A re-examination of the Pliensbachian and Toarcian Ostracoda of Zambujal, west-central Portugal

Pliensbachian and Toarcian Ostracoda first described by Exton (Geological Paper, Carleton University, Ottawa, 79: 1–104 1979) from the Lusitanian Basin, west-central Portugal have been re-examined. As a result, a greater diversity in the Ostracoda (80 species) is now recognized. Two species are newly described (Eucytherura zambujalensis sp. nov., Ektyphocythere mediodepressa sp. nov.) from the marls and calcareous shales of the Maria Pares Hill section near the village of Zambujal. Poor preservation precludes a complete taxonomic review of the present material. Five ostracod zones are proposed; Gammacythere ubiquita–Ogmoconchella gruendeli Zone, Poly cope cerasia–Polycope cincinnata Zone, Liasina lanceolata–Ogmoconcha convexa Zone, Bairdiacypris rectangularis–Kinkelinella sermoisensis Zone, and Cytherella toarcensis-Kinkelinella costata Zone. Although the ostracod assemblages possess strong similarities to those described from Northwest Europe, some of the Zambujal assemblages are dominated by the genus Polycope. A marked faunal turnover, in association with the extinction of the Metacopina occurs in the lower Subzone of the tenuicostatum Zone of Lower Toarcian age. These faunal events are discussed in relation to changing environmental conditions.

The Zambujal section is situated approximately 15 km southsouthwest of Coimbra in west-central Portugal, on the flanks of the Maria Pares Hill (48'2", 8'28'W) which overlooks the village of Zambujal (Fig. 1). The section extends along the road from Zambujal to Furadouro and covers the stratigraphical interval between the lowest Pliensbachian through to the top of the Toarcian.

SAMPLING
A total of 32 outcrop samples were collected during June 1973 by K. Hooper and W. Cox, from beds which had been numbered and described by Mouterde et al. (1964). Additional samples from the lower part of the tenuicostatum Zone (semicelatum Subzone) at Zambujal, Peniche, and Brenha have also been examined in order to detail the extinction of the metacopid Ostracoda. The ammonite zonation scheme, bed number, approximate thicknesses, and sample numbers are illustrated in Fig. 2. All of the samples excluding sample 117, which comprised a well indurated marl and could not be broken down, comprised marls or calcareous shales. Several preparation methods were tried (boiling, white spirit, hydrogen peroxide), with the latter proving the best method for breaking down the calcareous sediments. A dry weight of lOOg per sample was found to be the minimum weight needed to gain representative fossil assemblages (Exton, 1979).

