PAL 0
ELSEVIER
Palaeogeography, Palaeoclimatology, Palaeoecology 137 ( 1998 ) 127 151
Bivalve provinces in the Proto-Atlantic and along the southern
margin of the Tethys in the Jurassic
Chunlian Liu, Michael Heinze, Franz
T. F a r s i c h *
hTstitut /iTr Paldontologie der Universit6t Wiirzburg, Pleicherwall 1. D-97070 Wiir=burg. Germany
Received 29 August 1996: accepted 5 June 1997
Abstract
Using multivariate methods, the distribution pattern of Jurassic bivalve genera and subgenera within the ProtoAtlantic and along the southern margin of the Tethys is analysed based on an extensive literature survey and
examination of field collections. Four bivalve provinces are recognised within the Proto-Atlantic: (1) the Boreal
Bivalve Province in the north: (2) the Northern Transitional Province: (3) the Southern Transitional Province: and
(4) the Mediterranean Bivalve Province. The former two provinces are assigned to the Boreal Faunal Realm, the
latter two to the Tethyan Faunal Realm. The southwestern margin of the Tethys belongs to the Mediterranean Bivalve
Province, the southeastern margin to the Ethiopian Bivalve Province. The latter is only poorly defined by bivalve
genera/subgenera: in particular, its northern boundary is blurred, with the Arabian region occupying an intermediate
position between the Mediterranean and the Ethiopian bivalve provinces. Province boundaries based on bivalves do
not always coincide with those based on ammonites. The reason for this is probably different modes of life: ammonites,
as a rule, being active swimmers and bivalves being substrate-related and subjected to passive dispersal during the
larval stage. Diversity is not suitable to characterize bivalve provinces, due to differences in outcrop size, preservation
potential and sampling history, and due to differences in the degree of environmental heterogeneity within and
between regions. Migration patterns of Jurassic bivalves either reflect changes in the climatic condition (e.g. the
northward spread of Tethyan taxa in the late Early Jurassic and Middle Jurassic in the course of amelioration of the
climate) or prevailing current patterns. The origin of faunal provinciality is thought to be complex and a result of the
interplay of several factors, the importance of which varied throughout the Jurassic. © 1998 Elsevier Science B.V.
Keywords. palaeobiogeography; Jurassic: Bivalvia; Tethys; Proto-Atlantic; cluster analysis
I. Introduction
P r o v i n c i a l i t y in Jurassic m a r i n e f a u n a s is well
k n o w n . First d e m o n s t r a t e d by the d i s t r i b u t i o n
p a t t e r n o f a m m o n i t e s within N o r t h A t l a n t i c to
M e d i t e r r a n e a n areas by N e u m a y r (1883), such
* Corresponding author. Fax: +49 931 57705.
E-mail: ffanz'fuersich@rzr°e'uni-wuerzburg'de
0031-0182/98/$19.00 ,<) 1998 Elsevier Science B.V. All rights reserved.
PII S0031-0182(97)00096-5
p r o v i n c i a l i t y has since been recognised on a global
scale, with the r e c o g n i t i o n o f several faunal realms
a n d provinces (e.g. Arkell, 1956; H a l l a m , 1969,
1994; C a r i o u , 1973; Enay, 1980; W e s t e r m a n n ,
1981: Doyle, 1987; Riccardi, 1991; see Table 1 for
the P r o t o - A t l a n t i c ) . It was H a l l a m ( H a l l a m , 1977,
1983) who first e x p l o r e d in greater detail the extent
benthic bivalves m i r r o r e d the d i s t r i b u t i o n p a t t e r n
o f nektic a m m o n i t e s . H e recognized four bivalve
128
( i L m ~'r a~
Pa/~t~'(<,~'o,,,,ra/d~. Po/ac'(~/imafolok, y. P,t/dco~.'~o/o.q,l' 137 / 19~,%'/ 127 151
Table I
Faunal realms ~llld ptovillces in lhe Prolo-Atlantic based Oll
~.tll~nlOniles and bixal\cs
Realms
based on
anlmonitcs
Proxinces
based on
anlnlollites
Provinces
b~tsed on
bi~alxc>
Realms
based on
bikalvcs
Boreal
Bore~d
Subboreal
Boreal
Northern
Wvansitiomd
Boreal
Tcthynn
Submcditevvancan
Southern
Transitional
Mediterranean
"l'eth~nn
Mediterranean
set, we demonstrate that the origin of faunal
provincialily in the Jurassic cannot be related to
just a single factor but is the result of a complex
interplay of factors, the importance of which varied
throughout the Jurassic. In addition, by extending
Liu's ( Liu, 1995 ) study from the Proto-Atlantic to
the southern margins of the Tethyan Ocean we
unravel the relationship of Jurassic biwdves across
this ocean and assess the importance of the Tethys
as a physical barrier to benthic organisms.
2. Data base
provinces ill the Early Jurassic: ( 1 ) the European
Province: (2) tile East Asian Province: (3) tile
Southwest Pacific Province" and (4) the West
American
Province, supplemented
by the
Ethiopian Province from tile Mid Jurassic
onwards. Subsequent work on the palaeobiogeography of Jurassic biwdves dealt largely with the
Pacific or Arctic regions (e.g. Hayami. 1984. 1987,
1989: Crame, 1986, 1987, 1991: Damborenea,
1993) or discussed the distribution pattern of
particular taxa such as Lithiotis ( Nauss and Smith.
1988) or buchiids (Sha and Ffirsich. 1994).
Recently, Liu (1995) carried out an extensive
survey of Jurassic bivalves along the margins o["
the Proto-Atlantic to study their palaeobiogeographic relationships. This was possible because a
large new data base of the distribution pattern of
Jurassic bivalves in time. space and different types
of facies had become available in recent years
through taxonomic or palaeoccological studies
{e.g. Etirsich, 1977, 1982: Duff, 1978: Kelly, 1984,
Kelly, 1992: Ftirsich and Werner. 1988. 1989;
Oschmann. 1988; Clausen and Wignall, 1990:
Heinze. 1991). Thus, one of the weaknesses of
many palaeobiogeographic studies
an insufficient data base
is thought to have been at least
partially overcome.
The origin of limnal provinciality within the
,lurassic is still controversial: past explanations
include, amongst other Factors, salinity w~riations
and temperature gradients, physical barriers, and
sea level changes (e.g. Arkell. 1956: Halhu-n. 1969.
1977, 1983: Gordon, 1975: FClrsich and Sykes,
1977: Riccardi, 1991). Using an improved data
Many palaeobiogeographic studies are severely
hampered
by an
inadequate
data
base.
Fortunately,
century-long
palaeontological
research in the European Jurassic has produced a
high quality data base. Although, in the case of
bivalves, genera yield considerably less detailed
palaeogeographic
information
than
species
(Heinze, 1996), we have refrained from using
species as the basic taxonomic unit because evaluation of published taxonomic information is critical
for the quality of the data and cannot always be
carried out at the desired species level. At the
generic/subgeneric level, a taxonomic evaluation
of published inl\~rmation was feasible and resulted
in revised and accurate faunal lists for the respective localities. The Proto-Atlantic data matrix has
been published (Liu, 1995, appendix 2). Data
for tile southern Tethys has been deposited m
the library of the Inslitul ./fir Paliiomolo~ie
of Wtu'zburg University under the number
PIW1996VI. It has been compiled from the literature on the southern margin of the Tethyan Ocean
(mainly Sowerby, 1840; Stoliczka, 1865; Kitchin.
