1 Introduction

The genus Linderina Schlumberger is a widespread Eocene hyaline, larger benthic foraminifera (LBF), with records from the Caribbean and across much of the Tethys into South-East Asia, and potentially even into low latitudes in the southern hemisphere (Fig. 1b). The genus is characterized by a hyaline disc-shaped test composed of a layer of equatorial chamberlets arranged in a cyclical pattern as in a typical orbitoidal foraminifers but lacking lateral layers and chambers (Cole 1942; Neumann 1954; Freudenthal 1969; Loeblich and Tappan 1987; Ferràndez-Cañadell and Serra-Kiel 1999). Previous records, justified by the illustrated associated fossils, suggest that the stratigraphic distribution of the genus ranges from Lutetian to Priabonian (Freudenthal 1969; Loeblich and Tappan 1987; Ferràndez-Cañadell and Serra-Kiel 1999; Serra-Kiel et al. 2016; BouDagher-Fadel 2018). According to Renema (2007), the genus is confined to Bartonian to Priabonian beds in the Indo-Malayan region in SE Asia. Serra-Kiel et al. (1998) interestingly did not comment on the genus and its stratigraphic distribution in the currently widely used Paleocene–Eocene biostratigraphic scheme. The oldest occurrence of the genus Linderina in the Tethys was reported from the lower Lutetian beds of the Masoko Formation in Tanzania (Cotton and Pearson 2012).

Fig. 1
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a Generalized geological map of Central Anatolian Crystalline Complex (CACC) and its location in tectonic map of Turkey (simplified from Göncüoğlu et al. 1991 and Köksal et al. 2008). b Palaeogeographic distribution of the Eocene LBF genus Linderina (after Özcan et al. 2021) with the location of central Anatolia. Middle Eocene palaeogeography and palaeocurrent directions simplified after Huber and Sloan (1999), Bice et al. (2000), Huber et al. (2003), and Halliburton Neftex® product suite. c Simplified stratigraphy of the Kozaklı Basin north of Avanos, Nevşehir (from Gül et al. 2012). d Geological map of the area to the north of Nevşehir with the place of the study area (map simplified from Akbaş et al. 2011) AS: Ayhan Section

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We herein introduce Linderina from a ‘middle’ Ypresian carbonate sequence in Central Anatolia focusing on the description of its external and internal test features. The associated foraminifers are briefly discussed. We also illustrate some Bartonian Linderina specimens from NW Turkey to show that embryo types and the chamber arrangements identified in the Ypresian specimens also persist in specimens from stratigraphically much younger levels. A comprehensive taxonomic and phylogenetic synthesis of the genus based on the records from the various localities in the Neo-Tethys and also from the Eocene of Caribbean region (mainly from Jamaica) will be published separately.

