Lauraceae fossils from a volcanic Palaeocene oceanic island, Ninetyeast Ridge, Indian Ocean: ancient long-distance dispersal?

Abstract

Aim: Geological and fossil records are critical for historical biogeography studies. A plant fossil assemblage from a small, well-dated, transient late Palaeocene island was re-investigated with regard to regional geology and vicariance versus dispersal hypotheses. Location: Deep Sea Drilling Program Leg 22, Site 214 on the Ninetyeast Ridge (NER) in the mid-Indian Ocean region. Methods: Leaf cuticular material was recovered from residues from a previous palynofloral study of Site 214 sediments during the 1970s and identified. The palynoflora was reassessed. Results: The only leaf cuticular material recovered with stomata can be placed in crown-group Lauraceae. It is confirmed that the palynoflora reflects the presence of a low-diversity island flora in the late Palaeocene, comprising ferns and mostly herbaceous angiosperms with readily dispersible propagules, and perhaps austral podocarps. Other pollen taxa of almost certain local origin were arecoid palms and taxa related to Chloranthaceae. The strong overall similarity of the palynoflora to Australo-Antarctic and New Zealand assemblages is also confirmed. Main conclusions: Foliar fossils of Lauraceae demonstrate the occurrence of one of the world's largest, most widely distributed woody plant families on a late Palaeocene island. The presence of plants on this island could be explained by vicariance via a vegetated Upper Cretaceous Kerguelen Plateau, in part because crown-group Lauraceae may be at least this old. However, there are records of other taxa in the Kerguelen region that are anomalous with vicariance, plus evidence for a catastrophic biotic extinction event centred in the area in the latest Cretaceous. Plants were therefore most likely to have reached the island by means of dispersal. This suggests either the presence of presently unknown vegetated land nearby in the Kerguelen region in the late Palaeocene, or long-distance dispersal, probably from the Australian region. The dispersal of viable seeds could have been facilitated by birds or perhaps by ocean-surface drift with or without the assistance of ocean-going animals. The fossils allow that even small, short-lived islands could have acted as 'stepping stones' for biotic interchange between Australia and Africa, and perhaps other regions.