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Diversity and biogeographic distribution of Jurassic nautiloids of Kutch, India, during the fragmentation of Gondwana
Journal
of African
Earrh
Pergamon SO899-5362(00)00082-8
Scmces.
Vol. 31. No. 1. pp. 175-185. 2000 o 2000 Elsewer Scence Ltd All rights reserved. Printed I” Great Britam 0899-5362100 s- see front matter
Diversity and biogeographic distribution of Jurassic nautiloids of Kutch, India, during the fragmentation of Gondwana
Pakeontology
K. HALDER Division I, Geological Survey of India, CHQ, 15 A and 6, Kyd Street, Calcutta-700 016, India
ABSTRACT-Biogeographic distribution patterns of nautiloid species found in the Jurassic rocks of Kutch, India, are considered in relation to the fragmentation of Gondwana. The pattern of early diversity and endemism during Late Bathonian-Early Callovian times, followed later by a dominance of cosmopolitan forms particularly during the Oxfordian, is interpreted as a consequence of the rifting of Gondwana. The creation of a shallow basin immediately followed by drifting of the Indian Plate triggered early speciation events. The later development of sea routes made free fauna1 exchanges across the Tethys possible and resulted in a marked increase of pandemism in fauna1 association. Possible barriers to the dispersal of nautiloids are also briefly reviewed. o 2000 Elsevier Science Limited. All rights reserved. Rl%UMc-Le type de repartition biogeographique des especes de Nautildides dans les roches jurassiques de Kutch, Inde, est Btudie et consider6 dans I’optique de la fragmentation du Gondwana. Le type pr&oce de diversitg et d’enddmisme rbgnant au Bathonien superieur-Callovien infhrieur, suivi plus tard par la domination de formes cosmopolites surtout ZI I’Oxfordien, est interpret6 comme la consequence du rifting du Gondwana. La formation de bassins peu profonds, suivie immediatement par la derive de la plaque indienne, a declenchg les Bvenements prbcoces de spkciation. Le d&eloppement posterieur de voies marines a rendu possible les Bchanges de faune B travers la T&hys et provoque le pandemisme croissant des associations faunistiques. Les obstacles possibles a la dispersion des Natilo’ides sont aussi passes brievement en revue. 0 2000 Elsevier Science Limited. All rights reserved. (Received
519198: revised
version
received
1214199:
accepted
2514199)
INTRODUCTION A thick succession of sediments were deposited in Kutch during Bajocian to Albian times and were subdivided into four major litho-units. These are the Patcham, Chari, Katrol and Bhuj Formations in ascending order (Waagen, 1873-l 985; Nath, 1932; Mitra et al., 1979; Singh et al., 1982; Krishna, 1984). These are exposed as structural domes in several localities of Kutch including Jumara, the type section of the Chari Formation where the upper part of the Patcham Formation is also exposed. These two formations are noted for their abundance of marine invertebrate fossils. The Patcham and Chari Formations of Jumara and the
Chari Formation of some other localities like Keera, Jara and Jhura (Halder and Bardhan, 1996b; Fig. I) have yielded all the specimens that constitute the present study. They range in age from the Late Bathonian to Oxfordian (Bardhan et a/., 1994a). Nautiloids occur throughout this sequence, but are much rarer than the associated bivalves, ammonites or brachiopods and have received much less attention. However, recent systematic searches in the field areas have resulted in a reasonably large collection of nautiloids. Representatives come from all the horizons of the succession.
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K. HALDER
I
Procymatoceras
*
P.noet[ingi.
Figure 1. palieogeographic and oceans;
The
pictonicum l
Poracenoceras
l
P. jumarense
biogeographic distribution map is modified after blank: land areas
C P. of Enay
sp.
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A P.prahexagonum.
Y Pseudagonides
subbiongulatus.
Kutch nautiloid species during the and Cariou 11997). Hatched: marine
Current knowledge of taxonomy and distribution of Mesozoic nautiloids is unsatisfactory. Species were erected mainly on the basis of typological methods and their generic affiliations appear to be doubtful at times. They need thorough revision especially in the light of intraspecific variation and sexual dimorphism. More information is available from European countries, except for recent studies in Saudi Arabia (Tintant, 1987) and Kutch, India (Bardhan eta/., 1994b; Halder and Bardhan, 1996a, 1996b, 1997). Information is limited from other countries that were once part of Gondwana. The distribution pattern of nautiloid species in Kutch is related to the timing of Gondwana fragmentation. After Late Paleozoic rifting of the supercontinent, several land areas became sub-merged as a result of
176 Journal
A.
calloviense.
Late Bathonian. The continental platforms
a marine transgression. Fragmentation and separation of the continents followed during the Middle Jurassic (Haq et al., 1987; Bosellini, 1989) causing formation of new basins and creation of sea routes between distant regions. The opening of isolated basins is associated with characteristic biotic assemblages, especially of ammonites, leading to distinct biogeographic provinces which are variously called Realms or Fauna1 Provinces (Kaufmann, 1973). Kutch belongs to an Ethiopian or Indo-Madagascan Fauna1 Province, having similar assemblages to those found in countries like Ethiopia, eastern Africa and Madagascar that once bordered the Kutch Sea. The origin and temporal changes in the biogeographic distribution of endemic and pandemic nautiloid species in Kutch are seen in this light.
Diversity
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DISTRIBUTION OF NAUTILOIDS An overall similarity occurs in the composition of nautiloids at the generic level between the Ethiopian and Mediterranean Provinces. It is true also for other groups like ammonites, bivalves, gastropods, belemnites and brachiopods (Spath, 1927-l 933; Cox, 1940, 1952; Kitchin, 1900; Buckman, 1917; Das et al., in press). However, on closer inspection, the distribution pattern of nautiloids reveals that many new species had appeared in the Ethiopean Province during Bathonian-Callovian times, but this early differentiation in composition became blurred with the subsequent development of wellestablished sea routes. Interestingly, many other groups which invaded Kutch during this interval, especially Late Bathonian, underwent similar rapid diversification. Corals from the oldest exposed horizon of the Patcham Formation at Jumara gave rise to a multitude of species, of which about 65 are endemic (Gregory, 1900; Pandey and Fursich, 1993). Recently, Das et a/. (in press) reported 11 new species of gastropods, belonging to various genera from the Late Bathonian horizons of Kutch, which show a strong Tethyan affinity, particularly with that of Europe. Nautiloids, like other contemporary biota, used the newly formed sea routes and free fauna1 migration across the continents took place (Krishna and Cariou, 1990). It reveals, tf that the opening of a new basin initially tr speciation events and many new forms of _. -. - _ taxonomic groups appeared suddenly. The distribution of Kutch nautiloids is shown in the palasogeographic maps (Figs 1, 3 and 5) with the important forms figured (Figs 2,4 and 6). Their temporal distribution in Kutch, other countries of the Ethiopian Province and Europe, is shown in Fig. 7 for comparison. Changes in the distribution pattern through time are described here.
