What can insects, especially Chironomidae, tell us about austral ecological and biogeographic history?

Peter Cranston *1

1 Emeritus professor, The Australian National University, Canberra, ACT 200, Australia

Entomologists have been in the vanguard of southern hemisphere biogeographers. Views on origins of the austral biota have swung from endemic austral, to northern invasion, back to gondwanan vicariant, to dispersals from mixed sources. New Zealand insects have been claimed to represent gondwanan relicts dating to separation of Zelandia from Antarctica, and linked phylogenetically more to southern America than to neighbouring Australia. In perhaps the most extreme contrasting view, NZ diversity derives from neo-radiations with all extant biota derived by dispersal since the surmised Oligocene ‘drowning’ that eliminated all terrestrial biota. With increasing availability of phylogenetic estimates for a range of well-sampled austral insects, and with dating and tempo derived from increasingly sophisticated models of molecular evolution, we can re-assess these earlier views. Amongst the biogeographically-significant insects are the austral Chironomidae midges studied half a century ago (Brundin, 1966). With a good Cretaceous amber fossil record, and earlier wing compression fossils, a chronogram can now be presented for the tempo of the evolution of the ‘Brundin taxa’.  No NZ taxa are sister to Australians but to South Americans as Brundin argued. However dates for NZ nodes are too young for Gondwanan fragmentation (Cranston et al, 2010) but some straddle and pre-date the Oligocene. Northern hemisphere branches are very old (Pangean) or young – by colonisation from the south. Other Chironomidae represented in NZ are congruent but include sisters to Australians (recent dispersal, both ways). Other insects (and arthropods) challenge Oligocene drowning – an NZ biota persisted, at least in Nelson. Dating and placement of fossils is critical.

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