Palaeogeography, Palaeoclimatology, Palaeoecology 308 (2011) 160–170

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Palaeogeography, Palaeoclimatology, Palaeoecology

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Ostracod recovery after Permian–Triassic boundary mass-extinction: The southTibet record

Marie-Béatrice Forel a,⁎, Sylvie Crasquin b, Thomas Brühwiler c, Nicolas Goudemand c, Hugo Bucher c,Aymon Baud d, Carine Randon a

a UPMC Univ. Paris 06, CNRS, UMR 7207, Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements, T.46-56, E.5, case 104, 75252 Paris cedex 05, Franceb CNRS, UPMC Univ. Paris 06, UMR 7207, Centre de Recherche sur la Paléobiodiversité et les Paléoenvironnements T.46-56, E.5, case 104, 75252 Paris cedex 05, Francec Paläontologisches Institut und Museum der Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerlandd BGC, Parc de la Rouveraie 28, 1018 Lausanne, Switzerland

⁎ Corresponding author.E-mail address: marie-beatrice.forel@upmc.fr (M.-B.

0031-0182/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.palaeo.2011.02.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 December 2009Received in revised form 27 January 2011Accepted 20 February 2011Available online 24 February 2011

Keywords:OstracodsEarly TriassicMiddle TriassicSouth TibetTulong

Lower to Middle Triassic ostracods from the Tulong section, south Tibet, are described here for the first time.Samples from the first two stages of the Early Triassic (Griesbachian and Dienerian) are barren of ostracods;the following stage (Smithian) revealed low diversity ostracod faunas; a substantial diversification in taxabegan at the base of the fourth stage (Spathian) and developed into the first stage of the Middle Triassic(Anisian). Furthermore, exploration of additional feeding modes is developed in the Spathian and Anisian,with notable occurrence of filter-feeding taxa. High abundance of filter-feeding ostracods in Spathian andlower Anisian units indicates that benthic habitats became oligotrophic. These habitats typically harboredostracod faunas of Mesozoic affinities, suggesting that the evolutionary turnover of ostracods was fostered bythe declining input of nutrients from the Spathian on. Marked faunal similarities with other Tethyan areas,mainly the northern part of Tethys, are observed during Spathian and Anisian times.

Forel).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

As for many organisms, ostracods (Crustacea) were deeply affectedby the end-Permian extinction. During the Late Permian, the marineostracod faunas continued to be diversified and abundant in the areasnot affected by the regression such as South China (Crasquin et al.,2010), Southern Alps (Crasquin et al., 2008) and Turkey (Crasquin-Soleau et al., 2004a,b). Our knowledge of ostracods from the boundaryinterval has recently improved and shows abundance continuedthrough the extinction event (in South China: Crasquin-Soleau andKershaw, 2005; Crasquin-Soleau et al., 2006a; Forel et al., 2009;Crasquin et al., 2010; Forel and Crasquin, in press; in Southern Alps:Crasquin et al., 2008; in Turkey: Crasquin-Soleau et al., 2004a, b; SaudiArabia: Crasquin-Soleau et al., 2005). In contrast, Lower and MiddleTriassic marine ostracod faunas remain generally rare and well-datedfaunas are scarce. Lower Anisian deep water faunas are known fromRomania (Crasquin-Soleau and Gradinaru, 1996). Some neritic specieswere recognized (or just quoted) in the Early Triassic (Induan–Olenekian) and Early Anisian of Australia (Jones, 1970), Pakistan(Sohn, 1970), Nepal (Bunza and Kozur, 1971), Greece (Kozur, 1971;Ardens et al., 1979), Germanic Basin (Kozur, 1973), Israel (Hirsch andGerry, 1974; Honigstein and Crasquin, 2011), Kashmir (Agarwal, 1979,

1980, 1981; Agarwal and Kumar, 1981) and South China (Wang, 1978;Wei, 1981; Hao, 1992, 1994). A first evaluation of the ostracodevolutionary responses through the Permian–Triassic events waspublished by Crasquin-Soleau et al. (2007b), with evidence that atransitional interval between Palaeozoic and typically Meso-Cainozoicfaunas existed for this clade. Some forms of Mesozoic affinities werediscovered in the latest Permian mixed with typical Palaeozoic forms(Crasquin-Soleau et al., 2004a, b). The presence of survivors in the basalEarly Triassic was first discovered by Jones (1970) in the Perth Basin(Australia) and by Sohn (1970) in the Salt Range (Pakistan). It wasconfirmed in South China by Wang (1978) in Guizhou and NorthYunnan Provinces, Wei (1981) in Sichuan Province, and Hao (1992,1994) in Guizhou Province. Palaeozoic survivors were recognized in theWestern Taurus (Crasquin-Soleau et al., 2004a, b), in Sichuan Province(Crasquin-Soleau and Kershaw, 2005) and at the GSSPMeishan section,Zhejiang Province (Forel and Crasquin, in press). The only data forSmithian and Spathian ostracod faunas come from Guangxi Province inSouthChina (Crasquin-Soleauet al., 2006a). This interval is fundamentalfor the understanding of the recovery of the faunas after the end-Permian mass extinction.

Most ostracods are benthic dwellers and are considered as valuablepalaeoenvironmental markers. The ostracod faunas reported here bringa significant contribution for reconstructing the evolution of theecological upheavals of the benthos during Early and early MiddleTriassic times, and therefore allow new insights into palaeoenviron-mental conditions.