Late Bathonian The Late Bathonian of Kutch is characterised by the sudden appearance of several endemic nautiloid species. Paracenoceras calloviense (Oppel) (Fig. 2c) is among the earliest representatives of nautiloids, which appeared during the Late Bathonian in Kutch and continued till the Middle Callovian without any appreciable morphological change. It is believed to have originated in Kutch and has given rise to some endemic species iteratively (Bardhan et a/., 1994b). The species appeared in Mediterranean Europe only during the Callovian (Tintant, 1969). The species has also been described from the Early Callovian sequence of Madagascar (Spath, 1925). Another early species, i.e. Paracenoceras prohexagonum Spath (Fig. 2b), is, however, endemic
of Jurassic
nautiloids
of Kutch,
India
to the Ethiopian Province. The first record was from the Late Bathonian of Somalia (Spath, 1935). Later, P. prohexagonum and allied forms were described from the Late Bajocian to Early Bathonian sediments of Saudi Arabia (Tintant, 1987) and from the Late Bathonian of Kutch (Halder and Bardhan, 1997). Paracenoceras noetlingi Halder and Bardhan (Fig. 2f) is recorded from the Late Bathonian units of Kutch and Baluchistan (Noetling, 1896; Halder and Bardhan, 1997). P. jumarense (Waagen) (Fig. 2e), a ribbed descendant of P calloviense, is, however, restricted to a single horizon of a single locality: Jumara in Kutch. P. sp. A Halder and Bardhan (Fig. 2d), another endemic species of Kutch, is poorly represented and perhaps an iterative off-shoot of P. calloviense in the Late Bathonian (Bardhan et a/., 1994b; Halder and Bardhan, 1997). The other Bathonian genus of Kutch is Procymatoceras Spath, which is represented by P. sp. Spath (Fig. 2a). It shows wide intraspecific variability and homeomorphy with Cymatoceras Spath following Early Cretaceous radiation (Kummel, 1956, 1964). It is closely comparable, if not conspecific, with the Early Callovian Procymatoceraspictonicum of Europe (Tintant, 1970). This suggests a migration event after the establishment of a sea route between Kutch and Europe.
part of the Early Callovian sequence, Paracenoceras kumagunense (Waagen) (Fig. 4a), another endemic species, appeared in Kutch. (Waagen, 1873-l 875; Spath, 1927-l 933). It seems to be restricted to the Early Callovian and has been recorded from the contemporaneous horizon of Madagascar (Spath, 1925). Paracenoceras dorsoexcavatum Parona and Bonarelli (Fig. 4b, c), a Callovian dwarf species from Europe (Tintant, 1984) was recently found from this horizon of Kutch and from the Late Callovian of Saudi Arabia (Tintant, 1987). Another descendant of P. calloviense, P. sp. (Fig. 4d) has recently been recorded from Kutch (Halder and Bardhan, unpubl. data). Its highly inflated, depressed shell with wide flattened venter and angular ventrolateral margins makes it a very distinctive species, unique to Kutch. It shows a long temporal distribution from the latest Early Callovian to Late Callovian and has its acme in the Middle Callovian. The oldest record of Cymatonautilus Spath, a biostratigraphically important genus, comes from the latest Early Callovian in Kutch and is present elsewhere, as well as in Kutch, only up to the Middle Callovian (Halder and Bardhan, 1996b).
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K. HALDER
Figure
2. The Late Bathonian nautiloid species If) lateral views of Paracenoceras prohexagonum P. jumarense IFI 10060) (e) and the holotype
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of Kutch. (a) (Fl 100031 of P. noetlingi
Ventral view of (b), P. calloviense IGSI Type No.
Procymatoceras pictonicum IFI 102001; IFI 100511 (cl, P. sp. A IFI 10120) 2913) from Baluchisfan If).
IblId),
Diversity
I
and biogeographic
Procymotoceros
pictonicum.
l
+ P.dorso-excovatum. + Cymatonautilus Figure map areas.
3. The is modified
biogeographic after Enay
distribution
callignoni distribution and Cariou
of Jurassic
n Poracenoceros
P.SP.
nautiloids
colloviense
V Pseudaganides
.
of Kutch,
India
A P. kumogunense
subbiongubtus
C P.oganiticus.
4 C.pachygoster. of Kutch nautiloid (1997). Hatched:
Cymatonautilus is also reported from other countries of the Ethiopian Fauna1 Province, i.e. Madagascar and Saudi Arabia (Tintant, 1970, 1987). It also occurs in the northern border of Tethys in western Europe, i.e. Portugal, Spain, France and as far east as Austria (Tintant, 1981). It appears that the genus is longitudinally very restricted and has a wide latitudinal distribution, lying mainly within the palEosubtropical belts (Halder and Bardhan, 1996b). This short temporal and wide geographic extent makes Cymatonautilus a short-lived genus (sensu Ager, 1984). The Kutch species, C. co//ignoniTintant (Fig. 4f), according to Halder and Bardhan (1996b), is the
species during marine continental
the Callovian. platforms
and
The palzeogeographic oceans; blank:
land
oldest and appears to be endemic to the IndoMadagascan Fauna1 Province. They considered the older Paracenoceras prohexagonun as the ancestor from which Cymatonautilus was derived through peramorphosis (sensu McNamara, 1986). Cymatonautilus then migrated rather rapidly into other geographic areas in a manner compatible with the punctuation equilibrium model of evolution (cf. Gould and Eldredge, 1977). Another species from the same horizons of Kutch was recently found and identified as CymatonautiluspachygasterTintant (Fig. 4e) and was previously reported from the Early Callovian of Spain (Tintant, 1981).
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K. HALDER
Figure
4. The Callovian nautiloid species of Kutch. (a) Lateral view of the holotype of Paracenoceras kumagunense IGSI Type No. 1891); lb) and (cl lateraal and ventral views of P. dorso-excavatum (Fl 10205, Fl 10206J, respectively; (dJ ventral view of P. sp. (Fl 10127); (eJ and If) lateral views of Cymatonautilus pachygaster (Fl 102041 and C. collignoni IFI lOOOlJ, respectively; /gJ, (hJ lateral views of Pseudaganides subbiangulatus IFl 102031 and P. kutchensis I= P. aganiticus) IFI 102021, respectively.
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Diversity
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m Porocenoceros
hexogonum.
distribution
A
P. gigonteum
of Jurassic
nautiloids
OP. cf. lorioli
of Kutch,
c Pseudogonides
India
oqoniticus.
@P.off.ledonicus. Figure 5. The biogeographic map is modified after Enay areas.
distribution and Cariou
of the (1997).
