Paleobiogeography of Maastrichtian to early Eocene Ostracoda of North and West Africa and the Middle East Ashraf M. T. Elewa Geology Deptartment, Faculty of Science, Minia University, Egypt email:
[email protected]
ABSTRACT: Data on 103 species and subspecies of ostracoda from Maastrichtian to lower Eocene localities in North Africa, West Africa, and the Middle East were analyzed using correspondence analysis to interpret marine paleobiogeography in this wide area of the world during the time in question. Two main biogeographical provinces, the South Tethyan Province (STP) and the West African Province (WAP), were connected during this entire period through the Trans-Saharan Seaway. In addition, a third grouping was distinguished that combines species from both provinces, but with more similarity to the STP. R-mode cluster analysis based on the Jaccard coefficient of similarity demonstrated the distinctness of the first two provinces, and indicated that the third grouping represents an overlap of the Garra Type (GAT) and the Afro-Tethyan Type (ATT) of Bassiouni and Luger (1990). The study confirms the stability of ostracod habitats in this region. There was essentially no turnover across the K/T and Paleocene/Eocene boundaries, and faunal changes came about almost entirely due to lateral migration of certain genera. Statistical analysis of the available data shows that the most common genera are: Buntonia, Cytherella, Leguminocythereis, Bythocypris, Mauritsina, Ordoniya and Paracypris.
INTRODUCTION
Studies of the ostracod assemblages of North and West Africa and the Middle East began in the sixties of the previous century, when R. A. Reyment published detailed studies on the Upper Cretaceous to Lower Tertiary ostracods of Nigeria (Reyment 1960; 1963; 1966). These publications opened the way for research by numerous other authors working in the region, including Apostolescu (1961), Barsotti (1963), Esker (1968), Grekoff (1969), Bassiouni (1969a, 1969b, 1969c, 1969d, 1970, 1971), Al Sheikhly (1981, 1985), Boukhary et al. (1982), Donze et al. (1982), Foster et al. (1983), Carbonnel (1986), Carbonnel and Johnson (1989), Carbonnel et al. (1990), Bassiouni and Luger (1990), El-Sogher (1991), Elewa et al. (1999), Morsi (1999), Bassiouni and Morsi (2000) and Elewa (in press). In a very interesting paper, Keen et al. (1994) presented an excellent stratigraphical and paleobiogeographical overview of the Tertiary ostracods of North Africa and the Middle East, but without much attention to the ostracod fauna of West Africa. Moreover, they excluded the smooth or unornamented ostracods from their study, regardless of their occurrence even in great numbers in their material. The present paper deals with the paleobiogeographical and paleoecological implications of the ostracod faunas of this wide region by means of multivariate analysis, to obtain quantitative information about migration directions, as well identifying the environmental factors that affected migrations during this time. METHODS AND TECHNIQUES
This study is based on samples collected from various Egyptian localities (text-fig. 1), integrated with published accounts on the ostracod faunas of Egypt and the surrounding countries of North and West Africa and the Middle East (text-fig. 2). The author has examined 103 ostracod species and subspecies ranging in age from Maastrichtian to early Eocene. Of these, 46 species
TEXT-FIGURE 1 Location map of the studied areas (black squares) of Egypt.
are considered relevant in the regional study, after excluding rare species defined as taxa that have only been observed in fewer than three of the countries in this region. In addition, the data from countries with ostracod faunas that cannot be compared with those of Egypt (e.g., Saudi Arabia, Iraq, Pakistan) were omitted from the analysis. The resulting matrix of 46 species from 13 countries was subjected to correspondence analysis, based on presence/absence data, to generate a contingency table of frequencies with a constant row sum, to distinguish the paleogeographic provinces of the investigated countries. R-mode cluster analysis, based on the Jaccard coefficient of
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TEXT-FIGURE 2 The areas covered in the present study, with a late Paleocene paleogeography (after Keen et al. 1994).