1903: Daqu& 1910; Holdhaus, 1913: Douville,
1916, 1925: Cox. 1925, 1935a,b, 1940, 1952, 1965;
Stefanini, 1925. 1939; Weir, 1925.Weir, 1929;
Barrabe, 1929: Dietrich, 1933: Jaboli. 1959: Desio
et al., 1960: Freneix, 1965: Mongin. 1967: Hirsch.
1980: Parnes, 1981; Manivit et al., 1990; Jaitly
et al, 1995; Holzapfel, 1997 ). These literature data
were augmented by extensive collections of
biwdves, particularly flom Morocco, Tunisia,
western India, Jordan and the Sinai Peninsula,
(It Liu ut aL . Pakwo,eeo~raplo', Pakwoclimalolo,zv, Palaeoecolo,.,,y 137/199,¥2 127 151
made in the last few years by members of the
Wiirzburg Palaeontological Institute.
The data matrix was subdivided into time slices
corresponding to the eleven stages of the Jurassic
{Table 2). This should allow us to trace changes
in the palaeobiogeographic pattern through time.
Furthermore, the substrate in which a bivalve
occurred was noted in order to evaluate the importance of facies on the distribution pattern. As
detailed information on the facies is often not
available, only three major groups of facies have
been distinguished: fine-grained siliciclastics,
coarse-grained siliciclastics,
and
carbonates.
Table 2
Geographic areas, time slices and facies types in which the data
base has been grouped
Proto-Atlantic
Southern Tethyan Shelf
129
Despite this, a substrate-related analysis was possible only for some stages of the Jurassic.
The geographic areas selected for the study
range fl'om East Greenland to Morocco in a
north-south direction and from Mexico/Texas to
Kachchh (western India) in a northwest southeast
direction {Table 2).
Most palaeobiogeographic studies are beset by
the problem that the available information is not
evenly distributed among areas, time slices, and
facies types. This is particularly true when the
analysis is very detailed. As a result, the significance of the results may vary considerably. For
this reason we restrict our analysis of the southern
Tethys margin to the Bathonian and Callovian
time intervals, for which an adequate data base is
available.
3. Numerical methods
Geographic units
East Greenland
Scotland/northern
England
Southern England
Northern France
Southwest France
Spain
Portugal
Morocco
Texas Mexico
Tunisia
Arabia.'Near East
East Africa
West India (Kachchh)
Time .v/ices
Tithonian
Kimmeridgian
Oxfordian
Callovian
Bathonian
Bajocian
Aalenian
Toarcian
Pliensbachian
Sinemurian
Henangian
Callovian
Bathonian
Although palaeobiogeographic studies should
be based on abundance data, such data are hardly
ever available and must be substituted by presence absence data ( binary data), which are usually
sufficient for large-scale palaeobiogeographic
studies (e.g. Digby and Kempton, 1987; Shi, 1993).
For the present study, presence absence data were
used and treated by multivariate analysis. A comparison of various analytical approaches by Liu
(1995) showed that the Q-mode cluster analysis
with the weighted pair-group method using arithmetic averages ( W P G M A ) and based on the
Simpson Similarity Coefficient or, in the case of
the Pliensbachian, oll the Simple Matching
Coefficient (e.g. Simpson, 1960; Newton, 1990;
Shi. 1993 ) yielded the biologically most meaningful
results. (A detailed discussion of the methodology
is found in Liu, 1995.)
4. Results
fine-grained
siliciclastics
(clay. silt): T
coarse-grained
siliciclastics
(sand): S
carbonates: C
4.1. Bivalve prot'hlces m the Jurassic PromA l/antic
Based on the multivariate analysis discussed
above and based on characteristic taxa, four
bivalve provinces can be distinguished within the
l "~0
('. Liu el a[. " Palaeo,.,eoyraldly, PalauoclhJlaloh)gy, Pa[auo¢,co/o~l' 137 (199~'~') 127 151
Jurassic Proto-Atlantic (Liu, 1995; table I). They
are, from north to south: the Boreal Bivalve
Province, the Northern Transitional Province, the
Southern
Transitional
Province
and
the
Mediterranean Bivalve Province. To avoid confusion, Liu (1995) introduced the terms "Northern
Transitional
Province"
and
"Southern
Transitional Province" because their boundaries
only partly coincide with the ammonoid-defined
Subboreal and Submediterranean provinces. Not
all provinces can be recognized for each time
interval considered, and the boundaries of the
provinces change throughout the Jurassic. The
Boreal Bivalve Province and the Northern
Transitional Province are grouped in the Boreal
Realm, whereas the Southern Transitional
Province and the Mediterranean Bivalve Province
belong to the Tethyan Realm (Table 1). This
pattern corresponds to that of ammonite provinces
and realms within the Proto-Atlantic (e.g. Hallam,
1969, 1971, 1975). Apart from forming a cluster,
each province is characterized by several taxa
which are either restricted to it within the ProtoAtlantic or are at least conspicuous elements of
the fauna, occurring rarely outside that province
during the time interval under consideration.
Details of the cluster analyses and the resulting
bivalve provinces including their characteristic taxa
are given in Liu (1995). Four of her maps detailing
extent and boundaries of these provinces during
the Pliensbachian, Oxfordian, Kimmeridgian and
Tithonian are reproduced here in a modified way
( Figs. 1 and 4 -6) as they are the data base l\~r the
discussion. In addition, maps of the bivalve provinces during the Bathonian and Callovian that
include parts of the southeastern Tethys are given
in Figs. 2 and 3. These six time interw~ls are best
represented by data. Only for the Callovian can
all four provinces be distinguished. During the
other stages either the Northern o1" the Southern
Transitional Province is missing.
4.2. Bivalve dLs'tribulio, patterns aloHg tile South
Tethvan margi17
Figs. 7 and 8 provide dendrograms derived fl-om
the cluster analysis in which data from the ProtoAtlantic are combined with data from the southern
margin of the Tethys (Morocco, Tunisia, Arabia
and Middle East, East Africa and Kachchh) for
the Bathonian and Callovian. The analysis
( W P G M A ) has been run twice: once with the
geographic areas defined in Table 2 as the basic
units and once with data from the geographic
areas divided up according to facies types as
defined in Table 2. For the Bathonian, the den&ogram based on areas ( Fig. 7A) shows four clusters:
(1) East Greenland, (2) southern and northern
England, (3) Tunisia, Mexico and Kachchh, and
(4) northern and southwestern France, Portugal,
Morocco, Arabia, and East Africa.
While East Greenland can be interpreted as
representing the Boreal Bivalve Province and
northern and southern England as representing
the Southern Transitional Province there is little
meaning in differentiating between the remaining
two clusters, as adjacent data points often belong
to geographically widely separated areas. Together
they could be regarded as representing the
Mediterranean Bivalve Province. However, when
differences in facies are taken into account, the
cluster analysis results in four clusters with East
Africa and Kachchh forming a distinct grouping
that can be regarded as representing an Ethiopian
Bivalve Province. Surprisingly, Arabia does not
cluster with East Africa and Kachchh, but forms
an independent cluster related to northern Africa,
southern European areas, and northern France
(Fig. 7B). An analysis of lists of genera from the
various areas reveals that it is the presence of taxa
such as Fimbria, Pseudis'ocardht, Pterocardia,
Prorokhl, bTtegricardium and Gryphaeligmus in
Arabia, compared to areas further south, that led
to the separation of the two areas in the dendrogram. However, the local abundance of Eli gmus
in Arabia as well as in Kachchh and East Africa
suggests that the three areas should be combined
in a single province. Additional arguments for this
come fi-om an analysis of the distribution patterns
of some species. Thus, Aji'icogryplmea costelhtta is
known from Arabia as well as East Africa: and
also occurs rarely in Tunisia. Chhmlys curvivc,'ians
is another species restricted in the Bathonian to
Arabia and East Africa/Kachchh (see also Hallam,
1977: Heinze, 1996). Clearly, in the Bathonian,
Arabia seems to occupy an intermediate position
and could be placed with either East Africa and
Kachchh, to represent the Ethiopian Bivalve
C. Liu et al. / Palaeogeography Palaeoclimatoh~gy, Palaeoecology 137 (1998) 127 151
131
60 °
iiiiii!iiiiiiiiii:::::::
!if:: iii:
30 °
•
@
•
•
•
-~]
Weyla (Weyla)
Lithiotis
Opisoma
Pachymytilus
Pachyrisma (Durga)
Hipppopodium
~-..~i
::::!!iiiiiiiiiiii!i/
.?:iiiii!!iii!!iii
......