Kutch nautiloid species during Hatched: marine continental
fseudaganides Spath, on the other hand, showed altogether a different evolutionary and migrational history. It evolved in Europe during the Bajocian, and the Late Bathonian species P. subbiangulatus (d’orbigny) (Fig. 49) of Europe (Marchand and Tintant, 1973) migrated later into the Kutch Sea during the early Middle Callovian (Halder and Bardhan, unpubl. data). On entering into Kutch, it speciated quickly by the Middle Callovian to another species, I? kutchensis (Waggen) (Fig. 4h). This new species shows both stratigraphical and morphological overlap with the European ancestor. I? kutchensis has a remarkable morphological corres-pondence with the Late CallovianMiddle Oxfordian P aganiticus (Schlotheim) of Europe (Marchand and Tintant, 1973). Both species are small with narrow umbilicus, elevated section, convex to
the Oxfordian. platforms
and
The palzeogeographic oceans; blank:
land
flat flanks and narrow venter with a similar sutural pattern. It is likely that they are conspecific and represent two geographic variants. Their stratigraphical heterochroneity is resolved by the recent discovery of P. kutchensis from the Late Callovian horizon of Kutch (Halder and Bardhan, unpubl. data). It therefore appears that the species originated in Kutch during the early Middle Callovian and migrated during the Late Callovian time to Europe where it flourished.
Oxfordian The Oxfordian nautiloid fauna of Kutch comprises species of two genera, namely Paracenoceras and Pseudaganides. The former consists of three species: i) P. hexagonum (Sowerby) (Fig. 6a, b); 4 P. giganteum (d’orbigny) (Fig. 6~); and
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K. HALDER
0
Figure 6. hexagonum Pseudaganides
The Oxfordian nautiloid species of Kutch. (a) and IFI 101031; (cl-(e) lateral views of P. giganteum aff, ledonicus IFI 10201) (e).
I$) P. cf. /orioli(Loesch) (Fig. 6d) (Halder and Bardhan, 1996a). The latter is represented by a single species, i.e. Pseudaganides aff. ledonicus (de Loriol) (Fig. 6e). The interesting feature of the Kutch Oxfordian nautiloid assemblage is that its previous endemic character appears to be lost and instead is marked by pandemism. All the paracenoceratid species have been recorded from elsewhere, particularly in Europe (Fig. 5). P. hexagonum is reported from the EarlyMiddle Oxfordian of western Europe and Saudi Arabia (Tintant, 1987). P. giganteum is stratigraphically younger and is found from the Late Oxfordian of France. Pseudaganides ledonicus is a Middle-Late Oxfordian form from France and Switzerland (Marchand and Tintant, 1973). However, all these
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(bJ Lateral lFl 10121)
and fcl,
ventral P. cf.
views lorioli
of lFl
zii
Paracenoceras 10123) (d)
1
and
species have been found in a single horizon in Kutch, i.e. from the topmost conglomeratic part of the Dhosa Oolite Member (Mitra et al., 1979). This horizon is a condensed sequence deposited as a transgressive lag (Singh, 1989; Fursich et al., 1992) and the faunas of different ages are consequently mixed.
DISCUSSION From the above information on the distribution of the Jurassic nautiloids of Kutch, some facts immediately emerge. First, all endemic nautiloid species of Kutch appeared during Late Bathonian to Early Callovian times. This interval marks a rapid speciation event characterising many taxonomic groups. Second, several of the nautiloid forms that appeared during
Diversity
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this interval of time in Kutch are recorded in younger horizons in other provinces. Finally, nautiloid species largely became cosmopolitan from the Middle Callovian onwards, particularly during the Oxfordian (Fig. 7). This distribution pattern, when compared with the fragmentation of Gondwana, sheds light on the tectonic evolution of the basin. Sea-floor spreading between Africa and the Indo-Madagascan Block started during the Callovian-Early Oxfordian time (Coffin and Rabinowitz, 1987, 1988; Haq eta/., 1987; Bosellini, 1989). However, it was preceded by earlier phases of rifting and continental stretching that started during the Late Palaeozoic and persisted until the Jurassic (Bosellini, 19891, causing formation of a complex system of tectonic lows. The rifting and break-up of Gondwana, and Mesozoic eustatic sealevel rise (Haq et a/., 1987; Hallam, 1988) caused the sea to transgress over this complex system of depressions during Early to Middle Jurassic times. The oldest marine sediments of Early Jurassic age were recorded from East Africa and Madagascar (Bosellini, 1989). The Kutch Basin opened up during the beginning of rifting of the Indian Plate from West Gondwana in the Late Triassic (Biswas, 1982). Marine transgression in the basin started during the rift-drift transition phase of the Indian Plate (Biswas, 1991). The earliest record of marine sediments of Kutch is from the Bajocian-Bathonian (Singh et a/., 1982).
T I
T II
of Jurassic
nautiloids
of Kutch,
India
During this early stage, the newly opened basin acted as a centre of speciation and promoted endemism. In Kutch, at least eight new species appeared during the Late Bathonian-Early Callovian (Fig. 7). However, the development of shallow migrational pathways between this province and Europe did not take much time. At least two Indian, Late Bathonian forms appeared in Europe by the beginning of the Callovian. Later, migration in both directions was established, and during the Oxfordian, most of the species became common to both provinces. The migration route was presumably the Tethyan Shallow Shelf (Cariou, 1973).
CONCLUSION Kummel (1956) recognised three surges in the evolution of post-Triassic nautiloids during the Jurassic, Cretaceous and Early Tertiary. The first two had a much greater effect in shaping nautiloid history by producing more than ten genera each, whereas the last surge produced five (fig. 4: Kummel, 1956; figs 292, 293: Kummel, 1964; Teichert and Matsumoto, 1987). The species diversity shows an overall tendency to increase with minor fluctuations during the Mesozoic and Early Tertiary (fig. 5: Kummel, 1956). Even recent data support a sustained increase in species diversity throughout the Jurassic: 69 in the Early Jurassic; 79 in the Middle Jurassic; and 80
r
-
T
T-. ! ’ i
12: I
I
I-
I
I
31
I
3
I
I
:3
i
I
J
jK
LlMjNlO
-
I
Figure 7. The temporal distribution of Kutch nautiloid species in Kutch (I), in other areas of the Ethiopian Province (2) and in Europe 131. A: Procymatoceras pictonicum; B: Paracenoceras calloviense; C: P. prohexagonum; D: P. noetlingi; E: P. jumarense; F: P. sp. A; G: P. kumagunense; H: P. dorso-excavatum; I: P. sp.; J: P. hexagonurn; K: P. giganteum; L: P. cf. lorioli; M: Pseudaganides subbiangulatus; N: P. kutchensis (= P. aganiticusl; 0: P. aff. ledonicus; P: Cymatonautilus collignoni; 0: C. pachygaster.