GEOLOGY AND GEOLOGICAL SETTING
Portugal's Lower Jurassic sediments outcrop in two main regions; firstly in the Lusitanian Basin which is situated to the west of the Hesperian Massif, between 38' 30" and 41°N, and secondly 111 an east-west strip on the southern flank of the Algarve Massif. The Lower Jurassic sediments of the Zambujal section are situated close to the present day eastern margin of the Lusitanian Basin, close to the Hesperian Massif (Fig. 1). Throughout the Lusitanian Basin, the Lower Jurassic sediments are dominated by dolomites, limestones and marls, attaining a maximum Lhickness of over 600 m in the northwest of the region (Figueira da Foz).
The earliest Jurassic sediments in the Lusitanian Basin belong to the Hettangian Dagorda Formation, comprising greyish red marls with dolomites and evaporites (Fig. 3). This formation marks the initial marine transgression upon the terrestrial red lithologies of the Upper Triassic Silves Formation. The overlying Coimbra Formation consists of dolomites and dolomitic limestones and marks the onset of shallow marine conditions throughout the Lusitanian Basin. The upper boundary of this formation 1s highly diachronous, ranging in age from Upper Sinemurian (obtusum Zone) in northwestern outcrops (e.g. Sao Pedro de Muel) close to the basin axis, to Lower Pliensbachian (jarnesoni Zone) in southeastern outcrops (e.g. Tomar) close to the original basin margin (Mouterde et al., 1971). The interbedded shales, marls and limestones of the overlying informally named Brenha Formation constitute the remainder of the Lower Jurassic succession. The relative proportion of limestones to marl/shale within this formation varies according to the proximity to the paleomargin of the basin. In the vicinity of Tomar, towards the southeastern margin, the marls and shales do not form a significant component of the sediments until the Lower Toarcian. Predominantly argillaceous sedimentation commenced in the Upper Sinemurian towards the basin -Creambuff marl -Dark greybuff marl  Pliensbachian, with an apparent maximum water depth occurring in the margaritatus Zone of the Upper Pliensbachian. This corresponds to the most widespread episode of argillaceous sedimentation, and also to the common occurrence of bituminous shale beds in the central part of the basin (e.g. Sao Pedro de Muel and Peniche). These 'deep' water shales are abruptly overlain throughout the basin by limestones and indurated mark of the spinatum Zone. The position of the Pliensbachian-Toarcian boundary within the Portuguese succession is not well defined. The well indurated limestone and marl sequence containing spinatum Zone ammonites forms an easily recognizable lithostratigraphic unit throughout the Lusitanian Basin. A combination of lithostratigraphic and biostratigraphic criteria has apparently been used to place the boundary at the point where these hard lithologies are abruptly overlain by soft bluegrey shales containing abundant specimens of pyritized juvenile dactylioceratid ammonites. However, from the faunal lists provided by Mouterde (1955) for the Peniche coastal exposures, it would appear that a more acceptable boundary position is at the base of Mouterde's Bed 15e, within the limestone-marl unit, corresponding to the first appearance of Paltarpites paltus (K. Page, pers. comm., 1994). The underlying beds contain ammonites of the genus Tauromeniceras belonging to the emaciaticeras Zonule (K. Page, in press) which is believed to be equivalent in age to the upper part of the spinatum Zone, hawskerense Subzone in northern Europe. The occurrence' of soft blue-grey shales with dactylioceratid ammonites immedi-ately above the hard limestone-marl unit has been found in exposures as far apart as Peniche, Brenha and Zambujal. These assemblages are believed to occur within the lower part of the semicelatum Subzone of the tenuicostatum Zone. Sample was collected from these beds approximately 50 cm to 1 m above the top of the hard limestone-marl unit. At Zambujal the faunal lists of Mouterde et a/. (1964) provide no firm indication of the presence of the paltum Subzone and it is probable that in the basin margin locations a condensed sequence or a discontinuity occurs between the spinatum Zone and the semicelatum Subzone of the Lower Toarcian. The Toarcian succession predominantly consists of mark and shales up to the bijirons Zone; thereafter the frequency and thickness of interbedded limestones gradually increases.
In the Zambujal region, the Pliensbachian and Toarcian comprises a series of calcareous shales, mark and limestones representing the informally named Brenha Formation. The jamesoni-ibex Zones are dominated by interbedded buff to pale grey mark and limestones, with subordinate calcareous shales and claystones. The davoei and margaritatus Zone sediments become progressively more shaley and darker in colour. A marked lithological change can be seen at the top of the margaritatus Zone and into the spinatum Zone, with the occurrence of interbedded hard limestones and grey to buff coloured indurated marls. Within the lower part of the tenuicostatum Zone, an abrupt change in sedimentation witnesses the reappearance of dominantly pale grey to buff calcareous shales and marls, with subordinate limestone development. The interbedded nature of the shale-marl-limestone lithologies continues through the Toarcian and into the Aalenian, although the proportion of limestone to marl and calcareous shale increases, from the bijirons Zone upwards.

OSTRACOD BIOZONATION
Five ostracod zones are defined for the Pliensbachian and Toarcian of Zambujal, Portugal. All five ostracod zones comprise assemblage zones, using the most abundant ostracod taxa, particularly those which possess limited stratigraphical ranges. The proposed zonation is outlined in Fig. 4, in relation to the ammonite biostratigraphy. Accessory species are also included, where they are considered important within the assemblage. A complete ostracod range chart for the Pliensbachian and Toarcian of Zambujal is presented in Fig. 5.    2). In most cases.

