Danian—Selandian (D—S) planktic foraminiferal taxonomy and paleoecology, including the most problematic index-species of biochronological schemes, are here revised based on qualitative and quantitative data from the Caravaca and Zumaia sections, Spain. As a first step the morphological and microtextural diagnostic characters are revised in order to achieve appropriate morphological distinctions of the investigated species. The analysis of latitudinal preferences of the planktic foraminiferal species deduced from a comparison of their relative abundances at Caravaca (western Tethyan sub-tropical waters) and Zumaia (central North Atlantic warm temperate waters) in suite with a statistical comparison of quantitative stratigraphic distributions of the species at Caravaca have shown several divergences in their patterns of spatio-temporal distribution (i.e., in their latitudinal preferences and quantitative stratigraphic distributions). This analysis allowed taxonomical separation of 41 species from the following genera: Eoglobigerina, Subbotina, Parasubbotina, Globanomalina, Luterbacheria, Acarinina, Igorina, Morozovella, Praemurica, Chiloguembelina, and Zeauvigerina. Morphologically convergent species pairs such as Acarinina trinidadensis and Praemurica inconstans, Acarinina praecursoria and Acarinina uncinata, Morozovella conicontruncata and Morozovella angulata, or Morozovella cf. albeari and Igorina albeari, are well differentiated using aforementioned criteria. Since some of the species are index-taxa, the taxonomic refinements are essential to clarify and compare the planktic foraminiferal zonations from the Danian—Sealandian transition. A new lower/higher (L/H) latitude taxa ratio is proposed for paleoclimatic interpretations based on the paleoecological and quantitative studies. Fluctuations in L/H ratio in the Caravaca section suggest three climate warming events during the D—S transition, one of them probably occurring at the D—S boundary.
The Danian/Selandian (D—S) boundary has been defined in the Zumaia section, northern Spain, using a radiation of the calcareous nannofossil group, Fasciculithus, as event for global, marine correlation (Schmitz et al. 2011). The position of the D—S boundary in the planktic foraminiferal stratigraphic scale could not be clarified because taxonomic problems caused uncertainties in correlation of the Paleocene zones (Arenillas et al. 2008).
The reference biozonation for most of the current Paleocene planktic foraminiferal stratigraphiers is that of Berggren and Pearson (2005), which is based on the systematics of Berggren and Norris (1997) and Olsson et al. (1999). Some of their conclusions contradicted previous taxonomies, such as those of Luterbacher (1964), Stainforth et al. (1975), Blow (1979), Toumarkine and Luterbacher (1985), and Arenillas (1996), but they had the advantage of photographing the holotypes of most Paleocene species by means of Scanning Electronic Microscope (SEM). The SEM images prevent some subjective interpretations made from the original drawings and descriptions of these holotypes. However, these images reflect deficiencies in the preservation of many specimens, by impeding the observation of the original wall-texture and ornamentation, both of which are important features in planktic foraminiferal systematics.
In addition, reconciliation between splitter and lumper morphological taxonomies is still a long-awaited outcome. Until an invariable and unique species concepts exist, the number of species proposed for a given time interval (number of morphospecies) and the number of evolutionary species within anagenetic series (number of chronospecies within a given lineage) will continue to be a matter of arbitrary judgment. This subjectivity and discrepancy in the proposed taxonomies have caused and probably will continue to cause uncertainty in the biostratigraphic scales. The classic use of morphological and microtextural diagnostic characters for the distinction of planktic foraminiferal species has made it possible to show convincingly how many species there are in a certain time period. Morphostatistical analyses such as those by Arenillas and Arz (2007), and Arz et al. (2010), have proved valuable for morphospecies discrimination within planktic foraminifera. Paleoecological analysis and studies on the quantitative stratigraphic distributions of the species should also be valid in taxonomy, assuming that differences in their patterns of spatio-temporal distribution are criteria for the separation of species.
A revision of the D—S planktic foraminiferal taxonomy and paleoecology are documented herein. Divergences in latitudinal preferences of the species as well as statistical comparisons of their quantitative stratigraphic distributions are used as criteria for the taxonomical separation of species. These patterns of spatio-temporal distribution were inferred comparing relative abundances of the species at Caravaca and Zumaia (Spain) and analyzing their fluctuations in abundance across the D—S transition at Caravaca. The main purposes of the taxonomic refinements are to better clarify the stratigraphic position of the biozones established by the different planktic foraminiferal specialists for the D—S transition, and propone a new paleoclimatic ratio (lower/higher latitude taxa ratio), which is able to recognize fluctuations in temperature across the D—S transition.
Institutional abbreviation.—MPZ, Museo Paleontológico of the Universidad de Zaragoza, Aragon Government, Spain.
Other abbreviations.—D—S, Danian—Sealandian transition; FOD, first occurrence data; L/H, lower/higher latitude, LOD, last occurence data.