2 Regional geological setting and Eocene stratigraphy

The study area is located in the central part of the Central Anatolian Crystalline Complex (CACC, Fig. 1a), which is regarded either as the metamorphosed northern passive margin of the Tauride–Anatolide Platform facing the İzmir–Ankara–Erzincan branch of Neo-Tethys or a distinct terrane (see Okay 2008 for a discussion). Precambrian–Mesozoic metamorphic rocks form the basement of the CACC and ophiolitic rocks were thrust over it (Göncüoğlu et al. 1991). Upper Cretaceous or Cenozoic terrigenous to shallow-marine clastic-carbonate rocks lie unconformably on the metamorphic as well as on the granitic rocks (Seymen 1981; Göncüoğlu et al. 1991, 1997; Köksal and Göncüoğlu 1997; Öztaş 2000; Gül et al. 2012) (Fig. 1a, c, d). According to Köksal and Göncüoğlu (1997), basement rocks north of Avanos, near the close vicinity of the present study area, are overlain either by upper Cretaceous (?)–Paleocene Göynük Olistostrome or by a ca. 170-m-thick Paleocene clastic sequence of the Yeşilöz Formation (Fig. 1c). The Paleocene sequence is characterized by a lower, ca. 20-m-thick massive fluvial conglomeratic unit, and an upper, 150-m-thick mainly lacustrine to shallow-marine (?) sandstone–siltstone–shale–cherty limestone intercalation. The age of this part of the sequence was poorly dated as Paleocene depending on algae and non-diagnostic foraminifers (Köksal and Göncüoğlu 1997). The Ayhan Formation (partly equivalent to the Mucur Formation of Köksal and Göncüoğlu 1997) unconformably overlies the Yeşilöz Formation (Öztaş 2000; Gül et al. 2012). According to Öztaş (2000) and Gül et al. (2012), the Ayhan Formation is a transgressive sequence with coarse conglomeratic beds and sandstone–siltstone–shale alternations at its base and a carbonate succession with abundant LBF upwards. The carbonate sequence of the 51-m-thick Ayhan Formation contains mainly nummulitids, alveolinids, rotaliids and orthophragminids at its upper beds (Gül et al. 2012). The carbonate unit was poorly dated as lower–middle Eocene by Köksal and Göncüoğlu (1997) based on the presence of Nummulites sp., Alveolina sp. and Assilina sp. Öztaş (2000), in an unpublished report, documented a rather detailed fossil list from the Ayhan Formation. However, these foraminifers were identified at the genus level, and are far from establishing a precise age for the unit. The most diagnostic foraminifers reported Öztaş (2000) are Orbitolites complanatus, Alveolina sp., Actinocyclina sp., Assilina sp., Lockhartia sp., Nummulites sp., and Opertorbitolites sp. The author also cited the occurrence of genus Linderina without illustration and species assignment. Öztaş (2000) stated that the age of Ayhan Formation should be accepted as ‘middle Eocene’ considering the regional geological perspectives of the study area, although the identified fauna suggests late Ypresian. Contradictions between the age and regional geology were later discussed by Gül et al. (2012), providing more detailed fossil documentation from the Ayhan Formation. Gül et al. (2012) concluded that the age of the unit is upper Ypresian based on assemblages of alveolinids, nummulitids and orthophragminids. Moreover, the hemipelagic beds of the overlying Çevirme Formation contain E7-8 planktonic foraminiferal assemblages, suggestive of a Ypresian–Lutetian transitional age for the base of the unit, which is consistent with the LBF-based ages assigned to the carbonates below (Gül et al. 2012).

3 Materials and methods

Specimens of Linderina occur in the middle part of the Ayhan Formation near Ayhan village (also variously used as Ayhanlar), to 16 km northwest of Avanos (Nevşehir) in central Turkey (38°49′52.78ʺ N; 34°44′11.04ʺ E; Fig. 3). In addition to regular sampling from the hard carbonate levels, three samples (152, 156 and 158) were collected from the friable limestone beds. Sample 152 with Linderina was collected from the lower part, 15 m above the base of carbonate succession of the Ayhan Formation. Sample 156 in the upper part of the unit yielded mainly nummulitids and sample 158, collected from the uppermost part of the formation where a transition to deep-marine marls occurs, yielded abundant orthophragminids and sporadic nummulitids. These samples consist of matrix-free specimens. Morphometric measurements and counts of Linderina were carried out on axial and equatorial sections of the megalospheric specimens. The measurements and counts are listed in Table 1.

Table 1 Statistical data of Linderina Schlumberger
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4 Description of the Ayhan section, fossil content and age

The Ayhan Formation, widely exposed near Ayhan village to the north of Avanos, is a transgressive clastic-carbonate unit covering the basement or overlying the Paleocene clastic rocks (Öztaş 2000; Gül et al. 2012). The lower, 20-m-thick clastic part of the formation is represented by a massive conglomerate unit and overlying succession of mudstone and siltstone alternation without diagnostic fossils (Fig. 3). The carbonates of the Ayhan Formation are about 51-m thick and are characterized by thin-to medium bedded highly fossiliferous limestone and argillaceous limestone beds that pass into the hemipelagic marls of the Çevirme Formation (Figs. 2d, 3).

