lower paleolatitudes.These includePagiophyllum, which is commonlyassociated with the thermophilous pollen taxa

Classopollis (Vakhrameev 1991; Axsmith et al.2004; McElwain et al. 2005) andSagenopteris. Weestimated an average Jurassic paleolatitude, as aqualitative proxy for relative global paleotem-perature, for all nine fossil plant beds followingthe methods of Rees et al. (2000). This methodaverages the observed paleolatitudes that eachtaxon typically occupied in the whole Jurassicperiod, using an extensive biogeographical da-tabase of Mesozoic fossil plants (Rees et al.2000). The analysis indicates that the taxonomiccomposition of Bed 6 is more typical of muchlower paleolatitudes, thus supporting our sug-gestion that maximum regional temperaturesfor the entire RhaetianHettangian interval like-ly occurred in the earliest Hettangian, coinci-dent with Bed 6 (Fig. 8B).

Post Tr/J Boundary Fern Spike?

The earliest Jurassic macrofossil plant as-semblages (represented by Bed 6) are notablefor their high relative proportion of fern taxa.Bed 6 contains the highest abundance of fern

taxa compared with any of the other Triassicor Jurassic fossil plant beds: 49% comparedwith a maximum of 31% recorded in the Tri-assic. The high fern sum in Bed 6 is mainlydue to a high relative abundance of Osmun-daceae (Cladophlebis denticulata); however,three other fern families are also present (Dip-teridaceae, Matoniaceae, Marattiaceae). Field-

work in 20022004 shows that this coal, al-though thin, is laterally extensive across thewhole of the Jameson Land Basin, suggestingthat conditions favoring peat-forming vege-tation, such as high precipitation and/or re-

duced clastic input, must have prevailed atthis time (Fig. 3). Higher precipitation or aninvigorated hydrological cycle is an expectedconsequence of higher global temperatures,because of increased vapor holding capacityof the troposphere (Hay and DeConto 1999)and increased latent heat transfer from low tohigh latitudes (Ufnar et al. 2004).

Peat-forming vegetation and a high relativeproportion of fern (trilete) spores (89%) are re-corded from tropical paleolatitudes in the New-ark Basin of North America at the Tr/J bound-ary (Fowell and Olsen 1993; Olsen et al. 2002).

These coaly beds and the contained fern spikeare unique to the Tr/J boundary interval andhave been interpreted as representing an expan-sion of opportunistic disaster taxa followingthe catastrophic environmental effects of a me-teorite impact (Fowell and Olsen 1993; Olsen etal. 2002). It is not unusual for peat-forming veg-etation to contain a high proportion of fern taxa.It is intriguing, however, that the only coal de-posits in both basins occur within the negativeisotopic anomaly that characterizes the Tr/Jboundary globally. Further work is now re-quired to determine whether these high ferncompositions are merely a taphonomic artifactor whether they signify a truly unique ecologi-cal and/or climatic event across the NorthernHemisphere at the Tr/J boundary and in theearliest Jurassic.

Tr/J Vegetation Dynamics at Astarteklft

Our paleoecological results enable subdi-vision of the Tr/J vegetation dynamics at As-tarteklft into four distinct phases. In phase 1,prior to the negative isotopic excursion andCO2-induced global warming (Fig. 1), the Tri-assic vegetation of the Astarteklft region was

a rich and heterogeneous broad-leaved, gym-nosperm-dominated forest. Agathis-dominat-ed forests of New Zealand and Australia to-day, withPodocarpusand an understory of thecycad Lepidozamia hopei, are a good modernanalogue (White 1994). Podozamites, Ginkgo,Elatocladus, andBaieramost likely co-dominat-ed the canopy, whereas Pagiophyllum andAn-


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omozamites, and to a lesser extent Nilssonia andPseudoctenis, likely formed a sub-canopy. Dip-teridaceous ferns, Equisetites, and other rarelarge-leaved taxa such as Anthrophyopsis andMacrotaeniopteris made up the ground cover

(Appendix 3). No single taxon dominated thecanopy, sub-canopy, or ground-cover habitats,and the flora comprised a high percentage oftaxa with complex reproductive cycles requir-ing animal vectors for fertilization and dis-persal (cycads and bennettites). If our inter-pretations are correct, vines were also presentin low to rare abundances (Lepidopteris) (Ap-pendix 3).

In phase 2, generic richness, evenness, andstanding species richness began to declineand ecological composition changed marked-ly. These vegetation changes are indicative of

instability in local plant communities and co-incide with evidence for deterioration in cli-matic and atmospheric conditions (McElwainet al. 1999; Huynh and Poulsen 2005). The can-opy was first dominated by Podozamites, andStachyotaxus became an additionally impor-tant element at the boundary. We hypothesizethat a mid-canopy habit in the Triassic was al-most completely eradicated in the JamesonLand Basin, owing to high emigration and/orextinction of erect cycads and bennettites (Ap-pendix 3). Marked changes in dominance pat-terns of ground-cover taxa also occurred dur-

ing the Triassic to Jurassic transition as anabundance of dipteridaceous ferns gave wayto osmundaceous ferns. Phase 3 is character-ized by low evenness, depressed generic rich-ness, peak species extinction, and the mostcompositionally distinctive vegetation of theentire RhaetianHettangian interval, as indi-cated from detrended correspondence analy-sis. This phase coincides with the first peak ofthe main negative isotopic excursion and max-imum estimated pCO2and global temperature.