Material and methods
The D—S planktic foraminiferal taxonomic and paleoecological revision is based mainly on the Caravaca and Zumaia sections, Spain (Fig. 1). The Caravaca section is located in the Subbetic Zone of the Betic Cordilleras (western Tethys), to the south of the town Caravaca de la Cruz (southeastern Spain). D—S sediments belong to the Jorquera Formation, consisting of grey marls and calcareous marls of middle—lower bathyal depths (Ortiz et al. 2008). The planktic foraminiferal stratigraphic record of the Caravaca section was studied by Arenillas (1996) and Arenillas and Molina (1997). The Zumaia/Zumaya section is located in the western Pyrenees (paleo-Bay of Biscay, North-central Atlantic), to the northwest of the village of Zumaia (northern Spain). The D—S transition at Zumaia spans the upper part of the Danian Limestone Formation, consisting of greyish limestone-reddish marly couplets, and the lower part of the Itzurun Formation, of red to grey marls (Arenillas et al. 2008). According to Ortiz in Arenillas et al. (2008), D—S sediments were also deposited at middle—lower bathyal depths. The planktic foraminiferal stratigraphic record of the Zumaia section was studied by Arenillas (1996) and Arenillas and Molina (2000), and recently revised by Arenillas et al. (2008).
The revision takes into account previously reported taxonomies, mainly those of Beckmann (1957), Luterbacher (1964), Stainforth et al. (1975), Blow (1979), Toumarkine and Luterbacher (1985), Arenillas (1996), Berggren and Norris (1997), and Olsson et al. (1999). Most of the illustrated planktic foraminiferal specimens come from the Caravaca section. Specimens were obtained from samples disaggregated in tap water and diluted H2O2, then washed and sieved into 63–106 µm and ≥106 µm size fractions, and dried at 50°C. Representative splits of about 300 specimens of planktic foraminifera from ≥106 µm size fraction were chosen from each sample, using an Otto microsplitter, to obtain quantitative data (relative abundance of the species). All the representative specimens were selected and mounted on microslides for a permanent record and identification. These microslides were deposited in the Museo Paleontológico of the Universidad de Zaragoza, Spain, with repository numbers MPZ.
To analyse the latitudinal preferences of planktic foraminiferal species, previous paleobiogeographical and isotopic studies (e.g., Boersma and Premoli Silva 1989, 1991; Shackleton et al. 1985; Arenillas 1996; Olsson et al. 1999) and relative abundance were considered in both the Caravaca and the Zumaia sections. The average relative abundance of species was calculated for the stratigraphic intervals corresponding to the Acarinina uncinata and Morozovella cf. albeari zones in both sections (Tables 1, 2). Since the Caravaca section is situated at lower latitudes (subtropical Tethyan region), the species which are more abundant in Caravaca than in Zumaia were considered to be lower-latitude dwellers, preferably tropical-subtropical, whereas the rest were considered to be higher-latitude dwellers, i.e., widely cosmopolitan. The D—S transition in Zumaia is characterized by large cyclical fluctuations in planktic foraminiferal assemblages (Arenillas et al. 2008), from warmer to cooler, but the average relative abundances of species should reflect its latitudinal position.
Average relative abundances of the species in the Acarinina uncinata Zone from both Caravaca and Zumaia sections, and probable latitudinal preferences. Data obtained from Arenillas (1996), Arenillas and Molina (1997, 2000), and Arenillas et al. (2008). Note that the A. uncinata Zone of Caravaca stratigraphycally continues below with respect to the samples shown in Figs. 3 and 5, and Tables 4 and 5.
In addition, the quantitative stratigraphic distributions of the species across the D—S transition of Caravaca (Tables 3, 4) were compared to evaluate the difference in their patterns of temporal distribution, and to use these differences as criteria for the taxonomic separation of species. The cluster analyses based on Morisita's index measures have been used to find groupings that represent similar ecological requirements and/or biological behaviours, and as a method for taxonomic separation of those morphologically similar species.
Danian—Selandian taxonomic and biostratigraphic controversy
Planktic foraminiferal stratigraphic controversy.—Taxonomic problems have recently caused uncertainties in the D—S biostratigraphy (Arenillas et al. 2008; Sprang et al. 2009), which impeded the exact placement of the D—S boundary within planktic foraminiferal stratigraphic scales. Figure 2 shows the most probable correlation of some of these planktic foraminiferal zonations.
The D—S transition was initially divided into the Acarinina uncinata, Morozovella angulata, and Igorina pusilla zones, using the first occurrence data (FODs) of the nominate species to place their lower boundaries (Bolli 1966; Toumarkine and Luterbacher 1985; Canudo and Molina 1992). The upper boundary of the Igorina pusilla Zone was placed at the FOD of Luterbacheria pseudomenardii (= Planorotalites pseudomenardii according to Toumarkine and Luterbacher 1985). Although the transitional forms between these species and their ancestors appear to have caused confusion, the taxonomic concepts of A. uncinata (Bolli, 1957), M. angulata (White, 1928), and L. pseudomenardii (Bolli, 1957) have not changed over time, except for their generic assignment. Toumarkine and Luterbacher (1985) included A. uncinata in the genus Morozovella, and I. pusilla and L. pseudomenardii in the genus Planorotalites. The species I. pusilla (Bolli, 1957) became more problematic, so Berggren et al. (1995) and Arenillas and Molina (1997) excluded it as index species.
Average relative abundances of the species in the Morozovella cf. albeari Zone from both Caravaca and Zumaia sections, and probable latitudinal preferences. Data obtained from Arenillas (1996), Arenillas and Molina (1997, 2000), and Arenillas et al. (2008). Note that the M. cf. albeari Zone of Caravaca stratigraphycally continues above with respect to the samples shown in Figs. 3 and 5, and Tables 4 and 5.