Gammacytheve ubiquita-Ogmoconchella gvuendeli
abundances above 100 are rounded to the nearest 10. Sample numbers are the same as those used in Exton (1979), except for sample Z5 which is an additional sample.  (Boomer & Ainsworth, in press). Throughout much of Europe, the Metacopina become extinct within the tenuicostatum Zone. However, in the Mochras Borehole, North Wales, their extinction occurs in the lowermost falcferum Zone (Boomer, 1991). Remarks. This assemblage is initially marked by low diversity and low abundance ostracod assemblages (samples 120, 121) (Fig. 5), similar in composition to those described by Ainsworth (1986) and Boomer (1991)

OSTRACOD BIOSTRATIGRAPHY OF THE ZAMBUJAL SECTION
Eighty species of Ostracoda have been recognized from the Pliensbachian and Toarcian of Zambujal, Portugal by the present authors (Fig. 5). The marked increase in faunal diversity compared to the earlier study by Exton (1979) reflects the numerous studies undertaken on European Lias sequences since 1979. Many of the taxa from Zambujal have been described throughout northern Europe, albeit with discontinuous or differing stratigraphic ranges, reflecting either the geographical setting and/or facies variation.

Lower Toarcian
A marked change in the ostracod assemblage occurs within the lower part of the tenuicostatum Zone (sample Z5), with the extinction of the Metacopina, in association with an abrupt decline in faunal diversity and abundance. During the upper part of the tenuicostatum Zone (samples 120, 121), a slight increase in faunal diversity and abundance is noted (Fig. 5). This marked faunal turnover occurs throughout much of onshore and offshore northwest Europe during this time.
A further increase in diversity and abundance occurs within the falciferum Zone (samples 122, 123), with 12 species present. The assemblages are dominated by Bairdiacypris (B. triangularis Ainsworth, B. rectangularis Ainsworth), Pseudomacrocypris sp. A sensu Ainsworth and Kinkelinella sermoisensis (Apostolescu). Similar assemblages have been described from the Fastnet and North Celtic Sea Basins (Ainsworth, 1986;Ainsworth et al., 1989).  (Fig. 5). Many of these taxa are common throughout northern Europe during this time.

Upper Toarcian
Thirty-one species of Ostracoda have been recovered from the Upper Toarcian of Zambujal, of which 14 taxa have their originations (Fig. 5) The thouarsense Zone (sample 129) assemblage is very similar in composition to the underlying variabilis Zone. A marked increase in faunal diversity and abundance occurs at the base of the levesquei Zone (sample 130), with the occurrence of six new species (Fig. 5). Many of the taxa have been described from northwest Europe, especially in those studies of Ainsworth (1986) and Boomer (1991) from the Toarcian and Aalenian of the Fastnet Basin and Mochras Borehole, respectively. Praeschuleridea pseudokinkelinella Bate & Coleman is noteworthy for its higher stratigraphical range (upper bifronsthouarsense Zone) at Zambujal, compared with that in England (falcife rum-bifrons Zones).