The post Tr/J recovery interval, definedhere as phase 4, is characterized by a rebound

of generic richness at Astarteklft; however,standing species richness for the entire Jame-son Land Basin remained low throughout thisphase, owing to low species origination andlow numbers of range-through taxa. As Bed 6is the only coal swamp deposit at Astarteklft,it is not yet possible to evaluate whether thisapparent rebound in generic richness in Bed 6

is real or a taphonomic artifact. This will bepossible only when a pre-boundary bed fromthe same depositional environment has beenidentified for direct comparison. Most of theraw species richness in Hettangian plant beds

at Astarteklft is made up of singletons, sug-gesting extremely high species turnover dur-ing the post-boundary interval (Fig. 4E). Themacroecological characteristics of the plantcommunities such as evenness and composi-tion did not fully rebound to pre-excursionlevels until the end of phase 4 as reflected inBeds 7 and 8, which occur 46 m aboveandlikely on the order of 3 to 4 million years af-terthe Tr/J boundary, indicating a long re-covery period. It is noteworthy that the natureand tempo of vegetation dynamics spanningthe Tr/J boundary parallel those observed in

Carboniferous swamp forest spanning theWestphalian/Stephanian boundary (DiMicheleet al. 1996), hinting that there may be a com-mon macroecological response of plants toglobal climate change.

Significance of Results in the Broader

Context of Tr/J Boundary Events

Our paleoecological analysis reveals a closecoupling of long-term environmental changein the latest Rhaetian and marked shifts in therichness, evenness, and composition of terres-trial plant communities at Astarteklft. How-

ever, the terrestrial plant communities of theswamps, floodplains, and dry-ground envi-ronments show significant declines in biodi-versity and shifts in ecological compositionprior to the Tr/J boundary, when maximumenvironmental perturbations occurred (Fig.1). This pattern of more gradual or pulsed bio-diversity loss and compositional change is inmarked contrast to the tempo of macroecol-ogical change recorded across the K/T bound-ary, which was undoubtedly catastrophic(Wilf and Johnson 2004). A gradual tempo ofextinction and macroecological change prior

to the Tr/J boundary, as recorded here, is not,however, consistent with a catastrophic causalmechanism coincident with the boundary,such as a meteorite impact (Olsen et al. 2002).The absence of both shocked quartz (Moss-man et al. 1998) and a suitably aged crater(Hodych and Dunning 1992), and iridium val-ues that are more indicative of a mantle than


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extraterrestrial origin (Mossman et al. 1998)also suggest that such a causal mechanismwas rather unlikely.

Global cooling has also been invoked as apotential extinction mechanism because of an

increased abundance of conifer pollen at theboundary, which is interpreted as indicative ofclimatic cooling (Hubbard and Boulter 2000).However, the fact that dipteridaceous fernspores, which today have an exclusively trop-ical distribution, andClassopollispollen, a pre-sumed high-temperature indicator (Vakhra-meev 1991), are both found in the same pal-ynological associations as the conifer pollenstrongly argues against a global cooling hy-pothesis. The role of sea-level changes viahabitat loss in either regional or global Tr/Jmarine extinctions has been strongly chal-

lenged (Hesselbo et al. 2004) and lacks a mech-anistic explanation for gradual extinctions onland. Our paleoecological analysis moststrongly supports a central role for extinctionmechanisms that operated over hundreds ofthousands to millions of years, initiating en-vironmental change and instability of plantcommunities and perhaps of entire ecosys-tems. Climatic and environmental change as-sociated with the emplacement of the CentralAtlantic Magmatic Province (CAMP) is themost likely candidate. Peak volcanic activity ofCAMP (199.5 0.5 Ma) (Knight et al. 2004;

Marzoli et al. 1999, 2004) is temporally con-temporaneous with the Tr/J boundary (as de-fined by the 13C curve; 199.6 0.3 Ma) andwith minimum negative isotopic values fromorganic and inorganic carbon (Palfy 2000).

