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The Cretaceous^Tertiary (K^T) mass extinction in plankticforaminifera at Elles I and El Melah, Tunisia
Narjess Karoui-Yaakoub a;*, Dalila Zaghbib-Turki a, Gerta Keller b
a Faculte des Sciences de Tunis, De¤partement de Ge¤ologie Campus Universitaire, 1060 Tunis, Tunisiab Geosciences, Princeton University, Princeton, NJ 08544, USA
Received 1 July 1999; accepted 9 August 2001
Abstract
Planktic foraminiferal faunas across the K^T transition at Elles and El Melah in northwestern and northeasternTunisia, respectively, reveal patterns of species extinctions and species survivorship similar to those found at the El Kefstratotype and the Ain Settara sections. Slightly more than 2/3 of the species disappeared at or before the K^Tboundary event and slightly less than 1/3 survived into the Danian where most disappeared sequentially within zone P1a(Parvularugoglobigerina eugubina). Relative species abundance patterns reveal that the 13^16 K^T survivors dominated(80%) the assemblages in the latest Maastrichtian, whereas the K^T extinct species were rare and totaled less than 20%of the total assemblages.
The K^T survivors are generally small with little surface ornamentation and geographically widespread from low tohigh latitudes. In contrast, K^T extinct species are large, highly ornamented and geographically restricted to lowlatitudes. This indicates that the K^T mass extinction was selective, rather than random, and predominantly affectedthe less robust tropical species. With the exception of the opportunistic Guembelitria species which dominate the earlyDanian, most K^T survivor species suffered severely as is evident by the decreased species populations after the K^Tevent. Their eventual demise appears to have been related to post-K^T environmental changes and competition fromevolving Tertiary species. These results reveal a complex mass extinction pattern that in addition to the K^T impactevent is keyed to long-term environmental changes preceding and following this event. ß 2002 Elsevier Science B.V.All rights reserved.
Keywords: K^T boundary; mass extinction; Tunisia
1. Introduction
The Cretaceous^Tertiary (K^T) boundary massextinction in planktic foraminifera is well knownfrom the El Kef stratotype (e.g. Smit, 1982;Brinkhuis and Zachariasse, 1988; Keller, 1988,
1995; Keller et al., 1995; Ben Abdelkader, 1995;Ben Abdelkader et al., 1997) which for the mostpart has de¢ned the extent of the biotic catastro-phe in low latitudes. This is largely because the ElKef section has remained the most expanded andcontinuous sediment record of this boundaryevent known to date worldwide (Keller, 1996).In recent years, continued research in Tunisiahas uncovered several new K^T boundary transi-tions, including the deeper water El Melah section
0031-0182 / 02 / $ ^ see front matter ß 2002 Elsevier Science B.V. All rights reserved.PII: S 0 0 3 1 - 0 1 8 2 ( 0 1 ) 0 0 3 9 8 - 4
* Corresponding author. Fax: +216-1-871-666.E-mail address: [email protected] (G. Keller).
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www.elsevier.com/locate/palaeo
in the northeast, the Elles and Ain Settara sec-tions to the northwest and north of the KasserineIsland which were deposited at middle to outerneritic depths, and the inner neritic Oued Seljasection south of the Kasserine Island and at theedge of the Sahara platform (e.g. Karoui et al.,1995; 1996; Keller et al., 1998; Molina et al.,1998; Dupuis et al., 1998; 2002; Karoui-Yaa-koub, 1999; Zaghbib-Turki et al., 2000; 2001)(Fig. 1).
We report here on the planktic foraminiferalturnovers of two of these sections: Elles and ElMelah. El Melah is unique in that it represents adeeper more open ocean environment than any ofthe other sections and Elles appears to have anexpanded K^T transition equal to the El Kef stra-totype. The propitious discovery and analysis ofthese new K^T transitions (by N.K.-Y.) comes ata time when the El Kef stratotype is itself endan-gered by oversampling and agricultural encroach-ment. Alternative and equally continuous K^Tboundary transitions, like Elles, are therefore anecessary and welcome addition to El Kef.
The Elles sequence is located 75 km southeastof El Kef in a valley near the hamlet of Elles(Fig. 1) where sediments spanning from Campa-nian to lower Eocene are continuously exposed.The K^T transition outcrops in numerous placesand can be traced over hundreds of meters alongthe slopes of the valley. Therefore, oversamplingis not a concern, unlike at El Kef, nor is there anydanger of obliteration due to agricultural en-croachment because of the steep valley slopes.For biostratigraphers interested in the K^T massextinction as well as environmental changes be-fore and after this event, the Elles section o¡ersthe necessary continuous outcrops that are largelymissing at the El Kef and other Tunisian sections.
The late Cretaceous to early Tertiary sequenceat Elles was ¢rst reported by Pervinquiere (1903)who noted the expanded Cretaceous^Tertiarysedimentary record, which was later con¢rmedby Said (1978) who documented the biostratigra-phy based on ostracods and planktic and benthicforaminifera. Said’s study was done at about 1 mintervals and consequently did not specify the K^T boundary transition. Karoui-Yaakoub (1999)and this study re-examined this section and lo-
cated the K^T transition based on the same litho-logical changes and planktic foraminiferal extinc-tions and evolutions as at the El Kef stratotype(see also Karoui and Zaghbib-Turki, 1998),whereas Robin et al. (1998) identi¢ed the K^Tspinel horizon and Rocchia (Zaghbib-Turki etal., 2000) identi¢ed the Ir anomaly. In addition,ostracods of the Elles section have been re-exam-ined in a recent study by Said-Benzarti (1998).
This study details the planktic foraminiferalbiostratigraphy, mass extinction and faunal turn-over across the K^T transition at Elles from twodi¡erent sample sets of the same outcrop which ishere labelled Elles I. The ¢rst sample set detailsthe species ranges from the latest Maastrichtianthrough the Danian. The second sample set wassubsequently taken for direct comparison with El
Fig. 1. Paleogeography of Tunisia during the late Maastricht-ian and early Tertiary with paleolocations of the K^Tboundary sections (modi¢ed after Burollet, 1967).
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Kef for the interval from 0.5 m below to 0.8 mabove the K^T boundary. This study shows thesimilarities between El Kef and Elles in speciesextinctions, survivorship and relative abundancechanges and generally con¢rms the pattern previ-ously revealed at El Kef (e.g. Keller et al., 1995)and also con¢rmed at Ain Settara (see Luciani,2002). In addition, we present the biostratigraphyand species ranges at El Melah.