The alphanumeric zonation of Berggren et al. (1995) and Berggren and Pearson (2005) divided the D—S transition in two biozones: P2, equivalent to the A. uncinata Zone, and P3, equivalent to the M. angulata and I. pusilla zones. Theses authors subdivided P3 into two subzones: P3a and P3b, the P3a/P3b boundary being the FOD of Igorina albeari. The upper boundary of P3b (or P3b/P4 boundary) is also the FOD of Luterbacheria pseudomenardii (= Globanomalina pseudomenardii according to Olsson et al. 1999).
Arenillas (1996) and Arenillas and Molina (1997) used the FODs of Morozovella crosswicksensis and Igorina albeari to subdivide the previous P3 or M. angulata Zone into three biozones: M. angulata, M. crosswicksensis, and I. albeari zones. Since Olsson et al. (1999) considered M. crosswicksensis (Olsson, 1960) to be a junior synonym of Morozovella occlusa (Loeblich and Tappan, 1957), the lowermost Selandian M. crosswicksensis-type specimens (Blow 1979; Arenillas 1996) were renamed Morozovella cf. albeari by Arenillas et al. (2008). The M. crosswicksensis-typs specimens were interpreted by Olsson et al. (1999) and Sprang et al. (2009) as belonging to Igorina albeari, so P3a/P3b boundary corresponds to the M. angulatal M. cf. albeari boundary recognized by Arenillas and Molina (1997).
The biostratigraphical distributions of the most part of the D—S transition index-species are debatable. Figure 3 shows the stratigraphical position of the biozones proposed by Arenillas and Molina (1997, modified) and by Berggren and Pearson (2005) in the Caravaca section, taking into account that the former relied on the taxonomy of Arenillas (1996) and the latter on that of Olsson et al. (1999).
Planktic foraminiferal taxonomic controversy.—Taxonomic differences regarding the taxonomic concept of certain species, including some of the index-species, and their generic assignments, have emerged among specialists which in turn has caused biostratigraphic controversy. The D—S planktic foraminiferal taxonomy reported here (SOM_1: Supplementary Online Material available at http://www.app.pan.pl/archive/published/SOM/app57-Arenillas_SOM.pdf) build upon the work of Arenillas (1996), which was largely based on the work of Luterbacher (1964), Stainforth et al. (1975), Blow (1979), and Toumarkine and Luterbacher (1985). This taxonomic proposal is compared with that of Olsson et al. (1999) (Table 5).
Arenillas (1996) grouped the species of the D—S transition into the following genera: Eoglobigerina Morozova, 1959, Subbotina Brotzen and Pozaryska, 1962, Parasubbotina Olsson, Berggren, and Liu, 1992, Globanomalina Haque, 1956, Luterbacheria Canudo, 1994, Acarinina Subbotina, 1953, Igorina Davidzon, 1976, Morozovella McGowran, 1964, Praemurica Olsson, Berggren, and Liu, 1992, Chiloguembelina Loeblich and Tappan, 1956, and Zeauvigerina Finlay, 1939. This generic classification was shared by Berggren and Norris (1997) and Olsson et al. (1999), except for Luterbacheria which was included within Globanomalina.
Eoglobigerina includes the following species (Fig. 4): E. edita (Subbotina, 1953), E. fringa (Subbotina, 1950), E. cf. trivialis (E. trivialis Subbotina, 1953, sensu Blow, 1979), E. spiralis (Bolli, 1957), and E. tetragona Morozova, 1961. This proposal for Eoglobigerina is similar to that of Blow (1979), and partially shared by Olsson et al. (1999) who validated only the first four, considering E. fringa and E. tetragona junior synonyms of E. eobulloides (Morozova, 1959) and E. edita respectively. Arenillas (1996) used the species concept of E. trivialis sensu Blow (1979) (Fig. 4H, I), but this is a junior synonym of Subbotina triangularis (Fig. 4G) as shown by Olsson et al. (1999). The E. trivialis-type specimens of Blow (1979), here considered E. cf. trivialis, were probably included in Subbotina cancellata Blow, 1979 by Olsson et al. (1999).
Average relative abundances of the species in the Caravaca section from sample 1 to sample 9.5. In bold, lower latitude dwellers; * species that apparently changed their latitudinal preferences; ** insufficient data. Note that the change in latitudinal preference of Chiloguembelina cf. subcylindrica occurs in the Acarinina uncinata Zone but below sample 1.
Average relative abundances of the species in the Caravaca section from sample 9.75 to sample 14.25. In bold, lower latitude dwellers; * species that apparently changed their latitudinal preferences; ** insufficient data.
Comparison of taxonomies by Arenillas (1996 modified) and Olsson et al. (1999), and latitudinal preferences. (1) Taxonomy by Arenillas (1996 modified); (2) taxonomy by Olsson et al. (1999); (3) details of species SEM-images; (4) latitudinal preferences according to previous works; (5) latitudinal preferences proposed here; * near-shore dwellers; ** initially low latitude dwellers, finally higher latitude dwellers. Some species not were considered by Olsson et al. (1999), but they were probably included within the species concepts of Luterbacheria ehrenbergi (a), Praemurica uncinata (b, c), Morozovella praeangulata (d, f), and Chiloguembelina midwayensis (g).
