David J. Cantrill

Biodiversity and terrestrial ecology of a mid-Cretaceous, high-latitude floodplain, Alexander Island, Antarctica

The biodiversity and terrestrial ecology of the Late Albian Triton Point Formation (Fossil Bluff Group), Alexander Island, Antarctica is analysed to improve our understanding of polar biomes during the mid-Cretaceous thermal optimum. This formation was deposited on a high-latitude (75°S) floodplain and consists of two facies associations, a lower braided alluvial plain unit and an upper coastal meander-belt unit. Analysis of fossil plants in well exposed palaeosols reveals the existence of spatially complex plant communities. Braidplains supported patchy, low-density (91u2009trees/ha) stands of podocarp and taxodioid conifers on floodbasin substrates, and conifer–cycadophyte–fern–angiosperm thickets in riparian settings. Coastal meander-belts supported medium density (568u2009trees/ha) podocarp–araucarian conifer forests on mature floodbasin soils, and fern–angiosperm–ginkgo thickets in riparian settings. Growth-ring analysis indicates plants experienced stressful growing conditions on the braidplain characterized by high-frequency flood events, but more favourable growing conditions on the coastal plain. Additional vegetation disturbances were caused by arthropod–fungal attack, frost and wildfire. In terms of structure, composition, ecology and productivity these predominantly evergreen, broad-leafed conifer forests bear similarities to the extant temperate rainforests of New Zealand.

Taxonomy and palaeoecology of Early Cretaceous (Late Albian) angiosperm leaves from Alexander Island, Antarctica

Seven species of angiospermous leaves from the mid to Late Albian of Alexander Island, Antarctica provide further evidence of angiosperm radiation into high southern palaeolatitudes. The leaves have both palmate (three species) and pinnate (four species) venation. Entire margined leaves with brochidodromus venation are interpreted as belonging to the Magnoliidae, and possibly include members of the Laurales. Palmately veined forms representing the Laurales occur as do palaeoherbs. Other taxa have marginal teeth comparable to those found in the Rosidae. Palaeoecological analysis indicates that Hydrocotylophyllum alexandri sp. nov. was a herbaceous streamside coloniser; Gnafalea jeffersonii gen. et sp. nov. was a small shrubby plant growing adjacent to levee banks. The other angiosperms, Araliaephyllum quinquelobatus sp. nov., Timothyia trinervis gen. et sp. nov., Gnafalea binatus sp. nov., Ficophyllum palustris sp. nov., Dicotylophyllum lobatus sp. nov., occur infrequently in swamp deposits and probably represent a scattered understorey of trees and shrubs amongst a conifer and pentoxylalean overstorey.

Fossil woods from Williams Point Beds, Livingston Island, Antarctica: a Late Cretaceous southern high latitude flora

The wood flora from Williams Point, Livingston Island, contains 12 wood types of gymnosperm and angiosperm origin. Recent collections of material have increased the biodiversity of a postulated species-rich vegetation. The gymnosperm wood can be readily assigned to four form-genera: Araucarioxylon Kraus, Araucariopitys Jeffrey, Podocarpoxylon Gothan and Sahnioxylon Bose and Sah. This indicates a diversity of coniferous araucarian and podocarp trees alongside woods of uncertain affinity (Sahnioxylon; Bennettitales). Two angiosperm morphotypes are assigned to the organ genera Hedycaryoxylon Su¨ss (Monimiaceae) and Weinmannioxylon Petriella (Cunoniaceae). The remaining four taxa of angiosperm wood cannot be confidently placed in extant families as they exhibit features that suggest relationships with the Magnoliidae, Hamamelidae and Rosidae. This paper presents the first comprehensive taxonomic revision of the wood flora from Livingston Island and discusses the palaeoecology that prevailed at a latitude of about 60 degrees south during the Late Cretaceous. Newly described taxa are Araucarioxylon chapmanae sp. nov., Araucariopitys antarcticus sp. nov., Podocarpoxylon chapmanae sp. nov., P. verticalis sp. nov., P. communis sp. nov., Weinmannioxylon ackamoides sp. nov., Antarctoxylon livingstonensis gen. et sp. nov., A. multiseriatum gen. et sp. nov., A. heteroporosum gen. et sp. nov. and A. uniperforatum gen et sp. nov.

Cretaceous (Late Albian) coniferales of Alexander Island, Antarctica. 2. Leaves, reproductive structures and roots.

Coniferous foliage from the Albian of Alexander Island, Antarctica, is assigned to the Araucariaceae, Podocarpaceae, and Taxodiaceae based on attached or associated fertile remains. Araucarian foliage represented by Araucaria alexandrensis sp. nov. and A. chambersii sp. nov. is associated with ovulate cone scales described as Araucarites wollemiaformis sp. nov. and A. citadelbastionensis sp. nov., respectively. The Podocarpaceae is represented by Bellingshausium willeyii sp. nov. and the Taxodiaceae by Athrotaxites ungeri, both with attached cones. Sterile foliage is widespread belonging to the form genera Podozamites, Elatocladus, Brachyphyllum and Pagiophyllum. The conifers in this Albian southern high-latitude flora make up ca. 15% of the species diversity. Evidence from leaf litter distribution on palaeosols and leaf morphology suggest that the majority of conifers were large canopy-forming trees, although a few were probably small understorey shrubs.