2. Location and lithology
2.1. Elles section
The Elles section, located about 75 km south-east of the town of El Kef and near the hamlet ofElles, can be reached by the road from Maktar toSers. About 26 km along this road is a turno¡ toan unpaved road leading to the village of Elles.About 7 km along this road and prior to reachingthe village, there is a house on top of Campanianlimestones and behind it, the begin of a wide val-ley cut by the Karma river (usually dry). TheKarma valley is about 4 km long. Coming upthrough the valley (about 3 km), one passesthrough thick light gray late Campanian to earlyMaastrichtian limestones of the Abiod Formationwhich are rich in inoceramids, stegasters and ich-nofossils. This interval is followed by gray marlsinterspersed with about seven to eight 20 cm thickresistant marly limestone layers with fewer macro-fossils (e.g. Stegaster, Micraster and Holectypoids,see Li and Keller, 1998a; Zaghbib-Turki et al.,2000). Above this interval, gray marls of the ElHaria Formation continue for about 200 m wherethe valley forks. The right side fork has steepsides of gray marly shales with a few thin marlylimestone layers. The Elles I outcrop (Karoui-Yaakoub, 1999) is located in this valley about200 m past the fork on the left side wall. Addi-tional K^T outcrops, including Elles II (see Kelleret al., 2002) are located on the right side of theleft valley fork.
The K^T transition at Elles I is exposed on theleft side slope (of the right valley fork) about 12^14 m up from the valley £oor and marked by darkgray shales and clays. Near the base the slope is
covered by debris and only about 7^8 m are ex-posed below the K^T boundary. These latestMaastrichtian sediments consist of gray shales,marly shales and several thin resistant marly lime-stone layers between 4 and 7 m below the K^Tboundary. Although these resistant limestonelayers can be traced through both valley forks,the meter distance to the K^T boundary mayvary as a result of local erosion or topography.
The K^T transition is well marked by a 5^7 cmthick bioclastic packstone at Elles I. But the thick-ness of this bioclastic layer is laterally variableand may reach 25^30 cm in some outcrops, espe-cially in the valley to the left of the fork (e.g. EllesII, Keller et al., 2002). Above this bioclastic layeris a 0.5^1.0 cm thick clay layer followed by a 3^4mm thick rusty red layer associated with two thingypsum layers. This red layer marks the K^Tboundary event and contains altered spherules,spinels (enriched in Cr and Ni) and anomalousconcentrations of iridium (Robin et al., 1998;Zaghbib-Turki et al., 2000). Above this interval,the basal Danian consists of a 50^60 cm thickdark gray to black clay layer followed by 6 mof gradually lighter colored clayey shales andshales. Upsection, between 7.5 and 12 m grayshales are interbedded with several thin resistantmarly limestone layers followed by another 8 m ofmarly shales which grade into rhythmically inter-bedded limestone and marl layers.
2.2. El Melah section
The El Melah section is located in northeasternTunisia about 36 km west from the town of Ma-teur and 3 km southeast of the village of El Aoua-na. The section can be reached by taking the roadleading from Mateur to Sejnene. About 16 kmbefore Sejnene at a locality marked by the cross-ing of the Oued El Maleh river is a chalk quarry.From here onwards, the K^T section is reachedon foot by following the wide valley cut by theOued El Maleh river (rarely dry) through Campa-nian to early Maastrichtian light gray limestonesrich in megafossils followed by alternating graymarls and marly limestone layers (Abiod Forma-tion) similar to the Karma valley at Elles. Con-tinuing along the valley one traverses the gray
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marls of the El Haria Formation and after about2 km reaches the darker gray shales of the K^Tboundary transition. The K^T boundary horizonis marked by a thin rusty red layer similar to Ellesand El Kef. A dark organic-rich clay layer ofDanian age overlies the red layer. The 20 cmabove and below the K^T red layer are stronglybioturbated as evident by Thalassinoides burrows.Upsection, gray shales and marls alternate andthe ¢rst marly limestone is present at 1.80 mabove the rusty red layer that marks the K^Tboundary.
3. Methods
The Elles section was sampled at two di¡er-ent times. The ¢rst sample set (by N.K.-Y. andD.Z.-T.) shown here spans from 6 m below to 8 mabove the K^T boundary. Samples were taken atabout 25^30 cm intervals, except for the K^Ttransition from 1 m below to 2 m above theboundary where samples were taken at closer in-tervals of 5^10 cm and at 1^2 cm spacing acrossthe rusty red layer. The second sample set (G.K.)was taken during a subsequent visit when the sec-tion was trenched and resampled at 5 cm intervalsbetween 0.5 m below to 0.8 m above the K^Tboundary rusty red layer in order to obtain acomparable sample set to the El Kef stratotype(see Keller et al., 1995). The results of both Ellesstudies are described herein. The El Melah sectionwas generally sampled at 5^10 cm intervals ex-cept across the rusty red layer and organic-richclay where samples were taken at 2^5 cm inter-vals.
Laboratory procedures di¡ered somewhat forthe two sample sets. For the ¢rst study, sampleswere soaked in water and dilute hydrogen perox-ide and then washed through a 63 Wm screen. Thesame procedure was followed for the subsequentmore detailed K^T intervals at Elles and El Mel-ah, except that these samples were washedthrough a 38 Wm screen. The smaller size fractionwas used because previous studies have shownthat the earliest Danian species are often smallerthan 63 Wm (e.g. Keller, 1993; Keller et al., 1995).As a result, Danian species ranges, and conse-
quently the biozonations may di¡er dependingon the size fraction used.
For the ¢rst study the s 63 Wm size fraction ofeach sample was analyzed, a species census takenfor biostratigraphic analysis, determination ofspecies ranges and evaluation of the mass extinc-tion. The same method was followed for the ElMelah section. For the detailed K^T study at El-les and comparison with the El Kef stratotype,the s 38 Wm size fraction was quantitatively an-alyzed and relative abundances determined fromrepresentative sample splits of about 250^350specimens (Table 1).
Fig. 2. Planktic foraminiferal zonation of Keller et al. (1995)with some modi¢cations by N.K.-Y. and D.Z.-T. and com-parison with Berggren et al. (1995).