Subbotina (Fig. 4) includes the following species: S. triangularis (White, 1928), S. triloculinoides (Plummer, 1927), and S. compressaformis (Khalilov, 1956). This Subbotina classification is similar to that of Blow (1979), who recognized two subspecies in S. triangularis triangularis and S. t. cancellata, the last of which was raised to full species by Olsson et al. (1999). Although S. eocaenica (Terquem, 1882) must be considered as nomen dubium non conservandum, Blow (1979) and Arenillas (1996) retained this name for Subbotina with inflated-chambers occurring from the middle Paleocene to the Eocene (Fig. 4T). These forms are now re-named S. compressaformis (Fig. 4U, V). Olsson et al. (1999) included them in S. triloculinoides, although due to their different morphologies, stratigraphic ranges and patterns of spatio-temporal distribution, it appears reasonable to continue considering them as distinct species.
In the D—S transition, Parasubbotina (Fig. 4) includes the following species: P. pseudobulloides (Plummer, 1927), P. quadrilocula (Blow, 1979), and P. variospira (Belford, 1984). The second one was considered by Olsson et al. (1999) a junior synonymous of P. varianta (Subbotina, 1953), but Arenillas (1996) used the name P. varianta for lower—middle Danian forms with high rate of chamber enlargement. Arenillas (1996) also used the species Parasubbotina ferreri (Orue-Etxebarria and Apellaniz, 1991), but apparently this species is a junior synonym of P. variospira.
Praemurica raises more taxonomical problems, because Olsson et al. (1999) included some species that Blow (1979) and Arenillas (1996) considered as belonging to the genus Acarinina. According to the latter, Praemurica includes only Pr. inconstans (Subbotina, 1953) in the D—S transition, whereas Acarinina includes (Figs. 5–7): A. arabica (El Naggar, 1966), A. hansbollii (Blow and Banner, 1962), A. indolensis Morozova, 1959, A. praeaqua Blow, 1979, A. praecursoria Morozova, 1957, A. praepentacamerata (Shutskaya, 1956), A. trinidadensis (Bolli, 1957), and A. uncinata (Bolli, 1957). Olsson et al. (1999) considered that A. uncinata comes under the genus Praemurica, and A. trinidadensis and A. praecursoria are junior synonyms of Pr. inconstans and Pr. uncinata respectively. Nevertheless, Blow (1979), Toumarkine and Luterbacher (1985), and Arenillas (1996) showed that these species have a more or less developed muricate wall (instead of a cancellate wall as in Praemurica), which makes them more appropriate for grouping within the genus Acarinina. Arenillas (1996) considered that A. praeangulata (Blow, 1979) to be a more evolved morphotype of A. praepentacamerata, as both do not present peripheral muricocarina. Other authors considered that A. praeangulata was the first member of Morozovella (e.g., Berggren and Norris 1997; Olsson et al. 1999). In addition, Blow (1979) and Arenillas (1996) showed two evolutionary trends within Acarinina: one towards reducing the number of chambers (A. hansbollii and A. praeaqua), and one towards the trochospire raise and the aperture migration in intraumbilical position (A. indolensis and A. arabica), but these were not considered by Olsson et al. (1999).
Igorina includes the following species: I. albeari (Cushman and Bermúdez, 1949), I. pusilla (Bolli, 1957), and I. tadjikistanensis (Bykova, 1953) (Fig. 6). The most problematic one is I. pusilla. Bolli (1957) defined I. pusilla as a biconvex globorotalid with a smooth wall and angular axial margin, but no keel. Stainforth et al. (1975) and Toumarkine and Luterbacher (1985) indicated that well-preserved specimens of this species have a strong perforated wall, and included it in the genus Planorotalites. Davidzon (1976) proposed this species as belonging to his new genus Igorina, along with other species such as I. tadjikistanensis (Bykova, 1953) and I. laevigata (Bolli, 1957). Blow (1979) assigned to Globorotalia (Acarinina) convexa convexa Subbotina, 1953 the taxonomic concept of I. pusilla by Stainforth et al. (1975) and Davidzon (1976), noting that these specimens have a muricate wall. Finally, Arenillas (1996) and Olsson et al. (1999) set the current concept of Igorina (i.e., biconvex forms with a muricate wall), but they opposed the concepts of I. tadjikistanensis and I. pusilla.
Morozovella groups the following species in the D—S transition (Figs. 5–7): M. aequa (Cushman and Renz, 1942), M. angulata (White, 1928), M. conicotruncata (Subbotina, 1947), M. cf. albeari (= M. crosswicksensis Olsson, 1960, sensu Blow, 1979), M. lacerti (Cushman and Renz, 1946), and M. simulatilis (Schwager, 1883, sensu Luterbacher 1964). This taxonomic framework is based on the studies of Luterbacher (1964) and Blow (1979), but it differs substantially from that of Olsson et al. (1999) who only considered three species for this time interval: M. angulata and M. conicotruncata, in addition to M. praeangulata. However, Arenillas (1996) showed two trends within Morozovella developing through the D—S transition: one towards reducing the chamber number (M. lacerti and M. aequa) and one towards a compressed biconvex shape (M. simulatilis and M. cf. albeari). In the lower Selandian, transitional forms (Fig. 7P) between M. conicotruncata (Fig. 7N, O) and M. velascoensis (Cushman, 1925) are also encountered (Fig. 7Q, R). This variety (attributed to M. conicotruncata by Arenillas 1996) was considered a new species, Morozovella protocarina, by Corfield (1989).