Fig. 2
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Field photographs of the Ypresian Ayhan Formation and underlying and overlying units exposed to the north of Avanos, Nevşehir

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Fig. 3
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Stratigraphic column of the Ayhan Formation in the Ayhan section and distribution of the fossils. Thin-section microphotographs of important levels are shown alongside. The numbers on the right side of the column refer to sample numbers. nb Nummulites burdigalensis, od Orbitoclypeus douvillei, df Discocyclina fortisi, ao Alveolina aff. obtusa, al Alveolina sp., lo Lockhartia sp., op Operculina sp., pf planktonic foraminifers

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The foraminiferal assemblages of the Ayhan Formation are typical for the Ypresian of the Western Tethys (after Serra-Kiel et al. 1998). The genus Nummulites in samples 152 and 156 is represented by two species, N. burdigalensis de la Harpe (Fig. 4a–g), a granulated nummulitid and N. aff. globulus Leymerie (Fig. 4h–k), a radiate species with a large polar pile. The granulated specimens display granulation in the central part of the test and are uniformly distributed over the test surface but are lacking in the peripheral part of the test. Schaub (1981) arranged three subspecies in the N. burdigalensis stock; N. b. burdigalensis de la Harpe and N. b. kuepperi Schaub, both restricted to lower Cuisian (SBZ10), and N. b. cantabricus Schaub, confined to the middle Cuisian (SBZ11) (Serra-Kiel et al. 1998). According to Schaub (1981), the proloculus of megalospheric N. burdigalensis cantabricus is around 250–400 µm, notably bigger than that of N. burdigalensis burdigalensis with a proloculus diameter of 200–300 µm. Nummulites burdigalensis kuepperi; on the other hand, possesses notably smaller proloculus varying in diameter between 120 and 150 µm. Our specimens have an average proloculus diameter of about 244.2 µm based on 13 specimens, while proloculus diameter varies between 130 and 360 µm. These specimens are assigned to N. burdigalensis burdigalensis, a key subspecies for the SBZ10 Zone. The radiate Nummulites specimens possess a small proloculus varying in diameter between 110 and 125 µm, with an average of 117.5 µm based on 4 specimens. These specimens cannot be safely assigned to any species of genus Nummulites but show some affinity to N. globulus, which is an early Ypresian species. Thus, we assigned them to N. aff. globulus.

Fig. 4
figure 4

Nummulitids (a–k) and rotaliids (l–o) from sample 152. a–g Nummulites burdigalensis de la Harpe. a, c, e; external views, b, d, e; equatorial sections, g; axial section. h–k Nummulites aff. globulus Leymerie. h External view, i axial section, j, k equatorial sections. l, m Lockhartia tipperi Davies, vertical section. n, o R. cf. newboldi d’Archiac and Haime, vertical section. a, b 152-1, c, d 152-2, e, f 152-26, g 152-33, h 152-22, i 152-24, j 152-25, k 152-21, l 152-5, m 152-20, n 152-187, o 152-6

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Alveolinids are common in the lower and middle part of the Ayhanlar Formation (Fig. 5). The loose specimens from sample 152 are represented by Alveolina cf. histrica Drobne (Fig. 5a), A. decastroi Di Scotto (Fig. 5b), A. fornasinii Checchia-Rispoli (Fig. 5c), A. cf. reicheli Sirel and Acar (Fig. 5d), A. cremae Checchia-Rispoli (Fig. 5e), A. schwageri Checchia-Rispoli (Fig. 5f), A. cf. rugosa Hottinger (Fig. 5g), A. cf. cayrasi Dizer (Fig. 5h), A. cf. pinguis Hottinger (Fig. 5i, j) and various other alveolinids not identified here. According to Serra-Kiel et al. (1998), Alveolina histrica, A. decastroi and A. cremae are key alveolinid species for SBZ11, though the stratigraphic range of former species may extend into SBZ12 and A. schwageri is key for SBZ10. Alveolina fornasinii, on the other hand, is a key species for SBZ9 and 10. Sirel and Acar (2008) consider that A. cayrasi (A. cayrasensis of the authors) is, in fact, confined to SBZ11, not SBZ13 as reported by Serra-Kiel et al. (1998).