FAUNAL ANALYSIS
Changes in the faunal composition of the Ostracoda at the suborder-superfamily level in the Zambujal sequence are numerically illustrated in Fig. 6. From these data, a number of distinct 'episodes' with characteristic assemblages are noted, at least one of which is of global significance. To interpret these changes in faunal composition it is also necessary to integrate information both on diversity levels and rates of faunal turnover, as outlined in Fig. 7. It must be noted beforehand that the sampling strategy was strongly influenced by the sediment type (e.g. only mark and calcareous shales were processed). Futhermore, none of the data take into account the weight of the unprocessed sample.
The earliest sediments studied (iamesoni to davoei Zones, samples 103-1 09) yield quite diverse assemblages dominated by the Cytheracea and the Metacopina. During most of the succeeding margaritatus Zone interval, however, there are no Metacopina recorded. These assemblages are dominated (up to 90%) by the Cladocopina (as species of Polycope). This undoubtedly reflects some environmental shift at the site of deposition. Not only are the Cladocopina the most abundant faunal group as a percentage of the samples during this interval, but they are also numerically abundant in absolute terms.
No modern analogue is known for such assemblages. The Cladocopina occur in almost all marine environments, however, they are rarely encountered in large numbers (R. C. Whatley, pers comm.). Diverse cladocopine assemblages have been recorded from the Quaternary of the Arctic Ocean (Joy & Clark, 1974). Lower Jurassic assemblages from other European sections commonly record Polycope species, but never in such high numbers or in such dominance.
The peak in Polycope abundance declines steadily from the lower part of the margaritatus Zone (sample 111), with the uppermost margaritatus Zone (sample 11 5) interval seeing a return to the assemblages recorded in the lowest part of the sequence. These conditions continue through to the top of the spinatum Zone (sample 119). At this junction, specimen abundance sees a marked decrease. This in itself is perhaps not significant since similar fluctuations occur throughout the sequence, however, this decrease is concomitant with the onset of a marked diversity trough (Fig. 7b) and an increased extinction rate (Fig. 7c).
This event was described by Exton (1979) as being the point at which the Metacopina became extinct in the Lusitanian Basin and the timing of this event was thought to be consistent throughout much of northwest Europe. Subsequent studies (Boomer, 1991(Boomer, , 1992 have shown that the timing of this extinction event can be traced through to the tenuicostatum and even falciferum zones of the Early Toarcian in some more northerly sequences. As a result of more detailed sampling of the Zambujal and Peniche sections, in association with careful ammonite biostratigraphical control (K. Page, pers. comm.), we have established that the final extinction of the Metacopina in the Lusitanian Basin must have occurred during the earliest Toarcian (tenuicostatum Zone) and not at the Pliensbachian/ Toarcian boundary.
Whatever events brought about the demise of the Metacopina the conditions were not completely inimical to the survival of benthonic Ostracoda. The abundance remained low, with diversity halved during the tenuicostatum and lower falciferum Zones (samples Z5, 120-122), yet by the beginning of the Middle Toarcian (upper falciferum Zone, sample 123) the assemblages had recovered to their former species richness. From Fig 6, it would appear that the niche left by the Metacopina was filled initially by the Bairdiacea which had hitherto only been rarely recorded in the sequence. The Cladocopina also increase in importance during the Middle Toarcian, but by the Upper Toarcian (thouarense Zone, sample 129) they had begun to decrease and they, together with the Bairdiacea, had been replaced by the Platycopina. The success of the latter group is in accordance with the observations of Boomer (1991) and Boomer & Whatley (1992) where the demise of the Metacopina in the extensive Liassic sequence of the Mochras Borehole, led to the subsequent success of the Platycopina.
The Middle and Upper Toarcian intervals are known to be characterized by periods of low oxygen conditions throughout much of Northwest Europe with the success of the Platycopina during this time being attributed to their filter feeding mode of life bestowing a greater survival capability in reduced oxygen environments (Whatley, 1991). This is probably a simplification of the Lower Jurassic picture since many non-platycopids also survive these kenoxic periods. It should be noted, however, that no evidence of oxygen deficient conditions have been observed in the Toarcian sedimentary record of Portugal. It is almost incontrovertible that the loss of such an important and long ranging group as the Metacopina, led to a large niche availablity which the Platycopina were best able to take advantage of.