Blow (1979) and Arenillas (1996) named M. crosswicksensis the specimens assigned herein to M. cf. albeari. However, Olsson et al. (1999) considered that the former was a junior synonym of Morozovella occlusa (Loeblich and Tappan, 1957). According to Blow (1979), M. occlusa differs from M. crosswicksensis by having a muricate circum-umbilical rim, well-developed muricocarina and almost a muricae-free wall (Fig. 6). Since Olsson et al. (1999) synonymized both species, the lowermost Selandian M. crosswicksensis-type specimens were reconsidered to be M. cf. albeari by Arenillas et al. (2008). These latter wanted to note with this name the probable difficulty in distinguishing M. cf. albeari from I. albeari (e.g., specimen in Fig. 6J). According to Arenillas (1996), the first one evolved from the lineage M. angulata—M. simulatilis developing an ever more compressed biconvex shape, whereas the second one evolved from the lineage A. praepentacamerataI. tadjikistanensis—I. pusilla developing circumcameral muricocarina in the most evolved forms. The M. crosswicksensis-type specimens of Blow (1979) and Arenillas et al. (2008) were included in I. albeari by Olsson et al. (1999) and by Sprang et al. (2008), introducing a new focus of taxonomic and biostratigraphic controversy. According to the taxonomy by Arenillas (1996), I. albeari differs from M. cf. albeari by having sutures generally covered with muricae, poorly developed (or absent) circumcameral muricocarina and being generally masked by the dense muricae, and aperture tending towards an intraumbilical position (SOM_1). Nevertheless, the SEM image (by Olsson et al. 1999) of the I. pusilla holotype (Fig. 6C) might suggest another interpretation: although its preservation is poor, it could have a muricate wall and muricocarina, and be reassigned to Morozovella as was suggested by Yassini (1979). In this case, (1) the D—S M. crosswicksensis-type specimens could be attributed to Morozovella pusilla, and (2) the taxonomic concept of I. pusilla sensu Stainforth et al. (1975), Davidzon (1979), and Toumarkine and Luterbacher (1985) could now be attributed to Igorina convexa (Subbotina, 1953) in agreement with the opinion of Blow (1979) (Fig. 6D).
Globanomalina Haque, 1956 and Luterbacheria Canudo, 1994 group includes the following species (Fig. 8): G. compressa (Plummer, 1927), G. haunsbergensis (Gohrbandt, 1963), G. chapmani (Parr, 1938), and L. ehrenbergi (Bolli, 1957). Berggren and Norris (1997) and Olsson et al. (1999) considered both genera synonyms, since they belong to the same evolutionary lineage. However, Arenillas (1996) differentiated Luterbacheria from Globanomalina by the presence of a keel instead of an imperforate margin in the former, considering that similar criteria have been used for the separation of other genera (e.g., Praemurica—Acarinina from Morozovella). Haig et al. (1993) considered G. haunsbergensis to be a junior synonym of G. chapmani, although their holotypes differ in the number of chambers (Fig. 8C, D; see diagnostic characters in SOM_1). The keeled genus Luterbacheria includes other upper Paleocene species according to Canudo (1994) and Arenillas (1996), such as L. troelseni (Loeblich and Tappan, 1957) (Fig. 8I, J) and L. pseudomenardii (Bolli, 1957) (Fig. 8K, L), whose taxonomical separation is debatable as the former is a transitional form between L. ehrenbergi and L. pseudomenardii.
Globoconusa includes the following species (Fig. 8): Gc. daubjergensis (Brönnimann, 1953), and Gc. conusa Khalilov, 1956. Olsson et al. (1999) considered both species synonymous, but they can be distinguished by the spire height, Gc. conusa being higher. Arenillas (1996) named the latter Gc. kozlowskii (Brotzen and Pożaryska, 1961), but apparently it is a junior synonym of Gc. conusa.
Chiloguembelina contains the following species in the D—S transition (Fig. 8): Ch. crinita (Glaessner, 1937, Ch. midwayensis (Cushman, 1940), Ch. cf. subcylindrica Beckmann (1957), Ch. subtriangularis Beckmann (1957), and Ch. taurica Morozova, 1961. This taxonomy was mainly based on Beckmann (1957) and subsequent studies of Arenillas (1996) and Olsson et al. (1999). The latter did not consider Ch. cf. subcylindrica (which is similar to Ch. midwayensis but with chambers more inflated) in the Paleocene. Moreover, they attributed to Chiloguembelina morsei (Kline, 1943) the taxonomic concept of Ch. taurica. Arenillas et al. (2007) showed that Ch. morsei is a junior synonym of Ch. midwayensis.
A genus probably related to Chiloguembelina is Zeauvigerina (Fig. 8), which groups the following species in the D—S transition: Z. aegyptiaca Said and Kenawy, 1956, and Z. teuria Finlay, 1947. It is doubtful that this taxon is planktonic (Loeblich and Tappan 1987), although taxonomists have considered it to be closely related to Chiloguembelina (Beckmann 1957; Huber and Boersma 1994; Arenillas 1996). However, others (e.g., Olsson et al. 1999) indicated that Chiloguembelina waiparaensis Jenkins, 1966 is a Zeauvigerina appearing in the Cretaceous, thereby unlinking Zeauvigerina from Chiloguembelina and raising new doubts about its pelagic habitat. In addition, although they inferred that it was planktonic based on quantitative data (high relative abundance compared with co-occurring benthic foraminifera), Huber and Boersma (1994) showed that this species yields stable isotopic values close to those of benthic foraminifera. Therefore, these forms probably were benthic with meroplanktonic behaviour.