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K. HALDER in the Late Jurassic (main literature source includes Tintant, 1970, 1981, 1987; Halder and Bardhan, 1997; and personal observation). The appearance of new forms is related to the development of new basins during the Mesozoic. In the Jurassic, the Indian Ocean and North Atlantic Ocean were formed and the post-Triassic ‘ecological vacuum’ was quickly colohised by a single genus, Cenoceras, that crossed the Triassic-Jurassic boundary and diversified rapidly (Kummel, 1956, 1964). During the Cretaceous, the South Atlantic opened up between Africa and South America (Dietz and Holden, 1970), which facilitated the evolution and diversification of the Cretaceous Cymatoceras, and its contemporaries. genus, However, the Early Tertiary radiation of nautiloids was favoured by the disappearance of ammonites in the end-Cretaceous mass extinction (Ward, 1987). Development of new basins in the extreme north and south latitudes during the Tertiary and Quaternary due to the opening of the North Atlantic and the separation of the Australian Plate from Antarctica (Dietz and Holden, 1970) have had an effect on further nautiloid evolution. It appears that the distribution of post-Triassic nautiloids was largely controlled by climatic conditions. Despite the fact that the Mesozoic climate was warmer and more equable than present (Enay and Cariou, 1997), the absence of nautiloids in the remote northern and southern latitudes and their concentration in and near the Tethyan Realm are indicative of its stenotopic nature. In this context, it is interesting to note that present-day Nautilus is a warm water stenothermal (10-25”(Z) organism restricted to the relatively deeper reaches (100-300 m) of only the Indo-Pacific Sea (Saunders, 1987; Saunders and Ward, 1987; Hayasaka et al., 1987; Ward, 1987). Long-term lateral migration of Mesozoic nautiloids along the shallow shelf was hindered by shell-crushing predators such as brachysauran crabs, teleost fishes, spiny crustaceans etc., which arose suddenly during the Mid-Mesozoic (Vermeij, 1977) forcing molluscs, echinoids and other marine invertebrates to undergo changes in morphology and habitat. It is worth noting that Recent Nautilus is vulnerable to predation by octopuses and particularly by teleost fishes in shallow seas (Saunders et al., 1987). Nautiloids, as contemporaneous ammonites, responded to the emergence of strongly-jawed predators by coarsening ribs and by increasing the frequency of ornamented forms (Bardhan and Halder, inpress; Ward, 1981).
ACKNOWLEDGEMENTS S. Bardhan (Geological
184 Journal
(Jadavpur University) and T.C. Lahiri Survey of India) critically reviewed the
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manuscript and provided many valuable suggestions. The author is grateful to D. Mukherjee, also of Jadavpur University, for her help with drawings and photography. Kind permission, given by the Director General of the Geological Survey of India for publication of the manuscript, is duly acknowledged. The author also acknowledges two anonymous reviewers for their valuable suggestions.
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and biogeographic
distribution
FDrsich, F.T., Oschmann, W., Singh, I.B., Jaitley, A.K., 1992. Hardgrounds, reworked concretion levels and condensed horizons in the Jurassic of western India: their significance for basin analysis. Journal Geological Society London 149, 313-331. Gould, S.J., Eldredge, N., 1977. Punctuated equilibria: the tempo and mode of evolution reconsidered. Paleobiology 3, 115-I 51. Gregory, J.W., 1900. Jurassic fauna of Cutch: the corals. Palaeontologia lndica 9 (2). 12-196. Halder, K., Bardhan, S., 1996a. The Oxfordian (Upper Jurassic) nautiloid fauna of Kutch, western India. Neues Jahrbuch Geologie Palaontologie, Abhandlungen 201, 1732. 1996b. The fleeting genus Halder, K., Bardhan, S., CymatonauT;/us (Nautiloidea): new record from the Jurassic Chari Formation, Kutch, India. Canadian Journal Earth Sciences 33, 1007-1010. Halder, K., Bardhan, S., 1997. On some new Late Bathonian paracenoceratids (Nautiloidea) from Kutch, India and their evolutionary and biostratigraphic implications. Neues Jahrbuch Geologie Palaontologie, Monatshefte 1997, 543-561. Hallam, A., 1988. A re-evaluation of Jurassic eustasy in the light of new data and the revised exxon curve. In: Wilgus, C.K. et a/. (Ed?..), Sea Level Changes: An Iintegrated Appproach. Society Economic Palaeontologists Mineralogists, Special Publication 42, 261-273. Haq, B.U., Hardenbol, J., Vail, P.R., 1987. Chronology of fluctuating sea-levels since the Triassic. Science 235, 1156-1167. Hayasaka, S., Oki, K., Tanabe, K., Saisho, T., Shinomiya, A., 1987. On the habitat of Nautilus pompilius in Tanon Strait [Philippines) and the Fiji Islands. In: Saunders, W.B., Landman, N.H. (Eds.), Nautilus: The Biology and Paleobiology of a Living Fossil. Plenum Press, New York, pp. 179-200. Kauffman, E.G., 1973. Cretaceous Bivalvia. In: Hallam, A. (Ed.), Atlas of Palaeobiogeography. Elsevier Scientific Publishing Company, Amsterdam, pp. 353-383. Kitchin, F.L., 1900. The brachiopods of Kutch. Palaeontologia lndica 9 (3), l-87. Krishna, J., 1984. Current status of Jurassic stratigraphy of Kachchh, western India. International Symposium on Jurassic Stratigraphy 3, 730-741, Krishna, J., Cariou, E., 1990. Ammonoid fauna1 exchanges during Lower Callovian between the Indo-East African and Submediterranean Provinces: implications for the long distance east-west correlations. Newsletters Stratigraphy 23, 109122. Kummel, B., 1956. Post-Triassic nautiloid genera. Bulletin Museum Comparative Zoology, Harvard College 114 (7), 494p. Kummel, B., 1964. Nautiloidea-Nautilida. In: Moore, R.C. (Ed.), Treatise on Invertebrate Paleontology. Geological Society America University Kansas Press, Lawrence, pp. K383-K466. Marchand, D., Tintant, H., 1973. Etudes statistiques sur Pseudaganides aganiticus (Sclotheim) et diverses especes voisines. Bulletin Scientifique Bourgogne 28, 1 1 l-l 69. McNamara, K.J., 1986. A guide to the nomenclature of heterochrony. Journal Paleontology 60, 4-l 3. Mitra, K.C., Bardhan, S., Bhattacharya, D., 1979. A study of Mesozoic stratigraphy of Kutch, Gujarat, with special reference to rock-stratigraphy and bio-stratigraphy of Keera dome. Bulletin Indian Geologists Association 12 (11, 129-143.
