Early Cretaceous vegetation in a polar ecosystem—Palynology and zircon dating of the Koonwarra Fossil Bed, Victoria, Australia

Abstract

The Early Cretaceous Koonwarra site, Victoria, Australia, comprises a fossil bed considered to be a Konservat Lagerstätte famous for its myriad of fossil fish, feathers, invertebrates, and plants. Here we provide a vegetation reconstruction based on detailed palynology from a 21 m long drill core of which the basal 10 m represents the Koonwarra Fossil Bed (KFB) proper. Palynological analysis of the KFB revealed well-preserved assemblages, and c. 30,000 palynomorphs were identified, representing 138 taxa. The palynoassemblages are dominated by gymnosperm pollen, especially Alisporites (‘seed-ferns’), Araucariacites (Araucariaceae), Podocarpidites (Podocarpaceae) and monosulcate grain producers (most possibly Ginkgoales and Pentoxylales). Minor constituents include Eucommidiites and various arborescent gymnosperm pollen, including those produced by Cheirolepidiaceae such as Classopollis. The next most abundant groups are the fern spores, followed by lycophyte spores, both of which constitute a substantial portion of the palynoflora. Angiosperm pollen represents a minor but important element of the flora throughout the succession (1–6%). The LAICP-MS U–Pb dating of detrital zircon grains bracketing the Koonwarra Fossil Bed reveal a maximum dominant age of ≤ 114.1 Ma (latest Aptian) but with a few younger grains dated at 110 Ma.

The palynological analysis tentatively ascribes a slightly younger age to the Unit, with assemblages referred to the very base of the Coptospora paradoxa Zone, signifying an early Albian age, based on the presence of the indices Coptospora paradoxa and Stoverisporites lunaris, combined with abundance data of various groups. Comparisons with coeval successions from other Southern Hemisphere localities, indicated that the Koonwarra assemblages derived from a cool temperate lake-margin vegetation.