The principal factors influencing the biogeographical and bathymetric distribution, as well as the abundance of the planktic foraminiferal species, are both physical-chemical (temperature, nutrients, oxygenation, light, salinity, water density, turbidity, and pressure) and biotic factors (life cycles, algal symbionts, food supply, predations, and interspecific relationships) in the pelagic realm. Most physical-chemical factors are in function of depth. Fluctuations in species abundance across the stratigraphic series are related to changes in one or more of these factors. Finding connections between species abundance fluctuations and changes in a particular ecological/biological factor is difficult due to the complexity of interactions between the controlling factors. However, it is feasible to assume that divergences in the quantitative stratigraphic distributions between two morphologically similar species are related to different ecological requirements and/or biological behaviors and therefore both belong to two—reproductively isolated—species.
According to de Vargas et al. (2001), heterochronic reproductive behaviors associated with niche adaptation may be a common mode of speciation in planktic foraminifera. Norris (2000) indicated that divergences in the timing and depth of reproduction are two important factors in the speciation of planktic foraminifera and other pelagic groups (seasonal sympatric and depth parapatric speciation, respectively). Thus many planktic foraminiferal species may have had narrower geographic ranges and ecological requirements than has been suspected, and very slight morphological differences may distinguish related species adapted to significantly different environments. In addition, biogeographic, ecological, and genetic studies on living specimens suggest that morphological taxonomies have underestimated the number of pelagic species (Darling et al. 2000).
Paleobiogeographic, quantitative and isotopic studies have been used to identify the latitudinal and bathymetric preferences of Paleocene planktic foraminiferal taxa. Summarizing the paleoecological data and interpretations by Boersma and Premoli Silva (1989, 1991), Shackleton et al. (1985), Corfield and Cartlidge (1992), D'Hondt and Zachos (1993), Arenillas (1996), Berggren and Norris (1997), and Olsson et al. (1999), the D—S planktic foraminiferal genera may be grouped into low-middle latitude (or tropical-subtropical), shallow water dwellers (Praemurica, Acarinina, Morozovella, and Igorina), and middle-high latitude (or cosmopolitan), intermediate-deep water dwellers (Eoglobigerina, Subbotina, Parasubbotina, Globanomalina—Luterbacheria, and Chiloguembelina). Nevertheless, particular species have paleoecological requirements and biological behavior that may be different from the norm within their genera. More data on the latitudinal preferences of planktic foraminiferal species is needed to refine the paleoenvironmental indeces used in studies of paleoclimatic and paleoceanographic fluctuations. In addition, such evidence can be used as a criterion to taxonomically separate species that had previously been synonymized by their morphological similarity.
Testing the latitudinal preferences of the species.—Species latitudinal preferences have been inferred by comparing their relative abundance (Tables 1, 2) between Caravaca and Zumaia. It is noteworthy that sediments of both sections were deposited at relatively similar depths (middle-lower bathyal), so the bathymetric factor should not influence the paleoecological interpretations.
Results indicate that species preferring low latitudes in the A. uncinata Zone (Table 1) were E. cf. trivialis, E. edita, S. compressaformis, G. chapmani, L. ehrenbergi, Pr. inconstans, A. trinidadensis, A. uncinata, A. hansbollii, A. indolensis, A. arabica, A. praepentacamerata, A. praeaqua, I. tadjikistanensis, Ch. taurica, Ch. midwayensis, Ch. cf. subcylindrica, Z. teuria, Gc. daubjergensis, and Gc. conusa. Species preferring middle latitudes, therefore being more cosmopolitan, were E. fringa, E. tetragona, E. spiralis, S. triloculinoides, S. triangularis, G. compressa, G. haunsbergensis, P. pseudobulloides, P. quadrilocula, and A. praecursoria.
In the M. cf. albeari Zone (Table 2), species preferring low latitudes (tropical-subtropical) were L. ehrenbergi, Pr. inconstans, A. uncinata, A. praepentacamerata, I. tadjikistanensis, M. angulata, M. simulatilis, M. cf. albeari, M. lacerti, M. aequa, Ch. midwayensis, Ch. subtriangularis, Z. teuria, and Z. aegyptiaca. Species preferring middle latitudes (temperate) were S. triloculinoides, S. triangularis, S. compressaformis, G. haunsbergensis, G. chapmani, P. quadrilocula, A. hansbollii, A. praeaequa, M. conicotruncata, Ch. cf. subcylindrica, and Ch. crinita.
Table 5 summarizes the latitudinal preferences of the species according to previous studies and compares with the results obtained here. Although both interpretations are consistent, some unexpected differences are noteworthy. Except for Ch. crinita, chiloguembelinids are more abundant in Caravaca, suggesting that they strongly preferred low latitudes. This result is consistent with paleobiogeographic studies by Beckmann (1957) and Olsson et al. (1999), who found chiloguembelinids preferably occur outside high latitudes.