of Jurassic
nautiloids
of Kutch,
India
Nath, R., 1932. A contribution to the stratigraphy of Kutch. Quarterly Journal Geological Mining Metallurgical Society India 4, 161-174. Noetling, F., 1896. Fauna of the Kellaways of Mazar Drik. Palaaontologia lndica Memoire 16 (I), l-22. Pandey, D.K., Ftirsich, F.T., 1993. Contributions to the Jurassic of Kachchh, Western India. I. The coral fauna. Beringeria 8, 3-69. 1987. The species of Nautilus. In: Saunders, W.B., Saunders, W.B., Landman, N.H. (Eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Plenum Press, New York, pp. 33-52. Saunders, W.B., Spinosa, C., Davis, L.E., 1987. Predation on Nautilus. In: Saunders, W.B., Landman, N.H. (Eds.), Nautilus: The Biology and Paleobiology of a Living Fossil. Plenum Press, New York, pp. 201-212. Saunders, W.B., Ward, P.D., 1987. Ecology, distribution and population characteristics of Nautilus. In: Saunders, W.B., Landman, N.H. (Eds.), Nautilus: The Biology and Paleobiology of a Living Fossil. Plenum Press, New York, pp. 137-162. Singh, C.S.P., JaitlY, A.K., Pandey, D.K., 1982. First report of some Bajocian-Bathonian (Middle Jurassic) ammonoids and the age of oldest sediments from Kachchh, W. India. Newsletters Stratigraphy 11, 3740. Singh, I.B., 1989. Dhosa Oolite-a transgressive condensation horizon of Oxfordian age in Kachchh, western India. Journal Geological Society India 34, 152160. Spath, L.F., 1925. Jurassic Cephalopoda from Madagascar. Bulletins American Paleontology 2 (44), 141-170. Spath, L.F., 1927-1933. Revision of the Jurassic cephalopod fauna of Kachh (Cutch). Palaaontologia Indica, New series 9 (2), 945p. Spath, L.F., 1935. Jurassic and Cretaceous Cephalopda. The Mesozoic palaaontology of British Somaliland. Geology Palaeontology British Somaliland 2, 205-228. Teichert, C., Matsumoto, T., 1987. The ancestry of the genus Nautilus. In: Saunders, W.B., Landman, N.H. (Eds.), Nautilus: The Biology and Paleobiology of a Living Fossil. Plenum Press, New York, pp. 25-32. Tintant, H., 1969. Un cas de dimorphisme chez les Paracenoceras (Nautiloideal du Callovien. In: Westermann, G.E.G. (Ed.), Sexual Dimorphism in Fossil Metazoa and Taxonomic Implications. IUGS, Series Al, 167-184. Tintant, H., 1970. Les ‘Nautiles a c&es’ du Jurassuque. Annales Paleontologie (Invertebres) 55 (I), 53-96. Tintant, H., 1981. Un cas de parallelisme Bvolutif synchrone chez les Nautiles a c&es du Jurassique. Boletim Sociedade Geoldgica Portugal 22, 63-69. Tintant, H., 1984. Exemples de nanisme specifique chez les Nautiloides du genre Paracenoceras au Jurassique moyen. Geobios, Memoire Special 8, 403-410. Tintant, H., 1987. Les Nautiles du Jurassique d’Arabie Saoudite. Geobios, Memoire Special 9, 67-129. Vermeij, G., 1977. The Mesozoic marine revolution: evidence from snails, predators and grazers. Paleobiology 3, 245-258. Waagen, W., 1873-1875. Jurassic fauna of Kutch: the Cephalopoda. Palaeontologia Indica, Series 9 (I), 247~. Ward, P.D., 1981. Shell sculpture as a defensive adaptation in ammonoids. Paleobiology 7 (1). 96-100. Ward, P.D., 1987. The natural history of Nautilus. Allen and Unwin, Boston, 267~.
Journal
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Pergamon SO899-5362(00)00082-8
Scmces.
Vol. 31. No. 1. pp. 175-185. 2000 o 2000 Elsewer Scence Ltd All rights reserved. Printed I” Great Britam 0899-5362100 s- see front matter
Diversity and biogeographic distribution of Jurassic nautiloids of Kutch, India, during the fragmentation of Gondwana
Pakeontology
K. HALDER Division I, Geological Survey of India, CHQ, 15 A and 6, Kyd Street, Calcutta-700 016, India
ABSTRACT-Biogeographic distribution patterns of nautiloid species found in the Jurassic rocks of Kutch, India, are considered in relation to the fragmentation of Gondwana. The pattern of early diversity and endemism during Late Bathonian-Early Callovian times, followed later by a dominance of cosmopolitan forms particularly during the Oxfordian, is interpreted as a consequence of the rifting of Gondwana. The creation of a shallow basin immediately followed by drifting of the Indian Plate triggered early speciation events. The later development of sea routes made free fauna1 exchanges across the Tethys possible and resulted in a marked increase of pandemism in fauna1 association. Possible barriers to the dispersal of nautiloids are also briefly reviewed. o 2000 Elsevier Science Limited. All rights reserved. Rl%UMc-Le type de repartition biogeographique des especes de Nautildides dans les roches jurassiques de Kutch, Inde, est Btudie et consider6 dans I’optique de la fragmentation du Gondwana. Le type pr&oce de diversitg et d’enddmisme rbgnant au Bathonien superieur-Callovien infhrieur, suivi plus tard par la domination de formes cosmopolites surtout ZI I’Oxfordien, est interpret6 comme la consequence du rifting du Gondwana. La formation de bassins peu profonds, suivie immediatement par la derive de la plaque indienne, a declenchg les Bvenements prbcoces de spkciation. Le d&eloppement posterieur de voies marines a rendu possible les Bchanges de faune B travers la T&hys et provoque le pandemisme croissant des associations faunistiques. Les obstacles possibles a la dispersion des Natilo’ides sont aussi passes brievement en revue. 0 2000 Elsevier Science Limited. All rights reserved. (Received
519198: revised
version
received
1214199:
accepted
2514199)
INTRODUCTION A thick succession of sediments were deposited in Kutch during Bajocian to Albian times and were subdivided into four major litho-units. These are the Patcham, Chari, Katrol and Bhuj Formations in ascending order (Waagen, 1873-l 985; Nath, 1932; Mitra et al., 1979; Singh et al., 1982; Krishna, 1984). These are exposed as structural domes in several localities of Kutch including Jumara, the type section of the Chari Formation where the upper part of the Patcham Formation is also exposed. These two formations are noted for their abundance of marine invertebrate fossils. The Patcham and Chari Formations of Jumara and the
Chari Formation of some other localities like Keera, Jara and Jhura (Halder and Bardhan, 1996b; Fig. I) have yielded all the specimens that constitute the present study. They range in age from the Late Bathonian to Oxfordian (Bardhan et a/., 1994a). Nautiloids occur throughout this sequence, but are much rarer than the associated bivalves, ammonites or brachiopods and have received much less attention. However, recent systematic searches in the field areas have resulted in a reasonably large collection of nautiloids. Representatives come from all the horizons of the succession.