Panthalassa

Pangea

Palaeo-Tethys

Neo-Tethys

Panthalassa 3

2

14

Fig. 1. Palaeogeographical map during the Early Triassic (adapted from Golonka, 2002); 1, South Tibet; 2, South China; 3, Austria, Germany, Greece, Hungary, Romania, and Slovakia;4: Himalaya and Nepal.

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2. Geological setting

During the Early Triassic, South Tibet was located on the northernGondwanian margin, in the southern part of Tethys Ocean (Ogg andvon Rad, 1994; Ricou, 1994) or Neo-Tethys (Baud et al., 1996) (Fig. 1).

The Tulong section in south Tibet (N28°26′13–E86°08′09; 4430 m)is well known for its Lower Triassic succession. A first classic butincomplete section was described by Garzanti et al. (1998) and againby Shen et al. (2006); however, more recently, the lower part of thesection down to the real Permian–Triassic boundary was discoveredin new sections from the same area and thrust plate (Brühwiler et al.,2010a). These new sections are on tectonic strikeWest and East of theclassic section and allow establishing an almost complete successionfrom the Late Permian up to the Early Anisian. The completesuccession is divided by Brühwiler et al. (2010a) in several lithologicalunits labeled from I to VI. The classical section near the village ofTulong exposes only units III to VI, the lower unit being faulted againstthe Permian dark shales, resembling the Kuling Shales of the classicalsuccession of the Indian margin. This complete Lower Triassicsuccession is well dated by means of ammonoids and conodonts(Brühwiler et al., 2009, 2010a, b, submitted for publication; Fig. 2). It isbeyond the scope of this paper to re-describe the section in detail. Theshort following outline of the section is based on Brühwiler et al.(2009) and the high-resolution ammonoid age control for theSmithian part of the section has been established by Brühwiler et al.(2009, 2010a, b, submitted for publication).

Unit I consists of 3 m of carbonate rocks subdivided into twosubunits. Sub-unit Ia of Griesbachian age is composed of 2 m ofyellowish-reddish thin-bedded dolomitic limestones intercalatedwith few thin shale layers. Sub-unit Ib of Dienerian age is composedof 1 m of light gray dolomitic limestone.

Unit II is composed of faulted and tightly folded dark greenshales. No unfolded and continuous exposures could be found butthe thickness of this unit is estimated between 50 and 100 m. Itsage is comprised within the Dienerian to early Smithian timeinterval.

Unit III corresponds to 9 m of gray limestone beds alternating withthin shale intervals, subdivided into four sub-units. Sub-unit IIIa ofSmithian age is 1.60 m thick and is composed of red-weathereddolomitized calcarenite and gray packstones. Sub-unit IIIb of middleSmithian age is composed of 2 m of thin-bedded, platy to nodular,gray to light red limestones alternating with shale interbeds. Sub-unitIIIc is 4 m thick and composed of medium bedded gray limestonealternating with shales and very nodular marly limestone. The lowerand upper parts of sub-unit IIIc are middle Smithian and late Smithian

in age, respectively. Sub-unit IIId corresponds to 1 m of massive,highly nodular marly limestone of late Smithian age.

Unit IV of Early Spathian age comprises 4 m of shales subdividedinto a lower green part and an upper red part.

Unit V of Middle Spathian age is composed of 6 m of nodular, marlyand bioturbated limestones of “ammonitico rosso” facies.

Unit VI is one meter of thin-bedded yellowish nodular limestonesfollowed by less than 1 m of gray and yellowish limestone. The lastbeds contain a probably condensed ammonoid fauna of early andmiddle Anisian ages.

3. Materials and methods

Forty-six samples, distributed through the 35-m section, werestudied for ostracod analysis (Fig. 2). Ostracods are determined on thebasis of external and internal characters of the carapace and have to bereleased from the enclosing matrix. Because ostracod carapaces arecomposed of calcium carbonate and enclosed in calcareous rocks,extraction by means of acid is precluded. Samples processing isachieved by the ‘hot acetolysis’ technique allowing disaggregation ofdehydrated hard limestones and release of ostracod shells (Lethiersand Crasquin-Soleau, 1988; Crasquin-Soleau et al., 2005).

A lower Griesbachian fauna from Southwestern Tibet (Gyanyimasection, Burang area) has previously been described in Crasquin-Soleau et al. (2007a), yielding 14 species. The ostracod faunasdescribed here are the first ones recorded for Early and MiddleTriassic times in Tibet.

4. Results

4.1. Species richness

Ostracods were recovered from the interval comprised betweenthe top of subunit IIIa and the top of unit VI. Twenty nine samples outof 46 contained ostracods. However, six fossiliferous samples yieldedunidentifiable ostracod specimens (06Ti02, 06Ti05, 06Ti09, 06Ti10,06Ti16 and 06Ti24). Species richness and number of specimens foreach sample are given in Fig. 3a. A systematic description of the fauna(44 species belonging to 15 genera, including 7 new species and onenew genus) recovered in Tulong is presented in a separated paper(Forel and Crasquin, 2011).

Several samples from unit III are barren of ostracods (06Ti1, 06Ti3,06Ti12, 06Ti15 and 06Ti17). Productive samples belong to SubunitsIIIa and IIIb: when present, ostracod faunas recovered show lowabundance and low diversity. Peaks and drops both in numbers of

06Ti18b

06Ti17

06Ti16

06Ti13

06Ti09

06Ti05

06Ti22

06Ti35

06Ti32

06Ti27

06Ti18a

06Ti04

06Ti0306Ti0206Ti01

06Ti0606Ti0706Ti08

06Ti11

06Ti12

06Ti10

06Ti1406Ti15

06Ti19

06Ti20

06Ti21

06Ti23

06Ti24

06Ti2606Ti25

06Ti34

06Ti33

06Ti3106Ti30

06Ti29

06Ti28

TWB 35

TWB 4

TWB 9

TWB 25

TWB 32

Na 3Na 1

Na 12

TWB 14

TWB 22

Green shales

Red shales

Red dolomitic limestone

Grey limestone

ca. 50-100m

SAMPLES

Ow

enites

Claraia

Prionolobus

0

5

10

15

20

[m]

?