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Tab
le1
Rel
ativ
epe
rcen
tab
unda
nce
data
ofpl
ankt
icfo
ram
inif
era
acro
ssth
eK
^Tbo
unda
ryat
Elle
sI,
Tun
isia
(G.K
.da
tase
t)
Rel
ativ
epe
rcen
tab
unda
nces
ofpl
ankt
icfo
ram
inif
era
acro
ssth
eK
^Tbo
unda
ryat
Elle
sI,
Tun
isia
(s63
Wm
)
cmbe
low
K^T
boun
dary
cmab
ove
base
ofK
^Tbo
unda
rycl
ay
Bio
zone
sP
lum
mer
ita
hant
keni
noid
es(C
FI)
P0
(s63
Wm
)P
1a
Dep
th(c
m)
45^
5040
^45
35^
4030
^35
25^
3020
^25
15^
2010
^15
5^ 100^
50^
11^
33^
66^ 10
10^
1515
^20
20^
2525
^30
30^
3535
^40
40^
4545
^50
50^
5555
^60
60^
6565
^70
70^
75
Eog
lobi
geri
naed
ita
U1
UU
2U
1E
oglo
bige
rina
frin
gaU
11
UU
UU
11
E.
sim
plic
issi
ma
UU
UU
UU
2U
Par
vula
rugo
glob
iger
ina
eugu
bina
U1
U3
35
4
Par
vula
rugo
glob
iger
ina
long
iape
rtur
aU
1U
1U
23
1321
35
Glo
goco
nusa
conu
sa2
33
51
32
35
711
811
816
9G
.tr
ipar
tita
11
UU
UP
lano
rota
lites
plan
ocom
pres
sus
31
1
Woo
drin
gina
horn
erst
owne
nsis
U1
UU
Gue
mbe
litri
acr
etac
eaU
UU
UU
UU
UU
U2
3436
3129
3613
2820
2433
1930
2727
2116
Gue
mbe
litri
ada
nica
UU
U2
U5
814
17
33
310
99
35
6G
uem
belit
ria
irre
gula
ris
62
2123
141
11
27
87
63
95
Gue
mbe
litri
atr
ifol
iaU
1U
54
66
13
U3
312
103
78
UU
Glo
bige
rine
lloid
esas
pera
65
47
94
43
35
83
62
2U
44
93
33
43
32
Glo
bige
rine
llavo
lutu
sU
UU
UU
U1
23
UU
UU
G.
yauc
oens
is3
42
43
32
32
3U
UA
bath
omph
alus
may
aroe
nsis
U
Glo
botr
unca
naae
gypt
iaca
UU
UU
UU
UU
UU
U
G.
arca
UU
UU
UU
UU
UU
UG
.du
wi
UU
UU
UU
UG
.du
peub
lei
UU
UU
UU
UU
UU
G.
esne
hens
isU
UU
UU
UU
UU
UU
UG
.fa
lsos
tuar
tiU
UU
UG
.in
sign
isU
UU
UU
UU
UU
UG
.ro
sett
aU
UU
UU
UU
UU
UU
UG
lobo
trun
cani
taco
nica
UU
UU
UU
UU
UU
G.
pett
ersi
UU
UU
UU
UU
PALAEO 2757 1-5-02
N. Karoui-Yaakoub et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 178 (2002) 233^255 237
Tab
le1
(con
tinu
ed)
Rel
ativ
epe
rcen
tab
unda
nces
ofpl
ankt
icfo
ram
inif
era
acro
ssth
eK
^Tbo
unda
ryat
Elle
sI,
Tun
isia
(s63
Wm
)
cmbe
low
K^T
boun
dary
cmab
ove
base
ofK
^Tbo
unda
rycl
ay
Bio
zone
sP
lum
mer
ita
hant
keni
noid
es(C
FI)
P0
(s63
Wm
)P
1a
Dep
th(c
m)
45^
5040
^45
35^
4030
^35
25^
3020
^25
15^
2010
^15
5^ 100^
50^
11^
33^
66^ 10
10^
1515
^20
20^
2525
^30
30^
3535
^40
40^
4545
^50
50^
5555
^60
60^
6565
^70
70^
75
G.
stua
rti
UU
UU
UU
UU
UU
UG
.st
uart
ifor
mis
UU
UU
UU
UU
UU
Glo
botr
unca
nella
min
uta
UU
U1
UU
UU
UU
U
G.
peta
loid
eaU
1U
1U
UU
UU
2U
G.
subc
arin
atus
UU
2U
1U
UU
UU
UU
UU
U2
UU
Gub
leri
nacu
vilie
riU
G.
robu
sta
UU
UU
Hed
berg
ella
holm
dele
nsis
2U
22
23
32
12
22
22
U1
35
22
U2
1
H.
mon
mou
then
sis
33
23
33
41
11
52
32
22
33
77
32
31
1H
eter
ohel
ixco
mpl
anat
a2
22
31
49
55
2U
U1
UU
1U
U
Het
eroh
elix
dent
ata
1814
2424
1919
2322
2420
117
55
3U
19
1610
46
44
46
3H
.gl
abra
nsU
U1
UU
1U
UU
2H
.gl
obul
osa
44
24
93
39
46
192
2U
U1
14
1U
21
U1
UH
.m
orem
ani
U2
UU
UU
UU
UU
Het
eroh
elix
nava
rroe
nsis
1114
2410
1220
1811
1616
1228
2519
1524
6028
1833
1923
1821
198
13
H.
plan
ata
58
42
56
42
57
41
U1
U3
U2
UU
UU
UH
.pu
lchr
a2
UU
UU
1U
UU
UU
UU
3U
UU
H.
punc
tula
ta3
31
U1
U3
43
22
UU
H.
stri
ata
U2
1U
UU
U2
1U
Pla
nogl
obul
ina
braz
oens
isU
UU
UU
UU
UU
U
P.
cars
eyae
UU
U2
UU
11
UU
UP
.m
ulti
cam
erat
aU
UU
UU
UU
UU
UP
lum
mer
ita
hant
keni
noid
esU
UU
UU
UU
UU
U
P.
reic
heli
UU
UU
UU
UP
seud
ogue
mbe
lina
cost
ulat
a21
1916
1916
1712
1813
1726
47
32
17
42
21
1U
2U
2
P.
hari
aens
isU
UU
U3
UU
UU
UP
.ke
mpe
nsis
45
35
67
45
98
4U
3U
11
2U
UU
P.