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K. HALDER
I
Procymatoceras
*
P.noet[ingi.
Figure 1. palieogeographic and oceans;
The
pictonicum l
Poracenoceras
l
P. jumarense
biogeographic distribution map is modified after blank: land areas
C P. of Enay
sp.
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A P.prahexagonum.
Y Pseudagonides
subbiongulatus.
Kutch nautiloid species during the and Cariou 11997). Hatched: marine
Current knowledge of taxonomy and distribution of Mesozoic nautiloids is unsatisfactory. Species were erected mainly on the basis of typological methods and their generic affiliations appear to be doubtful at times. They need thorough revision especially in the light of intraspecific variation and sexual dimorphism. More information is available from European countries, except for recent studies in Saudi Arabia (Tintant, 1987) and Kutch, India (Bardhan eta/., 1994b; Halder and Bardhan, 1996a, 1996b, 1997). Information is limited from other countries that were once part of Gondwana. The distribution pattern of nautiloid species in Kutch is related to the timing of Gondwana fragmentation. After Late Paleozoic rifting of the supercontinent, several land areas became sub-merged as a result of
176 Journal
A.
calloviense.
Late Bathonian. The continental platforms
a marine transgression. Fragmentation and separation of the continents followed during the Middle Jurassic (Haq et al., 1987; Bosellini, 1989) causing formation of new basins and creation of sea routes between distant regions. The opening of isolated basins is associated with characteristic biotic assemblages, especially of ammonites, leading to distinct biogeographic provinces which are variously called Realms or Fauna1 Provinces (Kaufmann, 1973). Kutch belongs to an Ethiopian or Indo-Madagascan Fauna1 Province, having similar assemblages to those found in countries like Ethiopia, eastern Africa and Madagascar that once bordered the Kutch Sea. The origin and temporal changes in the biogeographic distribution of endemic and pandemic nautiloid species in Kutch are seen in this light.
Diversity
and biogeographic
distribution
DISTRIBUTION OF NAUTILOIDS An overall similarity occurs in the composition of nautiloids at the generic level between the Ethiopian and Mediterranean Provinces. It is true also for other groups like ammonites, bivalves, gastropods, belemnites and brachiopods (Spath, 1927-l 933; Cox, 1940, 1952; Kitchin, 1900; Buckman, 1917; Das et al., in press). However, on closer inspection, the distribution pattern of nautiloids reveals that many new species had appeared in the Ethiopean Province during Bathonian-Callovian times, but this early differentiation in composition became blurred with the subsequent development of wellestablished sea routes. Interestingly, many other groups which invaded Kutch during this interval, especially Late Bathonian, underwent similar rapid diversification. Corals from the oldest exposed horizon of the Patcham Formation at Jumara gave rise to a multitude of species, of which about 65 are endemic (Gregory, 1900; Pandey and Fursich, 1993). Recently, Das et a/. (in press) reported 11 new species of gastropods, belonging to various genera from the Late Bathonian horizons of Kutch, which show a strong Tethyan affinity, particularly with that of Europe. Nautiloids, like other contemporary biota, used the newly formed sea routes and free fauna1 migration across the continents took place (Krishna and Cariou, 1990). It reveals, tf that the opening of a new basin initially tr speciation events and many new forms of _. -. - _ taxonomic groups appeared suddenly. The distribution of Kutch nautiloids is shown in the palasogeographic maps (Figs 1, 3 and 5) with the important forms figured (Figs 2,4 and 6). Their temporal distribution in Kutch, other countries of the Ethiopian Province and Europe, is shown in Fig. 7 for comparison. Changes in the distribution pattern through time are described here.
Late Bathonian The Late Bathonian of Kutch is characterised by the sudden appearance of several endemic nautiloid species. Paracenoceras calloviense (Oppel) (Fig. 2c) is among the earliest representatives of nautiloids, which appeared during the Late Bathonian in Kutch and continued till the Middle Callovian without any appreciable morphological change. It is believed to have originated in Kutch and has given rise to some endemic species iteratively (Bardhan et a/., 1994b). The species appeared in Mediterranean Europe only during the Callovian (Tintant, 1969). The species has also been described from the Early Callovian sequence of Madagascar (Spath, 1925). Another early species, i.e. Paracenoceras prohexagonum Spath (Fig. 2b), is, however, endemic
of Jurassic
nautiloids
of Kutch,
India
to the Ethiopian Province. The first record was from the Late Bathonian of Somalia (Spath, 1935). Later, P. prohexagonum and allied forms were described from the Late Bajocian to Early Bathonian sediments of Saudi Arabia (Tintant, 1987) and from the Late Bathonian of Kutch (Halder and Bardhan, 1997). Paracenoceras noetlingi Halder and Bardhan (Fig. 2f) is recorded from the Late Bathonian units of Kutch and Baluchistan (Noetling, 1896; Halder and Bardhan, 1997). P. jumarense (Waagen) (Fig. 2e), a ribbed descendant of P calloviense, is, however, restricted to a single horizon of a single locality: Jumara in Kutch. P. sp. A Halder and Bardhan (Fig. 2d), another endemic species of Kutch, is poorly represented and perhaps an iterative off-shoot of P. calloviense in the Late Bathonian (Bardhan et a/., 1994b; Halder and Bardhan, 1997). The other Bathonian genus of Kutch is Procymatoceras Spath, which is represented by P. sp. Spath (Fig. 2a). It shows wide intraspecific variability and homeomorphy with Cymatoceras Spath following Early Cretaceous radiation (Kummel, 1956, 1964). It is closely comparable, if not conspecific, with the Early Callovian Procymatoceraspictonicum of Europe (Tintant, 1970). This suggests a migration event after the establishment of a sea route between Kutch and Europe.
part of the Early Callovian sequence, Paracenoceras kumagunense (Waagen) (Fig. 4a), another endemic species, appeared in Kutch. (Waagen, 1873-l 875; Spath, 1927-l 933). It seems to be restricted to the Early Callovian and has been recorded from the contemporaneous horizon of Madagascar (Spath, 1925). Paracenoceras dorsoexcavatum Parona and Bonarelli (Fig. 4b, c), a Callovian dwarf species from Europe (Tintant, 1984) was recently found from this horizon of Kutch and from the Late Callovian of Saudi Arabia (Tintant, 1987). Another descendant of P. calloviense, P. sp. (Fig. 4d) has recently been recorded from Kutch (Halder and Bardhan, unpubl. data). Its highly inflated, depressed shell with wide flattened venter and angular ventrolateral margins makes it a very distinctive species, unique to Kutch. It shows a long temporal distribution from the latest Early Callovian to Late Callovian and has its acme in the Middle Callovian. The oldest record of Cymatonautilus Spath, a biostratigraphically important genus, comes from the latest Early Callovian in Kutch and is present elsewhere, as well as in Kutch, only up to the Middle Callovian (Halder and Bardhan, 1996b).