0

5

25

a

b

II

a

b

c

d

a

b

V

VI

IV

III

I

nai

hta

pS

nai

htim

SA

nis

ian

Die

n.

EA

RLY

TR

IAS

SIC

(T

ulo

ng

Fo

rma

tio

n)

M. T

R.

Grie

sb

ach

ian

PE

RM

IAN

Hellenites

Prionites

Wasatchites

Glyptophiceras

NordophicerasColumbites

Xenoceltites

arctoceratid A

arctoceratid B

GymnitesPseudodanubites

AMMONOID FAUNASUNITS

albanitid n. gen.

Fengshanites

Paragoceras

KulingShales

Gyronites

Pseudoceltites

Ostracods record presented on Fig.3

Shales

Shales with phosphatic nodules

Laminated limestone

Legend

Nodular, laminatedlimestone

Dolomitic limestone

Ammonoids

Orthoceratids

Bivalves

Crinoids

Brachiopods

Foraminifera

Gastropods

Ostracods

Stromatactis cavities

White, recrystallisedlimestone

Fish teeth

Fig. 2. Lithostratigraphy, microfossil content and samples positions (from Brühwiler et al., 2009, 2010a, submitted for publication).

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163M.-B. Forel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 308 (2011) 160–170

species and specimens are observed, reaching 4 species and 31specimens in sample 06Ti08.

Samples from subunit IVa, 06Ti18a and 06Ti18b, are barren. Theostracod record largely improves at the transition between units IVband V (concomitantly with a change from clastic to carbonatesediments). Species richness peaks with a maximum of 13 speciesin sample 06Ti21. This peak is followed by a sharp decrease untilsample 06Ti24, which is also barren.

In the coming discussions, it is noteworthy that units III and IVyielded too few species and specimens for a meaningful use ofstatistical tools. However, ostracod faunas are poorly known withinthis critical time interval and these newly recovered assemblages arenevertheless worth being described in detail. The upper part of units Vand VI show a rediversification, numbers of specimens and speciesreaching a second peak in sample 06Ti30, with 389 specimensbelonging to 11 species.

4.2. Ostracod assemblage composition

Ostracods discovered in Tulong belong to six families which varyalong the section as shown in Fig. 3b. The most common families areBairdiidae and Healdiidae, which constitute together between 43%and 100% of the assemblages. Bairdiidae species constitute alwaysmore than 20% of the faunas and Healdiidae species (genusHungarellaand Torohealdia?) are more than 10% when present. Cytheruridae(genus Citrella and Cytheropteron?) is the third most representedfamily, and occurs exclusively in the upper part of the section, fromsample 06Ti27 to 06Ti35.When present, species from this family formbetween 11 and 22% of the assemblages. Sigilloidea (genus Micro-cheilinella) species have been found in 4 samples from units V and VI(samples 06Ti21, 06Ti27, 06Ti28 and 06Ti33). When present, they arebetween 7 and 14% of the assemblages. Cytherissinellidae (genusCallicythere) are present in 4 samples from units V and VI (samples06Ti21, 06Ti22, 06Ti26 and 06Ti33). In those samples, they arebetween 7 and 17% of the species. One species belonging to thesuperfamily Cypridoidea (genus Spinocypris) has been found insample 06Ti27, where it is 11% of the assemblage. Finally, one speciesof the order Palaeocopida has been recovered in 5 samples (06Ti07,06Ti20, 06Ti31, 06Ti33 and 06Ti35) where it constitutes between 11and 100% of the assemblages.

5. Discussion

5.1. Preservation state

Epigenesis, recrystallisation, dissolution and crushing affect thepreservation of ostracods and therefore may alter their palaeobiolo-gical and palaeoecological significance. Convexity, thickness, archi-tecture of the carapaces play an important role in the preservationstate of ostracod shells (Lethiers, 1979). Except for several specimens,in most cases, the preservation is poor; specimens recovered presentrecrystallization zones and crushed parts. This observation did nothowever preclude determinations at the specific level. Moreover, allthe specimens found in Tulong area are represented by closedcarapaces, indicating limited transport and/or burial in soft substra-tum (Oertli, 1971).

Samples from Units III and IV yielded very few specimens andspecies, samples 06Ti15 to 06Ti19 being barren. It is not possible todetermine whether the barren parts lack ostracods because ofdiagenesis, preservation biases, initial absence/complete eradicationof ostracods populations by environmental events. However, accord-ing to Brühwiler et al. (2009, 2010a, b), unit III is highly fossiliferous,indicating that complete deletion through diagenesis or selectivepreservation of the faunas can be excluded.

5.2. Palaeobathymetric setting

All the ostracods found here are typical of intertropical warmwaters (Crasquin-Soleau et al., 1999, 2004a). Palaeoenvironmentalpreferences of Late Palaeozoic and Lower Triassic ostracod families(Peterson and Kaesler, 1980; Costanzo and Kaesler, 1987; Melnyk andMaddocks, 1988a, b; Crasquin-Soleau et al., 1999) encountered inTulong are summarized here:

a. Palaeocopids and thick shelled ornamented Bairdiidae: highenergy proximal zone.

b. Cytherissinellidae, Cytheruridae, and Healdiidae: median zonewith euryhaline environments in shallow to very shallow waters.

c. Smooth Bairdiidae: distal zone, open carbonate environments withnormal salinity.

