29 July 2008

The different meanings of 'Gondwanan'

Tuatara, leiopelmatid frogs, weta (Orthoptera), peripatus (Onychophora), southern beech (Nothofagus) and kauri (Agathis australis) and other New Zealand species are often referred to as Gondwanan taxa. What does this mean? Gondwanaland was a large landmass made up of the southern continents that slowly broke apart through Jurassic and Cretaceous time. The New Zealand area began to break away from Eastern Australia and Antarctica around 83 million years ago in a large chunk of land the size of India, called Zealandia. Slowly this landmass sank. All that remains of this small continent are New Zealand, New Caledonia, Norfolk, Chatham Islands and the subantarctic islands. The term ‘Gondwanan’ is given to species whose distribution recalls this ancient history.

In a series of papers, Adrian Paterson (Lincoln University) and Steve Trewick (Massey University) have been questioning the relevance of Gondwanaland and its influence on modern New Zealand species 1,2,3. In an upcoming paper in Philosophical Transactions of the Royal Society B special issue on evolution on Pacific Islands, they, along with Julia Goldberg (a PhD student from Massey University), explore what the term ‘Gondwanan’ actually means for New Zealand species.

The term ‘Gondwanan’ seems to have been used by scientists in at least three different ways.

First, a taxon is Gondwanan if its lineage has been continuously present in New Zealand since Zealandia broke away from Gondwanaland.

Second, a taxon is Gondwanan if it is descended from a lineage that was present in Gondwanaland prior to break-up but arrived in New Zealand after Zealandia broke away from Gondwanaland and sank (dispersed and colonised).

Third, a taxon is Gondwanan if species are currently found in former Gonwanaland landmasses but with no expectation of a common historical process to explain different instances of this pattern (which could be because of either of the two meanings above).

For example, penguins are found in New Zealand, Australia, southern Africa, South America and Antarctica and certainly demonstrate a Gondwanan distribution (third meaning). Penguins have a fossil history to at least 62 Myr ago and molecular analyses suggest a much earlier origin in the Gondwanaland landmass, implying that they are found in the New Zealand region by virtue of having been in Gondwanaland when Zealandia rifted away (first meaning). Fossil and molecular data all suggest that modern living penguin species all share a recent ancestor from about 25-30 million years ago. Living penguin genera of Eudyptes, Eudyptula and Megadyptes may have colonized New Zealand from other former Gondwanan landmasses since that time (second meaning). Indeed, Eudyptula, the blue penguin, is found in Australia and New Zealand with molecular evidence for recent contact between these populations. Thus, the modern distribution of penguins could be described as Gondwanan in all three senses.

Does it really matter; it’s all Gondwanan isn’t it? Science is about being precise, removing ambiguity and these three meanings suggest different things. Meaning three simply acknowledges that there is a group of species that share a certain southern hemisphere pattern. Meanings one and two provide different explanations for the origin of this pattern with meaning one implying species have been continuously in New Zealand for 80 million years and meaning two implying that the species could have arrived at any time over the last 80 million years. Understanding the difference will allow biogeographers (scientists that study where species are found) to be better able to tell the complicated story of the history of the New Zealand’s biota.

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