Fig. 5
figure 5

Alveolinids from sample 152. All axial sections. a Alveolina cf. histrica, 152-65. b A. decastroi Scotto Di Carlo, 152-17. c A. fornasinii Checchia-Rispoli, 152-18. d A. cf. reicheli Sirel and Acar, 152-14. e A. cremae Checchia-Rispoli, 152–9. f A. schwageri Checchia-Rispoli, 152-69. g A. cf. rugosa Hottinger, 152-15. h A. cf. cayrasi Dizer, 152–56. i, j A. cf. pinguis Hottinger, i 152-59, j 152-60

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Orthophragminids occur only towards the upper levels of the Ayhan Formation, being most common in sample 158, 32 m above the sample 152 (Fig. 6). This sample yielded an assemblage of Orbitoclypeus douvillei (Schlumberger), Discocyclina fortisi (d’Archiac), Discocyclina archiaci (Schlumberger), and D. augustae Weijden. Orbitoclypeus douvillei, with an average deuteroconch diameter of 146.0 µm based on 5 specimens, is assigned to O. douvillei douvillei (Fig. 6a–e), the earliest member of O. douvillei lineage. This subspecies has a narrow stratigraphic distribution in the lower part of upper Ypresian, and is accepted as a marker subspecies for SBZ10, although it extends to the lowermost part of SBZ11 (Less 1998; Özcan et al. 2007; Zakrevskaya et al. 2011). Discocyclina fortisi is the most abundant orthophragminid species in the studied sample. With an average deuteroconch diameter of 588.6 µm based on 25 specimens, this species was assigned to Discocyclina fortisi fortisi (d’Archiac), corresponding to the most primitive stage of D. fortisi lineage. This subspecies, ubiquitous in the lower Eocene shallow-marine deposits of Turkey, is solely confined to SBZ10 in the Tethys (Less 1998; Özcan et al. 2007; Zakrevskaya et al. 2011). Discocyclina augustae is rare in our material, and only four specimens were studied. These specimens with an average deuteroconch diameter of 101.25 µm were assigned to D. augustae sourbetensis Less, the earliest member of the species in upper Ypresian and early Lutetian (SBZ10-13) (Zakrevskaya et al. 2011). Discocyclina archiaci is represented only by one specimen with a deuteroconch diameter of 550 µm. This specimen is tentatively assigned to D. archiaci cf. archiaci (Schlumberger), a subspecies with a narrow stratigraphic range in lower upper Ypresian (SBZ10) (Özcan et al. 2007; Zakrevskaya et al. 2011).

Fig. 6
figure 6

Equatorial sections of orthophragminids from sample 158 in the upper part of the Ayhan Formation. a–e Orbitoclypeus douvillei douvillei (Schlumberger). f–h Discocyclina fortisi fortisi (d’Archiac). i–k D. augustae sourbetensis Less. l D. archiaci cf. archiaci (Schlumberger). a, b 158-1, c, d 158-5, e 158-4, f 158-18, g 158-19, h 158-17, i 158-28, j 158-23, k 158-24, l 158-15

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The marly beds in the lower part of the Çevirme Formation (samples 160–163) contain planktonic foraminifera that were assigned to E7 and 8 (after Pearson et al. 2006) by Gül et al. (2012), suggesting that the age of the basal part of the Çevirme Formation is late Ypresian–early middle Eocene. Therefore, combining the information from the aforementioned taxa, we concluded that the age of the Ayhan Formation is early late Ypresian.

5 Linderina Schlumberger from the Ayhan Formation

5.1 Measurement and terminology

The measurements of the test features and parameters related to the embryonic apparatus and the following chambers/chamberlets are illustrated in Fig. 7 and tabulated in Table 1. All specimens are deposited in the paleontological collections of the Geological Engineering Department of İstanbul Technical University and prefixed EO/.

