Why moa? part 1

Everyone knows that mammals were replaced by birds in the indigenous fauna of New Zealand.

And many will continue to assume that this is because this archipelago has simply been so isolated that mammals have never reached it.

However, how many realise that terrestrial mammals were indeed present in what is now New Zealand for tens of millions of years?

And who has thought through what this means for our explanations of the prevalence of birds in New Zealand?

At first glance, it seems to make sense that birds colonised New Zealand when mammals could not - for the simple reason that birds can travel on the wind across hundreds of kilometres of water.

However, the more one looks at this premise the more simplistic it seems.

There are at least six reasons to doubt that the usurpation of mammalian niches by birds in New Zealand is owing to a 'founder effect' caused by the isolation of New Zealand.

• Land mammals can be assumed to have been present right from the start, when what is now New Zealand was still part of Gondwana at the time of the dinosaurs.
• Land mammals are also known to have been present when what is now New Zealand was a separate entity called Zealandia, even after this low-lying landmass was nearly submerged by geological forces (https://en.wikipedia.org/wiki/Saint_Bathans_mammal and https://www.newscientist.com/article/dn10773-fossils-reveal-new-zealands-indigenous-mouse/ and https://www.researchgate.net/publication/6640844_Miocene_mammal_reveals_a_Mesozoic_ghost_lineage_on_insular_New_Zealand_southwest_Pacific).
• Bats are known to have adopted a semi-terrestrial way of life as long as 15 million years ago in what is now New Zealand (https://en.wikipedia.org/wiki/New_Zealand_lesser_short-tailed_bat), giving them more than enough time to evolve into land mammals here.
• Pinniped mammals have also been present on the shores of what is now New Zealand for millions of years (https://www.theguardian.com/world/2020/nov/11/new-species-of-extinct-monk-seal-identified-from-fossils-in-new-zealand and https://en.wikipedia.org/wiki/Arctocephalus_forsteri), giving them time to become land mammals.
• Moa and kiwi are flightless, owing either to flightless ancestors having reached what is now New Zealand or to volant founders having been drastically modified by evolution in-situ. Either way, mammals should have been able to evolve accordingly: if birds can lose their wings, why can bats and pinnipeds not also produce similarly drastic modifications of their limbs?
• Even in Australia, where fully terrestrial mammals have been successful, flightless birds have occupied certain niches (e.g. https://en.wikipedia.org/wiki/Emu and https://en.wikipedia.org/wiki/Genyornis and https://en.wikipedia.org/wiki/Dromornis). Indeed, the most massive land animal (besides the human species) in Australia at the time of European arrival was a species of cassowary (https://en.wikipedia.org/wiki/Cassowary), not a species of kangaroo.

So, should we perhaps update our conceptual framework accordingly?

Could there be ecological, as opposed to accidentally historical, reasons for the failure of land mammals in New Zealand? Is it possible that the avian ancestors had competitive advantages such that they prevailed on this archipelago in the face of (still poorly documented) mammalian competition - and would have done so regardless of any isolation?

to be continued...

Publicado el diciembre 26, 2021 07:26 TARDE por milewski