similarity, was carried out on the data matrix to confirm the results of the correspondence analysis and to define the existing groups more clearly. Finally, an attempt was made to reach an appropriate conclusion about the paleoenvironments of each of the distinguished provinces based on the paleoenvironmental significances of the ostracod assemblages contained within these provinces. The studied ostracod species have wide geographic and stratigraphic ranges, and show no clear extinction pattern in the upper Paleocene to lower Eocene interval, as a general trend. In addition, almost all of the index forms in the Maastrichtian fauna survived into the Paleocene. The correspondence analysis program is based on the method of Benzécri (1973) and is contained on a CD-ROM disk included with the textbook written by Reyment and Savazzi (1999) on the aspects of multivariate statistical analysis in geology. The cluster analysis program used herein is included with the PAST statistical package, version 0.35 of February, 2001. Statistical analysis of the investigated data
The 103 species and subspecies investigated here are included within 42 genera and 17 families according to the following statistics. Trachyleberididae is the most diverse family, with species representing 17.5% of the total assemblage (text-fig. 3). From text-figure 4, it is clear that the most common genera in the studied population are Buntonia (13.6%), Cytherella (5.8%), Leguminocythereis (5.8%), Bythocypris (4.8%), Mauritsina (4.8%), Ordoniya (4.8%) and Paracypris (4.8%). Text-figure 5 illustrates that the smooth (unornamented) species represent only 15.5% of the total assemblage, although individuals of these species occur in great abundance within the studied material. These observations lead to the following notes:
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1. Buntonia, which represents warm water conditions (Carbonnel and Johnson 1989), is more abundant in West Africa. 2. Cytherella, as an indicator of reduced oxygen marine conditions (Whatley 1991), has higher abundances in the Middle East and North Africa. 3. Leguminocythereis, ranging over large areas of West and North Africa, represents shallow water environments, with relatively easy passage through the shallow waters of the Trans-Saharan Seaway (Keen et al. 1994). In the present study, this genus prevails more in West Africa than in the Middle East and North Africa. 4. Bythocypris, Mauritsina, Ordoniya and Paracypris are considered by Bassiouni and Luger (1990) to represent the outer shelf to upper bathyal marine environments. These species are predominant in the Middle East and North Africa. Paleobiogeography and paleoecology
Forty-six selected species in 13 countries (Table 1) were the subject of correspondence analysis to define an appropriate paleobiogeographical history for the studied areas. The results are summarized in Table 2 and the values of the second to fifth latent vectors are shown in Table 3. Table 2 shows that the first four latent roots account for more than 63 % of the trace that could be suitable for constructing a paleobiogeographical pattern for the ostracod faunas of the studied areas. As usual, the first latent vector is lost because of the effects of the contingency table and the effect of scaling, therefore, the second latent vector denotes of the first real and interpretable latent vector. I use here standard statistical terminology and not that of Benzécri (1973) in which the term “inertia” replaces “variance”.
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TEXT-FIGURE 3 Pie chart of the studied ostracod families.
TEXT-FIGURE 6 The second vs. third latent vectors.
TEXT-FIGURE 4 Pie chart of the studied ostracod genera.
TEXT-FIGURE 5 Pie chart of the studied ornamented vs. unornamented ostracod genera.
The biplots of the second vs. third, fourth and fifth latent vectors (text-figs. 6, 7 and 8) clarify that the second latent vector could successfully divide the data into three main groups. Investigation of these groups (text-fig. 6) showed that the first group with the highest values of the second latent vector is related to the countries of North Africa (Tunisia, no. 7 and Algeria, no. 12) and the Middle East (Jordan, no. 10 and Israel, no. 11). In this group, the most significant ostracod species are Mauritsina coronata (Esker) (no. 48), M. jordanica nodoreticulata Apostolescu (no. 49), Ordoniya ordoniya (Bassiouni) (no. 52), O. hasaensis (Bassiouni) (no. 53), O. bulaqensis (Bassiouni) (no. 54), Megommatocythere denticulata (Esker) (no. 55), Protobuntonia nakkadi Bassiouni (sp. 50), Reticulina proteros Bassiouni (no. 51) and Xestoleberis tunisiensis Esker (no. 56). It is noteworthy that this assemblage is contained within the South Tethyan Type of Bassiouni and Luger (1990) or the South Tethyan Province of the present study (STP). The second group in the graph text-figure 6 represents Egypt (no. 9), in which chararacteristic species are Leguminocythereis sangalkamensis (Apostolescu) (no. 16), Soudanella laciniosa triangulata Apostolescu (no. 19), Buntonia attitogoensis Apostolescu (no. 31), Paracypris nigeriensis Reyment (no. 41), Paracypris jonesi Bonnema (no. 45), Reymenticosta bensoni (Damotte and Donze) (no. 46), Cytheropteron lekefense Esker (no. 47), and Uroleberis glabella Apostolescu (no. 59). These species are common to the South Tethyan Province (STP) as well as to the third group in the graph, which belongs to the Afro-Tethyan Type (ATT) of Bassiouni and Luger (1990). From the same graph, it is clear that the ATT or the West African Province (WAP of the present study) contains all the remaining studied species. Notice that Libya (no. 8) has a strong affinity to the WAP, thus confirming the opinion of Barsotti (1963). From the paleoecological point of view, we notice in the graph that the second latent vector groups the species according to their water depth. This could be a separation between the species of the outer neritic to upper bathyal habitat (STT or STP) and those of the epicontinental habitat (ATT or WAP), in the sense of Bassiouni and Luger (1990) and Keen et al. (1994).