The Upper Toarcian (samples 129-134) sequence appears to show a steadily increasing dominance of Platycopina. In the youngest sample (134) examined, the dominance ( fig. 7d) is almost at its greatest in the sequence, diversity is decreasing and abundance increasing. This suggests increasing environmental stress where one group or one species is best adapted to survive and reproduce. Description. Carapace of medium size, subtriangular in lateral view, subovate in dorsal view. Anterior margin asymmetrically rounded, extremity slightly below mid-height. Posterior margin rounded subtriangular in left valves, subtriangular in right valves, extremity below mid-height. Dorsal margin slightly convex to straight in left valves, straight with prominent cardinal angles in right valves. Posterior margin convex, tapering towards posterior. Maximum length below mid-height, maximum height at anterior cardinal angle, maximum width behind mid-length. Left valve larger than right valve, overlapping right valve dorsally and ventrally. Sexually dimorphic, male dimorph more elongate than female. Carapace strongly calcified. Ornament of strongly developed open, longitudinal ribbing with poorly defined cross-ribs forming subrounded to subovate reticulation. Mid-laterally, ribbing short and thickened, often discontinuous, forming a vertically aligned depression, extending from mid-valve region towards apex of triangular ribbing. This depression is bounded by vertically aligned elements of ribbing. Eye spot moderately well developed. Eye sulcus less well defined. Inner lamella moderately broad, line of concresence coincides with inner margin. Radial pore canals not observed. Hinge antimerodont; right valve terminal dentate  Description. Carapace very small, subtriangular in lateral view, inflated in dorsal view. Anterior margin slightly asymmetrically rounded, extremity immediately below mid-height. Posterior margin subtriangular, extremity above mid-height. Dorsal margin slightly sinuous. Posterior margin weakly convex, tapering upwards towards posterior margin. Maximum length above mid-height, maximum height at anterior cardinal angle, maximum width behind mid-length at posterior termination of postero-ventral nodes. Left valve slightly larger than right. Carapace moderately strongly calcified. Ornament of moderately coarse polygonal reticulation, most strongly developed centrally, weakening towards the margins. Three oblique ribs developed anteriorly, the uppermost rib joins onto a welldeveloped eye node, the central rib extends from below midanterior margin, extending to ribbed postero-dorsal node, while the ventro-lateral rib extends from antero-ventral margin along poorly inflated alae terminating immediately below ventral margin at a prominent postero-ventral node. Well formed dorsal flange extends from anterior to posterior cardinal angles in right valves. Inner lamella broad anteriorly and posteriorly. Inner margin coincides with line of concresence. Radial pore canals not observed. Hinge lophodont. Muscle scar pattern not observed. Dimensions. Length 360-385 pm, height 18&195 pm. Distribution. At present, only recovered from the type locality, samples 104-1 13, beds 23-33, jamesoni-margaritatus Zones, Lower to Upper Pliensbachian. Remarks. This species is similar to Eucytherura transversiplicata (Bate & Coleman, 1975) recorded from the Toarcian and Aalenian of northwest Europe, but differs in the possession of a ribbed postero-dorsal node and a very prominent posteroventral node. Eucytherura batei (Ainsworth, 1986), is also similar, but can be distinguished by the absence of the oblique central rib, postero-ventral node and postero-dorsal node.

CONCLLJSIONS
In the light of more recent studies on Lower Jurassic Ostracoda, a re-examination has been undertaken on the Pliensbachian and Toarcian Ostracoda first described by Exton (1979)  3. An analysis of the ostracod assemblages from the Zambujal sequence has revealed fluctuations in the faunal composition which almost certainly reflect environmental changes, although at this point their interpretation remains uncertain. One distinct episode in the Late Pliensbachian led to the temporary dominance of the Cladocopina, a steady rise which is followed by a steady fall. The significance of this is uncertain since no modern of fossil analogues are known. The most significant change in faunal composition is the event which brought about the extinction of the Metacopina during the semicelatum subzone, the lower subzone of the tenuicostatum Zone of Early Toarcian age. This extinction event is similar to that which occurs throughout much of Northwest Europe at this time. Further work is required to fully understand the causes of this important event. The niche vacated by the Metacopina appears to have been initially filled by the Bairdiacea and later the Platycopina.