iii!iiiii
x~
0°
Boreal BivalveProvince
Mediterranean BivalveProvince
[~
Southern Transitional Province
Pliensbachian
Fig. 1. Pliensbachian bivalve provinces and occurrence of some characteristic taxa in the Proto-Atlantic. Base m a p after Barron et al.
( 1981 ), modified after Ziegler (1988). Dashed line: realm boundary based on ammonites.
Province, or with North Africa and southern
Europe to represent the Mediterranean Bivalve
Province.
For the Callovian, the dendrogram based on
areas (Fig. 8A) shows the Mediterranean Bivalve
Province discussed in the context of the ProtoAtlantic to extend eastwards to include Tunisia,
whereas Arabia, East Africa and Kachchh belong
to a separate cluster which, again, can be interpreted as representative of the Ethiopian Bivalve
Province. As an anomaly, Portugal is also grouped
with this cluster. Based on facies types (Fig. 8B),
the same grouping can be recognised, except that
what we regard as the Ethiopian Bivalve Province
is split into two subclusters, largely on the basis
of facies (carbonates versus siliciclastics). As an
oddity, southwestern France groups with Arabia,
East Africa and Kachchh.
Thus, an Ethiopian Bivalve Province is discernible for both the Bathonian and Callovian. it is
more difficult to define the northern boundary of
this province and to define the province by genera
or subgenera (see discussion below).
5. Discussion
5.1. Acuity o f resolution. genera versus species as
data base
There can be little doubt that a palaeobiogeographic analysis carried out at the species level
C Lit~ ~'t dl.
132
Pa/a~'o'..,~'~,,..,r~qdO'
Pa/acoc/iHmlo/~yl'.
Pa/ae~J~'co/o.~,l' 137 .~ 199~, ', 127 151
6 0 ,~
....
iiiiiiiiiiiiii/~,
3(1'
•
Ceratomyopsiv
Opis I Trigonopis )
i 7i i i i i i i i i i i i i i i
\ \
.............
\
•
::i+i!!iiii:
.....
30:
Borea[ Bixalve Province
Mediterranean
Bivalve Province
Ethiopian Bivalve Province
S o u t h e r n t r a n s i t i o n a l Prox incc
•
.~. . . .
Fig. 2. Balhonian bixal\c provinces of the Proto-Atlnlltic and southern margin of the Tethys. and distribution of some characteristic
taxa. Base map al'ter Barl-Oll el al. (19gl). modilicd after Zieglcr ( 198gL Dashed line=realm boundar> based on Zltllnlonilcs.
yields more detailed information than that carried
out at the generic 'subgeneric level, if the taxonomic
basis is sound. Nevertheless an argunlent l\~r
restricting the analysis to genera subgenera is that
such a sound taxonomic data base is at present
not feasible (see also Hallam, 1977). Although this
is generally true, distract progress has been made
in Recent decades on the taxonomy of Jurassic
bivalves from previously poorly documented areas
(e.g. Fthsich, 1982: Damborenea, 1987a,b: Yin
and F/]rsich. 1991; Aberhan, 1994: Jaitly el al.,
1995: Holzapfel, 1997) or of particular groups
such as the pectinids (Johnson. 1984) ol- the bakevelliids (Muster, 1995). Thus. while accepting
Hallam's (Hallam, 1977) argumel'lt Oll the whole.
Heinze (1996), in comparing bivalve species flom
the Ethiopian faunal province with those fl-om
Europe, was able to demonstrate that the distinctness o f the Ethiopian faunal province was consider-
ably enhanced compared to an analysis at the
generic level. This is particularly evident when the
nunlber of endemic species is considered ( Fig. 9).
However, the necessary information is not yet
available to carry out such an analysis for larger
areas and we have to be content with the cruder
picture offered by genus-based data sets.
5.2.
Biva/vc versus altmtonite pazterns
Ammonites have received far more attention as
palaeobiogeographic indicators in the Jurassic than
bivalves. This is understandable because, being the
main biostratigraphic tools ill the Jurassic, they
were studied m much greater detail than bivalves.
Nevertheless, as Hallam (1977) remarked, bivalves
are the most diverse and abundant macroinvertebrate group in the Jurassic, occurring in a wide
range of environments. Moreover, being nearly
('. Liu et al,
Pa/aeo~,,eo:,,rap/s
' Palaeoclimatolo%y,
Pa/aeoecolo<v
137 : 1998) 127
151
133
60 °
30 °
®
•
•
•
•
X
arcomytilus
Arca(Eonavicula)
arctica
Lopatinia
Modiolus (Strimodiolus)
Praebuchia
~]
Boreal Bivalve Province
-[7~
"))i i i i i i :
. i i i 2 i i
i i i i i i i
................
" i i i i
Ethiopian Bivalve Province
[III]
Northern transitional Province
~]
Southern transitional Province
i i i i i i i i i t
........
........
...iiii.
Mediterranean Bivalve Province
1~
0°
i i i i i ....
" .............
i i i i i 7 i i i i
i i i i i i i i i i(
,
i. . . . . . . . .
'
30 °
"
'"-*)iiiiii'
Fig. 3. C a l l o v i a n bivalve p r o v i n c e s o f the P r o t o - A t l a n t i c a n d s o u t h e r n m a r g i n o f the Tethys, a n d d i s t r i b u t i o n o f s o m e c h a r a c t e r i s t i c
t a x a . Base m a p a f t e r B a r t o n el al. ( 1981 ). m o d i f i e d a f t e r Z i e g l e r ( 1 9 8 8 ) . D a s h e d l i n e = r e a l m b o u n d a r y b a s e d o n amn3oniles.
exclusively benthic, bivalve faunas can be expected
to show regional differences in addition to their
distinct facies dependence. However, this facies
dependence might overprint and obscure the influence of larger-scale biogeographic factors such as
climate, physical barriers and tectonic processes.
Along the margins of the Proto-Atlantic, biogeographic boundaries based on ammonites
and bivalves coincide remarkably well during
the Bathonian, Oxfordian, Kimmeridgian and
Tithonian (Figs. 2 and 4 6). In contrast, the
boundary between the Boreal and Tethyan Realm
differs drastically in the Pliensbachian ( Fig. 1 ) with
the boundary based on ammonites crossing
through southern Spain. whereas that based on
bivalves was situated in northern France. During
the Callovian (Fig. 3), the discrepancy was less
pronounced; the realm boundary based on a m m o nites was situated in northern Spain, while that
based on bivalves was in northern France.
Apparently, the southward spread of Boreal
ammonites in the Callovian, after the physical
barrier of the Mid North Sea High had become
ineffective, was not mirrored by a similar spread
of Boreal bivalves, For the Pliensbachiam and to
a lesser degree also for the Callovian, a feasible
explanation for the discrepancy in the ammonite
and bivalve distribution pattern could be that, on
the whole, bivalves spread passively during their
planktic larval stage whereas ammonites, as nektic
and nektobenthic organisms, also spread actively.