The taxonomic diversity of each group is calculated and therespective percentages of these three groups all along the section aregiven in Fig. 3c and the list of corresponding species belonging to eachgroup is presented in Table 1. Palaeobathymetric setting is estimatedby the constitution of assemblages regarding those three groups. Eachbathymetrical zone is marked by relative dominance of thecorresponding group (in number of species), relative abundances ofthe other groups give precisions about the positioningwithin the zonedetermined by the dominant group.

The presence of smooth Bairdiidae in subunits IIIa, IIIb and base ofIIIc tends to indicate a distal setting, with environmental instabilitiesshown by the presence of proximal zone species.

Unit V is characterized by the relative dominance (more than50%) of distal zone species (except for samples 06Ti20, 06Ti26 and06Ti29), median zone species being never less than 20% (except for06Ti23) and proximal forms being always more than 30%, whenpresent. This indicates an outer shelf setting for the whole of unit V(except sample 06Ti24 yielding no ostracod). Furthermore, the baseof this unit, from samples 06Ti20 to 06Ti23, records an increase ofsmooth Bairdiidae, reaching 100% of the fauna in sample 06Ti23: thebase of unit V corresponds to a deep outer shelf setting. The top ofthe unit, from samples 06Ti25 to 06Ti29, presents relatively lowerproportions of smooth Bairdiidae, between 67 and 25% of thefaunas: the top of unit V corresponds to a shallow outer shelfsetting. Brühwiler et al. (2009) note the disappearance of neogon-dolellid conodonts between samples 06Ti25 and 06Ti26, possiblyindicating a return to shallower and/or warmer waters. All formsbeing typical of warm intertropical waters, this shallowing trend isrecognized in ostracod fauna by the relative diminution of outershelf forms between these two samples. The ostracod data from thebase of unit V are consistent with the interpretation by von Radet al. (1994) of a deep outer shelf plateau under well oxygenatedconditions.

In faunas found in unit VI, smooth Bairdiidae are still present butless dominant. This decrease of smooth Bairdiidae corresponds to arelative increase in proximal and median species. This trend indicatesa shallow outer shelf platform and follows the shallowing recorded inthe top of Unit V.

5.3. Palaeoenvironmental setting other than bathymetry

Ostracods communities found in Tulong can be separated into 2groups:

• Communities dominated by Bairdiidae.• Communities dominated by Healdiidae, mostly genus Hungarella.

Cytherissinellidae (genus Callicythere) are extremely under repre-sented and cannot be included in this reconstruction. The distributionof families is figured in Fig. 3b.

Healdiidae specimens are very abundant in units V and VI. Theirrelative abundance, in terms of specimens, is comprised between 15%

a

b

c

d

a

b

V

VI

IV

III

naisinA

nai

hta

pS

nai

htim

S

cissairT.di

MCI

SS

AIR

TY

LR

AE

Hellenites

NordophicerasColumbites

Brayarditescompressus

Gymnites

albanitid n. gen.

Fengshanites

Paragoceras

Tu

lon

g F

orm

ati

on

Greenshales

Redshales

0

5

10

15

m

06Ti18b

06Ti17

06Ti16

06Ti13

06Ti09

06Ti05

06Ti22

06Ti35

06Ti32

06Ti27

SAMPLESAMMONOID

FAUNASUNITS

06Ti18a

06Ti04

06Ti0306Ti0206Ti01

06Ti0606Ti0706Ti08

06Ti11

06Ti12

06Ti10

06Ti1406Ti15

06Ti19

06Ti20

06Ti21

06Ti23

06Ti24

06Ti2606Ti25

06Ti3406Ti33

06Ti3106Ti30

06Ti29

06Ti28

Na 12

Pseudoceltitesmultiplicatus

Nammalitespilatoides

Glyptophicerassinuatum

Pseudodanubites

Nyalamitesangustecostatus

Wasatchitesdistractus

80604020 1000

Ostracod families distribution (%)

b)

PalaeocopidsCypridoideaCytheruridae

MicrocheilinellidaeCytherissinellidae

Healdiidae

Bairdiidae

λ’ H'% of Healdiidae specimens

d)

80604020 100080604020 1000

Palaeobathymetric affinities (%)

% of proximal zone species

% of median zone species

% of distal zone species

c)Number of species

Number of specimens

a)

0 2 4 6 8 10 12 14

0 100 200 300 400

164M.-B.Forel

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Palaeogeography,Palaeoclimatology,Palaeoecology

308(2011)

160–170

Table 1List of species belonging to each palaeobathymetric setting.

