Wednesday, February 06, 2008

Evolution: Gondwana vs New Zealand

Gondwana* was a large land mass that tended to reside in the southern hemisphere from Cambrian times to mid-Jurassic times (c.500 million years ago to c.160mya). It consisted, in the main, of Antarctica, Africa, South America, India, Australia, and New Zealand. For part of its existence, it was united with its northern equivalent into the supercontinent Pangea.

Through this time, Gondwana experienced a number of climate changes, due in part to the movement of Earth's techtonic plates which formed then ultimately broke up the continent.
Africa moved away first, followed by South America and India (the latter has been a particularly fast mover, crashing spectacularly into Asia to form the Himalayas).

Of course, disintegration took many millions of years, so the dates are approximate. The dispersal (and later speciation) of fauna is not entirely stopped by such splits. For some time after complete separation, islands are typically scattered between the land masses. This more or less allows for some island hopping from one land mass to another, typically through "rafting" - the carriage of fauna across on masses of vegetation, often after storms. Richard Dawkins treats this well in The Ancestor's Tale, and makes the point that even if such an event sounds unlikely, given the time scales involved - millions of years - it's unlikely that it wouldn't happen. And all it takes for a population to be established is one pregnant female.

I always thought that New Zealand broke away from Australia, because they seem to be such a neat fit. However, New Zealand is usually cited as splitting from Antarctica/Australia earlier. Ultimate separation was achieved around 82mya. Dawkins says Australia was finally sufficiently free of Antarctica to obviate island hopping around 55mya, although estimates vary a fair bit; Wikipedia suggests it was still freeing itself at 40mya.
The point about this is that New Zealand was isolated for substantially longer than Australia. At the time of the dinosaur-extinction K-T meteor - 65mya - New Zealand was already isolated, and Australia probably was not.

Various sources suggest New Zealand eroded since isolation, by up to 80% - some claim it disappeared below the waves altogether for a time, although this seems unlikely. The question is around the sustainability of populations in isolation. This is mainly relevant for large terrestrial tetrapods. Conventional wisdom is that the number of unique flightless birds in New Zealand reflects the absence of terrestrial predatory species until human introduction in the past thousand years.

There remains a few species in New Zealand's islands that are "anomalous" to the global narrative, including:
  • the living Tuatara - only lizard-like - actually a distant lizard relative called a sphenodont.
  • fossil dinosaurs and crocodilia from well before NZ's isolation
  • and the newly-discovered sb mammal or "waddling mouse" from NZ's very recent evolutionary past, c.16 million years ago (main discussions here and here), interpreted as non-therian, ie egg-laying.

These need to be drawn into the narrative one way or another.

Related discussions:

*Gondwana means "land of the Gonds"; the term Gondwanaland is both redundant and obsolete.


Dawkins, R (2004): The Ancestor's Tale. Phoenix, London.

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