palp
ebra
12
U2
UU
UU
UU
2U
PALAEO 2757 1-5-02
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Tab
le1
(con
tinu
ed)
Rel
ativ
epe
rcen
tab
unda
nces
ofpl
ankt
icfo
ram
inif
era
acro
ssth
eK
^Tbo
unda
ryat
Elle
sI,
Tun
isia
(s63
Wm
)
cmbe
low
K^T
boun
dary
cmab
ove
base
ofK
^Tbo
unda
rycl
ay
Bio
zone
sP
lum
mer
ita
hant
keni
noid
es(C
FI)
P0
(s63
Wm
)P
1a
Dep
th(c
m)
45^
5040
^45
35^
4030
^35
25^
3020
^25
15^
2010
^15
5^ 100^
50^
11^
33^
66^ 10
10^
1515
^20
20^
2525
^30
30^
3535
^40
40^
4545
^50
50^
5555
^60
60^
6565
^70
70^
75
Pse
udot
extu
lari
ade
form
isU
UU
UU
UU
UU
UU
U
P.
eleg
ans
UU
UU
UU
UU
UU
Rac
emig
uem
belin
afr
ucti
cosa
UU
UU
UU
UU
UU
R.
inte
rmed
iaU
UU
UU
UU
UU
R.
pow
elli
UU
UU
UU
UU
Ros
ita
cont
usa
UU
UU
UU
UU
UU
R.
plic
ata
UU
UU
UU
UU
UU
R.
wal
¢sch
ensi
sU
UU
Rug
oglo
bige
rina
hexa
cam
erat
a3
23
33
21
22
13
U
R.
mac
roce
phal
aU
UU
UU
UU
U1
1U
R.
rotu
ndat
aU
UU
UU
UU
UU
UU
R.
rugo
saU
2U
UU
1U
U3
21
UR
.sc
otti
UU
UU
UU
UU
UU
Sha
ckoi
nasp
.U
UZ
euvi
geri
nasp
.U
UU
UU
Tot
alnu
mbe
rco
unte
d35
633
338
834
740
839
133
741
042
937
437
056
346
139
334
013
578
202
333
412
376
369
414
296
263
324
275
63W
m,
exce
ptfo
rth
eP
0in
terv
al(0
^20
cmab
ove
the
K^T
boun
dary
)w
here
few
non-
rew
orke
dan
din
situ
spec
imen
sar
epr
esen
tan
dm
ost
ofth
efa
unal
asse
m-
blag
eis
inth
esm
alle
r38
^63W
msi
zefr
acti
on.
Not
eth
atU
=6
2%or
very
rare
spec
imen
sfo
und
inse
arch
ing
the
sedi
men
tre
sidu
esbe
yond
the
split
used
for
the
faun
alco
unt.
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4. Biostratigraphy
The biozonation of Keller et al. (1995) is usedin this study with some modi¢cations by N.K.-Y.and D.Z.-T. (e.g. combining zones P1b and P1c asParasubbotina pseudobulloides (P1b) zone and us-ing the Abathomphalus mayaroensis zone to markthe latest Maastrichtian, Fig. 2). The alternativebiozonation of Berggren et al. (1995) is shown forcomparison. Stratigraphically important speciesare illustrated in Plates I and II.
A new classi¢cation of Paleocene planktic fora-minifera was recently published based on wall tex-ture (Olsson et al., 1999). This study generallyfollows this new classi¢cation scheme and newgeneric assignments with exceptions as noted be-low.
We retain the species name Eoglobigerina fringafor the small four-chambered early Danian mor-photype which Olsson et al. (1999) now consideras part of a very widely varying Eoglobigerinaedita population. We also retain the name Parvu-
larugoglobigerina longiapertura for the very com-pressed ¢ve- to eight-chambered forms with longslit-like apertures which Olsson et al. (1999) in-clude within the Parvularugoglobigerina eugubinapopulation. In addition, Olsson et al. also consid-er all triserial species as variants of a very hetero-geneous population of Guembelitria cretacea. Weretain the species name Guembelitria irregularisfor morphotypes with irregularly stacked cham-bers, Guembelitria danica for morphotypes withhighly regularly arranged chambers and elongatespires and Guembelitria trifolia for the very shortspired forms. These morphologies are easily dis-tinguished from G. cretacea and their relativeabundance distributions suggest di¡erent ecologica⁄nities (see Keller et al., 2002, for further dis-cussion).
4.1. Plummerita hantkeninoides zone
This zone spans the range of P. hantkeninoidesand was originally de¢ned by Masters (1984,1993) and subsequently by Pardo et al. (1996).At Elles I the range of P. hantkeninoides spansthe last 7 m of the Maastrichtian and 10 m atElles II (located in the left fork of the valley) ascompared with 6 m at El Kef (Li and Keller,1998a). The range of this excellent latest Maas-trichtian marker species spans the last 300^400kyr of the Maastrichtian, or most of chron 29rbelow the K^T boundary, as estimated from thepaleomagnetic record at Agost (see Pardo et al.,1996; Groot et al., 1989). This species is easilyidenti¢ed by its long apical spines and is commonin Tunisian sections where its stratigraphic range(generally s 6 m) provides a good estimate of the
Plate I. All specimens from Elles I, Tunisia.
1^3 Abathomphalus mayaroensis (BOLLI)4 Rugoglobigerina reicheli (BROº NNIMANN)5 Plummerita hantkeninoides (BROº NNIMANN)6^7 Guembelitria cretacea (CUCHMAN)8 Guembelitria danica (HOFKER)9 Geumbelitria trifolia (MOROZOVA)10 Eoglobigerina moskvini (SHUTSKAY)11 Eoglobigerina fringa (MOROZOVA)12^14 Globoconusa conusa (CHALILOV)15 Eoglobigerina pentagona (MOROZOVA)
Previous taxonomy Revised taxonomy (Olssonet al., 1999)
Subbotina pseudobulloides Parasubbotinapseudobulloides
Subbotina varianta Parasubbotina variantaEoglobigerina trivialis Subbotina trivialisGloboconusa conusa Parvularugoglobigerina
extensaChiloguembelina waiparaensis Zeauvigerina waiparaensisPlanorotalites compressa Globanomalina compressaGlobigerina taurica Praemurica tauricaMorozovella inconstans Praemurica inconstansMorozovella trinidadensis jun.Synonym of
Praemurica inconstans
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completeness of the latest Maastrichtian interval.The P. hantkeninoides zone replaces the Abathom-phalus mayaroensis zone for the top part of theMaastrichtian.