Proximal zone species Median zone species Distal zone species

Bairdia jeancharlesi Forel, 2011b Callicythere subovata (Zheng, 1976) Acratia goemoeryi Kozur, 1970Bairdia letangea Forel, 2011b Callicythere cf. lysi Crasquin-Soleau, 2004 Acratia cf. symmetrica Hao, 1992Bairdia sp.F Callicythere cf. mazurensis (Styk, 1972) Acratia sp.ATriassicindivisa tibetinella Crasquin, 2011b Citrella ampelsbachensis (Kozur & Bolz, 1971) Acratia ? sp.B

Cytheropteron ? sp.A Bairdia anisica Kozur, 1970Hungarella tulongensis Crasquin, 2011b Bairdia anisicaforma Monostori, 1994Hungarella sp.A Bairdia balatonica Méhes, 1911Hungarella sp.C Bairdia fengshanensis Crasquin-Soleau, 2006Hungarella sp.D Bairdia finalyi (Méhes, 1911)Hungarella sp.E Bairdia cf. cassiana rotundidorsata Monostori, 1995Hungarella ? sp. Bairdia sp.CTorohealdia ? sp.A Bairdia ? sp.D

Bairdia sp.EBairdia cf. Bairdia(Urobairdia) sp.1 sensu Monostori, 1995Bairdiacypris anisica Kozur, 1971Bairdiacypris combea Forel, 2011bPetasobairdia collini Forel, 2011bUrobairdia ? sp.cf.Bythocypris sp.B sensu Bolz, 1971Liuzhinia guangxiensis Crasquin-Soleau, 2006Liuzhinia larmea Forel, 2011bLiuzhinia sp.BMicrocheilinella sp.AMicrocheilinella sp.BMicrocheilinella sp.CSpinocypris vulgaris Kozur, 1971

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(sample 06Ti21) and more than 80% (sample 06Ti26), with theexception of sample 06Ti23 yielding no specimen (Fig. 3d). Such afauna rich in Healdiidae is rare and allows new insights intopalaeoenvironmental conditions.

The modifications of ostracod assemblages are here studied interms of the Shannon diversity index H′ (Shannon andWeaver, 1949),defined as:

H′ = − ∑S

i=1pi ln pi

with

S total number of species.pi relative abundance of the ith species in the sample.

H′ is widely used in studies of diversity patterns and is relativelyinsensitive to sample size (Stirling and Wilsey, 2001). This indexallows deeper analysis of ostracod assemblages compared to speciesrichness (Fig. 3a), taking into account the number of specimens ofeach species. To further reduce the effect of sample size, we comparedH′ with another representative diversity measure, the Simpson indexof dominance λ (Simpson, 1949) whose high values traduce lowdiversity. To work with an index which values parallel the diversity(high values traducing high diversity), we use here λ′:

λ = ∑ p2i� �

λ′ = 1−λ = 1− p2i� �

H′ and λ′ have been calculated for all the samples yieldingostracods and are presented in parallel with percentages ofHealdiidae specimens in Fig. 3d. However, biases are hard to

Fig. 3. Lithostratigraphy and ostracod fauna evolutions in Tulong. Panel a: Evolution of numpercentage of ostracod families. Panel c: Evolution of the percentage of proximal, median aindices HVand λV.

estimate and make it statistically inappropriate to consider exactcalculated values of those indexes.Wewill here preferentially drawattention on the relative values and evolution trends of H′ and λ′.

Two general observations can be made from Fig. 3d:

• H′ and λ′ show similar trends.• H′ and λ′ are negatively correlated to the relative abundance ofHealdiidae: low number of Healdiidae specimens corresponds tohigh diversity index and vice versa.

Comparison of H′, λ′ and the percentage of Healdiidae indicatesthat thriving of Healdiidae occurred in low diversity assemblageswhereas communities dominated by Bairdiidae are characterizedby higher diversity. These variations of H′ and λ′ show deepmodifications of populations that do not respond to changingbathymetrical settings but to other environmental parameters.

Whatley (1990, 1991) established a direct relationship betweennutrition and respiration in certain post-Palaeozoic ostracods and ithas been used as a proxy for water oxygenation. Ostracods show twomodes of nutrition and respiration: deposit-feeding and filter-feeding.Filter-feeders create a permanent and enhanced water circulationover their ventral respiratory surface by means of their numerousbranchial plates. Filter-feeding ostracods have thus a comparativeadvantage to deposit-feeding forms when oxygenation is low. Thesetraits have been demonstrated to be relevant in the Palaeozoic(Lethiers and Whatley, 1994). They showed that dominance of filterfeeders correlates with falling levels of oxygen, and reciprocally.

Bairdiidae are inhabitants of normal marine settings, and morespecifically of well oxygenated waters (R. Maddocks, personalcommunication, 2010). As shown by the H′ and λ′ trends, com-munities documented in Tulong which are dominated by thisfamily record a normal marine setting without environmentalstress.

Healdiidae belong to Metacopina that became extinct duringToarcian (Lower Jurassic) (Boomer et al., 2008). The unknown soft

ber of species (species richness S) and number of specimens. Panel b: Evolution of thend distal zone species. Panel d: Evolutions of percentages of Healdiidae specimens, of

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parts of metacopine ostracods make it difficult to reconstruct theiranatomy and hence, to make inferences about their physiology andbiology. However, based on the similarity of morphologies of hardparts of Metacopina and Platycopina (Palaeozoic to Recent), Lethiersand Whatley (1994) postulated that metacopine were filter-feedingostracods. Assuming that Healdiidae are filter-feeding ostracods,periods and/or areas of their abundance can be interpreted as oxygenimpoverished settings. However, recent works question the modelsestablished by Whatley (1990, 1991) and Lethiers and Whatley(1994) (e.g. Swanson et al., 2005; Brandão, 2008; Brandão and Horne,2009; Horne et al., submitted). Moreover, the extinction of Metaco-pina during a period of widespread dysoxia is contradictory with thefundamentals of these models (Boomer et al., 2008). In this context,the relative abundance of Metacopina as an indicator of oxygencontent remains ambiguous.

