Evolution of the Antarctic bryozoan biota as a response to environmental and climatic changes: (Eocene, La Meseta Formation, Seymour Island, Antarctic Peninsula)

The Eocene bryozoans reveal a spectacular diversification in the stratigraphical column of the LMF, Seymour Island, showing a great variation in the colony growth-forms and taxonomy enhanced by a great radiation of a new taxa (Hara 2001).

The very base of the sandy, transgressive series in the lowermost part of the LMF (Telm1) includes loosely encrusting (membraniporiform), and unizooidal, flexible articulated or rooted colonies (catenicelliform), which are either taxonomically and morphologically different from the overlying fauna. At present such bryozoans are widely distributed in the tropical-warm temperate latitudes particularly deposited in the shallow-water settings (Hara 2015).

The massive, hemisperical cerioporine cyclostomes, reminiscent of the Cretaceous in the Northern Hemisphere and differently-shaped multilamellar cheilostomes represented by numerous new taxa are dominant biota in the lower part of Telm1-2 (Hara 2001, 2002).

The free-living lunulitiform, disc-shaped colonies, which occur in the middle part of the LMF (Telm4-Telm5), are characteristic for the warm, shallow-self environment with a temperature range of 10 to 29°C. Environmentally, lunulitids are absent when the bottom sediments is lower than 10-12°C. At present they inhabit the circumpolar to warm-temperate waters (Hara et al. 2018). They have bimineralic skeletons, with the traces of aragonite, which is indicative for the temperate shelf environment, sandy and often shifting substrate.

The bryozoans from the upper part of the LMF (Telm6-Telm7) are scarce, either represented by in-situ lepraliomorph biostrome layer up to 5 cm thick or poorly-preserved sole fragments of the bryozoans associated with penguins and fish remains.

Changes in the biotic composition of the diversified bryozoan biota of the late early Eocene-late Eocene in the stratigraphical column of the LMF mark a distinct environmental and climatic events, referred to EECO, MECO, and EOT for the upper part of this formation.

The isotopic δ¹⁸O analyses of the bryozoan skeletons from the lower part of the La Meseta Fm. show the temperature range from 13.4°C to 14.6°C (according to the equation given by Anderson & Arthur 1983; unpublished Hara 2021; what is consistent with isotopic data of other marine macrofaunal fossil records (see Ivany et al. 2008).

Anderson T.F., and Arthur M.A.1983. Stable isotopes of oxygen and carbon and their application to sedimentologic and paleoenvironmental problems. SEPM Short Course, 10: 1-151.

Hara U. 2001. Bryozoans from the Eocene of Seymour Island, Antarctic Peninsula. Palaeontologia Polonica 60: 33-155.

Hara U., 2002. A new macroporid bryozoan from Eocene of Seymour Island, Antarctic Peninsula, Polish Polar Research, 23: 213-225.

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

Hara U., Mors T., Hagstrom J. & Reguero M. A., 2018. Eocene bryozoans assemblages from the La Meseta Formation of Seymour Island. Geological Quarterly, 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. Geological Society of America Bulletin, 120: 659-678.