Ammodiscus tenuissimus Grzybowski, 1898

Fig. 20. 1 - Type figure of Ammodiscus tenuissimus from Grzybowski (1898);
2 - Paratype specimen from the Grzybowski Collection
(scanned from a light microscope photograph by W. Kuhnt)

ORIGINAL DESIGNATION: Ammodiscus tenuissimus Grzybowski, 1898

TYPE REFERENCE: Grzybowski, J., 1898. Otwornice pokładow naftonośnych okolicy Krosna. Rozprawy Wydziału Matematyczno-Przyrodniczego, Akademia Umiejętności w Krakowie, serya 2, vol. 33, p. 272, pl. 10, fig. 35. See also: Kaminski, M.A. & Geroch, S., 1993. A revision of foraminiferal species in the Grzybowski Collection. Grzybowski Foundation Special Publication, 1, 239-323 [Lectotype].

TYPE SPECIMEN: Not originally designated. Specimens from Grzybowski's samples are deposited in the micropaleontological collections of the Jagiellonian University, Kraków, and are registered as UJ-132-P 1/37 and UJ-132-P 1/49. The lectotype specimen designated by Kaminski & Geroch (1993) is from the Potok H-34, 265 m (UJ-132-P 1/37). Additional specimens are from the Potok H-33 well, 170 m and 207 m.

TYPE LEVEL: Eocene, Silesian Unit of the Polish Carpathians.

TYPE LOCALITY: Grzybowski (1898) listed this species in samples from 13 exploration wells drilled in the vicinity of Krosno, Poland. The lectotype is from the Potok oil field near Krosno.

DIAGNOSTIC FEATURES: Test circular or slightly elliptical, thin, and may have a globular proloculus. Coiling planispiral, slightly involute, comprised of as many as seven whorls. Whorl increases in diameter slowly at first, later more rapidly. Outermost whorl is noticeably wider than previous whorls, and overlaps the previous whorl. Specimens have a depressed coil suture. Wall thin, finely agglutinated with a smooth finish.

SIZE: The syntypes preserved in the Grzybowski Collection attain a diameter of 1 mm.

SYNONYMS: None verified.

OBSERVED OCCURRENCES: Ammodiscus tenuissimus Grzybowski is common species, and is widely reported from the Upper Cretaceous and Paleocene of the Alpine-Carpathian region. Grzybowski (1898) originally described it from the Eocene of the Silesian Unit of the Polish Carpathians. The lectotype is from a sample of Early Eocene age (probably the earliest Eocene Trochamminoides assemblage). Grzybowski (1901) reported it from the Late Cretaceous to Paleogene Inoceramus beds of the Gorlice region. Jurkiewicz (1967) reported it as common from the Paleocene and Eocene of the Polish Carpathians. Schröder-Adams & McNeil (1994) reported it from the upper Eocene to upper Miocene of the Beaufort-Mackenzie Basin. Cicha et al. (1998) reported it from the Kiscellian to Egerian (mid Oligocene to Early Miocene) of the Austrian Molasse Basin. In the North Atlantic literature, this species has been widely confused with Ammodiscus planus. We illustrated specimens as Ammodiscus planus from the Paleocene of the Lizard Springs Formation of Trinidad.

KNOWN STRATIGRAPHIC RANGE: Late Cretaceous to Eocene.

BATHYMETRY: Middle bathyal to abyssal

REMARKS: This species bears superficial resemblance to Ammodiscus planus Loeblich, 1940, first described from Turonian shallow marine sediments (lagoonal facies) in Texas. Ammodiscus tenuissimus differs mainly in its larger dimensions and finely finished wall, and is the preferred term for compressed specimens of Ammodiscus from deep-water (predominantly flysch-type) assemblages.

Numerous authors have applied the designation Ammodiscus tenuissimus (Gümbel, 1862) to small Early Cretaceous ammodiscids (e.g. Bartenstein & Brand, 1951; Neagu, 1975; Geroch & Olszewska, 1990). However, according to Seibold & Seibold (1955), Gümbel's originals are indeed calcareous and rightly belong in the genus Spirillina. This would resolve the question of whether or not Grzybowski's species is a junior homonym of Gümbel's species. Early Cretaceous forms reported by various authors as A. tenuissimus (Gümbel) probably belong in a different species.

ILLUSTRATIONS: Plate 20 - Ammodiscus tenuissimus Grzybowski
Fig. 1a,b Lectotype, Silesian unit of the Polish Carpathians, near Krosno Poland, Potok H-34 well, 265 m (UJ-132-P 1/37), transmitted light in immersion & reflected light, 2a,b. Paralectotype; Fig. 3. Metatype, Potok H-33 well, 207 m (UJ-132-P 1/49), reflected light; Fig. 4. Paleocene, Lizard Springs Formation, Trinidad; Fig. 5,6. Paleocene, North Sea, Shell 9/23-1 well, reflected light; Fig. 7. Eocene, Labrador Margin, Roberval K-92 well, 2150 m.