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TEXT-FIGURE 7 The second vs. fourth latent vectors.
TEXT-FIGURE 8 The second vs. fifth latent vectors.
The same figure shows a trend of negative association between the second latent vector and the third one at the third group (WAP). Notice that the highest values of the third vector represent species that prefer warm water conditions such as several Buntonia species (nos. 20, 21, 24 and 25), and others. According to Carbonnel and Johnson (1989), Buntonia may be regarded as typical of infralittoral-circalittoral tropical waters. This negative association in the graph could be attributed to a negative relation between the depth and the temperature factors, where temperature (third latent vector) decreases as the depth (second latent vector) increases. The graph shows that the second factor, temperature, was effective primarily on the fauna of the West African Province (WAP).
teiskotensis (Apostolescu) (no. 42) and Nucleolina tatteuliensis (Apostolescu) (no. 43). Although there is not enough known about the environmental conditions of the species of these two groups, the results of this analysis indicate the following:
From text-figure 7, showing the plot of the second vs. fourth latent vectors, one observes a co-occurrence of the outer neritic to upper bathyal ostracod species like: Leguminocythereis lokossaensis Apostolescu (no. 27) and Dahomeya alata anteroglabrata Bassiouni and Luger (no. 58), from the outer to middle shelf Esna Type (EST) of Bassiouni and Luger (1990), and the epineritic forms such as the Buntonia species (nos. 20, 21, 24, 25 and 26) as well as others of the West African Province such as Leguminocythereis cf. L. lagaghiroboensis Apostolescu (no. 28) and Isohabrocythere teiskotensis Apostolescu (no. 36). This indicates the effect of the turbulent water conditions on the distribution of the studied assemblage. Finally, from the graph in text-figure 8, which shows the plot of the second vs. fifth latent vectors, it is clear that the fifth latent vector arranges the species according to their abilities to tolerate reduced oxygen conditions. This is most evident in the WAP, where Senegal (no. 1) is very close to Leguminocythereis senegalensis Apostolescu (no. 14), Buntonia virgulata Apostolescu (no. 15) and Reticulina sangalkamensis (Apostolescu) (no. 16). In contrast, NW Nigeria (no. 5) and Niger (no. 13) are close to Leguminocythereis bopaensis (Apostolescu) (no. 34), Bairdia ilaroensis Reyment and Reyment (no. 37), Trachyleberis teiskotensis (Apostolescu) (no. 38), Alocopocythere
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1. The first group, containing Reticulina sangalkamensis (Apostolescu), inhabits deeper environments (middle to outer shelf) than the second, which contains Nucleolina tatteuliensis (Apostolescu), a species that indicates shallow euhaline shelf conditions (Bassiouni and Luger 1990). 2. The rarity of Buntonia spp. in Senegal (Table 1), as compared with the other regions, may indicate low oxygen conditions in that area (Carbonnel and Johnson 1989). 3. As mentioned above, Cytherella, which could tolerate reduced oxygen (Whatley 1991), is more abundant in the Middle East and North Africa than the WAP. Therefore, the WAP, generally, reflects well oxygenated waters. 4. As to Togo (no. 3), although the genus Buntonia is relatively rare in this country, Togo is close to the second group because its ostracod fauna generally indicates shallow euhaline shelf (e. g. Buntonia sehouensis Apostolescu, Isohabrocythere teiskotensis Apostolescu and Soudanella laciniosa laciniosa Apostolescu). Kaiho (1991) stated that the low oxygen events occurred coincidentally with episodes of oceanic warming and sluggish deep water circulation. Thus, the fifth latent vector may indicate the degree of oxygenation at depth, where the association from Senegal (no. 1 in graph 8) comprised of species inhabiting deep marine conditions reflects low oxygen levels. This is also the case in the South Tethyan Province (STP). In contrast, the West African Province (WAP) reflects shallow marine conditions associated with well oxygenated waters.
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TEXT-FIGURE 9 Dendrogram resulted from R-mode cluster analysis, based on the Jaccard coefficient of similarity (the paired-group method), of the 46 ostracod species used in the present study.
