Cretaceous (Late Albian) coniferales of Alexander Island, Antarctica. 1: Wood taxonomy: a quantitative approach

Abstract

Silicified conifer woods are very common in the mid-Cretaceous (Late Albian, 100 Ma) Triton Point Member of the Neptune Glacier Formation (Fossil Bluff Group), SE Alexander Island, Antarctica. These occur as up to 7 m high in situ tree trunks and stumps rooted in carbonaceous palaeosols and as allochthonous logs and wood fragments in fluvial channel and sheet sandstone facies. Sixty-eight wood samples were examined in this study and were classified in terms of five form taxa using a quantitative approach. Araucarioxylon (1.5% of specimens) is characterised by dominantly multiseriate, alternately arranged bordered pitting on radial tracheid walls and by 1–4 araucarioid cross-field pitting. Araucariopitys (11.8% of specimens) is characterised by dominantly uniseriate tracheid pitting with subordinate biseriate, alternate tracheid pitting and by 1–4 araucarioid cross-field pitting. Podocarpoxylon sp. 1 (63.1% of specimens) is characterised by contiguous, uniseriate tracheid pitting and 1–2 podocarpoid cross-field pits. Podocarpoxylon sp. 2 (22.1% of specimens) is similar to P. sp. 1, differing only in that ray height is lower, tracheid pits are dominantly spaced more than one pit diameter apart and abundant axial parenchyma is present. These first four taxa all possess growth rings with subtle boundaries. Taxodioxylon (1.5% of specimens) is characterised by 1–2 seriate, oppositely arranged, bordered tracheid pitting, 1–2 taxodioid cross-field pitting and very marked ring boundaries. These woods were derived from large trees with basal stump diameters of up to 0.5 m and probable heights of up to 29 m. Data from leaf traces suggest that Araucariopitys and Podocarpoxylon sp. 1 and sp. 2 (97% of specimens) were evergreen with leaf retention times of >5 years. These predominantly evergreen conifer forests grew in a mild, high latitude (75°S) environment during the mid-Cretaceous greenhouse climate phase.