In other words, the distribution pattern of bivalves
should be much more influenced by current patterns than that of ammonites. This would imply
that northward-directed currents dominated in
the area of western Europe, at least in the
Pliensbachian and Callovian, and caused the
discrepancy in the boundaries of faunal realms.
134
C Liu e t a / . '
PalaeoL, eo¢~rap/ly, Palaeoclimcttolo~+v, Pa/aeoecohs~,,y 137 ( 1 9 9 8 ) 127 151
iiiiiii
iiiiiiii
I 6°0
~iiiiiiiiii
iiiiiiiiiiiilJ
.......
iiiiiiiiiii
iiiiiiii..
!iiiiiiiii!iiiiiiii!.:!:i
iiiiiiiiiiiiiiiiii~
30°
iiiiiiiiiiiiiiiiiiiiiiiii~
®
A rcomytilus
•
A rctica
[]
Discoloripes
Lopatinia
•
Buchia (Buchia)
•
Praebuchia
\
" " "
~
Boreal BivalveProvince
~
Mediterranean BivalveProvince
-[[~
Northern Transitional Province
..........................
Fig. 4. Oxfordian bivalve provinces and occurrence of some characteristic taxa in the Proto-Atlantic. Base map after Barron et al.
( 1981 ), moditied after Ziegler (1988). Dashed line= realm boundary based on ammonites.
5.3. The distinctness o]the Ethiopian Bivalve
Province
As the cluster analysis for the Bathonian
Callovian time interval shows, the Ethiopian
Bivalve Province is less distinct than other bivalvebased provinces. The reason for this lies in the
transitional nature of the Arabian region which,
in the Middle Jurassic, apparently was a spreading
centre not only for ammonites (e.g. Enay and
Mangold, 1982; Enay et al., 1987) but also for
some bivalves which originated in that area and
migrated both westward along the southern margin
of the Tethys towards North Africa and southern
Europe, and southward into the I n d o M a l a g a s y
Gulf (see below). With respect to diagnostic
genera/subgenera, Heinze (1996) has shown that
the province does not include true endemic
genera/subgenera. Thus, Eligmus occurs abun-
dantly, but is by no means confined to the
Ethiopian Province. A characteristic and widespread genus is the arcoid hutogmmmatodon,
which managed, however, to cross the Tethyan
Ocean by the Bathonian, as is evidenced by its
occurrence in northern Tibet. The well known
Bathonian "6~)rbula" lyrata, a new genus that
awaits description, is abundant in nearshore
Bathonian sediments of Kachchh, but also occurs
from Madagascar to Morocco and contributes to
the blurred boundary of the Ethiopian Bivalve
Province.
Less
common
is
the
oyster
AJ)'icogo,phaea, which is known from the Middle
East and Somalia and also rarely occurs in Tunisia.
The recently erected subgenus hutoweyla is known
only from the Callovian of Kachchh (Jaitly et al.,
1995), but is very rare: it may represent a true
endemic.
In contrast, Heinze (1996) showed that the
C. Liu el al. Pa&eogeography,Palaeoclimatology, Palaeoecology137 (1998) 127 151
135
ii 160o
i!:
"
• Eomiodon~ " i ~i i i i i i i i i i i i i i i i i i i i i i i ~
• Laternula
• Arctica
[] Discoloripe
• Buchia(Buchia) X~::~X~
• Praebuchia
X~K:x.~
Modiolus (Strimodtolus)
[~
Boreal BivalveProvince
~]
Mediterranean BivalveProvince
r~lq
Northern Transitional Province
.i
30°
iiiiiiiiiiiiiiiiiiiii
0°
Fig. 5. Kimmeridgian bivalve provinces and occurrence of some characteristic taxa in the Proto-Atlantic. Base map after Barron
et al. (1981), modified after Ziegler (1988). Dashed line=realm boundary based on ammonites.
Ethiopian Bivalve Province can be well defined at
the species level. For example, in the Bathonian
and Callovian of Kachchh about 25% of the
bivalve species are endemic. Together with an
additional 10% of the species, which are restricted
to the Ethiopian Bivalve Province, this province
has a high degree of distinctness. An example is
the pectinid Chlamys curvivarians, which is confined in the Bathonian and Callovian to the
Ethiopian Province and only rarely occurs outside
of it, in Tunisia, during the Late Callovian and
Oxfordian (see also Hallam, 1977). The limid
Plagiostoma harronis, according to Hallam (1977)
another characteristic endemic species known from
Somalia and Abyssinia (Daqu& 1905; Weir, 1929),
is, in fact, Late Jurassic in age and may well be a
synonym of another, more widespread species of
Plagiostoma.
A comparison of the Ethiopian Bivalve Province
with the Ethiopian Ammonite Province (often also
called the Indo-Malagasy Province) is less straightforward because the boundaries of the latter are
less clearly defined. Moreover, based on ammonites, some authors differentiate between an
Erythrean Province (Arabia and the Middle East)
and an Indo East African Province (Gill and
Tintant, 1975; Krishna and Cariou, 1990).
According to Cariou et al. (1985), the Ethiopian
Ammonite Province in the Bathonian extended as
far north as Egypt, Israel and Lebanon ("faunes
arabo-malgaches"), but several elements characteristic of the region migrated westward across
northern Africa (e.g. Enay and Mangold, 1982),
thus blurring the boundary. Based on bivalves, the
Arabian region occupied an intermediate position
between the Mediterranean and the Ethiopian
Bivalve Provinces. According to the cluster analysis, the province boundary runs between Arabia
136
I'<tl~co<gco<Wap/. ', Palacoclimalolo~' Palacuccotok,.' 137 ~ 199<~7 127 151
C Liu cl al.
\
.
®
•
[]
•
•
~]
Eomiodon
A rcom.vtilus
Arctoti~
Discoloripe;
Buchia (Buchi
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
7
30°
.........
Mo,liolu., ( S t r t ~ / u s ~ i
Camptonectes (mclearnia) "q~ i "
Pty{ homya (Ptrchom~a) N N
i;;iil;iiiiiiiiiiiiiil.J"~"~,_
Boreal BivalveProvince
~ ' ~ Medilerranean Bivah'cProvince
-~m
Northern Transitional Province
Fig. 6. T i t h o n i a n bix.ah.e pro,Anccs and occurrence o1 some characteristic taxa m the Proto-Atlantic. Base m a p after Barron ct al.
{ 19811, modilied after Ziegler (19881. Dashed line=realm boundary based on ammoniles.
and East Africa (Fig. 2). In the Callovian, the
boundaries of the ammonite- and bivalve-based
provinces coincide and run north of Egypt and the
Middle East (Fig. 3). Thus, on the whole,
ammonite- and bivalve-based Ethiopian provinces
show the same dilemma: a poorly defined northern
boundary due to the marginal position of the
spreading centre ( Middle East).
5.4. Diversity patterns
Ever since it was recognised that the diversity
of organisms exhibits distinct latitudinal gradients
both in terrestrial and marine realms (e.g. Fischer.
1960: Stehli et al., 1967 in the case of bivalves),
diversity has been regarded a biogeographic tool
that should be easily applicable to the fossil record.
Hallam (1972) was the first to investigate latitudinal diversity trends of Jurassic bivalves m a
semi-quantitative study of bivalve genera of the
margins of the Proto-Atlantic. from Greenland to
Morocco. He demonstrated that, contrary to
expectations, there was an increase in the overall
diversity of bivalves northwards. Ftirsich and
Sykes (1977) investigated diversity trends of
Oxfordian bivalves in the same area and lbund a
northward decrease of the overall diversity which
they interpreted as resulting from a decrease m
environmental heterogeneity in the same direction.