4.2. Abathomphalus mayaroensis zone
The total range of A. mayaroensis has long beenused to de¢ne the late Maastrichtian. However, inrecent years there has been a shift towards replac-ing the top of this zone by the Plummerita hant-keninoides zone (Plate I). There are two majorreasons for this zonal change. Firstly, the rangeof A. mayaroensis is rather long (about 2 myr)and therefore the zone provides poor biostrati-graphic resolution to determine how complete,or incomplete, the late Maastrichtian is. For ex-ample, although intrazonal hiatuses may bepresent within the 2 myr interval of the range ofthe zonal index, this could not be determinedfrom the presence of the A. mayaroensis zone.But by identifying the short-ranging P. hantkeni-noides, it can be determined whether the top ofthe Maastrichtian is present. Secondly, it is wellknown that A. mayaroensis is generally morecommon in high latitudes and in deeper wateropen ocean environments, and rare or sporadi-cally present in low latitudes and shelf regions.For this reason, the ¢rst and last appearances ofA. mayaroensis are often diachronous and henceunreliable for global correlations (e.g. Masters,1984, 1993; Keller, 1988; Olsson and Liu, 1993;Pardo et al., 1996).
Nevertheless, in this study N.K.-Y. and D.Z.-T.prefer to use the last appearance of Abathompha-lus mayaroensis to mark the latest Maastrichtian,
rather than the Plummerita hantkeninoides zone,because they did not ¢nd P. hantkeninoides inthe last 10 cm below the red layer in their sampleset of Elles I, whereas A. mayaroensis was present(Fig. 3). However, P. hantkeninoides is present inthis interval in the second more closely spacedsample set of the same outcrop (Fig. 4), as wellas in the nearby Elles II outcrop. Besides the con-sistent and common occurrence of A. mayaroensisreported by N.K.-Y. and D.Z.-T. is very atypicaland possibly due to particular and intensivesearch of the species in the below K-T boundarysamples. In our sample set of the same outcrop,this species is only observed at 40^45 cm belowthe K^T boundary (Table 1). A similarly rare andsporadic occurrence of A. mayaroensis is observedin the nearby Elles II outcrop (see Keller et al.,2002), as well as at El Kef and Ain Settara. AtAin Settara, isolated occurrences of A. mayaroen-sis are reported at 14 m below the K^T boundaryand just below the K^T boundary (Dupuis et al.,1998; Molina et al., 1998; Luciani, 2002). At ElKef, this species is only reported as isolated oc-currence at 20 m below the K^T boundary (Liand Keller, 1998a,b,c).
4.3. K^T boundary and mass extinction
This boundary is de¢ned by the coincidence ofseveral characteristic lithological and geochemicalcriteria: a lithological change from marls or shalesto dark gray or black clay; a 2^4 mm thin rustyred layer at the base of the clay; the presence ofspherules, spinels and anomalous concentrationsof iridium and other plantinum group elementsin the rusty red layer. Paleontological criteria in-
Plate II. All specimens from Elles I, Tunisia.
1^4 Parvularugoglobigerina eugubina (LUTTERBACHER and PREMOLI SILVA)5 Chiloguembelina midwayensis (CUCHMAN)6 Chiloguembelina taurica (MOROZOVA)7^8 Praemurica inconstans (SUBBOTINA)9 Parasubbotina pseudobulloides (PLUMMER)10^11 Praemurica trinidadensis (BOLLI)12, 16 Subbotina triloculinoides (PLUMMER)13 Praemurica taurica (MOROZOVA)14^15 Globoconusa daubjergensis (BROº NNIMANN)
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clude the extinction of all ornate large tropicaland subtropical species, including all globotrunca-nids, racemiguembelinids and rugoglobigerinidsbelow the red layer and organic-rich clay layer.This extinction horizon is followed by the ¢rstappearance of Danian species at or near thebase of the organic-rich clay (e.g. Globoconusaconusa, Eoglobigerina fringa, Eoglobigerina edita,Eoglobigerina eobulloides, Woodringina horner-stownensis, Plates I and II; Keller et al., 1995).The survivors are Cretaceous ecological general-ists, characterized by small size, little or no testornamentation and wide geographic ranges. TheK^T boundary is marked by these criteria in boththe Elles I and El Melah sections, as also observedat El Kef and Ain Settara (Keller, 1988; Keller etal., 1995; Molina et al., 1998; Luciani, 2002).
4.4. P0 zone
This zone spans the part of the basal Danianorganic-rich black clay layer from the extinctionof the tropical^subtropical species group (at therusty red layer) to the ¢rst appearance of Parvu-larugoglobigerina eugubina and/or Parvularugoglo-bigerina longiapertura (Plate II). In many earlierstudies zone P0 was considered to span the organ-ic-rich clay layer (e.g. Smit, 1982; Keller, 1988;Olsson and Liu, 1993; Molina et al., 1998). How-ever, new studies based on the smaller 38^63 sizefraction suggest that P0 may be restricted to thelower part of this black clay layer. At El Melah,zone P0 is condensed, about 10 cm thick, andspans the dark clay interval. At Elles I, zone P0spans the clay layer (66 cm) in N.K.-Y.’s sampleset, but is restricted to the lower part of this in-terval (20^25 cm) in G.K.’s sample set due to thepresence of tiny P. eugubina and P. longiaperturain the smaller size fraction as discussed below.Zone P0 is characterized by abundant triserialspecies (Guembelitria cretacea, Guembelitria trifo-lia, Guembelitria danica, Plate I).
4.5. P1a zone
This range zone spans from the ¢rst appearanceof Parvularugoglobigerina eugubina and/or longia-pertura to the extinction of these taxa. At El Mel-ah, the ¢rst P. eugubina (lumped with P. longia-pertura) were observed 10 cm above the K^Tboundary red layer and zone P1a spans 140 cm(Fig. 3, Plate II). At Elles I, the P1a zone is 5.6 mthick and comparable to the 4.5 m observed at ElKef (Keller, 1988, Fig. 6). Zone P1a can be sub-divided into P1a(1) and P1a(2) based on the ¢rstappearance (FA) of Subbotina pseudobulloides(Fig. 2).