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K. HALDER
Figure
2. The Late Bathonian nautiloid species If) lateral views of Paracenoceras prohexagonum P. jumarense IFI 10060) (e) and the holotype
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of Kutch. (a) (Fl 100031 of P. noetlingi
Ventral view of (b), P. calloviense IGSI Type No.
Procymatoceras pictonicum IFI 102001; IFI 100511 (cl, P. sp. A IFI 10120) 2913) from Baluchisfan If).
IblId),
Diversity
I
and biogeographic
Procymotoceros
pictonicum.
l
+ P.dorso-excovatum. + Cymatonautilus Figure map areas.
3. The is modified
biogeographic after Enay
distribution
callignoni distribution and Cariou
of Jurassic
n Poracenoceros
P.SP.
nautiloids
colloviense
V Pseudaganides
.
of Kutch,
India
A P. kumogunense
subbiongubtus
C P.oganiticus.
4 C.pachygoster. of Kutch nautiloid (1997). Hatched:
Cymatonautilus is also reported from other countries of the Ethiopian Fauna1 Province, i.e. Madagascar and Saudi Arabia (Tintant, 1970, 1987). It also occurs in the northern border of Tethys in western Europe, i.e. Portugal, Spain, France and as far east as Austria (Tintant, 1981). It appears that the genus is longitudinally very restricted and has a wide latitudinal distribution, lying mainly within the palEosubtropical belts (Halder and Bardhan, 1996b). This short temporal and wide geographic extent makes Cymatonautilus a short-lived genus (sensu Ager, 1984). The Kutch species, C. co//ignoniTintant (Fig. 4f), according to Halder and Bardhan (1996b), is the
species during marine continental
the Callovian. platforms
and
The palzeogeographic oceans; blank:
land
oldest and appears to be endemic to the IndoMadagascan Fauna1 Province. They considered the older Paracenoceras prohexagonun as the ancestor from which Cymatonautilus was derived through peramorphosis (sensu McNamara, 1986). Cymatonautilus then migrated rather rapidly into other geographic areas in a manner compatible with the punctuation equilibrium model of evolution (cf. Gould and Eldredge, 1977). Another species from the same horizons of Kutch was recently found and identified as CymatonautiluspachygasterTintant (Fig. 4e) and was previously reported from the Early Callovian of Spain (Tintant, 1981).
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K. HALDER
Figure
4. The Callovian nautiloid species of Kutch. (a) Lateral view of the holotype of Paracenoceras kumagunense IGSI Type No. 1891); lb) and (cl lateraal and ventral views of P. dorso-excavatum (Fl 10205, Fl 10206J, respectively; (dJ ventral view of P. sp. (Fl 10127); (eJ and If) lateral views of Cymatonautilus pachygaster (Fl 102041 and C. collignoni IFI lOOOlJ, respectively; /gJ, (hJ lateral views of Pseudaganides subbiangulatus IFl 102031 and P. kutchensis I= P. aganiticus) IFI 102021, respectively.
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Diversity
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m Porocenoceros
hexogonum.
distribution
A
P. gigonteum
of Jurassic
nautiloids
OP. cf. lorioli
of Kutch,
c Pseudogonides
India
oqoniticus.
@P.off.ledonicus. Figure 5. The biogeographic map is modified after Enay areas.
distribution and Cariou
of the (1997).
Kutch nautiloid species during Hatched: marine continental
fseudaganides Spath, on the other hand, showed altogether a different evolutionary and migrational history. It evolved in Europe during the Bajocian, and the Late Bathonian species P. subbiangulatus (d’orbigny) (Fig. 49) of Europe (Marchand and Tintant, 1973) migrated later into the Kutch Sea during the early Middle Callovian (Halder and Bardhan, unpubl. data). On entering into Kutch, it speciated quickly by the Middle Callovian to another species, I? kutchensis (Waggen) (Fig. 4h). This new species shows both stratigraphical and morphological overlap with the European ancestor. I? kutchensis has a remarkable morphological corres-pondence with the Late CallovianMiddle Oxfordian P aganiticus (Schlotheim) of Europe (Marchand and Tintant, 1973). Both species are small with narrow umbilicus, elevated section, convex to
the Oxfordian. platforms
and
The palzeogeographic oceans; blank:
land
flat flanks and narrow venter with a similar sutural pattern. It is likely that they are conspecific and represent two geographic variants. Their stratigraphical heterochroneity is resolved by the recent discovery of P. kutchensis from the Late Callovian horizon of Kutch (Halder and Bardhan, unpubl. data). It therefore appears that the species originated in Kutch during the early Middle Callovian and migrated during the Late Callovian time to Europe where it flourished.
Oxfordian The Oxfordian nautiloid fauna of Kutch comprises species of two genera, namely Paracenoceras and Pseudaganides. The former consists of three species: i) P. hexagonum (Sowerby) (Fig. 6a, b); 4 P. giganteum (d’orbigny) (Fig. 6~); and
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K. HALDER
0
Figure 6. hexagonum Pseudaganides
The Oxfordian nautiloid species of Kutch. (a) and IFI 101031; (cl-(e) lateral views of P. giganteum aff, ledonicus IFI 10201) (e).
I$) P. cf. /orioli(Loesch) (Fig. 6d) (Halder and Bardhan, 1996a). The latter is represented by a single species, i.e. Pseudaganides aff. ledonicus (de Loriol) (Fig. 6e). The interesting feature of the Kutch Oxfordian nautiloid assemblage is that its previous endemic character appears to be lost and instead is marked by pandemism. All the paracenoceratid species have been recorded from elsewhere, particularly in Europe (Fig. 5). P. hexagonum is reported from the EarlyMiddle Oxfordian of western Europe and Saudi Arabia (Tintant, 1987). P. giganteum is stratigraphically younger and is found from the Late Oxfordian of France. Pseudaganides ledonicus is a Middle-Late Oxfordian form from France and Switzerland (Marchand and Tintant, 1973). However, all these
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(bJ Lateral lFl 10121)
and fcl,
ventral P. cf.
views lorioli
of lFl
zii
Paracenoceras 10123) (d)
1
and
species have been found in a single horizon in Kutch, i.e. from the topmost conglomeratic part of the Dhosa Oolite Member (Mitra et al., 1979). This horizon is a condensed sequence deposited as a transgressive lag (Singh, 1989; Fursich et al., 1992) and the faunas of different ages are consequently mixed.