Fig. 7
figure 7

Axial (a) and equatorial (b and d) sections of Linderina Schlumberger with test parameters used here in the description of the genus (see Table 1 for the explanation). The specimen illustrated in b represents Morphotype 2 with two auxiliary chambers formed at the 4th growth stage (line drawing of the embryonic apparatus and early chamber arrangement of the same specimen shown in c), and specimen in d represents Morphotype 1 with a single auxiliary chamber (line drawing of the embryonic apparatus and early chamber arrangement of the same specimen shown in e). Specimen in a is from the Ayhan Formation. Specimen in b is from the early Bartonian Soğucak Formation in Gizliliman section in Gökçeada (NW Turkey), and specimen d is from Bartonian beds of the Şevketiye Formation in Biga Peninsula (NW Turkey). See Özcan et al. (2010) and Özcan et al. (2018a, b) for the paleontologic and stratigraphic information on the Eocene sequences in these regions. Numbers denote growth stages. a 152-136, b, c GIZ11-51, d, e ADA6-3

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5.2 Description of Linderina

The test of genus Linderina in the studied material is typically discoidal with a lobate outline due to large, arc-shaped or radially elongated cup-shaped chambers/chamberlets (as observed in the equatorial layer) forming the main equatorial layer of the genus (Fig. 8a–e). The arcuate chamberlets may be easily seen in externally wet specimens (Fig. 8a–c). Externally and in axial sections, two types of tests can be differentiated with transitional forms between them: (a) almost flat to slightly irregular specimens with a thickened massive layer confined to the central part of the test (Fig. 9f, g) and (b) biconvex, stout specimens, notably thick in their central part of the test (Fig. 9a–e). The test surface is smooth, and piles are absent. The thickened central part of the test, however, displays a ‘pitted’ surface showing the presence of notably large pores (Fig. 8a–c), well observed in the axial sections (Fig. 9). The test diameter varies between 0.82 and 2.1 mm, with an average of 1.4 mm based on 47 specimens (Table 1). Based on 11 specimens, the thickness of the test in the center (T1) and near the periphery (T2) of the test ranges between 0.2 and 0.64 mm and 0.17 and 0.27 mm, with sample averages of 0.475 and 0. 228 mm. The height of the chambers (HEL1 and HEL 2 as observed in the axial sections) ranges between 110 and 210 µm in the early stage and 150 and 235 µm at the peripheral part of the test, with sample averages of 153.9 and 194.5 µm, respectively.

Fig. 8
figure 8

External views of Linderina Schlumberger specimens from the Ayhan Formation, SBZ10, upper Ypresian, showing the variation in the shape of the test. Note that in some specimens, thick massive layer is located only in the central part of the test (a, b), while in others, it covers almost the whole surface of the test (c–e) (also see Fig. 9 for a comparison of their axial sections). Some radially elongated cup-shaped chamberlets are easily seen in the peripheral parts of the wet specimens. a 152-112, b 152-109, c 152-107, d 152-103, e 15-111

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Fig. 9
figure 9

Axial and slightly off-centered axial sections of Linderina Schlumberger from the Ayhan Formation, SBZ10, upper Ypresian. Note the variation in the extent of the massive layer on the lateral sides of the test. a 152-118, b 152-119, c 152-123, d 152-136, e 152-122, f 152-135, g 152-185

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Axial sections reveal that the chamberlets are positioned either along a flat or a slightly curved plane (Fig. 9f). The chamberlets of the successive cycles also seem to be offset from this plane and overlapping each other on the lateral sides of the test (Fig. 9e, g).

The embryonic apparatus of all megalospheric specimens consists of 3 chambers of almost similar shapes (Figs. 10, 11, 12) (but different sizes), encircled by a thin embryonic wall. These chambers are not perfectly circular in outline but rather sub-rounded with occasionally sharp margins. They are separated by almost flat to slightly curved walls, producing an overall appearance of a ‘Y’ shape (Figs. 10, 11, 12). The walls separating the embryonic chambers are thin and occasionally not well observed due to crystal overgrowth. These three chambers form a distinctive structural entity, an embryonic apparatus that can be relatively easily distinguished from the later chambers. Two types of chamber arrangement were observed in the early nepionic stage of the studied specimens that led to the differentiation of two morphotypes; Morphotypes 1 and 2.

