Early angiosperm diversification in the Albian of southeast Australia: implications for flowering plant radiation across eastern Gondwana
Introduction
It is now widely accepted that angiosperms evolved and rose to dominance through the Early Cretaceous with the group appearing earlier in the low to mid latitudes and spreading polewards (Friis et al., 2011). While this pattern is well documented for the Northern Hemisphere (Friis et al., 2011) it is less so in the south (Drinnan and Crane, 1990, Cantrill and Poole, 2012). Indeed, the early macrofossil record of flowering plants in Australia is limited to angiosperms (leaves and flowers) from the Koonwarra Fish Beds (Aptian) (Taylor and Hickey, 1990) and the Winton Formation (Albian–Cenomanian) (McLoughlin et al., 2010). Palynofloras through the Cretaceous interval are more widely sampled due to interest by petroleum companies in the hydrocarbon potential of rocks of this age. However, despite extensive sampling relatively few studies of the angiosperm content have been made in Australia and it has been suggested that angiosperms were not an important component of the vegetation until well into the Late Cretaceous (e.g. Dettmann, 1973, Burger, 1990, Burger, 1993).
Recent investigations of the youngest exposed (upper Albian) strata in the Otway Basin have yielded spore–pollen assemblages with angiosperm grains that are both diverse and abundant. This study documents the diversity and discusses the implications.
Section snippets
Geological setting
During the Late Jurassic–Early Cretaceous, a series of complex fault-controlled grabens developed along the southern margin of Australia (Edwards et al., 1996, Duddy, 2003). These fault-controlled structures formed the Great Australian Bight, Duntroon, Otway, Bass and Gippsland basins, collectively referred to as the Southern Rift System (Stagg et al., 1990). The Otway Basin is an east–west oriented half-graben, initially formed along the Gippsland–onshore Otway basins during the
Palynology
Previously examined slides from Rotten Point, Johanna Beach and Cat Reef by Burger (1987) and Dinosaur Cove by Wagstaff and McEwen Mason (1989) were used in this study with new samples from Castle Cove, Milanesia Beach, Wreck Beach and Devils Kitchen. The method of pollen processing undertaken by Burger (1987) for Felton (1992) at the Rotten Point, Johanna Beach and Cat Reef areas is not disclosed in the unpublished report of Burger's (1987) findings or in the thesis of Felton (1992).
Palynostratigraphy
Thirty-one samples from the upper Eumeralla Formation of the Otway Basin (see Supplementary Table 2) contained sufficiently abundant spore–pollen assemblages to be counted. All of the samples contained well preserved and abundant assemblages (see Supplementary Table 3) that allowed their assignment to palynostratigraphic zonations.
Angiosperm pollen
The results of this palynological study indicate that angiosperms first appear in the upper Eumeralla Formation of the Otway Basin in the Phimopollenites pannosus Zone. The relative abundance and diversity of angiosperm pollen increased from the earliest part of the P. pannosus Zone to the Upper P. pannosus Subzone (Fig. 8, Fig. 9). A total of twenty-three angiosperm species, including one newly described species, Tricolpites tortuosus, is present in the 14 samples that yielded angiosperm
Species descriptions
Species of biostratigraphic significance in this study are not re-described but are illustrated in Plate I, Plate II, Plate III, Plate IV. In addition, reference is given in the text to those authors whose descriptions have contributed to the original description and any variation noted from published descriptions discussed. The taxa have been divided into broad taxonomic groups as defined by Burger (1993) and are dealt with alphabetically within these groups. The descriptions are based on the
Palynostratigraphy
The refinement of the Albian biostratigraphic zonations and ties to international zones and stages in this investigation provides constraints on angiosperm radiation across eastern Gondwana and Australia. The Crybelosporites striatus, Coptospora paradoxa and Phimopollenites pannosus zones can be confidently assigned in the sections examined in the Otway Basin, based on the previously discussed indicators. However, the lack of Appendicisporites distocarinatus, Hoegisporis uniforma and Amospollis
Conclusions
Early Cretaceous strata, west of Cape Otway in southeastern Australia, are assigned to the C. striatus Zone, C. paradoxa Zone, P. pannosus Zone and the Upper P. pannosus Subzone. The reliable indicators in the Otway Basin of these zones are C. striatus, B. holodictyus, C. paradoxa, T. trioreticulosus, C. cuneiformis, P. grandis, P. pannosus, P. jubatus and P. majus. A new Upper Phimopollenites pannosus Subzone of late Albian age is defined based on the presence of Appendicisporities
Acknowledgements
We would like to thank Geoscience Australia, the Melbourne Museum and Queensland Museum for the loan of slides and reference material. We would particularly like to thank John Laurie, David Pickering, Kristen Spring and Mary Dettmann who facilitated these loans. We would also like to thank Global Geolab Limited for altering their standard processing techniques. We are also grateful to Malcolm Wallace and the two anonymous reviewers for constructive and helpful comments on the manuscript. The
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