Recognising the important influence of litcies oll
diversity (species richness), F~rsich and Sykes
(1977) then continued to look at diversity trends
within particular facies types and could not
discriminate a consistent pattern. They explained
the lack of distinct diversity trends within specific
facies types as due to the influence of regional
environmental instability and differing environmental heterogeneity with respect to other environ-
C Liu el al.
Pa/aeogco~rap/y,
[
1.0
A
r-
137
Palaeoc/imato/og. ', Palaeoeco/o:,tv 137 (1998) 127 151
i
0.5
t
0
Arab
N-France
Portugal
Morrocco
"Tethys" Bivalve Province
SW-France
E-Africa
Tunisia
Mexico
t_
Kachchh
N-England
S.Traasifional Province
I-S-England
Boreal Bivalve Province
Greenland
B
Boreal Bivalve Province
I
1.0
~
05
I
0
[- Greenland (S)
S-England (C)
S-England (S)
S.Transitional Provine.
S-England (T)
N-England (C)
N-England (T)
N-England (S)
Tunisia (T)
Tunisia (C)
Morocco (T)
Mediterranean Bivalve Province
Morocco (C)
SW-France (C)
Portugal (C)
N-Fraaee (C)
Arab (T)
]
_]
t
Arab (C)
E-Africa (T)
E-A fric~ (C)
Ethiopian Bivalve Province
Kacl~hh (T)
Kachchh (S)
Kachckh (C)
Fig. 7. Dendrograms of a Q-mode cluster analysis with the WPGMA, based on the Simpson Similarity Coefficient, fbr Bathonian
bivalve genera/subgenera of the Proto-Atlantic and South Tethys margin. (A) Taking 12 areas into consideration (see Table I: Spain
has been omitted due to insufficient data) three provinces can be recognized. Note that the Ethiopian Bivalve Province, which clearly
shows up in (B) cannot be separated by means of the cluster analysis. (B) Using areas and facies types four provinces are distinguished.
C - carbonates; S - sandstones" T - clay and siltstones.
138
C. Liu
et
al. ' Palaeogcography, Palaeoclintatology, Palaeoccolo~L'137 (19982 127 151
A
1.0
05
I Kachchh
E-Africa
Arab
Ethiopian Bivalve Province
1t
Portugal
V Morocco
Mediterranean Bivalve Province Mexico
Tunisia
~ N-France
S.Transitional Province
SW-Franee
~ N-England
N.Transitional Province
S-England
-Greenland
Boreal Bivalve Province
B
I
1.0
I
Kachchh(T)
Kachchh(S)
E-Africa(T)
-
S-England (C)
Ethiopian Bivalve Province
Boreal Bivalve Province
~.
Greenland(S)
i
Ethiopian Bivalve Province
015
S-England(S)
S-England(T)
N-England(S)
N-England(C)
N-England(T)
N.Transitional Province
VlediterraneanBivalve Province
0
Morocco(T)
Morocco(C)
Tunisia(T)
Tunisia(C)
Portugal(C)
I
Arab(T)
Arab (C)
SW-France(C)
E-Africa(C)
Kachchh(C)
Fig. 8. Dendrograms of a Q-mode cluster analysis with the WPGMA, based on the Simpson Similarity Coefficient, for Callovian
bivalve genera :subgenera of the Proto-Atlantic and South Tethys margin. (A) Talking 12 areas into consideration (see Table I; Spain
has been omitted due to insufficient data) five provinces can be recognized. (B) Using areas and titcies types five provinces are
distinguished. Due to insufficient data Spain and northern France have not been included in the analysis. ("--carbonates:
S-sandstones; T-clay and siltstones. Note that the Ethiopian Bivalve Province is split in two clusters, depending on Facies.
139
C Liu el aL /' Palaeogeoeraphy, Palaeoclh72atolog~y, Palaeoecology 137 (1998) 127-151
BIVALVE DIVERSITY
n taxa
Z 120
8o
uJ
tarnilies
,o
GENERA
SPECIES
GENERA
SPECIES
4O
Fig. 9. Comparison of numbers of endemic genera/subgenera
and species in the Bathonian and Callovian of the Ethiopian
Bivalve Province.
mental parameters than substrate. Incidentally,
Oxfordian ammonites showed a clear northward
decrease in diversity across the same area both at
the generic and the species level. A generic decrease
in diversity from the Tethyan Realm to the
' B o r e a l ~ t l a n t i c Province" is also shown by Middle
Jurassic to Late Cretaceous belemnites (Doyle,
1987). However, belemnite diversity increases
again from his Boreal-Atlantic to his Arctic
Province.
Fig. 10 shows patterns in species richness,
expressed by the number of taxa, at the
generic/subgeneric and the familial level for the
Bathonian,
Oxfordian
and
Kimmeridgian.
Whereas overall bivalve diversity decreases northward from northern France to East Greenland, in
the Oxfordian and Kimmeridgian, corroborating
the findings of Farsich and Sykes (1977), the
Bathonian diversity pattern differs in showing low
values for northern France and thus no regular
decrease northward. This shows again the limitations of the use of diversity patterns. Most likely
the relatively low Bathonian diversity in northern
France reflects the low environmental heterogeneity, nearly exclusively shallow carbonate platform environments, and not so much a latitudinal
pattern. The fact that for the Bathonian,
Oxfordian, and Kimmeridgian of northern France
and southern England generic diversity values are
higher than in East Greenland probably reflects
i 40
S
~
A
A
A
A N
Fig. 10. Changes in bivalve diversity (expressed by the number
of genera and families, respectively) from Northern France
to East Greenland for the Bathoniam Oxfordian and
Kimmeridgian.
both the expected poleward decrease of diversity
and low environmental heterogeneity in East
Greenland, because carbonate regimes are not
present.
The reason that the diagram in Fig. 10 does not
include data from southern Europe, Morocco and
Texas/Mexico is the lack of sufficient data for most
of these areas, which automatically leads to low
(and misleading) diversity values. Similarly, lack
of sufficient data also forced us to restrict the
diversity analysis of the southern Tethys margin
to the Bathonian Callovian time interval.
When looking at bivalve diversities along the
southern margin of the Tethys, from Kachchh in
the south to Tunisia in the north during the
Bathonian and Callovian (Fig. 11), there appears
to be a weak trend towards lower values in the
north. However, the low values of Tunisia are
largely responsible for this 'trend'. In the
Bathonian, sediments of southern Tunisia pre-
140
('. I.iu et a/.
Pak~eoyeo,,,raldU'. Pa/ucoc/in~alo/oL,3'. Pa/aeoecolovy 1.77 ~ 199X; 127 151
apply also to some other groups such as belcmnites
(Doyle, 1987).
BIVALVE DIVERSITY
n taxa I
[,° I
5.5.
t
40
"El
S
t
- -
- -
m-,-
.,,-.4a
i
--.,~
•
,~
1
%%~ %% 4%%,%%
NW
Fig. 11. Changes in bival,,e diversity, expressed by the number
o1"genera and IEmilies respecti\,eb, along the southern margin
of tile Tcthys. The slight decrease in dix,ersity in a northx~estward direction is an artefacl caused b3 limited reformation from
some areas, especially Tunisia.
dominantly represent marginal marine, high stress
environments (Holzapfel, 1997). In the Callovian.
shallow nearshore environments prevail, still to a
large extent influenced by fluctuations in salinity.
