Australia: The Land Where Time Began

A biography of the Australian continent 

Biogeography – Panbiogeography, Tectonics and Evolution

Clade ages are the basis of modern analyses, which are calibrated by using the known fossil record, though in most groups this is misleading and identifications in the fossil record, though extant groups, that were morphology-based, were often misconstrued. Alternatively, the best of the molecular data - the clades and their distribution - are synthesised with revolutionary developments in hard rock geology – the radiometric dates for tectonic events, in a panbiogeographic approach. Calibration of a range of nodes with tectonic events is involved in this synthesis, though this process can be complicated by the tectonic events often being episodic or of long duration, such as some of the main events in Australasia, including the continental crust rifting in the formation of the Tasman and Coral Sea basins. In an event that lasted from 84 Ma to 55 Ma the rifting zone propagated northwards, with the result that the communities of biota were gradually separated. The actual breakup was the final step in a long extension process, and it is suggested by many distributions that much of the differentiation associated with rifting resulted from the pre-breakup deformation and magmatism. Also important in biogeography are failed rifts such as the Moonlight Tectonic Zone in New Zealand, in southern Africa the Lebombo Monocline, and in central Africa, the Cretaceous rift system.

Crustal shortening and thickening have led to other events in the region of the Tasman Sea and Coral Sea, and some of these processes are of long duration. A collision along the northern margin of the Australian plate in New Guinea in the Palaeocene, 58 Ma, the collision in the Eocene, 44-34 Ma, that is preserved in New Caledonia, and in the Oligocene, 26-25 Ma, in the North Island of New Zealand are suggested by Heads¹ to all possibly be part of a single collisional event that migrated to the south along the plate boundary (Glen & Meffre, 2009). Here, subduction zone rollback towards the Pacific has resulted from collision between forearc crust and continental fragments; new basins being created and arc-continent collision.

As the uppermost geological stratum life has proven to be stickier than believed, with populations surviving episodes of marine flooding, volcanism, glaciation, desertification, uplift, subsistence, as well as other events. Remnants of weedy taxa have also persisted in gullies, cracks in rocks, on nunataks, underground and many other habitats that are regarded as marginal. Even local tectonic changes, such as faulting, are capable of affecting living populations of groups in a non-weedy phase of their evolution leading to differentiation, at the other extreme. In New Zealand, New Caledonia, New Guinea and the Philippines, strike-slip movement, which developed under transpressional regimes, and has also led to biogeographic differentiation and disjunction.

Sources & Further reading

Heads, Michael, 2014, Biogeography of Australasia: A Molecular Analysis, Cambridge University Press