For re-interpretation of theWhatley (1990, 1991) and Lethiers andWhatley (1994)models, Horne et al. (submitted) studied the ostracodfaunas of Cenomanian–Turonian Boundary. In the context of theiranalysis, they estimated sizes of particles on which Platycopida arelikely to feed according to the spacing between the main screen setaeand setules on the mandibulae and maxillulae of Cytherella rwhatleyiBrandão, 2008 and Keijcyoidea infralittoralis Tsukagoshi, Okada, andHorne, 2006. It appears that the platycopid filter screen is adapted tocollect nanoplankton (2–20 μm) and probably at least the largerpicoplankton (0.2–2 μm) (Fenchel, 1988) whereas deposit feeders donot have the filter screen to feed on such small particles. Becausemesotrophic waters are dominated by nano- and picoplankton andbecause mesotrophic/eutrophic conditions produce abundant micro-plankton (20–200 μm; Fenchel, 1988; Zohary and Robarts, 1992;Caron et al., 1999), Horne et al. (submitted) came to the conclusionthat the signal of filter-feeding ostracods could be related tooligotrophic waters. In Tulong, communities dominated by Healdiidae(Units V and VI) show structural modification by thriving ofHungarella specimens within few species, in accordance with lowerH′ and λ′. These populations seem to record oligotrophic setting at thebase of the water column. However, the proliferation of specimensamong few species of Hungarella traduces a relative biological stress.Beyond the consideration of feedingmodes, Cronin and Raymo (1997)yielded the only study available clearly mentioning the link betweenlong-term diversity variations of deep-sea benthic ostracods andnutrients. They consider that decreased or more variable food supplymay increase stress, increasing competition for limited resources. Ifthis interpretation is valid for neritic ostracods, it could be in goodagreement with the biological stress recorded by Spathian andAnisian assemblages of Tulong. The observed high abundance withinfew species would thus traduce their higher competitive potential. Sofar, the genus Hungarella (Healdiidae) has been studied mainly fromUpper Triassic faunas (e.g. Bolz, 1971; Urlichs, 1971; Neale, 1983;Crasquin-Soleau and Depeche, 1993) but no information on itsenvironmental requirements is available.

Horne et al. (submitted) also consider that the filter feeders signalcould be indicative of low oxygen conditions not in bottom waterswhere ostracods live but in the water column above. Brühwiler et al.(2009) state that neither ammonoids nor conodonts indicate a stressof the pelagic compartment in Tulong. The oligotrophic conditions ofbottomwaters were not caused by dysoxic conditions higher up in thewater column (Units V and VI).

5.4. Palaeobiogeographical and palaeoceanographical relationships

Ostracods are benthic microorganisms with benthic larvalstages: ostracods' palaeogeographical links can be interpreted interms of water masses circulation but also of properties of thesewater masses, notably constant salinity, temperature. The migra-tion over long distances of benthic ostracods may have taken placein constant environmental conditions on their route.

Several indices allow measurement of the similarity of faunas fromdifferent areas. These methods have been applied to fossil ostracods ofdifferent periods in several studies (e.g. Schudack, 1996; Schudack andSchudack, 1997; Crasquin-Soleau et al., 2001; Crasquin-Soleau et al.,2006b). We chose here to follow the choice of Crasquin-Soleau et al.(2001, 2006b) to use the Johnson index of provincialism (Johnson,1971), defined by:

PI = C= 2e

where:

C number of common species between compared arease number of endemic species in the area where they are fewer

This index allows the calculation of similarity of assemblages taken 2by 2 in terms of composition: the higher his value, the closer theassemblages.

Forty-five species have been recognized inTulong: 33 are endemic tothe Tulong section, 9 during the Smithian, 25during the Spathian and19during the Anisian. 11 are known from other areas and/or other ages(Fig. 4). Some of the species found are figured on Plate 1 (see moredetails in Forel and Crasquin, 2011). The known stratigraphicalextensions of these species are reported on Fig. 4. The occurrence inthe Tulong section of 10 of these previously described species allowsenlarging their longevity.

During Smithian, 2 species (Acratia goemoeryi and Bairdia finalyi)were recognized in other areas. The first one is known from the LateAnisian of Hungary (Kozur, 1970), the Induan–Early Ladinian ofGreenland, Himalaya, Austria, Slovakia, Hungary (Kozur, 1971) andthe Early Anisian of Romania (Crasquin-Soleau and Gradinaru, 1996).The only comparison point could be the work by Kozur (1971).Unfortunately, Kozur did not provide data on the number of species inthe different cited areas and did not refer to original data sources. Nopalaeobiogeographical index could be calculated.

For the Spathian, Tulong shares 5 common species (Bairdiafengshanensis, B. finalyi, Bairdiacypris anisica, Liuzhina guangxiensis andSpinocypris vulgaris) with Guangxi (Crasquin-Soleau et al., 2006a)which are the only data available on this time slice.

During Anisian, there are 3 species (Bairdia anisica, B. balatonica, andBairdiacypris anisica) in commonwith Germany, Hungary and Romania.

Table 2 summarizes Johnson indices calculated for Spathian andAnisian in Tulong. All the indices are very low but indicate thatcommunications exist along the margins of the Palaeo-Tethys andbetween Neo and Palaeo-Tethys (at least during the Spathian and theAnisian).