It is therefore not surprising that regional diversity
values do not match those from other areas such
as Kachchh where fully marine conditions prevailed and environmental heterogeneity was high
(pers. observations). In addition, limited exposure
and far less extensive sampling of Jurassic rocks
of East and Northeast Africa probably contributed
considerably to the observed pattern. In Kachchh,
in contrast, bivalve faunas have been collected and
described systematically over the last decade (e.g.
Jaitly et al., 1995: Pandey et al., 1996) and provide
a much more comprehensive data base than that
of other regions.
In conclusion, diversity gradients are a poor
tool for defining provinces when applied to Jurassic
bivalves, partly due to the lack of sufficient data,
partly due to the role of environmental heterogeneity. Similar conclusions were reached by
Hallam (1977), when he compared the latitudinal
diversity curve of Recent bivalves with Jurassic
data from different palaeolatitudes. These findings
Di,vwrsal patterns
Biogeographic units are rarely static. Temporal
changes ill lhe position of province boundaries arc
caused to a large extent by expansions or contractions of the occupied ecospace, m other words by
dispersal. For a detailed analysis of dispersal patterns high stratigraphic resolutiol~ is a prerequisite.
In the case of bivalves, detailed stratigraphic information is often missing and migration analyses
have to be carried out at the stage level. Even at
this crude level, the northward dispersal of several
bivalve taxa of Tethyan affinity, starting in the
Aalenian and extending throughout the Bathonian,
is a conspicuous feature of the Proto-Atlantic.
Fig. 12 shows some representative taxa. such as
Arcomrlilu.s, Pachyrisma, Actinostreon, E/i~mus,
Triclfites and Dacrvom)'a. In the case of Eli t,mus,
dispersal from the southern margin of the Tethys
to northern France via Morocco look place within
the Bathonian. thus reaching the limit of the
stratigraphic resolution.
This "Tethyan Spread" of biwflves can be correlated with the regional amelioration of climate at
that time (e.g. Frakes et al., 1992), which is
corroborated by the existence of tropical carbonate
platl\~rms across large areas of France. In addition,
the Mid North Sea High served as an efficient
barrier (e.g. Ziegler, 1982, 1988) preventing cool
currents from the north fi-om entering western and
southern European shell" seas.
Larger-scale migration patterns are very difficult
to establish because of collection failure and
insufficient temporal resolution. The few examples
shown in Fig. 13 are among the best and demonstrate that several taxa migrated westward along
the southern margin of the Tethys, some of them
as far as the Proto-Atlantic. The genus El(~,,mus,
for example, apparently originated in the Late
Bajocian of the Middle East (Israel) and moved
southwards into East Aft'lea by the Middle
Bathonian to reach Kachchh by the Late
Bathonian. In the other direction, the genus
reached northern Aft'lea (Tunisia) and France by
the Late Bathonian. (The record of Eli~,,mus from
C. Liu et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 127 151
NORTHWARD
DISPERSAL
OF TETHYAN
BIVALVES
Bath
Baj
Aal
Hie
• ...,-..-......,~• °
Dacryomya
Bath
Bai
Aal
Plie
Bath
Ba]
Aal
Pile
Bath
Baj
Aal
Pile
Trichites
m •
Eligmus
•~
u
..._--~•
Actinostreon
.--~•
~ArcO
~
~
•
- - ' ~ MA
O
Bath
Baj
Aal
Plie
Pachyrisma •
mytilus
141
R
J'
sP
A
sw-F
,t
N-F
••
A
S-E
Bath
Aal
Plie
*
N-E
N
Fig. 12. The northward dispersal of Tethyan bivalves (the Tethyan Spread) during the Early and Middle Jurassic is interpreted as
being a result of climatic amelioration. Plie=Pliensbachian; Aal=Aalenian; Bq/=Bajocian; Bath=Bathonian; Mor= Morocco;
Sp = Spain: S W-F= southwestern France; N-F= northern France; S-E= southern England; N-E = northern England.
the Bajocian of Morocco by Dresnay (1963) has
to be regarded as doubtful, due to the lack of
biostratigraphic control.)
Examples
of migrating species include
Falcimytilus jurensis and Chlamys curvivarians. The
first occurrence of the former is in the Bathonian
of Kachchh. By the Callovian, the species had
migrated to Tanzania, Arabia, the Middle East
and Tunisia and reached the margins of the ProtoAtlantic ('Europe') by the Oxfordian. The oldest
record of C. curvivarians is from the Bajocian of
Somalia (Stefanini, 1939). In Kachchh the species
is known from Bathonian and Callovian strata.
By the Callovian it had spread to Arabia and
Israel; by the Oxfordian it had reached Tunisia.
Another example of migration is 'Corbula'
lymta, a relatively large, equivalved corbulid
occurring characteristically, but not exclusively, in
waters of reduced salinity (Mongin, 1967; own
observations). The earliest record is from presumably Lower Bathonian strata of Kachchh (Fiirsich
et al., 1994). From there it migrated to North
Africa, where it is known from the Upper
Bathonian of Morocco (Mongin, 1967), and the
Callovian of Tunisia (Holzapfel, 1997). The
species is also well known from the Bathonian of
Madagascar (e.g. Newton, 1895; Besairie, 1936;
Nicolai, 1950). Due to limited stratigraphic resolution it is, however, not clear whether it qriginated
in Madagascar or in Kachchh.
Some species, such as Propeamussium pumilum,
Spondylopecten palinurus, Eopecten velatus and
Radulopecten scarburgensis, occur in Europe earlier
than in areas of the Ethiopian Bivalve Province.
Although these species appear to have migrated
southward (Johnson, 1984), the fact that they are
not known during this critical time interval from
the southern margin of the Tethys (Algeria, Tunisia
142
C Liu el al.
Pahteo~eo%ral)hy, Pahteoclimatolo,~v, Palaeoecolo%y 137 (1998) 127 151
60°,
.......
:i:ii:~iCr~i:::"
~'i'ii:'
60°
:
i
30 °
i
°° 6
0°
Eligmus
30°
.
. ....
'~!
\\
-~
30:
Fig. 13. Dispersal patterns of the bivalves "CorhuN" Ivrala and Falc#nytilus/ureusis (A) and (71hmLv.s curvivarians and El&,mus (B)
from western India to Europe along the southern margin of the Tethys. The black arrows indicate the general dispersal direction.
Bj-Bajocian: B - B a t h o n i a n : E B - E a r l y Bathonian: M B = Middle Bathonian: L B - L a t e Bathonian; C - C a l l o v i a n : O - O x f o r d i a n .
and Libya) favours migration across the Tethys at
a more easterly position rather than migration
along the southwestern shelf areas of the ocean.
This would agree with the clockwise current
pattern envisaged to have existed in the western
part of the Tethyan Ocean in the Middle Jurassic
(see below; Heinze, 1996). Moreover. some of
these bivalves, such as Eopecten velatus and
Propeamussium pumilum, are known from Europe
and the Andean Basin of South America from
strata of the same age (Aberhan, 1994). Their
centre of origin is therefore still uncertain.
The preferred northwestward migration of
bivalve taxa along the southern margin of the
Tethys, from Arabia to North Africa, into Europe
in the Middle Jurassic corresponds to the generally
assumed current pattern within the Tethys (e.g.
Parrish and Barron, 1986). In these models an
E W directed equatorial current splits up into a
westward-directed northern equatorial current and
a southward-directed southern equatorial current
at the northeast margin of Africa. The latter, by
reaching south into the Malagasy Gulf, could have
facilitated the migration of taxa such as Eligmus
from the Arabian/Middle East region to East
Africa and Kachchh (Heinze, 1996). On the other
hand, this southward-directed current must have
been accompanied by some undercurrents moving
in the opposite direction. This can be postulated
on the basis of the northward-directed migration
of some taxa from Madagascar and Kachchh.