Fig. 10
figure 10

Equatorial sections of Linderina Schlumberger from the Ayhan Formation, SBZ10, upper Ypresian, and interpretation of the chamber arrangement. a–d Morphotype 1, e–h Morphotype 2. Note the large and irregular peripheral chamberlets in Morphotype 1. a, b 152-4, c, d 152–110, e, f 152-125, g, h 152-159

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Fig. 11
figure 11

Equatorial sections of Linderina Schlumberger Morphotype 1 from the Ayhan Formation, SBZ10, upper Ypresian, and interpretation of the chamber arrangement. Note the large, irregular and radially elongated chamberlets. a, b 152-108, c, d 152-102, e, f 152-107

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Fig. 12
figure 12

Comparison of equatorial sections of Linderina Schlumberger Morphotype 1 (a–c) and Morphotype 2 (d–f). Note the remarkable difference in the size of the trilocular embryonic apparatus. Chamberlets in Morphotype 2 are more regular and smaller than those of Morphotype 1. a 152-112, b 152-111, c 152-100, d 152-127, e 152-131, f 152-132

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Morphotype 1: Some specimens possess a single large chamber with two basal stolons, formed at the 4th budding step from which the nepionic growth starts. This chamber is actually an ‘auxiliary chamber’, easily recognized by its relatively large size compared to embryonic chambers and surrounding chamberlets. This chamber connects through basal stolon(s) to two chamberlets that form the 5th budding step. The chambers around the embryonic apparatus close at the 7th or 8th budding step, and cyclical growth start in the equatorial plane (Figs. 7d, e, 10a–d, 11, 12a–c). The size of the embryonic apparatus (parameter E1 + E2), based on 22 specimens, ranges between 125 and 370 µm with an average of 268.8 µm. The parameter E3 ranges between 140 and 270 µm with an average of 212.7 µm. The early equatorial chamberlets are characteristically arcuate in shape, but later, they are axially elongated, and some irregular, long chamberlets appear along the peripheral part of the test (Figs. 10a, b, 11a, c, 12c).

Morphotype 2: Some specimens possess two chambers (actually chamberlets) formed at the 4th budding step from which the nepionic growth starts. The auxiliary chamberlets are invariably unequal in size (Figs. 10e, 12d–f). It appears that there is retardation in chamber formation after the small 4th auxiliary chamberlet. From this, only one chamber is formed in the 5th budding step (thus, a total 5 chamberlets overall). Closing chamberlets are formed at the 7th or 8th budding steps. The size of the embryonic apparatus (parameter E1 + E2), based on 7 specimens, ranges between 135 and 220 µm with an average of 185.7 µm. The parameter E3 ranges between 100 and 155 µm with an average of 130.0 µm. The equatorial chamberlets are characteristically arcuate in shape throughout the ontogenetic development. In addition, these chambers are smaller (compared to Morphotype 1), so the cyclical pattern of the equatorial chamberlets appears tight. These specimens lack axially elongated or cup-shaped chamberlets.

