Ultrastructure of ostracods as a stratigraphical tool for the subdivision of the Senonian sequence in Israel

Ultrastructural size variations in pitted and ornamented Senonian ostracods from Israel are valuable to distinguish subzones in full successive sections. Pit and pore sizes of different genera are inversely correlated. Peaks of pit diameters from different locations are indicative of certain levels and therefore applicable to biostratigraphic correlations.


INTRODUCTION
The Senonian ostracods from the lower part of the Mount Scopus Group (Flexer, 1968) in Israel and the biozonation of the Senonian strata are described by Honigstein (1 984). The following ostracod assemblage zones, based on the first occurrence of diagnostic species, were established in the Late Coniacian to Campanian sequence. The Phyrocythere lata (S-1) Zone in the Late Coniacian (--?Early Santonian); the Cythereis rosenfeldi rosenfeldi ( S -2 ) and the Limburgina miarensis (S-3) Zones in the Santonian; the Leguminocythereis dorsocostatus (S-4) and the Brachycythere beershevaensis ( S -5 ) Zones mainly in the Lower Campanian. Ornamentation is one of the most important taxonomic features in ostracod valves. Within the species, the ornamentation is, in general, remarkably constant (Liebau, 1969). Strongly ornamented forms occur exclusively in marine species, especially among those living in littoral and shallow neritic environments (van Morkhoven, 1962). Reticulation patterns are used by various authors for classification of the Trachyleberididae (e. g. Pokorn9, 1969;Liebau, 1969;Damotte, 1976). The details of the sculpture of the leftiright valves of one carapace are normally intraspecifically constant (Liebau, 1975). Benson (1974Benson ( , 1975 compares the reticulation of ostracods with engineering structures. The rigid system of ornamentation must be strong enough to preserve the original shape from externally induced mechanical stresses (pressure, turbulation, etc. . .). The decrease in ornamentation, or smoothing factor, may be controlled by the temperature of the biotype (Carbonnel, 1972;Ishizaki, 1975). Hartmann & Kuhl (1978) observed in both smooth and reticulated species a decrease in fine sculpture with a higher level of calcification. The calcium content (or density of calcite grains) of the valves may depend on both the activity of outer epidermal cells and the calcium content in the water (Okada, 1981). Low calcified valves are more strongly ornamented and in high calcified carapaces the structures are weaker (Kiihl, 1980). He suggested that the calcification is dependent on the different sediments where the ostracods live, and the seasonal ostracod populations. Shapes of pits and meshes are almost determined by patterns of the epicuticle before ecdysis, namely before calcification, and they are probably independent of calcium content in the water (Okada, 1981). Gottner (1980) presented a positive correlation between the pit size and the salinity of brackish water ostracods.
The soft ostracod body is encased in a bivalved, calcareous shell, which if completely closed would severely limit the ability to sense the external environment. However, ostracods possess pore systems which are open to the exterior. These pores mostly have tactile receptors (setae or hairs) extending from the pores. In Trachyleberididae and Hemicytheridae, the two pore types (simple and sieve-type pores) coexist in valves of the same ostracod (Sandberg & Plusquellec, 1969). The simple pores are narrow, relatively uniform throughout the shell and have distal setae. Muller (1894) and Hartmann (1966) discussed in some Cytheracea the size variation in normal pores: narrow canals with long, thick hairs acting as touch receptors and wide canals with finer setae for more delicate sensory functions. Other functions of these pores are secretion of the glands (Omatsola, 1970;Puri, 1974) and chemo-or thermotropic operations (Omatsola, 1970). Jorgenson (1970 reported the size of the pore diameter of Trachyleberididae as being approximately 10 microns. The sievetype pore may function as a photoreceptor and has a surrounding sieve plate and as well as a mostly central opening for a seta. As in normal pores, the function of the hair is that of a touch receptor. Muller (1894) assumed that these pores may serve as eyes in ostracods without eye-spots, however, the bristles are not sensitive to light. Liebau (1978) suggested that the pores may also serve as a hearing device. The setae in the pores probably sense vibrations in the water, as do the ciliated cells in the ears of mammals. Some authors used pore types and bristles for taxonomic classification (Puri & Dickau, 1969;Sandberg & Plusquellec, 1969;Puri, 1974;Keyser, 1980). Studies by Rosenfeld & Vesper (1977) and Rosenfeld (1977) show that the form of the sievetype pores is an indication of palaeosalinity. In Recent ostracods, the differentiation between the two main types of pores (normal and sieve-type) is easy. Only in very few Senonian specimens from Israel are the fine sieve-type pores preserved, more often they are filled with sediment and the presence or absence of sieve plates cannot be determined.
Reticulation and tuberculation are related to the pore system; these features are genetically fixed and related to sense-organ systems (Liebau, 1969). The pattern of pores on the carapace appears consistent within the species (Puri & Dickau, 1969). Even ostracods of the same genus have similar uniform pore patterns, whereas the reticulation is a specific feature (Liebau, 1969;Okada, 1982). The evolution of the pores and ornament within a species are correlated (Liebau, 1978) and the network, to which the pore arrangements are related, is composed of meshes which remain in the same positions.
In the present study, an attempt is made for the first time, to measure and compare the size of ornamentation (pits) with the diameter of pores in certain groups of Senonian ostracods and to use these data for stratigraphic purposes.