Thus, while the large-scale current pattern does
explain westward and southward-directed migration along the southern margin of the Tethys, the
C. Liu et al. / Palaeogeogrophy, Palaeoclimatology, Palaeoecology 137 (1998) 127-151
current pattern within the Malagasy Gulf was
probably complex and the existence of undercurrents or smaller, wind-induced gyres enabled the
larvae of some taxa to migrate in the opposite
direction.
5.6. Causes of provincialism
Whereas the existence of a pronounced faunal
provincialism in the Jurassic is beyond doubt, its
origin is far from clear. In the past, explanations
for the distribution pattern of organisms, usually
ammonites, included changes in salinity, temperature differences, physical barriers, facies, and sea
level changes, to name but the most important (see
Hallam, 1975 for an extensive review). In the case
of bivalves, Hallam (1977) favoured eustatic sea
level changes as the main cause of their provincialism. All these explanations have one thing in
common: they are monocausal. It is, however,
highly unlikely, that for a period of at least 70
million years the distribution pattern of a group
of organisms has been governed by a single factor.
Potential factors governing the distribution of
marine organisms are either biological, such as
larval type, life habits, reproductive strategies, and
competition, or else abiotic. The latter include:
salinity, facies, temperature, marine currents,
marine connections, physical or physiological barriers, sea level changes, and global or regional
tectonic processes. It may be assumed with some
confidence that the influence of biological factors,
although certainly not negligible, remained relatively constant throughout the Jurassic; they are
therefore not further discussed. In contrast, the
various abiotic factors appeared to have differed
in importance through time. For example, the
formation of the Mid North Sea High in the Late
Bajocian and its continued existence in the
Bathonian (e.g. Ziegler, 1982, 1988) surely contributed significantly to faunal provinciality by severing connections between northernmost parts of the
Proto-Atlantic and areas further south. As a result,
this physical barrier formed the boundary between
the Boreal and the Tethyan Realm and between
the Boreal Bivalve Province and the Southern
Transitional Province. Moreover, the barrier enabled warm water species of Tethyan origin, such
143
as the bivalve Eligmus, to spread comparatively
far north.
The opening of marine connections, usually in
the form of straits or seaways, in turn facilitated
the faunal exchange with the Proto-Atlantic
and the Tethys. For example, the existence of
the so-called Hispanic Corridor since the
Pliensbachian (e.g. Hallam, 1983; Smith, 1983;
Smith and Tipper, 1986; Riccardi, 1991 ) increased
the faunal similarity between the Proto-Atlantic
and parts of the Tethys/Palaeo-Pacific distinctly,
whereby migration of Palaeo-Pacific bivalves
through the corridor eastwards dominated over
migration of Tethyan taxa in the opposite direction. A classic example is the sudden appearance
of the Palaeo-Pacific genus Weyla in the ProtoAtlantic as far north as southwestern France
(Fig. 14; Damborenea and Mancefiido, 1979). By
the Kimmeridgian, the influence of Palaeo-Pacific
bivalves on the faunas of Mexico had become so
great that in the various statistical analyses the
area formed a distinct cluster and no longer represents part of the Mediterranean Bivalve Province
(Fig. 5; see also Liu, 1995).
The importance of marine currents in governing
faunal distribution has been outlined above. As
Heinze (1996) showed, marine currents were not
only responsible for migration of bivalves along
the southern Tethys margin westward and southward, but probably also for the spread of some
taxa (e.g. Mytiloperna patchamensis, Eligmus rollandi and Indogrammatodon) from the southern
margin to the northeastern margin of the Tethys
and vice versa (e.g. Buchia and Linotrigonia).
Differences in the position of ammonite- and
bivalve-based province boundaries in the ProtoAtlantic, such as, for example, during the
Pliensbachian and Callovian, most likely also
reflect the influence of currents; passive dispersal
of planktotrophic larvae of bivalves surely producing a different pattern than actively swimming
ammonites.
Hallam (1977) assumed changes in sea level to
be the dominant control of diversity, extinction
and endemism of Jurassic bivalves, arguing that
certain other important biogeographic factors were
more or less constant. In Fig. 15 the degree of
endemism and the diversity (expressed by the
144
('. Liu el al. ,; Palaeo~Zeo~raldly, Palaeoclimatolo~zy, Palaeoecology 137 (1998/ 127 151
Distribution of
Weyla
)
SW-France I~
Spain I~
->>>>>>>>;
i:i:!:i:i:i:i:i:i:
Portugall~
......................................
,.....:...........................
Morocco •
i!iiiiii!i!!iiiiiiii!!!!iiiiiiii!!!iiiii!!!!iiiiiiill
i•!iiiii•i••!!iiii••!•i!iiii•••!!!iiiii•!!iii•i•••iiiii•i••••iiii•i•••••iiiiii•
Mexico •
Andean
Basin
ii••iiii••!!iii•!!iii•i••i!i•i••i!iiii!iiii••••!iiii•iiiii••iiiii•!iiii••!iiiii•iiii••iiii••iiii•••iiiii
time
Hettangian
Sinemurian
Pliensbachian
Toarcian
t
Hispanic Corridor open
Fig. 14. The sudden appearance of H}_'3'/a ill tile Proto-Atlantic and neighbouring shelf seas has been related to the opening of the
so-called Hispanic Corridor in Central America. Data from Damborenea and Mancefiido (1979) and R. Schmidt-El:fing (pers.
commun., 1996).
number of genera/subgenera) of bivalves within
the Proto-Atlantic is plotted against the sea level
curves of Hallam (1988), Haq et al. (1987) and
Vail and Todd ( 1981 ). The curves expressing diversity and degree of endemism are more or less
inversely correlated. On the whole, a high degree
of endemism goes hand in hand with low diversity
values. Theoretically, a high global sea level should
facilitate faunal exchange between areas, reduce
the degree of endemism and, due to larger shelf
areas, should lead to an increase in overall species
diversity (e.g. Hallam, 1977; Doyle, 1987).
Conversely, a low sea level leads to isolation of
basins and produces endemic taxa. However, this
relationship only partly holds true when sea level
curves and the degree of endemism are compared
(Fig. 15). Thus, the high sea level of the Toarcian
is mirrored by a low degree of endemism among
bivalves, and the same relationship is seen in the
Oxfordian (when Hallam's curve is taken as a
model). During the general rise in sea level from
the Hettangian to the Toarcian, the degree of
endemism gradually declines. The low degree of
endemism in the Bathonian, however, is not
accompanied by a high sea level.
The diversity curve with peaks in the
Pliensbachian, Bathonian and Kimmeridgian corresponds neither to high levels of endemism nor
C. Liu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 127 151
Stage
Diversity
Degree of endemism
Sea level rise
145
(Number of genera/subgenera)
Tithonian
Kimmeridgian
Oxfordian
)
Callovian
Bathonian
Bajocian
Aalenian
Toarcian
Pliensbachian
Sinemurian
f
Hettangian
A
B
C
I
I
50
40
I
30
SE = 5%
I
20 %
I
I
I
I
150
100
50
0
Fig. 15. Comparison of sea level curves, degree of endemism and diversity of bivalves in the Jurassic. The three sea level curves are
after Hallam (1988) (A), Haq et al. (1987) (B) and Vail and Todd ( 1981 ) (C). Diversity is expressed by the number of genera/subgenera
within the Proto-Atlantic; degree of endemism is expressed by the percentage of endemic taxa. Taxa were regarded as endemic when
occurring only in one area (as defined in Table I) of the Proto-Atlantic. SE=standard error.
to sea level highstands. Part o f the explanation is
that the data for some o f the stages, such as the
Hettangian, Aalenian, and Bajocian, are too
meagre to be discussed in this context. The diversity trends shown in Fig. 15 are therefore difficult
to interpret. It seems that only during some
intervals o f the Jurassic sea level exerted a m a j o r
influence on the b i o g e o g r a p h y o f bivalves.
