Palaeoenvironmental and climatic events (EECO-EOT) in the bryozoan fossil records of the Early Cenozoic of Antarctica

The earliest Cenozoic Antarctic bryozoan fossil records (late Early Eocene) are well documented from the shallow-marine–estuarine clastic succession of the lower part (Telm1-2) of the La Meseta Formation of Seymour Island. In the 800-meters thick stratigraphical profile of the LMF in the basal facies of the (Telm1), the earliest – late Early Eocene bryozoans are represented by the internal moulds of the loosely encrusting, unizooidal, flexible articulated or rooted colonies belonging to cheilostome buguloids and catenicelloideans, which are taxonomically and morphologically different from the overlying fauna. At present, representatives of (Beanidae, Catenicellidae, Savignyellidae and Calwelliidae widely occur in the tropical-warm temperate latitudes in the shallow-marine settings (Hara, 2015). Higher in Telm1 the most common are spectacular in size, massive multilamellar colonies, showing a great variety of shapes dominated by cheilostome celleporiforms and cyclostome cerioporids (Hara, 2001). The stable isotopic δ¹⁸O analyses of the bryozoan skeletons from the lower part of the LMF show the temperature range from 13.4 to 14.6°C (Hara, 2022), what is consistent with the isotopic data of other marine macrofaunal fossil records (Ivany et al., 2008).

The distinct free-living lunulitiforms bryozoans, for the first time reported from Antarctica from the middle part of the LMF (Telm4-6, Cucullaea I-II; Ypresian/Lutetian) are represented by the disc-shaped colonies - characteristic for the temperate warm, shallow-shelf environment, with the bottom temperature, which are never lower than 10 to 12°C. The skeletons of Lunulites, Otionellina, and Uharella are formed by the intermediate-Mg calcite (IMC) with the 4.5 mol% MgCO₃. Their bimineralic zoaria (with the traces of aragonite, calcite and strontium apatite) are indicative for the sandy, temperate shelf environment (Hara et al., 2018).

Contrary to occurrence of the rich bryozoans of the (Telm1–2), the Late Eocene bryozoans from the upper part of the LMF (Telm6–7), are represented by the scarce lepraliomorphs accompanied by the crustaceans, brachiopods and gadiform fish remains. The bryozoan-bearing horizon is composed of the single taxon tentatively assignated to Goodonia terminating the occurrence of the bryozoans, showing a sharp decline in their biodiversity between the lower and upper part of the formation (Hara, 2001), what is consistent with the overall pattern of Eocene cooling up to around 10,5°C in Telm6 and 7.

References

Hara U. 2001 – Bryozoa from the Eocene of Seymour Island, Antarctic Peninsula. Palaeontologia Polonica. III, 60: 33–156.

Hara U. 2015. Bryozoan internal moulds from the La Meseta Formation (Eocene) of Seymour Island, Antarctic Peninsula. PPR, 36, 25-49.

Hara U., 2022 – Geochemistry of the fossil and Recent bryozoan faunas in the natural diagenetic environments and their significance for the reconstruction of biota and climatic regimes in Cenozoic. Archive PGI-NRI, nr. 5210/2022.

Hara U., Mors T., Hagstrom J., Reguero M.A., 2018 – Eocene bryozoans assemblages from the La Meseta Formation of Seymour Island, Antarctica. Geol. Quar., 62: 705–728.

Ivany L.C., Lohmann K.C., Hasiuk F., Blake D.B., Glass A., Aronson R.B., Moody R.M., 2008 – Eocene climate record of the high southern latitude continental shelf: Seymour Island, Antarctica. Geol. Soc. Amer. Bull., 120, 5–6: 659–678.