MATERIAL AND METHODS
Material for the measurements of pore and pit sizes was selected from about 30 sections (wells and exposures; Fig. 1) from different parts of the country. All the studied material is stored in the collections of the Geological Survey of Israel, Jerusalem, under the Ostracode Laboratory Nos. T-. The ultrasonic cleaning of valves was only used in very soiled specimens, where the details of the structures could not be observed. The cleaning procedure normally leads to a weakening and flattening of the ornamentation in pitted specimens. The structures were photographed at different magnifications using the scanning electron microscope (S.E.M.) of the Hebrew University, Jerusalem, and the Geological Survey of Israel, and the diameters were measured on the photographs. At a later stage of the study, measurements were taken directly from the S.E.M. screen. From each sample, an average of four specimens were selected for examination. The pores of two different groups of Trachyleberididae and Hemicytherididae (reticulated and pustulated type) were measured on different parts of the valve (minimum about 10 pores/ specimen) and the results averaged. Ostracods of the pustulated type belong to the species Phyrocythere lata Honigstein, 1984 and Veenia fuwwarensis Honigstein, I984 (both subspecies), whereas the reticulated group is formed by Cythereis cretaria Bold, 1964, and its subspecies, Anticythereis judaensis Honigstein, 1984 and Ventrocythereis sinaiensis Honigstein, 1984. From the same samples, however, a comparable amount of specimens of Brachcythere is taken (B. angulatu Grekoff, 1951, B. cf. B. ekpo Reyment, 1960, andB. beershevaensis Honigstein, 1984). Here, the pit diameters were measured from the anterior and central area of the valve (for oblong pits, measurements are approximate). In general, pores in Brachycythere were not possible to observe, as they are mostly very narrow and filled with marl. Pores and pits were only clearly distinguished in a few specimens (see P1. 1, figs. 4,5

RESULTS
Preliminary examination of pore and pit sizes show that the same average values were obtained for female or male specimens as for left or right valves of adult ostracods from the same samples (cf. Liebau, 1975). This applies to all ostracod groups observed (reticulated/ pustulated types + Bruchycythere). Pore size is nearly uniform throughout the entire carapace. Within the same specimens there were only small variations in the diameter data observed.
For example, in specimens of Cythereis cretaria and Anticythereis judaensis from Ayyalon-3 ( Fig. 1 ), Ostracode Lab. No. T-7559, the single measurementsof pores range between 4.7 and 6.Opm, the average diameter for each specimen (8-1 0 poresispecimen) being between 5.2 and 5 . 6~m ; the average diameter for all specimens in the sample is, therefore, about 5.4pm. A total of 200 pits in the central areas of 25 specimens of Bruchycythere angulata from Nahal Massor 3, Location No. 55, Ostracode Lab. No. 7305 (Figs. 1,10) were measured, the results of which are shown in the following The pore and pit diameter averages are consistent for each sample and are therefore reliable for data processing. The total range in pore size of all studied specimens from different locations and stratigraphic levels is 3 -7 p m and in pit diameter 2-1 1 pm. These ranges are greater than the possible error of measurements under the S.E.M., which is about 10%).
A control experiment with 191 specimens of Brachycythere shows a normal distribution of the pit diameter (Fig. 2), the average of four specimens/sample (minimum 10 pitsispecimen measured) is proved to be sufficiently valid. The standard deviation of average measurements of the pits is given in Fig. 5.
The pits of Brachycythere are much larger in the central than in the anterior zone (PI. 1, fig. 6). The results of the ornamentation measurements on different area:$ o n the carapace yielded congruent curves. The maxima and minima, respectively, occur in the same sample of the pit size measured both in the central and anterior area (Fig. 3). Thus, pit sizes ofBracliycythere in the peripheral Lones oi the valves were neglected. The diameter ot the pores is almost constant in ostracods, both in reticulated and pustulated forms from the same stratigraphic level (Fig. 4). Therefore, both types were used together for this study. Diagrams of pore versus pit diameter show a mirror-like picture, but with incongruent amplitudes. For the same stratigraphic level, every  ' .
. ' . ' . Data from different locations (Figs. 3-6; 9-10) demonstrate that the size of ornamentation is dependent on certain stratigraphic horizons. The ostracod assemblage zones S-1 to S-5 (Honigstein, 1984) were superimposed on these results and only small deviations are shown for different locations (e.g. top of the S-3 Zone in Fig. 6: Bir-es-Saqati-1 and relatively large pit sizes in the S-2 Zone of Nahal Massor 3, Fig. 10). Therefore, average curves were drawn for the studied material from each region and all assemblage zones (Fig. 7). Only insignificant variations in the amplitudes were observed. This gave rise to the construction of a comprehensive diagram for all sections measured (Fig. 8). Specimens of Brachycythere show low pit diameter peaks in the assemblage Zones S-2 and S-4, medium values in the S-3 Zone and high maxima in the S-5 Zone (not only for the large species B. beershevaensis). The different ostracod assemblages of the S-2 and S-4 Zones allow an easy distinction between the peaks of similar amplitude. The minima, except in the S-5 Zone, are nearly of the same value. Every assemblage zone (except the S-l Zone, where only few ostracods were measured) can be divided into two or three parts according to the variation in pit diameter in full successive sections. The pore diameters in trachyleberidid (and hemicytheridid) ostracods gave, for nearly all samples, mirror-like results to those of the pits ofBrachycythere, but the amplitudes are smaller and single peaks for certain stratigraphic levels cannot be identified. Also, the ornaments of the pitted species are easier to recognise (larger size and better preservation). Therefore, pores should only be measured for completion of data intervals, where Brachycythere is lacking.