The apparent endemism of Tulong ostracods during the Smithiansuggests the isolation of this area. This observation is in accordancewithBrayard et al. (2009) who concluded that current activity was reducedwithin Tethys during Smithian time, with branches of the PanthalassicNorth Equatorial Current bathing north and south parts of Tethys.However, the low number of Smithian species (9) may also lead to anapparent endemism: more data are needed on Neo-Tethys faunasduring Smithian. The Spathian stage is regarded as a time of substantialoceanographic change with a relatively weak circulation bathing theNeo-Tethys (Fig. 6 in Brayard et al., 2009). Tibet was situated on thesouthern part of Neo-Tethys Ocean, so according to this view, bathed byhydrodynamically weak water masses. During the Early Triassic, theGuangxi Province was located on the eastern side of South China Blockand bathed by Panthalassic waters. The presence of species commonwith Guangxi indicates faunal exchanges between western PanthalassaandNeo-Tethys during the Spathian. Values of the Johnson index are thehighest during Spathian part of the Tulong record, thus recording anincreasingmixing of the ostracod faunas. For the Anisian, Table 2 showssimilarities between Tulong and Germany, Hungary and Romania, as

Bai

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Bai

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Norian

Rhaetian

Spathian

Smithian

Dienerian

Griesbachian

Changshingian

Capitanian

Late Triassic

Anisian

Ladinian

Permian

Early Triassic

Middle Triassic

Olenekian

Induan

Carnian

Acr

atia

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Fig. 4. Modified global stratigraphic distribution of already known species. Black squares: literature based distribution. Gray squares: new distribution including Tulong data.

167M.-B. Forel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 308 (2011) 160–170

well as with other Palaeo-Tethyan areas. This suggests exchangepathways between Palaeo-and Neo-Tethys at that time.

5.5. Ostracod recovery

The ostracod extinction and recovery global patterns through thePermian–Triassic boundary events have been established by Cras-quin-Soleau et al. (2007b). Among the fossil organisms documentedduring the Permian–Triassic interval, brachiopods are the closest toostracods in terms of mode of life. The comparison of patterns ofextinction/recovery described for brachiopods (Chen et al., 2005) andfor ostracods (Crasquin-Soleau et al., 2007b) shows that:

• The end of the extinction phase occurred during the latestChanghsingian, in bed 24e of Meishan section, Permian–TriassicGSSP (Crasquin et al., 2010).

• The presence of Palaeozoic survivors until the Spathian and EarlyAnisian in Tulong, shows that the turnover stage was longer forostracods than for brachiopods (Chen et al., 2005).

• Mesozoic forms are present in the Late Permian indicating that theturnover phase began earlier for ostracods than for brachiopods(Chen et al., 2005).

• The radiation stage seems to be coincident for ostracods andbrachiopods during Anisian.

The Lower Triassic ostracod record from the Tulong sectionconfirms previous studies showing that lower Triassic assemblagesare a mixture of Upper Palaeozoic and Lower Mesozoic representa-tives. Because Palaeozoic types are still found in the Early Anisian, ourdata are in accordance with the Crasquin-Soleau et al. (2007b)conclusion of the final turnover from Palaeozoic to Mesozoic faunasoccurring later, during the Late Anisian.

Palaeozoic forms are here found from the Smithian to the Anisian.Typical Palaeozoic genera found are:Acratia (A. cf. symmetricaHao, 1992in the Smithian, A. goemoeryi Kozur, 1970 in the Smithian and the

Spathian and A. sp.A in the Spathian) andMicrocheilinella (M. sp.A, sp.Bin the Spathian, M. sp.C in the Spathian and the Anisian). The typicalPalaeozoic Palaeocopida order is also found with the new genusTriassicindivisia Crasquin, 2011b. Mesozoic representatives are numer-ous and also found all along the section. The typical genera are: Liuzhinia(L. sp.A, sp.B and L. guangxiensis Crasquin-Soleau et al., 2006a),Hungarella (H. sp.A, sp.B, sp.C, and H. tulongensis Crasquin, 2011b),Torohealdia? (T. ? sp.A), Petasobairdia (P. collini Forel, 2011b), Calli-cythere (C. subovata Zheng, 1976, C. cf.mazurensis (Styk, 1972) and C. cf.lysi Crasquin-Soleau et al., 2004a, b), Cytheropteron? (C. ? sp.A),Spinocypris (S. vulgaris Kozur, 1971), Citrella (C. ampelsbachensis(Kozur and Bolz, 1971)).

The important thriving of Mesozoic filter-feeding ostracod speci-mens (Hungarella) seems to support the interpretation of anoligotrophic environment, however showing signs of biological stress.In Tulong, this characteristic allowed the important development ofMesozoic ostracods. These data are in accordance with the idea ofBoomer et al. (2008) that “Metacopina, or at least some of theirspecies, are opportunistic species capable of exploiting newly createdenvironments in the absence of significant competition”. The faunasfrom the Spathian and Anisian of Tulong record the transition of twostructurally different communities. This modification was allowed byshifting environment (eutrophy/mesotrophy to oligotrophy) notfavorable to the initial fauna (composed of Bairdiidae and welldiversified) but favorable to the new one (mainly composed ofHealdiidae and less diversified). The new environmental conditions,stressful for deposit-feeding ostracods, allowed not only the presencebut the thriving of filter-feeding forms, belonging to the Mesozoicfauna. In the context of the recovery following the end-Permianextinction, these new data show a double impact of conditions onostracods according to their feeding mode: deleterious on depositfeeders but favorable for filter-feeders.