The ¢rst appearances of Parvularugoglobigerinaeugubina and/or Parvularugoglobigerina longiaper-tura di¡er in the two di¡erent sample sets anddi¡erent size fractions analyzed at Elles I. In the¢rst sample set (long section, Fig. 4), based onthe s 63 Wm size fraction, P. eugubina (lumpedwith P. longiapertura) ¢rst appears at 66 cm abovethe K^T boundary near the top of the organic-rich clay. But in the second sample set based onthe 38^63 Wm size fraction (Fig. 5), the ¢rst P.longiapertura and P. eugubina appear at 20^25cm and 40^45 cm, respectively, above the redlayer and K^T boundary. This suggests that inthe early evolutionary range P. longiaperturaand P. eugubina remained morphologically smallspecies (6 63 Wm), but generally increased in sizeabove the organic-rich clay layer. It further sug-gests that the origination of P. longiapertura mor-photype may precede that of P. eugubina, thoughthis was not observed at El Kef by Keller et al.(1995).
4.6. P1b zone
This zone marks the interval from the last ap-pearance of Parvularugoglobigerina eugubina tothe ¢rst appearance of Parasubbotina varianta(Keller et al., 1995). At El Melah, P. varianta is¢rst observed at 250^300 cm above the K^Tboundary and zone P1b spans 100 cm (Fig. 3).At Elles I, the ¢rst appearance of P. varianta isnearly coincident with the disappearance of P.eugubina which is marked by a major faunal turn-over just below a limestone layer (Figs. 3, 6). A
Fig. 3. Planktic foraminiferal species ranges across the K^Ttransition at El Melah based on the s 63 Wm size fraction.
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Fig.
4.P
lankticforam
iniferalspecies
rangesacross
theK
^Ttransition
atE
llesI
basedon
thes
63Wm
sizefraction
analyzedby
N.K
.-Y.
Note
thedi¡
erencesbe-
tween
thespecies
censusdata
ofthe
two
separatesam
plesets
inF
ig.6A
andB
.
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Fig.
5.P
lankticforam
iniferalspecies
rangesacross
theK
^Ttransition
atE
llesI
basedon
thes
38Wm
sizefraction
anda
di¡erent
sample
setand
analyzedby
G.K
.N
otethe
di¡erences
between
speciesranges
andspecies
censusdata
arepartly
attributableto
di¡erences
insam
plingprocedures,
laboratoryand
analyticaltechniques
(e.g.size
fractionanalyzed),
reworking
andbioturbation
(possiblesam
plingof
burrows),
andtaxonom
icview
s(e.g.
identi¢cationas
well
aslum
pingor
splittingof
species).See
textfor
discussion.
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Fig.
6.(A
)R
elativespecies
abundancesof
theindigenous
Cretaceous
plankticforam
iniferalacross
theK
^Tboundary
atE
llesI
inthe
s63
Wmsize
fraction.N
otethat
allof
thesespecies,
which
areconsidered
extinctat
ornear
theK
^Tboundary,
aretropical
tosubtropical
andrare
tofew
oronly
sporadicallypresent
nearthe
endof
theM
aastrichtian.T
heircom
binedtotal
abundanceis
lessthan
20%of
theC
retaceousassem
blage.T
hus,the
K^T
boundarym
assextinction
selectivelyelim
inatedthese
subtropicalto
tropicalecological
specialists.(B
)R
elativespecies
abundancesof
Cretaceous
survivorsand
evolvingearly
Tertiary
plankticforam
ini-fera
inlatest
Maastrichtian
andearly
Danian
sediments
atE
llesI.
Faunal
countsare
basedon
thes
63Wm
sizefraction,
exceptfor
theP
0interval
where
most
ofthe
speciesare
dwarfed
andpresent
onlyin
thesm
aller38^63
Wmsize
fraction.N
otethe
unusuallyhigh
abundanceof
biserialspecies
inthe
earlyD
anianis
consid-ered
reworked.
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Fig.
6.(continued).
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similar faunal turnover is observed at Ain Settarawhere Molina et al. (1998) also note the FA of P.varianta near the extinction of P. eugubina andnearly coincident with the simultaneous appear-ance of seven new species and disappearance ofsix. At El Kef, a major faunal turnover also co-incides with the extinction of P. eugubina, in ad-dition to an abrupt increase in the size of Danianspecies (Keller, 1988). This suggests that zone P1bis condensed or missing in these sections.
Though earlier studies of El Kef and Ain Set-tara considered the early Danian interval to becomplete, the abrupt faunal turnover marked byseveral species extinctions (e.g. Parvularugoglobi-gerina eugubina, Parvularugoglobigerina longiaper-tura, Globoconusa conusa) and originations (e.g.Globanomalina planocompressa, Parasubbotinavarianta, Subbotina trivialis, S. triloculinoides, Glo-boconusa daubjergensis, Praemurica inconstans,Plates I and II), the abrupt decline in species pop-ulations, and the abrupt change to large morpho-types (s 150 Wm, see ¢g. 6 in Keller, 1988) allsuggest the presence of a hiatus. Hiatuses arecommon in Danian sections at the P1a/P1b inter-val as noted in sections worldwide (MacLeod andKeller, 1991). In Tunisian sections, this hiatus isapparently also present, though the stratigraphicextent may vary depending on paleodepth, topog-raphy and currents. For example, the shallow Sa-hara platform section at Oued Selja has a hiatusthat spans from P1c(1) to near the top of P1a(Keller et al., 1998). This errosive event may ex-press itself as a short hiatus or condensed intervalat the Ain Settara, Elles and El Kef localities tothe north.
4.7. P1c zone
This zone spans the interval from the ¢rst ap-pearance of Parasubbotina varianta to the ¢rst ap-pearance of Praemurica trinidadensis. At Elles thisinterval spans about 10 m (Fig. 6), whereas at ElMelah zone P1c spans 5 m (Figs. 3, 4). Zone P1ccan be subdivided based on the ¢rst occurrence ofPraemurica inconstans which at Elles and AinSettara nearly coincides with the extinction of Par-vularugoglobigerina eugubina. As noted above themajor faunal turnover at this interval suggests a
hiatus with P1b and the lower part of P1c(1) miss-ing.
Alternative zone P1b: N.K.-Y. and D.Z.-T.prefer to use a Parasubbotina pseudobulloides(P1b) zone to mark the interval from the extinc-tion of Parvularugoglobigerina eugubina to the¢rst appearance of Praemurica trinidadensis ; thiszone is equivalent to zones P1b and P1c of Kelleret al. (1995) (Fig. 2).