DISCUSSION From the above information on the distribution of the Jurassic nautiloids of Kutch, some facts immediately emerge. First, all endemic nautiloid species of Kutch appeared during Late Bathonian to Early Callovian times. This interval marks a rapid speciation event characterising many taxonomic groups. Second, several of the nautiloid forms that appeared during
Diversity
and biogeographic
distribution
this interval of time in Kutch are recorded in younger horizons in other provinces. Finally, nautiloid species largely became cosmopolitan from the Middle Callovian onwards, particularly during the Oxfordian (Fig. 7). This distribution pattern, when compared with the fragmentation of Gondwana, sheds light on the tectonic evolution of the basin. Sea-floor spreading between Africa and the Indo-Madagascan Block started during the Callovian-Early Oxfordian time (Coffin and Rabinowitz, 1987, 1988; Haq eta/., 1987; Bosellini, 1989). However, it was preceded by earlier phases of rifting and continental stretching that started during the Late Palaeozoic and persisted until the Jurassic (Bosellini, 19891, causing formation of a complex system of tectonic lows. The rifting and break-up of Gondwana, and Mesozoic eustatic sealevel rise (Haq et a/., 1987; Hallam, 1988) caused the sea to transgress over this complex system of depressions during Early to Middle Jurassic times. The oldest marine sediments of Early Jurassic age were recorded from East Africa and Madagascar (Bosellini, 1989). The Kutch Basin opened up during the beginning of rifting of the Indian Plate from West Gondwana in the Late Triassic (Biswas, 1982). Marine transgression in the basin started during the rift-drift transition phase of the Indian Plate (Biswas, 1991). The earliest record of marine sediments of Kutch is from the Bajocian-Bathonian (Singh et a/., 1982).
T I
T II
of Jurassic
nautiloids
of Kutch,
India
During this early stage, the newly opened basin acted as a centre of speciation and promoted endemism. In Kutch, at least eight new species appeared during the Late Bathonian-Early Callovian (Fig. 7). However, the development of shallow migrational pathways between this province and Europe did not take much time. At least two Indian, Late Bathonian forms appeared in Europe by the beginning of the Callovian. Later, migration in both directions was established, and during the Oxfordian, most of the species became common to both provinces. The migration route was presumably the Tethyan Shallow Shelf (Cariou, 1973).
CONCLUSION Kummel (1956) recognised three surges in the evolution of post-Triassic nautiloids during the Jurassic, Cretaceous and Early Tertiary. The first two had a much greater effect in shaping nautiloid history by producing more than ten genera each, whereas the last surge produced five (fig. 4: Kummel, 1956; figs 292, 293: Kummel, 1964; Teichert and Matsumoto, 1987). The species diversity shows an overall tendency to increase with minor fluctuations during the Mesozoic and Early Tertiary (fig. 5: Kummel, 1956). Even recent data support a sustained increase in species diversity throughout the Jurassic: 69 in the Early Jurassic; 79 in the Middle Jurassic; and 80
r
-
T
T-. ! ’ i
12: I
I
I-
I
I
31
I
3
I
I
:3
i
I
J
jK
LlMjNlO
-
I
Figure 7. The temporal distribution of Kutch nautiloid species in Kutch (I), in other areas of the Ethiopian Province (2) and in Europe 131. A: Procymatoceras pictonicum; B: Paracenoceras calloviense; C: P. prohexagonum; D: P. noetlingi; E: P. jumarense; F: P. sp. A; G: P. kumagunense; H: P. dorso-excavatum; I: P. sp.; J: P. hexagonurn; K: P. giganteum; L: P. cf. lorioli; M: Pseudaganides subbiangulatus; N: P. kutchensis (= P. aganiticusl; 0: P. aff. ledonicus; P: Cymatonautilus collignoni; 0: C. pachygaster.
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K. HALDER in the Late Jurassic (main literature source includes Tintant, 1970, 1981, 1987; Halder and Bardhan, 1997; and personal observation). The appearance of new forms is related to the development of new basins during the Mesozoic. In the Jurassic, the Indian Ocean and North Atlantic Ocean were formed and the post-Triassic ‘ecological vacuum’ was quickly colohised by a single genus, Cenoceras, that crossed the Triassic-Jurassic boundary and diversified rapidly (Kummel, 1956, 1964). During the Cretaceous, the South Atlantic opened up between Africa and South America (Dietz and Holden, 1970), which facilitated the evolution and diversification of the Cretaceous Cymatoceras, and its contemporaries. genus, However, the Early Tertiary radiation of nautiloids was favoured by the disappearance of ammonites in the end-Cretaceous mass extinction (Ward, 1987). Development of new basins in the extreme north and south latitudes during the Tertiary and Quaternary due to the opening of the North Atlantic and the separation of the Australian Plate from Antarctica (Dietz and Holden, 1970) have had an effect on further nautiloid evolution. It appears that the distribution of post-Triassic nautiloids was largely controlled by climatic conditions. Despite the fact that the Mesozoic climate was warmer and more equable than present (Enay and Cariou, 1997), the absence of nautiloids in the remote northern and southern latitudes and their concentration in and near the Tethyan Realm are indicative of its stenotopic nature. In this context, it is interesting to note that present-day Nautilus is a warm water stenothermal (10-25”(Z) organism restricted to the relatively deeper reaches (100-300 m) of only the Indo-Pacific Sea (Saunders, 1987; Saunders and Ward, 1987; Hayasaka et al., 1987; Ward, 1987). Long-term lateral migration of Mesozoic nautiloids along the shallow shelf was hindered by shell-crushing predators such as brachysauran crabs, teleost fishes, spiny crustaceans etc., which arose suddenly during the Mid-Mesozoic (Vermeij, 1977) forcing molluscs, echinoids and other marine invertebrates to undergo changes in morphology and habitat. It is worth noting that Recent Nautilus is vulnerable to predation by octopuses and particularly by teleost fishes in shallow seas (Saunders et al., 1987). Nautiloids, as contemporaneous ammonites, responded to the emergence of strongly-jawed predators by coarsening ribs and by increasing the frequency of ornamented forms (Bardhan and Halder, inpress; Ward, 1981).
ACKNOWLEDGEMENTS S. Bardhan (Geological
184 Journal
(Jadavpur University) and T.C. Lahiri Survey of India) critically reviewed the
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manuscript and provided many valuable suggestions. The author is grateful to D. Mukherjee, also of Jadavpur University, for her help with drawings and photography. Kind permission, given by the Director General of the Geological Survey of India for publication of the manuscript, is duly acknowledged. The author also acknowledges two anonymous reviewers for their valuable suggestions.
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