Eoglobigerina and Globanomalina—Luterbacheria were considered essentially cosmopolitan (Arenillas 1996; Olsson et al. 1999), but data suggest that E. cf. trivialis, E. edita, and L. ehrenbergi preferred low latitudes. This result supports the suggestion that these species can be taxonomically separated from the morphologically similar species E. tetragona, E. spiralis, and G. haunsbergensis respectively, as was interpreted by Arenillas (1996). It suggests that high-spired Eoglobigerina and Globanomalina prefer temperate latitudes.
Among the acarininids and morozovellids, A. praecursoria and M. conicotruncata are more abundant in Zumaia, suggesting that (i) they preferred temperate latitudes, (ii) both species can be taxonomically differentiated from the morphologically similar species A. uncinata and M. angulata respectively, and (iii) acarininids and morozovellids with many chambers in the last spire whorl preferred temperate latitudes.
Finally, it also is noteworthy that some species apparently changed their latitudinal preferences throughout the D—S transition. These are S. compressaformis, A. hansbollii, A. praeaqua, and Ch. cf. subcylindrica. Data suggest that they began as low latitude dwellers and ended up living at higher latitudes. Nevertheless, the relative abundances of the four species in Caravaca and Zumaia are very similar in the initial warmer period, suggesting that although they evolved in lower latitude waters, they soon adapted to cooler, higher latitudes.
Quantitative stratigraphic distributions as taxonomic criteria.—The Fig. 3 shows the quantitative stratigraphical distribution of planktic foraminiferal species across the D—S boundary at Caravaca (data in Tables 3, 4), according to Arenillas (1996) and Arenillas and Molina (1997). These quantitative data were used to compare the patterns of temporal distribution of the species and to evaluate their differences. Applying cluster analyses based on Morisita's index measures (Fig. 9), species group into clusters that represent similar quantitative stratigraphic distributions (i.e., more or less coincidence of the maxima and minima in relative abundance). These analyses have been applied in both the A. uncinata (Fig. 9A) and M. cf. albeari zones (Fig. 9B), and the obtained clusters represent groups with similar ecological requirements and biological behaviours. If this interpretation is assumed, results indicate that the clustering involves an uncharacteristically poor result, grouping species of different latitudinal preferences almost indistinctly. Results suggest that the fluctuations in species relative abundance over time are controlled by several physical-chemical and biotic factors (in addition to the temperature) in a complex interaction.
Nevertheless, divergences in the quantitative stratigraphie distribution between two morphologically similar species could be interpreted as: (i) they have different ecological requirements and/or biological behaviours, and (ii) both could belong to two reproductively isolated species. Comparing the “splitter” taxonomy by Arenillas (1996, modified) and the “lumper” taxonomy by Olsson et al. (1999), the species pairs most conflictive by their morphological similarity to be taxonomically differentiated are: S. compressaformis from S. triloculinoides, E. cf. trivialis from S. triangularis, E. spiralis and A. arabica, G. haunsbergensis from L. ehrenbergi, A. trinidadensis from Pr. inconstans, A. praecursoria from A. uncinata, M. conicontruncata from M. angulata, M. lacerti from M. aequa and Ch. cf. subcylindrica from Ch. midwayensis. Their distant position within the cluster dendrogram suggests that the species comprising each pair are in fact different. This finding supports the hypothesis that slight morphological differences may distinguish closely related species adapted to different environments as suggested by Norris (2000) and Vargas et al. (2001). In the D—S stratigraphie interval, the comparison of other conflictive species pairs cannot be made, such as M. cf. albeari vs. I. albeari, M. cf. albeari vs. M. occlusa, I. pusilla vs. I. albeari, L. ehrenbergi vs. L. troelseni, and L. troelseni vs. L. pseudomenardii, since this comparison must be carried out in the Selandian— Thanetian (upper Paleocene) interval. As some of them are index-species and their taxonomic definition is uncertain, their diagnostic characters are also included in SOM_1.
A case-study from the Danian-Selandian transition of the Caravaca section
Taxonomical and paleoecological study allows a lower/higher (L/H) latitude taxa ratio to be proposed (Tables 3, 4). The L/H ratio is the abundance in percentage of tropical-subtropical taxa with respect to the total, i.e., L/H = [L/(L+H)] × 100, where L = relative abundance of species preferring lower latitudes, and H = relative abundance of species preferring higher (temperate-high) latitudes. Its fluctuations approximately reflect the variations of the temperature at the ocean surface, which is linked to the local climate.
Fluctuations in L/H ratio across the D—S transition at the Caravaca section are shown in Fig. 10. This figure also includes the quantitative stratigraphic distributions of the most abundant genera at Caravaca in order to compare results. The interval from meter 14.50 to 15.25 shows abundance values similar to those of the first 10 meters of the Caravaca section, suggesting reworked levels.
According to this L/H curve, three stratigraphie levels stand out above the rest: meters 10.5, 12.0, and 13.5. In the three cases, the L/H ratio reaches maximum values (above 60%), suggesting climate warming events. Other relevant horizons are recorded at meters 3.5 and 8.0 (in the middle part of M. angulata Zone), the first characterized by a slight decrease of the L/H ratio and the latter by a very slight increase. Meter 3.5 (in the upper part of the A. uncinata Zone) is also characterized by a decrease in Acarinina (from 34% to 24%), suggesting climate-cooling event. Meter 10.5 (in the basal part of the M. cf. albeari Zone) is also characterized by a relevant increase of Morozovella (from 19% to 41%). A similar Morozovella acme-horizon was also identified at Tethyan sections such as Sidi Naseur, ElKef, Elles, and Aïn Settara in Tunisia, Gebel Aweina in Egypt, and Ben Gurion in Israel (Arenillas 1996, 2008; Arenillas and Molina 1997). According to Arenillas et al. (2008), this horizon could correspond to a climate-warming event.