The occurrence of the fern Hausmannia Dunker (Dipteridaceae) in the Cretaceous of Alexander Island, Antarctica

Hausmannia papilio Feruglio is a widespread component of Albian fossil floras on Alexander Island, Antarctic Peninsula. Hausmannia cannot be separated from extant Dipteris and is best regarded as a foliage form genus of the latter. Palaeoecological reconstructions suggests that Hausmannia papilio was a stream-side dweller and pioneer species.

Early Cretaceous fern foliage from President Head, Snow Island, Antarctica

Fertile fern foliage described as Lophosoria cupulatus sp. nov. contains the distinctive spore Cyatheacidites annulatus. The fossil foliage is morphologically similar to the Early Cretaceous form-genera Gleichenites and Microphyllopteris, whose time and space distribution in Gondwana matches that of the dispersed spore Cyatheacidites. Some of the Cretaceous southern hemisphere material assigned to Gleichenites and Microphyllopteris is probably allied to families such as the Lophosoriaceae rather than the Gleicheniaceae as had been previously supposed. The nearest living relative, Lophosoria quadripinnata, grows within a mean annual temperature range of 8–22° C, and a mean annual precipitation range of 195–1977 mm. The presence of Lophosoria cupulatus at palaeolatitudes of 55–65° S implies that during the Aptian the southern high latitudes were a minimum of 12° C warmer than the present day.

The fossil record of Cunoniaceae: new evidence from Late Cretaceous wood of Antarctica?

Fossil angiosperm wood from Upper Cretaceous sediments of Livingston Island and James Ross Island in the northern Antarctic Peninsula region is identified as having the combination of anatomical characters most similar to modern Cunoniaceae. The material is characterised by predominantly solitary vessels, opposite to scalariform intervessel pitting, scalariform perforation plates, heterocellular multiseriate and homocellular uniseriate rays, diffuse axial parenchyma. Anatomically, the specimens conform most closely to the fossil organ genus Weinmannioxylon Petriella which has been placed within the Cunoniaceae. The presence of Weinmannioxylon in Late Cretaceous sediments suggests that taxa within or stem taxa to the Cunoniaceae might have been a notable component of the forest vegetation that covered the Antarctic Peninsula during the Late Mesozoic and may therefore represent the earliest record of this family.

A petrified cycad trunk from the Late Cretaceous of the Larsen Basin, Antarctica

Cetricycas antarcticus gen. et sp. nov. occurs in the Late Coniacian to Campanian Lachman Crags Member of the Santa Marta Formation, Marambio Group, James Ross Island, Antarctica. The petrified trunk comprises a thick pith containing thin and thick walled parenchymous cells, vascular traces surrounded by a thin layer of loose secondary wood. Medullary bundles arise inside the wood zone and pass outwards through multiseriate medullary rays. The trunk anatomy indicates assignment to the Cycadales, and the presence of medullary bundles indicates the affinities with a clade comprising Lepidozamia, Macrozamia, and Encephalartos. This clade is presently confined to Australia and Africa, but fossil representatives extend the geographic range to India, South America and now Antarctica.

Late Jurassic (Kimmeridgian-Tithonian) macrofossil assemblage from Jason Peninsula, Graham Land: evidence for a significant northward extension of the Latady Formation

New exposures of fossiliferous sedimentary rocks at Cape Framnes, Jason Peninsula (6557S, 6033W) are assigned to the Middle-Late Jurassic Latady Formation of the south-eastern Antarctic Peninsula region. A sequence of fine to coarse-grained sandstones of unknown thickness has yielded a molluscan and plant macrofossil assemblage rich in the following elements: perisphinctid ammonites, belemnopseid belemnites, oxytomid, trigoniid and astartid bivalves, and bennettitalean fronds and fructifications. The overwhelming age affinities are with the Kimmeridgian-early Tithonian part of the Latady Formation, as exposed on the Orville and Lassiter coasts. The Cape Framnes sedimentary rocks help to constrain the age of a major sequence of acid volcanic rocks on Jason Peninsula, and show that the Latady Basin was geographically much more extensive than recognized previously. It was the principal depositional centre of Middle-Late Jurassic sedimentation in the Antarctic Peninsula back-arc region and in areal extent may have rivalled the essentially Cretaceous Larsen Basin.

Tectonic and climatic controls on a Mesozoic forearc basin succession, Alexander Island, Antarctica

The Cretaceous Fossil Bluff Group on Alexander Island, on the west side of the Antarctic Peninsula, contains a remarkably complete record of the evolution of a forearc basin. The latest (Aptian-Albian) stages in the basin history are recorded in a well-exposed succession at the southern end of the island, where a series of nunataks provide exposure of over a thousand metres of shallow marine and continental deposits. An abrupt facies shift from upper shoreface marine facies to braided fluvial deposits is interpreted as the record of regional uplift in the volcanic arc. This event coincides with the Palmer Land deformation event which may be related to a mid-Cretaceous mantle plume. A gradual reduction in depositional gradient and a return to shallow marine conditions towards the top of the exposed section is interpreted as a consequence of erosion of the arc and subsidence within the basin. Palaeocurrent data and facies distributions indicate that the continental deposits formed a fan- shaped wedge at least 30 km in diameter in the southern part of the forearc basin. Fossil plants indi- cate that the palaeoclimate was warm and humid throughout the period of deposition. Mapping and facies analysis of the upper part of the Fossil Bluff Group in southern Alexander Island has resulted in a revision of the stratigraphic terminology for the area. The Triton Point Member, formerly part of the Neptune Glacier Formation, has been raised to formation status and two members (the Citadel Bastion Member and the Coal Nunatak Member) and a Bed (the Upper Coal Nunatak Sandstone Bed) are defined here within the formation.