Type of wall: Microperforate, bilamellar, the wall is 4-5 μm thick, the inner layer is usually very thin (0.4-0.5 μm), the outer layer has a submicron granular (blocky) texture (Pl. 17.1, Fig. 21), ototara-type wall (see Chapter 15, this volume); surface ornamentation consists of fine but distinct, discontinuous costae on the youngest chambers, becoming faint on last chambers, with costae aligned with the long axis of the test; the wall is evenly perforated by pores that are usually about 0.50-0.75 μm in diameter, but can exceed 1 μm as a result of dissolution, pore channels are straight.

Test morphology: Test biserial, elongate, subtriangular in outline, periphery rounded; chambers increasing moderately to rapidly in size, usually 11-12, up to 14 in adult specimens; sutures depressed, perpendicular to slightly oblique to the growth axis; aperture a low, moderately narrow to broad, symmetrical arch centered or slightly off-center from the base of the final chamber, bordered on one side by a narrow collar that thickens away from its attachment point on the chamber face. Apical angle is larger than in C. cubensis, generally about 55-60°.

• Chiloguembelina cubensis (Palmer).—Beckmann, 1957:89, text-fig. 14 (7, 8) [lower Oligocene, Cipero Fm., San Fernando, Trinidad].—Hornibrook, 1990:368, pl. 1, fig. 1 [lower Oligocene, Cuba].—Spezzaferri and Premoli Silva, 1991:237, pl. 10, fig. 5a [lower Oligocene Subzone P21a, ODP Hole 538A, Gulf of Mexico].—Leckie and others, 1993:123, pl. 1, fig. 14 [lower Oligocene Subzone P21a, ODP Hole 803D, Ontong Java Plateau, western Pacific Ocean].—Hernitz Kucenjak and others, 2006:24, pl. 4, figs. 11, 12 [lower Oligocene Zone O4, Jihar-5 well, sample 190-200 m, Palmyride region, Syria]. [Not Palmer, 1934.]

DISTINGUISHING FEATURES.— Distinguished from Chiloguembelina cubensis by having a more rapidly flaring test outline, greater apical angle of about 55-60°, a more rapid increase in chamber size and generally a shorter and wider test. It differs from Chiloguembelina ototara and C. andreae n. sp. by the presence of fine, mostly discontinuous costae that are aligned parallel to the elongate axis of the test.

DISCUSSION.— The exceptionally well-preserved material from the Adriatic Sea and Syria enabled detailed study of the abundant biserial planktonic foraminifera. Two morphologically significant groups with costate surface ornamentation that clearly differ in shape have been observed. The first of these is the commonly identified species C. cubensis. The other group differs from C. cubensis by the growth rate of chambers, total number of chambers, apical angle and length and width of the test. The specimens with a more rapidly flaring test outline are placed in Chiloguembelina adriatica. This species appears in the uppermost Eocene Zone E16 and disappears at the end of Zone O4, with sporadic occurrences in Zone O5. In contrast, Chiloguembelina cubensis has a longer stratigraphic range with its lowest occurrence (LO) in the middle Eocene and highest common occurrence (HCO) at the top of Zone O4, above which it becomes rare and sporadic up to the uppermost Oligocene.

PHYLOGENETIC RELATIONSHIPS.— Probably evolved from C. cubensis during the latest Eocene.

STRATIGRAPHIC RANGE.— Uppermost Eocene (Zone E16) to the end of Zone O4, with sporadic occurrences in Zone O5 (this study).

GEOGRAPHIC DISTRIBUTION.— Cosmopolitan. Beckmann (1957:89, text-fig. 14:7, 8) was the first to illustrate biserial morphotypes with a costate surface based on two morphotypes from lower Oligocene sediments (Cipero Formation) in Trinidad. Morphotypes with a shorter and wider test and greater apical angle most probably belong to the herein described species C. adriatica n. sp. Hornibrook (1990:368, pl. 1, fig. 1) also figured costate specimens from the lower Oligocene of Cuba, and Leckie and others (1993; pl. 1, fig. 14) provided SEM images of the same morphotype from lower Oligocene of the Ontong Java Plateau. In the Adriatic Sea and Syria this form is very common in Zone O1 and O2 and comprises up to 30% of the biserial planktonic populations in the small size fractions (63-125 μm).

STABLE ISOTOPE PALEOBIOLOGY.— No data available.

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