5.3 Comparison of Morphotypes 1 and 2

Two morphotypes from the Ayhan Formation are differentiated basically by the notable differences in (a) size of the trilocular embryonic apparatus, (b) peri-embryonic chamber arrangement due to a different number of auxiliary chambers in both types, and (c) irregularity and size of the chamberlets (Fig. 12). Morphotype 1 has notably larger embryons than Morphotype 2. This feature is also observed in other Linderina populations from Bartonian and Priabonian of Turkey (unpublished data of EÖ). A single auxiliary chamber follows the embryonic apparatus in Morphotype 1 and two such chamberlets in Morphotype 2. This causes the formation of quite different chamber arrangements in the nepionic stage of both morphotypes. The auxiliary chamberlets in Morphotype 2 are distinctly unequal in size, and smaller ones can be easily missed in the equatorial sections if the section is not well oriented with respect to embryonic apparatus. The nepionic and neanic chamberlets in Morphotype 2 display a regular arrangement, and they are almost all arcuate in shape (Figs. 10e, g, 12d–f). They are relatively smaller compared to those of Morphotype 1. In addition, the chamberlets in Morphotype 1 are more irregular especially at the late ontogenetic stages, and some are radially elongated so that they appear cup-shaped (Figs. 8a, b, 11a, b). Moreover, some chamberlets appear to be distinctly large and elongated perpendicular to the growth direction in the equatorial section (Figs. 10c, 11a). In the studied material majority of the specimens belong to Morphotype 1.

6 A comparison of Linderina with Bartonian record in NW Turkey

As part of ongoing research, Linderina has been extensively studied from Bartonian and Priabonian deposits in Turkey (unpublished data of EÖ). A comparison of Ypresian Linderina from Ayhan Formation with those from the Bartonian of NW Turkey reveals the similarities in the structure of the trilocular embryonic apparatus and peri-embryonic chamber arrangement. Specimens from the Gizliliman section in Gökçeada, and Şevketiye section from Thrace and Biga Peninsula in NW Turkey (see Özcan et al. 2010, 2018a, b for stratigraphic framework and fossil content), reveal the occurrence of both morphotypes in the Bartonian (Fig. 13a–e). These specimens predominantly belong to Morphotype 2 with small embryonic apparatus and two auxiliary chamberlets (Fig. 13a–c; also see Fig. 7b and d). This is in contrast to the predominance of Morphotype 1 in the Ypresian Ayhan section. Our preliminary data show that in the Bartonian specimens, chambers are more regularly arranged, and large and irregular chamberlets recorded in the Ypresian specimens are not observed. It also appears that chamber formation in the Bartonian specimens is more stabilized, without much deviation in the growth plane. A detailed morphometric study of the genus from various localities in the Tethys and the Central America/Caribbean region is pending to test the evolutionary features of the genus and concept of nepionic acceleration, specifically the size of the embryonic apparatus and features of the chamberlets in the main layer.

Fig. 13
figure 13

Embryonic apparatus and early equatorial sections of Linderina Schlumberger Morphotype 1 and 2 in the Bartonian deposits of NW Turkey (unpublished data of EÖ; see Özcan et al. 2010 and 2018a for the stratigraphy of Eocene units in NW Turkey). a DER10-3, b DER2-8, c DER2-12, d ADA6-3, e ADA6-2

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7 Conclusions

Linderina occurs in the lower part of the Ayhan Formation, which was previously interpreted as either a lower–middle Eocene or middle Eocene shallow-marine carbonate unit, marking a major transgression over the basement rocks in central Anatolia. Age of the Ayhan Formation and stratigraphic level with Linderina is firmly constrained in this study as early upper Ypresian by the occurrence of typical Tethyan LBFs (nummulitids, alveolinids, orthophragminid and rotaliids). The unit, in fact, was correctly interpreted to consist of Ypresian fauna by Öztaş (2000) but incorrectly dated as middle Eocene because of the traditional belief that the Eocene sequence in central Anatolia mainly belongs to the middle Eocene. This notion, however, was lately challenged by Gül et al. (2012) by presenting that a major phase of the transgression in the Kozaklı Basin occurred in the Ypresian.

Linderina from Ayhan section in central Anatolia is represented by two morphotypes differentiated by the different size of the megalospheric embryonic apparatus and different peri-embryonic chamber arrangements. Currently, taxonomic approach to these forms is pending since a detailed study of the genus from the Tethys and Central America/Caribbean region, prompted by insufficient previous information on the internal test features of the genus, is in progress. However, our present data show that stratigraphic range of the genus extends into ‘middle’ Ypresian, although dominant occurrence of the genus is within the middle Eocene as recorded in previous studies.