5. Analytical reproducibility
Can faunal and biostratigraphic analyses of oneworker be reproduced reliably by another work-er? Perhaps most of the controversies surroundingthe K^T boundary mass extinction in plankticforaminifera, or indeed in all fossil groups, maybe resolved if uniform species concepts and thesame analytical methods could be employed byall workers. Unfortunately, this is still an unreal-ized dream among paleontologists. As a result,the data set of one worker is generally di⁄cultto reproduce by another worker. Nevertheless,there is signi¢cant progress with very similar re-sults obtained when the same analytical tech-niques and species concepts are employed (e.g.compare Keller et al., 1995, 2002, with Luciani,1997, 2002).
The El Kef foraminiferal blind test was de-signed to test the reproducibility of planktic for-aminiferal species census data and thereby resolvethe continuing controversy regarding the mass ex-tinction pattern. This test failed. It was initiatedduring the Snowbird II Conference (1988) to re-solve a disagreement between Smit and Keller re-garding the mass extinction pattern at El Kef inparticular. Smit (1982, 1990) argued that plankticforaminifera exhibit a pattern of abrupt and si-multaneous disappearance for all but one species(Guembelitria cretacea) at the K^T boundary withno reductions of species or species abundancesbelow the boundary. He interpreted this extinc-tion pattern as indicative of a geologically instan-taneous mass kill e¡ect as a result of a bolideimpact. Keller (1988) and Keller et al. (1995) re-ported local disappearances of about 10% of theCretaceous species within the last 50 cm below the
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K^T boundary and about 30% of the Cretaceousspecies surviving into the Danian. She interpretedthis progressive mass extinction pattern as the re-sult of late Maastrichtian environmental changeswith accelerated extinctions of tropical species atthe K^T boundary possibly due to a bolide im-pact and the survivorship of ecological generalistsinto the Danian. Many recent publications havesupported the Keller scenario of survivorship,though the percentage of species extinctions be-fore the K^T boundary remains a controversialissue (e.g. Luciani, 1997; Apellaniz et al., 1997;Molina et al., 1998). The blind test failed to gen-erate closely comparable results of replicate sam-ples and hence failed to resolve the controversy(see Canudo, 1997; Masters, 1997; Olsson, 1997;Smit and Nederbragt, 1997; Keller, 1997; Mac-Leod, 1996a,b; 1998).
MacLeod (1998) (p. 43) noted that although theblind test failed to resolve the controversy, it high-lighted the need for studies that address the issueof paleontological data reproducibility at both theempirical and theoretical levels. He noted thatpaleontologists must either improve the reproduc-ibility of systematic results, or develop methods toincorporate relatively high levels of uncertainty inboth qualitative and quantitative analyses. Weagree with this assessment, but note that the ma-jor problems may well be due to systematics andthe di¡erent conceptual approaches betweenlumpers and splitters. This report o¡ers anotherexample of this problem.
In this study, N.K.-Y. with D.Z.-T. and G.K.independently analyzed two di¡erent sample setsof the same Elles I section for a narrow intervalspanning the K^T boundary (Figs. 4, 5). Themethods employed di¡er (species census vs. quan-titative analyses of assemblages) and species con-cepts also di¡er for a number of species. There-fore, the results vary in detail, but are comparablein general. For example, N.K.-Y. identi¢ed 56Cretaceous species at Elles I (Fig. 4). 76% (43species) of these are in common with G.K.’s spe-cies list of the same section, but from a di¡erentsample set (Fig. 5). The species which are notrepresented in G.K.’s study are generally includedin closely related species morphologies and can beattributed to the lumper-splitter e¡ect (e.g. Clavi-
hedbergella^Shackoina, R. milamensis^R. rotunda-ta^R. penny^R. robusta, G. orientalis^G. arca,G. acuta^G. robusta, Globigerinella volutus^Globi-gerinelloides aspera, P. nuttali^Pseudotextulariadeformis^P. riograndensis^Planoglobulina brazoen-sis, P. excolata^Pseudoguembelina costulata,G. havanensis^G. petaloidea), and four were notobserved (P. glabrata, P. cornuta, P. acervuli-noides, G. mariei).
G.K. identi¢ed 61 Cretaceous species for thesame section (di¡erent sample set). 66% (41 spe-cies) of these are also present in N.K.-Y.’s study.Of the 20 species which are not present inN.K.-Y.’s study, all but one species (Zeuvigerina),are probably lumped with other species and hencecan be attributed to the lumper-splitter e¡ect (e.g.Globigerinelloides volutus^G. yaucoensis^Globige-rinelloides aspera, G. subcarinatus^G. petaloidea,Heterohelix dentata^H. globulosa^H. moremani,G. stuartiformis^G. falsostuarti^G. stuarti, Plano-globulina brazoensis^P. riograndensis, H. gla-brans^H. pulchra^Heterohelix complanata, G.arca^G. orientalis^G. esnehensis, Pseudotextulariadeformis^P. nuttali, Shackoina^Clavihedbergella).
Because of the lumper-splitter dichotomy, ahigh degree of reproducibility between workersis only possible if they share the same taxonomicconcepts. Since this is frequently not the case,analytical reproducibility even in the same sectionhas very high variability as seen in the example ofthe Elles I section. Therefore, one would assumethat the reproducibility would be better, even be-tween di¡erent sections and regardless of the spe-ci¢c worker, as long as the same taxonomic con-cepts are used. For example, a study by Luciani(1997, 2002) of the Erto section in Italy and AinSettara section in Tunisia shows very similar re-sults to those earlier obtained from the El Kefand Elles sections by Keller et al. (1995, 2002).This suggests that these workers have indepen-dently used the same species concepts based solelyon comparisons of published illustrations.
Many factors in£uence the reproducibility ofspecies range data, including ¢eld sampling tech-niques (e.g. sample spacing, size of samples, sam-ple contamination at outcrops), methods of sam-ple processing (e.g. mechanical breakage, acidtreatment causing dissolution, sieve size used),
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sample analysis (size fraction analyzed, time spentsearching for rare species, qualitative vs. quanti-tative analysis, problems in recognizing reworkedspecies), systematics (species concepts of outlierspecies) and ecological variations in species distri-butions. Given all these variables, it is perhapsmore surprising that the mass extinction patternis so consistent, than that there are still some ar-guments over the ¢ner points of the mass extinc-tion pattern.