DlSCUSSlON AND CONCLUSIONS
The variations in the pore diameter seen in some genera of Trachyleberididae and Hemicytherididae are related to the size of ornamentation of the examined species of Brachycythere. An evolutionary trend is excluded and so the palaeoecology may have influenced these size changes. Some attempts were made in this study to combine the ultrastructural data with palaeoenvironmental factors.
Stable isotope studies (oxygen and carbon isotopes) were abandoned after the rather disappointing results with Recent ostracods, reported by Durazzi (1977).

Explanation of Plate 1
Different pit diameters in Brachycythere Scale bar = 2 0 microns; tigs. 1-5, same magnification       Bandel & Hoefs (1975) mentioned, by studying gastropods, that metabolic processes during carbonate precipitation seem to affect the isotopic composition of the shells. This makes the determination of water (palaeo-) temperature and (palaeo-) salinity impossible. Biological mechanisms may, therefore, affect the calcification of ostracod shells. The structure of an ostracod carapace and its building process is very complex (Rosenfeld, 1979;Okada, 1982). In addition, in the studied material, single valves, necessary for isotopic analyses, are relatively rare.
By the usual washing technique for foraminifera and ostracods (sieve mesh size 76 p m ) , the fraction, which contains the highest number of specimens, is lost (fraction finer than 64pni). Thus, the determined plankton/ benthos ratio, a factor indicating depth of deposition, is not accurate (Black, 1980;C. Benjamini, Beersheva IJniveraity, pers. comm.). Therefore, the relationship bctwccn this ratio and the ultrastructural data of ostracods havc not been examined in the present study. Starinsky & Hexer (1969) and Flexer & Starinsky ( I 970) mentioned phosphate contents in Senonian rock\ a\ a lactor related t o plankton/benthos ratio and cmploycd thi\ new factor as a palaeobathymetric indicator. An increase in t h e Mg/Ca ratio in the environment c a u x \ greater ornamentation of ostracod tests and ;I clccrca\c in thi\ factor results in a smoothening of ornaments (Peypouquet et al., 1980). Phosphate, Mg and C a contents were analysed in about thirty samples of two sections (Rewaya-3: wel!, Galilee and Nahal Massor 3: surface, Negev) by the Geochemical Division of the G.S.I., Jerusalem. Their results were superimposed on the data of pit diameter measurements ofBruchycythere from the same samples. The diagram for Rewaya-3 ( Fig.  9) shows rather conformable trends of the curves of pit sizes and the MgiCa ratio; phosphate contents gave incomparable results. However, the data for Nahal Massor 3 (Fig. lo), where more samples were taken in a narrower stratigraphic interval, did not agree with the observations of the Rewaya-3 diagram. It seems that changes in the ornamentation of ostracods cannot be correlated with phosphate content (depth) neither with the amount of magnesium in the rocks (dolomitisation).
Preliminary studies were carried out by the author on Recent ostracods from the Gulf of Naples. In the depth interval of 30-200m, a steady increase in the pit size of Bosquetinu dentutu (Muller, 1894) with water depth could be observed. This species is similar to the Senonian Brachycythere in both ornamentation and taxonomic position. The similarity would therefore suggest an increase in pit diameter in Bruchycythere with water depth. The pore size changes, which show inverse correlation with the pit size in the Senonian species  trachylleberidid ostracods examined here. The Recent genera, PterygocythPreis, Carinocytlzereis and Acantliocytlzertpis, were found to yield nearly the same pore size in all depth intervals, but it i.i; possible that a n insufficient number o f :specimens were studied for definitive conclusions t o he drawn.
The results of the ultrastructural observations of the pore size of Trachyleberididae and Hemicytheridae and the pi1 diameter in Brachycythere were proved to be valuable lor biostratigraphic purposes. The size variations of the pores are inversely correlated with those o f the pits. The size ranges are consistent in certain stratigraphic levels frqm all parts of Israel. The method might be useful in order to differentiate between subzonesand improve the boundary lineation of the assemblage zones in full successive sections. For the application of the size cariations as a stratigraphic tool, identification t o specific level in ostracods is unnecessary. Only a few specimens of pitted forms are required. It is noteworthy that this method can be used even when age diagnostic ostracods are atxent. Further, more detailed work o n Recent ostracods may indicate the ecological factor(s) responsible for the combined changes in pit and pore sire.