The dynamic of ostracod diversity in Tulong can be confronted toother groups' data of Brühwiler et al. (2010b) about evolutionary trends

Plate 1. Ostracods from the Smithian–Anisian interval from the Tulong section, South Tibet. Scale bar is 100 μm. 1: Bairdia finalyi (Méhes, 1911): right lateral view, sample 06Ti11 2:Bairdia anisicaforma Monostori, 1994: right lateral view, sample 06Ti30 3: Bairdia balatonica Méhes, 1911: right lateral view, sample 06Ti35 4: Bairdia anisica Kozur, 1970: rightlateral view, sample 06Ti32 5: Bairdiacypris anisica Kozur, 1971: right lateral view, sample 06Ti21 6: Acratia goemoeryi Kozur, 1970: right lateral view, sample 06Ti28 7: Liuzhiniaguangxiensis Crasquin-Soleau, 2006a: right lateral view, 06Ti29 8: Liuzhinia sp.B: right lateral view, sample 06Ti27 9:Spinocypris vulgaris Kozur, 1971: right lateral view, sample06Ti27 10: Callicythere subovata Zheng, 1976: right lateral view, sample 06Ti22 11: Callicythere cf.mazurensis (Styk, 1972): right lateral view, sample 06Ti21 12:Microcheilinella sp.A:right lateral view, sample 06Ti21 13: Citrella ampelsbachensis (Kozur and Bolz, 1971): right lateral view, sample 06Ti31 14:Hungarella sp.A: right lateral view, sample 06Ti34 15:Hungarella sp.B: right lateral view, sample 06Ti21 16:Hungarella sp.C: right lateral view, sample 06Ti20.

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in Smithian ammonoids of the Northern Indian Margin. They recognizethree main features for ammonoids of Smithian age: radiation duringearly Smithian, a high turnover rate during the middle Smithian andextinction in the late Smithian. The samples used for ostracod analysis aremiddle and late Smithian in age. The very low abundance/diversityassemblages recovered, and the absence of ostracods at the Smithian/Spathian transition (Fig. 3a)maypossibly reflect the sameenvironmentalchanges that led to the end-Smithian ammonoid extinction (Brühwiler etal., 2010b). Moreover, the work of Galfetti et al. (2007) on the EarlyTriassic stratigraphy from the South China Block and the northern IndianMargin reveals a shift from suboxic conditions in the late Smithiantoward well-oxygenated waters from the Spathian onwards. Althoughammonoidsdonot bring informationabout thebaseof thewater column,the conclusions of Galfetti et al. (2007) are strikingly correlated with theabsence of ostracods from samples of late Smithian age and to theirimportant rediversification from the Spathian onwards in Tulong.Moreover, our observations are in accordance with Crasquin-Soleau etal. (2006a) and Galfetti et al. (2007) statement of significant increase inabundance anddiversity of themicrofauna (mainly ostracods). Galfetti etal. (2007) hypothesized that the major and global climatic change

Table 2Provincialism indices of ostracod faunas during Spathian and Anisian.

Germany Guangxi Hungary Romania

Tulong Anisian – – 0.11 0.02Spathian – 0.15 – 0.1

occurring near the Smithian–Spathian boundarywas probably linked to adrop of the atmospheric CO2 content.

6. Conclusions

(1) Ostracods are reported for the first time from the Early Triassic(Smithian and Spathian) and early middle Triassic of SouthTibet. When present, assemblages of Middle Smithian age areof low abundance and low diversity and no ostracods wererecovered from strata of late Smithian age. Assemblages ofSpathian and Anisian ages show important rediversification.The scarcity of specimens recovered from the Smithian to earlySpathian interval makes it difficult to assess local environmen-tal changes with confidence.

(2) Distal species (Bairdiidae) are always dominant indicatingoffshore environments. Relative proportions of proximal andmedian forms show a relatively shallow shelf setting in lateSpathian. Faunas from the beginning of the Anisian record ashallower water setting than those from the Spathian.

(3) The assemblages do not present the same palaeoecologicalcharacteristics all along the section. Healdiidae specimensconstitute an important part of Spathian and Anisian assem-blages, interpreted as indicating nutrients depletion at thewater–sediment surface.

(4) Significant faunal exchanges between southern Tibet and SouthChina took place during the Spathian. Anisian assemblagesshow similarities with Germany, Hungary and Romania,

169M.-B. Forel et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 308 (2011) 160–170

indicating the establishment of pathways and connectedsurface between Palaeo-Tethys and Neo-Tethys.

(5) Palaeozoic holdovers range as high as the early Anisian inTulong indicating that completion of the turnover betweenPalaeozoic and Mesozoic faunas took place later in the Anisian.Thriving of typical Mesozoic forms under oligotrophic envi-ronmental conditions shows the importance to consider therecovery of ostracods as a function of their mode of life andtype of respiration/nutrition.

Acknowledgments

The authors wish to thank Dr. Patrice Moix (Lausanne, Switzerland)for his assistance in the field. This work is part of the IGCP 572“Restoration of Marine Ecosystems following the Permian–Triassic MassExtinction: lessons for the present” and was funded by ATM (ActionsTransversales duMuseum) “Biodiversités actuelles et fossiles”. Fieldworkin southern Tibet was supported by the Swiss NSF (project200020_1277716 to H.B.). T.B., N.G. and H.B. thank Prof. Wan Xiaoquiao(University ofGeosciencesBeijing) forhishelpwith logistics.Weareverygrateful to thank Pr David J. Horne (Queen Mary University, London,UnitedKingdom) andan anonymous reviewerwhose remarks improvedan earlier version of this work. We thank Dr. Stephen Kershaw (BrunelUniversity, United Kingdom) for his constructing comments. We alsothank Martine Fordant (UPMC) for the processing of samples and thepreparation of ostracods and Alexandre Lethiers (UPMC) for thedrawings.

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