Ash (1986) suggested that there is little ev-idence for loss of higher taxonomic ranks ofplants across the Tr/J boundary. Our analysisin part confirms this, as only one family (Pel-taspermaceae) that goes extinct in Greenlandis a confirmed global extinction. An absence offamily-level extinction may be an artifact oftaxonomy (Kerp et al. 2006). However, genus-

level floral extinctions at Astarteklft werealso moderate at 17%, despite exceptionallyhigh species turnover (80%) (Fig. 4B). Usingthese data, one could argue that terrestrialplants do not undergo mass extinction accord-ing to the strictest definition of the term (in thesense of Sepkoski, [1981]). In contrast, paleo-ecological changes at Astarteklft indicate

that terrestrial vegetation in this region likelyunderwent major structural and composition-al change. This pattern of resilience amonghigher taxonomic ranks in spite of ecologicalupheaval has been observed for marine inver-

tebrates (Droser et al. 1997, 2000). The vege-tation changes at Astarteklft represent thesecond highest level of paleoecological changeof four possible levels developed by Droser etal. (1997), and far exceed normal backgroundecological change. These include likely loss ofthe mid-canopy habit, a drastic decline inabundance of reproductively specialized taxa(cycads and bennettites) across the entire Ja-meson Land Basin and a marked change inplant dominance. Our data suggest that thesechanges cannot be explained by taphonomicdifferences between Tr/J depositional envi-

ronments (Fig. 9).We hypothesize, on the basis of evidence for

geographically extensive macrofossil and pal-ynological turnover (80% of species) acrossthe entire Jameson Land Basin (Harris 1937;Pedersen and Lund 1980), that the nature andmagnitude of paleoecological changes record-ed at Astarteklft reflect wider regional veg-etation change. Changes in the source and/orstructure of plant primary productivity canhave a cascade effect throughout the entireecosystem (Vermeij 2004). How exactly thesemacroecological changes in the vegetation ofAstarteklft scale up to the whole JamesonLand region and how they affected the well-documented faunal extinction at the Tr/Jboundary remain important avenues of futureresearch.

Conclusions

1. Terrestrial plant biodiversity (richnessand evenness) shows evidence for a gradualdecline at both genus and species levels in thelatest Triassic prior to the Tr/J boundary. Thispattern suggests that terrestrial plant com-munities at Astarteklft show signs of insta-

bility prior to maximal global climatic and at-mospheric change. We conclude therefore thatrelatively minor changes in CO2and regionaltemperature had a disproportional negativeeffect on the stability of terrestrial plant com-munities. Alternatively, some as yet undetect-ed biotic or abiotic factors triggered instabilityin advance of the Tr/J boundary.


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2. Post-Tr/J boundary plant communities arecharacterized by an extremely high relative pro-portion of taxa occurring in one bed only (onaverage 40%), compared with 4% in Triassicplant communities. This indicates exceptionally

high species-level flux in the post-boundary in-terval. These data, together with a long time de-lay in the recovery of evenness, suggest a longpost-Tr/J recovery period.

3. Our paleoecological analysis revealscomplete replacement of the dominant Trias-sic plant taxa with new dominants in Jurassicplant communities that cannot be explainedsimply by taphonomic factors. The Triassicdominants did not go extinct at the Tr/Jboundary but instead persisted as ecologicallyrare components of Jurassic plant communi-ties. Similarly, the new Jurassic dominants

were recruited from rare occurrences withinTriassic ecosystems.

4. Ecological rarity, large leaf size, andcomplex reproductive biology were the pri-mary ecological/biological traits identified toincrease extinction risk for terrestrial planttaxa in the Jameson Land Basin at the Tr/Jboundary. All three determinants of extinc-tion risk match expectations based on modernmeta-population theory and ecophysiologicalmodels.

5. The gradual nature of terrestrial plantbiodiversity decline and compositionalchange recorded in East Greenland prior tothe Tr/J boundary does not support a singlecatastrophic extinction mechanism coincidentwith the boundary as defined by the isotopicexcursion, but rather invokes more gradualcausal agents of plant extinction.

6. Our study of vegetation dynamics span-ning the Tr/J boundary suggests that the tax-onomic severity of this extinction was decou-pled from the ecological severity.

7. If the paleoecological changes at Astar-teklft were a basin wide phenomenon, as ishighly probable according to preliminarydata, they are likely to have contributed sig-nificantly to Tr/J vertebrate extinctions by al-tering the dominant sources of primary pro-ductivity available to consumers.

Acknowledgments

We thank the National Geographic Society(7038-01), the Comer Foundation of Science

and Education (#13), the Field Museum, the

Royal Society of London, the United KingdomNatural Environment Research Council, andan EU Marie Curie Excellence Grant (MEXT-

CT-2006-042531) for funding. Special thanks

to E. C. Meeker and to R. H. and P. O. Schnadigfor additional funding support through theField Museum Womens Board Field Dream

Program. We thank I. Glasspool and D. Sun-derlin for fieldwork carried out in 2004, whichprovided preliminary data on South Tancre-

diaklft. Many thanks to the Danish PolarCentre, Constable Pynt Airbase staff, and R.

Hines for logistical support with fieldwork inEast Greenland. P. Wagner and I. Glasspool

are thanked for comments on earlier versionsof the manuscript. We thank D. Cantrill, W. A.DiMichele, and one anonymous reviewer for

their extremely thorough and constructive re-views. This paper is a contribution to Inter-

national Geoscience Programme Project 458.

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