Reticulophragmium rotundidorsatum (Hantken, 1875)

Fig. 127-1. Type figure of H. rotundidorsatum, from Hantken (1875);
2. Lectotype of R. rotundidorsatum, after Horváth (2002).

ORIGINAL DESIGNATION: Haplophragmium rotundidorsatum Hantken, 1875

TYPE REFERENCE: Hantken, M., 1875. Die fauna der Clavulina szaboi-Schichen Theil 1 - Foraminiferen. K. Ungar. Geol. Anst., Mitt. Jahrb. Budapest, Bd. 4, Heft 1, p. 12. pl. 1, fig. 2.

TYPE SPECIMEN: The lectotype designated by Horváth (2002) is deposited in the Geological & Paleontological Department of the Hungarian Natural History Museum, Budapest (M.99.40). Additional syntypes are housed in the paleontological collections of the Natural History Museum, Vienna.

TYPE LEVEL: Not originally designated. Reported from the lower Clavulina szaboi Beds (Early Oligocene) and the Kiscell Clay (NP 24) of Hungary. The lectotype is from the upper part of the Kiscell Clay Formation (Zone P21a).

TYPE LOCALITY: Hanken reported the species from Ofen [Budapest], Nagy-Kovácsi, and Budakeszi, Hungary. The lectotype is from Budapest, Újlak (the former Holzpach brickyard).

DIAGNOSTIC FEATURES: Test free, planispirally coiled, involute, with 7 1/2 to 9 chambers in the last whorl, and a shallowly depressed umbilicus. Test is as thick as it is broad, with a broadly rounded periphery. Chambers increase in thickness rapidly as added, with alveoles evenly distributed throughout the chamber lumen. Sutures straight, may be somewhat incised. Aperture is usually an interiomarginal slit, but largest specimens may have supplementary areal pores. The test wall is finely agglutinated and relatively thin, and the test is often compressed.

SIZE: Eocene specimens are typically 0.30 - 0.70 mm in diameter; Oligocene specimens from Hungary range up to 3 mm in diameter.


OBSERVED OCCURRENCES: Reticulophragmium rotundidorsatum was originally described from Oligocene epicontinental deposits in Hungary (Hantken, 1875). In the Hungarian basin it occurs from the upper Eocene to the upper Oligocene (Sztrákos, 1979, 2002; Horváth, 2002). In Romania it has been reported (as Cyclammina rotundidorsata) to occur in large numbers in the upper Oligocene Lăpuş Valley Beds in the Preluca region (Popescu & Iva, 1971). It has also been reported (as Cyclammina rotundidorsata) from the Upper Eocene of the Outer Flysch Carpathians (Geroch & Nowak, 1984), from the Oligocene and Lower Miocene of Leg 38 DSDP sites in the Norwegian-Greenland Sea (Verdenius & Van Hinte, 1983), and from the Eocene-Oligocene of the North Sea and Labrador Margin (Gradstein & Berggren, 1981). Braga et al. (1975) reported it as Haplophragmoides rotundidorsatum from the upper Eocene of the Possagno section in Italy. Wenger (1987) recorded it from the Eggenburgian [Early Miocene] of the Bavarian Molasse deposits. Cicha et al. (1998) recorded its range as Late Eocene to Eggenburgian in the Austrian Molasse Basin. In Bulgaria it is reported from the middle-upper Eocene (Darakchieva, 1999). In the Arctic, Schröder-Adams & McNeil (1994) reported it as Reticulophragmium rotundidorsatus from the Oligocene to Upper Miocene of the Beaufort-MacKenzie Basin. In the North Sea basin, Charnock & Jones (1997) reported its first undoubted occurrence [as Cyclammina (Reticulophragmium) rotundidorsata] in the lower Oligocene just above the FO of Karreriella seigliei (29/2a-6SY well, SWC at 4453') but noted that presumedly caved specimens are found in the Eocene. These authors reported it usually disappears in the upper Oligocene (Zone P21), but noted a single occurrence from lower Miocene Zone N4. In Venezuela, it was reported (as Cyclammina rotundidorsata) from the basal part of the Pecaya Formation (upper Oligocene) of the Falcon Basin by Diaz de Gomero (1977). Preece (1999) illustrated specimens from the Miocene of offshore Cabinda (West Africa).
We have observed R. rotundidorsatum in the Eocene to Miocene of the central North Sea and Norwegian Sea, in the Oligocene at ODP Site 985 in the central Norwegian Sea, in the Oligocene at ODP Site 1148 in the South China Sea (Kuhnt et al. 2002), in the Upper Eocene to Upper Miocene of the Labrador and northern Grand Banks margins, and in the Middle Miocene of the Gulf of Mexico offshore. In the northern North Sea, its average last occurrence is in Zone NSR7A of Early Oligocene age, but it extends into the Middle Miocene part of Zone NSR9A in the Central Graben, e.g., the Shell 30/19-1 well, 5232 ft (swc).