M a n y Jurassic bivalves are strongly faciesdependent (e.g. Hallam, 1969, 1971, 1977; Ftirsich,
1976). Correspondingly, the distribution pattern
o f Jurassic bivalves is related to facies, a fact that
had been noticed as early as the end o f the last
century (Nikitin, 1886; O r t m a n n , 1896) and is
clearly shown by the clusters in Figs. 7B and 8B,
where regions with the same facies type show the
greatest preference for clustering together. As a
result, bivalve diversity will be lower where, due
to a particular climatic or bathymetric situation,
the heterogeneity o f facies will be lower. This is
the case in deeper shelf settings where fine-grained
siliciclastics (clay, silt and marl) prevail. This
a r g u m e n t has been used by Fiarsich and Sykes
(1977) to explain the lower overall bivalve diversity
in Boreal environments in the Jurassic. Thus,
finally, the distribution pattern o f bivalves is
146
C. Liu et aL/ Palaeolzeography, Palaeoclimatolo~v. Pahwoecolo~,:v 137 (1998) 127 151
strongly influenced by the palaeogeographic setting, which changed considerably in the course of
the Jurassic.
The importance of palaeosalinity, promoted by
Hallam (1969) in the late sixties as the decisive
factor for regional differences between faunas in
northwestern Europe, has been overestimated. This
was recognised by Hallam as early as 1971.
Changes in salinity may have played some role
where, due to marked regression or regional uplift
and relatively humid conditions, marginal marine
environments occurred on a regional scale. An
example is the Bathonian Great Estuarine Group
of Scotland and the 'Upper Estuarine Series' of
Eastern England, which formed in connection with
the uplift of the Mid North Sea High (e.g. ZiegleL
1982) and in which brackish and hypersaline environments were widespread. Another example is
the Late Tithonian of Western Europe, in which
widespread brackish and hypersaline conditions
were the result of the late Tithonian regression.
Temperature gradients have been regarded by
several authors (e.g. Neumayr, 1883: Uhlig, 1911;
Enay, 1973; Riccardi, 1991 ) as the main cause of
Jurassic faunal provincialism. As has been pointed
out repeatedly (e.g. Hallam, 1975, 1985: Frakes,
1979) latitudinal temperature gradients in the
Jurassic were less steep than at present. For this
reason, Hallam (e.g. Hallam, 1975; see also Doyle,
1987) discounted temperature as the decisive factor
causing faunal provincialism. On the other hand,
the reduced solar radiation at higher latitudes must
have resulted in some temperature gradient, and
recent palaeoclimatological studies (e.g. Kemper,
1987; Frakes and Francis, 1988, 1990) suggest that
the polar regions in the Jurassic were not as warm
as has been assumed in the past. Arguments for
the existence of an Austral Faunal Realm at high
latitudes of the southern hemisphere, matching the
Boreal Realm of the northern hemisphere, were
put forward by Crame (1987). The presence of
bivalves such as Retroceramus, Inocerantus,
Anopaea, Arctotis, and Aucellina point to a climatic
zonation at high latitudes in the southern hemisphere. Similarly, the bipolar distribution pattern
of buchiid and inoceramid bivalves (e.g. Crame,
1987; Sha and Ffirsich, 1994) is indicative of
temperature-induced faunal provinciality (see also
Crame, 1986). The fact that many large and thickshelled bivalves, such as Diceras, Pachyrisma,
Pachymegalodon, Pterocardia and Pachymytilus,
are restricted to low latitudes further supports
some climatic control of Jurassic bivalve faunas
(see also Hallam, 1977, 1994). Thus temperature
must have contributed to t:aunal provincialism,
but
and this is the point
it was not the
only factor.
In conclusion, the distribution pattern of
Jurassic bivalves cannot be explained by a single,
overruling factor, but most likely is the result of a
combination of several factors, the importance of
which varied considerably throughout the Jurassic
(Fig. 16). This attempt at interpreting Jurassic
bivalve provincialism admittedly is not very spectacular. However, perhaps it lies closer to the truth
than earlier, monocausal explanations.
6. Conclusions
From the above the following conclusions can
be drawn:
( 1 ) Among various multivariate analytical methods, the Q-mode cluster analysis, with the weighted
pair-group method using arithmetic averages and
based either on the Simpson Similarity Coefficient
or the Simple Matching Coefficient, produced the
most meaningful results when applied to a data
matrix of Jurassic bivalve genera/subgenera from
13 areas and three facies types within the ProtoAtlantic and the South Tethys margin.
(2) Based on the cluster analysis and the distribution pattern of characteristic taxa, four bivalve
provinces can be distinguished within the ProtoAtlantic. From north to south these are: (a)
the Boreal Bivalve Province: (b) the Northern
Transitional
Province;
(c)
the
Southern
Transitional Province; and (d) the Mediterranean
Bivalve Province. The first two provinces belong
to the so-called Boreal Realm, the latter two to
the Tethyan Realm.
(3) Along the southern margin of the Tethys,
from North Africa to Kachchh and Madagascar,
C. Liu et al. /Palaeogeography, Palaeoclimatology, Palaeoecology 137 (1998) 127 151
147
Main factors governing the distribution pattern
of Jurassic bivalves in the Proto-Atlantic
Late Jurassic
Middle Jurassic
]
Early Jurassic
Fig. 16. Main factors controlling the distribution pattern of Jurassic bivalves in the Proto-Atlantic and their relative importance
through time.
two provinces are recognised in the BathonianCallovian; these are the Mediterranean Bivalve
Province and the Ethiopian Bivalve Province. The
latter province, and in particular its northern
boundary, is difficult to define on the basis of
genera/subgenera, but quite distinct when based
on species.
(4) The boundaries of provinces based on
ammonites and bivalves do not always coincide.
This can be explained by the differing mode of life
of the two groups (active swimming versus benthic
and passive drifting during the larval stages of
ammonites and bivalves, respectively).
(5) Diversity patterns are a poor tool for defin-
ing and characterizing Jurassic bivalve provinces,
partly due to lack of sufficient data, partly due to
the role of environmental heterogeneity.
(6) Within the Proto-Atlantic, several Tethyan
bivalve taxa migrated northwards between the late
Early Jurassic and the Middle Jurassic as a result
of climatic amelioration. In agreement with an
assumed E - W directed equatorial current, bivalves
show a preference for migrating in a northwestward direction along the southern margin of the
Tethys, from western India/Madagascar to northwest Africa and Europe.
(7) Faunal provincialism within Jurassic
bivalves is explained as the result of several factors,
148
C. Liu el al. / Palaeogeo~zraphy, Palaeocl#,atology, Palaeoecology 137 (1998) 127 151
the importance of which varied considerably
throughout the Jurassic.
Acknowledgements
We would like
Chris McRoberts,
the manuscript.
from reviews by
A.L.A. Johnson,
thanks financial
to thank Martin Aberhan and
Wt~rzburg, for critically reading
The paper greatly benefitted
A. Hallam, Birmingham, and
Derby. We acknowledge with
support by the Deutsche
Forschungsgemeinschq/t (grants Ful 31/12-1 and
Fu131/15-1).
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