6. K^T faunal turnover
The K^T mass extinction pattern in plankticforaminifera at Elles I analyzed for the two sam-ple sets di¡ers in some details. For example, theN.K.-Y. sample set (Fig. 4) shows an unusualpresence of many large tropical species abovethe K^T boundary (zones P0 and P1a(1)), whichare known to be extinct at or near the boundary(P. multicamerata, P. palpebra, Rugoglobigerinahexacamerata, R. scotti, P. riograndoensis, P. ele-gans, P. nutalli, G. aygyptiaca, Racemiguembelinafructicosa, R. rugosa, P. kempensis). Their pres-ence is likely a result of reworking and bioturba-tion as suggested by discolored, abraided and bro-ken tests, as well as the common presence ofThalassinoides burrows above and below the redlayer. In the second sample set where burrowswere carefully avoided wherever possible (Fig. 5),only a few of these tropical species are present atthe base of P0 and are attributed to reworking.Though P. kempensis and Pseudoguembelina cos-tulata are consistently present in the early Danianat Elles I as well as the nearby Elles II and AinSettara sections (Keller et al., 2002; Luciani,2002). Thus these species may also have survivedthe K^T event.
The overall extinction patterns of both Elles Istudies are similar to those observed in other Tu-nisian sections. Some species (V16%) are veryrare or sporadically present and not observed torange up to the K^T boundary. Their true extinc-tion level can not be determined. This is especiallyevident in that the species with early disappearan-ces di¡er between the two sample sets analyzed(Figs. 4, 5). For many species, single isolated oc-
currences are reported within the last 10 cm belowthe K^T boundary, an interval that consists of aforaminiferal packstone. At Elles II, where thisforaminiferal packstone is about 25^30 cm thick,cross-bedding is apparent and indicates a trans-ported and probably reworked older assemblage.For this reason, no conclusions can be drawn re-garding the extinction level of species that areonly sporadically present below this foraminiferalpackstone.
Nevertheless, the overall mass extinction pat-tern at Elles I is similar to that observed at ElKef (Keller et al., 1995), Ain Settara (Luciani,2002), Mexico (Keller et al., 1994; Lopez-Olivaand Keller, 1996; Keller, 1996), Italy (Luciani,1997) and other localities with tropical to sub-tropical planktic foraminiferal assemblages.Nearly 2/3 of the species disappeared at or nearthe K^T boundary and about 1/3 of the speciessurvived into the early Danian. The K^T extinctspecies group (which here includes species whichare rare and appear to disappear earlier) consistsof ecological specialists which includes all tropicaland subtropical species. These ecological special-ists are generally characterized by highly orna-mented, large multiserial or keeled morphologies.Their combined relative abundance is less than20% of the total planktic foraminiferal assemblag-es (Fig. 6A,B). In contrast, the K^T survivorgroup consists of ecological generalists. They arecharacterized by small biserial, triserial, trochospi-ral or planispiral morphologies with little surfaceornamentation. Ecological generalists dominatethe latest Maastrichtian oceans and their com-bined relative abundance may exceed 80%(Fig. 6B).
Above the K^T boundary, triserial species(Guembelitria) dominate (V80^90%) in the small38^63 Wm size fraction, but are relatively few(6 10%) in the larger s 63 Wm size fraction.This is likely a result of the high stress post-K^T boundary environment (MacLeod, 1993; Mac-Leod and Keller, 1994; MacLeod et al., 2000).Curiously, Cretaceous survivor species are rathercommon in zone P1a above the K^T boundary,especially the relative abundance of Heterohelixnavarroensis which appears to increase. In fact, apeak of the latter species reaches 60% at the P0/
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P1a boundary (Fig. 6B). This is very unusual. Ingeneral, survivor species strongly decrease in theearly Danian zones P0 and P1a in all Tunisiansections, including at Elles II which was collectedonly 200 m distant from Elles I (see Keller et al.,2002). The anomalous abundance of small Creta-ceous survivors at Elles I can therefore not beattributed to di¡erential local ecological condi-tions, but is likely due to locally abundant re-worked sediments as also suggested by the com-mon presence of obviously reworked largetropical species (Fig. 4).
7. Discussion and conclusions
The progressive extinction pattern at Elles I isoverall similar to that observed previously for thesame interval at outcrops at El Kef and Ain Set-tara (Keller, 1988, 1996; Keller et al., 1995; Mo-lina et al., 1998; Luciani, 2002), as well as insections in Israel, Spain, Italy, Lattengebirge,Mexico and Brazil (Canudo et al., 1991; Kellerand Benjamini, 1991; Peryt et al., 1993; Kelleret al., 1993, 1994; Lopez-Oliva and Keller,1996; Abramovich et al., 1998; Luciani, 1997;Apellaniz et al., 1997). Although in each of theselocalities the overall extinction pattern of special-ists and survivorship of generalists is similar, thereare enough di¡erences to fuel a continuing con-troversy about the nature of the K^T mass extinc-tion, whether progressive or instantaneous. Onepart of the controversy centers over whether thereare pre-K^T extinctions that foreshadow stressfulenvironmental changes during the latest Maas-trichtian, or whether a bolide impact at the K^Tboundary was solely responsible for the mass ex-tinction. The other part centers over the natureand number of Cretaceous species survivorship.
These arguments are unlikely to be solvedbased on a narrow interval spanning the K^Tboundary. Most K^T studies have concentratedon an interval of 50^100 cm below and abovethe boundary, or included at most the topmostfew meters of the Maastrichtian, and few haveexamined environmental and faunal changes dur-ing the entire late Maastrichtian. However, stableisotope studies demonstrate profound climatic
changes during the last 500 kyr of the Maastricht-ian (Stott and Kennett, 1990; Barrera and Keller,1990, 1994; Barrera, 1994; Li and Keller,1998a,b). High resolution studies are needed todetermine the nature of the progressive biotic ef-fects associated with climatic and environmentalchanges before the K^T boundary event.
Acknowledgements
We gratefully acknowledge Dr. N. Ben Haj Ali,Director of the Geological Survey of Tunisia,Tunis, and Dr. H. Ben Salem, for their tremendouslogistical and transportation support for the fieldexcursion and Workshop of May 1998 on TunisianK^T boundary sections. Fieldwork for the Ellesand El Melah sections was a joint effort withThierry Adatte and Wolfgang Stinnesbeck and wegratefully acknowledge their collaboration. Wethank H.P. Luterbacher and one anonymousreviewer for their comments and suggestions. Thisstudy was supported by NSF-INT 95-04309.
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