Meter 12.0 (in the lower part of the M. cf. albeari Zone) is characterized by another smaller increase in Morozovella (from 40% to 53%), mainly by biconvex morozovellids such as M. cf. albeari. Finally, meter 13.5 (in the lower part of the M. cf. albeari Zone) is also characterized by a slight increase in Morozovella (from 47% to 60%). As above, both horizons could correspond to climate-warming events. The latter could be related to the D—S boundary of the Zumaia stratotype. Arenillas et al. (2008) suggested that the D—S boundary at Zumaia corresponds to a low intensity hyperthermal event, since it coincides with the base of a red marly interval and a negative carbon isotopic excursion (CIE-DS2) similar to the P/E boundary also recorded at Zumaia (Schmitz et al. 1997, 1998; Arenillas and Molina 2000). However, unlike Caravaca, the Morozovella abundance decreases at Zumaia (Arenillas et al. 2008), suggesting the action of other paleoenvironmental factors in addition to surface oceanic temperature on the local planktic foraminiferal assemblages, such as fluctuations in sealevel, trophic conditions and/or salinity. Another hypothesis is that the D—S boundary at Caravaca is below or above meter 13.5, coinciding with the decline in the L/H ratio of meters 12.75 or 14.0.
In addition to classical morphological and microtextural diagnostic criteria, divergences in patterns of spatio-temporal distribution (i.e., in latitudinal preferences and quantitative stratigraphic distributions) have been applied as criteria for the taxonomical separation of the Danian—Selandian transition planktic foraminiferal species. These patterns have been inferred by comparing the average relative abundance of the species between Caravaca and Zumaia, and by using the quantitative stratigraphic distribution of the species at Caravaca. Morphologically convergent species pairs such as Acarinina trinidadensis and Praemurica inconstans, Acarinina praecursoria and Acarinina uncinata, Morozovella conicontruncata and Morozovella angulata, or Morozovella cf. albeari and Igorina albeari, are now well taxonomically differentiated. The analysis allowed to recognize 41 planktic foraminiferal species from the D—S transitional interval. This taxonomic revision also suggest that the lower boundary of the Subzone P3b of Berggren and Pearson (2005), marked by the FOD of I. albeari, is approximately equivalent to the lower boundary of the M. cf. albeari Zone of Arenillas and Molina (1997) and Arenillas et al. (2008), marked by the FOD of M. cf. albeari (= M. crosswicksensis sensu Blow 1979 and Arenillas 1996), since the former grouped both morphospecies in the taxonomic concept of I. albeari.
Paleoecological analysis indicates that the species preferring lower latitudes (or tropical-subtropical waters) were Eoglobigerina cf. trivialis (= E. trivialis sensu Blow 1979), E. edita, Globanomalina chapmani, Luterbacheria ehrenbergi, Praemurica inconstans, Acarinina trinidadensis, A. uncinata, A. praepentacamerata, A. indolensis, A. arabica, A. praepentacamerata (= A. praeangulata), Igorina tadjikistanensis, Morozovella angulata, M. simulatilis, M. cf. albeari, M. lacerti, M. aequa, Chiloguembelina taurica, Ch. midwayensis, Ch. cf. subcylindrica, Zeauvigerina teuria, Z. aegyptiaca, Gc. daubjergensis, and Globoconusa conusa. Species preferring higher latitudes (temperate), therefore being more cosmopolitan, were Eoglobigerina fringa, E. tetragona, E. spiralis, Subbotina triloculinoides, S. triangularis, Globanomalina compressa, G. haunsbergensis, G. chapmani, Parasubbotina pseudobulloides, P. quadrilocula, Acarinina praecursoria, Morozovella conicotruncata, and Chiloguembelina crinita. The analyzed data also suggest that S. compressaformis (= S. eocaenica sensu Blow 1979), Acarinina hansbollii, A. praeaqua, and Chiloguembelina cf. subcylindrica began as species preferring tropical-subtropical waters and ended up being more cosmopolitan.
Taxonomical and paleoecological study allows a lower/higher (L/H) latitude taxa ratio to be proposed. At Caravaca, three maxima (meters 10.5, 12.0, and 13.5) in the L/H ratio can be recognized, suggesting three climate warming events during the D—S transitional interval. The maximum warming event in meter 13.5 might be related to the D—S boundary because it has been found in the middle part of the M. cf. albeari Zone in the same interval as the D—S boundary in Zumaia.
Marius Dan Georgescu (University of Calgary, Canada) and Michael A. Kaminski (King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia) are thanked for the review of the manuscript. Eustoquio Molina (Universidad de Zaragoza, Spain) and HansPeter Luterbacher (retired) are also thanked for his continued support and suggestions. The author is grateful to Richard Stephenson (Zaragoza, Spain) for English corrections. This research was funded by the Spanish Ministerio de Educación y Ciencia projects CGL2007-63724/BTE and CGL2011-23077 (co-financed by ERDF funds), and the Aragonian Departamento de Educación y Ciencia (DGA group E05).