KNOWN STRATIGRAPHIC RANGE: Late Eocene to Late Miocene.

BATHYMETRY: Bathyal. We have not observed this species in abyssal ODP holes, in contrast to the modern species C. orbicularis (see below).

REMARKS: Reticulophragmium rotundidorsatum appears suddenly in Late Eocene flysch-type assemblages without any antecedent forms known to us. The lack of precurser forms suggests that the species may have migrated into bathyal assemblages from shallow water, since rotund cyclamminids are known from shallow-water Campanian deposits in France (Hofker, 1959) and in Antarctica and Argentina (Brian Huber & Norberto Malumian, personal communication to MAK). Many calcareous benthic species enlarged their bathymetric ranges into deeper water during the Late Eocene climatic cooling (Tjalsma & Lohmann, 1983; Miller et al., 1987), and we suspect that these paleoceanographic changes also affected agglutinated forms.
Specimens of R. rotundidorsatum from Eocene horizons are significantly smaller than Neogene specimens. Silicified specimens from the North Sea and Poland have straight sutures that appear as distinct dark bands when wet. In these specimens, the alveoles are aligned in rows parallel to the sutures. There are ~4 rows of alveoles in the last chambers. Specimens from Poland have a low apertural face without any visible areal pores. In these specimens, the apertural face is often collapsed flat. Our well-preserved specimens from ODP Site 985 (central Norwegian Sea) as well as the specimens illustrated from the Beaufort Sea by Schröder-Adams & McNeil (1994) have a low apertural face with no trace of supplimentary apertures. For this reason, we prefer to keep this form in the genus Reticulophragmium.
Specimens from the Miocene of the Labrador Margin are much larger (around 1.2 mm) and have a somewhat higher apertural face with larger agglutinated grains and small, poorly visible pores. Largest specimens have 10 chambers and incised sutures in the last whorl. It seems that these specimens are transitional to the modern species Cyclammina orbicularis Brady. In his revision of the Challenger monograph, Charnock & Jones (1990, 1997) regarded C. orbicularis as a junior synonym of R. rotundidorsatum, but we prefer to separate the two forms. In our view, the (?)Miocene to Recent species C. orbicularis differs in its much larger dimensions, spherical test, and more oblique sutures in the umbilical region. Although no evolutionary transitions have been documented, it appears likely that C. orbicularis evolved from R. rotundidorsatum during the Middle - Late Miocene by acquisition of a more spherical test and oblique sutures.

Fig. 127-2. Internal structure of R. rotundidorsata,
specimens from the Oligocene of Transylvania, from Popescu (1972)

ILLUSTRATIONS: Plate 127 - Reticulophragmium rotundidorsatum (Hantken)
Fig. 1a,b. Oligocene, "Autotype" from the Hantken Collection, (NHM, Wien), Kiscel marls, Budapest; Fig. 2. Upper Eocene, Silesian Unit of the Polish Carpathians, photo courtesy of G.D. Jones; Fig. 3a,b. Oligocene, Topotype from the Kiscell marls, Budapest, Specimen identified by Prof. L. Majzon, Cushman Collection; Fig. 4. Oligocene, ODP Site 985, Norwegian Sea, detail of aperture. Fig. 5a-c. Lower Miocene, MacKenzie Bay sequence of the Beaufort Sea, Orvilruk O-3, 2985-300 m.