Cluster analysis as a tool for defining groups within the studied data
In this study, R-mode cluster analysis based on the Jaccard coefficient of similarity using the paired-group method was applied to the data matrix. By means of this technique, three ostracod groups could be distinguished (text-fig. 9): Group I (text-fig. 9) is represented by a group of species that are common in the South Tethyan Type of Bassiouni and Luger (1990), which is named in the present study as the South Tethyan Province or STP. Group II (text-fig. 9) consists of a large number of species, most of which consistently occur in Egypt, which mainly represent a combination of the Garra Type (GAT) and the Afro-Tethyan Type (ATT) as well as the Esna Type (EST) and the South Tethyan Type (STT) of Bassiouni and Luger (1990). This group appears to represent the overlap of the GAT and the ATT of Bassiouni and Luger (1990). Notice, from the graph (text-fig. 9), the strong similarity between this group and the first group (group I) which contains species that are attributed, in the present study, to the South Tethyan Province (STP). Group III (text-fig. 9) consists of the species that consistently occur in the Afro-Tethyan Type (ATT) of Bassiouni and Luger (1990) (e. g. the West African Province of the present study; WAP). The results obtained by the aid of cluster analysis show the same three groups that were apparent in the correspondence analysis. The cluster analysis, however, could more accurately place the studied species at their true positions within the dis-
criminated groups. On the other hand, correspondence analysis successfully identifies the major environmental factors in the study area. SUMMARY AND CONCLUSIONS
Among the Maastrichtian to early Eocene ostracoda from the study area (text-fig. 2), the most common genera, according to samples and published reports, are Buntonia, Cytherella, Leguminocythereis, Bythocypris, Mauritsina, Ordoniya and Paracypris (text-fig. 4). The data distinguish between two major geographical provinces connected through the Trans-Saharan Seaway (text-figs. 6, 9): the South Tethyan Province (STP) and the West African Province (WAP). The ostracods of Egypt exhibit a strong affinity to both provinces but with more similarities to the South Tethyan Province (text-fig. 9). These results support the continued existence, up through Late Paleocene time, of a shallow marine transcontinental connection, i.e., the Trans Saharan Seaway of Reyment and Reyment (1980) and Reyment (1981), between the southern Tethys and the Guinean Province through Mali and NW Nigeria, as initially proposed by Barsotti (1963). In reviewing the paleoenvironmental factors affecting the distribution of the ostracod assemblages (water depth, water temperature, water turbulence and oxygen variation with water depth), it is worth noting that the interpretation of Cytherella as an indicator of reduced oxygen (Whatley 1991, and others) is substantiated by the fact that Buntonia species, which indicate normal oxygen conditions according to Carbonnel and Johnson (1989), are rare in the samples having Cytherella species
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TABLE 1 The studied data matrix.
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TABLE 2
TABLE 3
Summary of correspondence analysis.
Latent vectors of the studied 13 countries and 46 species.
Although ostracods have a strong tendancy towards endemism, the migration of epineritic species of North and West Africa was in both directions as indicated from cluster II (text-fig. 9) which shows a mixture of species from these two areas. This observation may be attributed to the change of water depth as indicated by the second latent vector (text-fig. 6). In contrast, the movement of several species indicates that migration around West Africa was dominantly one-directional from east to west, apparently due to the change of water depth, temperature, oxygenation as well as direction of sea currents (second, third, fourth and fifth latent vectors of text-figures 6, 7 and 8). Of course, paleoenvironments must have changed to some extent in this large region during the more than 20 million years from Maastrichtian to early Eocene time. Nevertheless, if we accept that the cited sources have correctly identified the most commonly collected ostracod taxa, then virtually all change in the local assemblages can be shown to be caused by changes in distribution, rather than origination or extinction in the studied lineages. This would appear to indicate either that these species are unusually tolerant of environmental change, or that environmental change in the area of study had little effect in the habitats occupied by Ostracoda. In either case, the studied material does not yield itself to a chronological subdivision, and the trends and grouping identified herein must be seen as consistent throughout the space and time covered by this study. ACKNOWLEDGMENTS
The author is much indebted to Prof. Dr. M. A. Bassiouni of the Geology Department, Faculty of Science, Ain Shams University, Egypt, and Prof. Dr. R. A. Reyment of the Swedish Natural History Museum, Sweden, for their kind help throughout the course of the present study. I am also grateful to Dr. J. Van Couvering, the Editor of the Micropaleontology Press, USA, and an anonymous reviewer, for suggestions to improve the manuscript. I would like to thank Dr. A. M. Morsi of the Geology Department, Faculty of Science, Ain Shams University, Egypt, for placing at my disposal some of his reference papers. REFERENCES AL SHEIKHLY, S. A. J., 1981. Maastrichtian – upper Eocene Ostracoda of the subfamily Trachyleberidinae from Iraq, Jordan and Syria. Unpublished Ph. D. Thesis, University of Glasgow, 229 p. ———, 1985. The new genus Ordoniya (Ostracoda) from the Paleogene of the Middle East. Proceedings of the Second Jordan Geological Conference, Amman, pp. 238-273. APOSTOLESCU, V., 1961. Contribution à l’étude paléontologique (Ostracodes) et stratigraphique des bassins cretacés et tertiaires de
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Manuscript received June 1, 2002 Manuscript accepted October 10, 2002
