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Re: emu history



----- Original Message ----- 
From: "John Bois" <jbois@umd5.umd.edu>
To: "Chris Hart" <s366978@student.uq.edu.au>
Cc: <dinosaur@usc.edu>
Sent: Wednesday, January 14, 2004 11:07 PM
Subject: Re: emu history


> Do you know (is it known) if NG has been intermittently connected to
> Australia--or was there only one separation event?  Did common
> ancestors of emu and cassowaries migrate to NG, evolve allopatrically, and
> then cassowaries migrate to Australia-or did both species evolve in
> Australia?
> Thanks, John Bois.


Tectonically, New Guinea is not seperated from Australia - they are both
part of the same craton.  The shallow basin between N. Australia and NG
(composed of the Arafura Sea, the Gulf of Carpentaria, and the Torres
Strait) has been recognisable since at least the Jurassic, and has been
variously inundated and dried-out as sea levels have fluctuated.  According
to the palaeo maps I have in front of me, the Arafura Sea was inudated
during the latest Eocene (35Ma), but at that time the land bridge between
Cape York and New Guinea across the Torres Strait was still in place.  Given
that the northern part of the craton seems to have started collision with
the Pacific plate at about 25Ma, resulting in the uplift of the New Guinea
highlands, and assuming that the associated downwarping was responsible for
lowering the Torres Strait to the extent that it became inundated, I would
guess that New Guinea and Australia became isolated at some point during the
Miocene, and then intermittently connected again during the Plio-Pleistocene
glacial maxima.  I'll see if I can find some actual data on this, though.

As for what Tertiary casuariformes were doing, who knows?  With the amount
of actual fossil data available, it is possible to construct all sorts of
plausible scenarios.  However, if cassowries dispersed between New Guinea
and Australia during the Pleistocene (which would seem to be the least
remarkable hypothesis), then the parent population should in theory be
identifiable through its higher levels of genetic diversity, although if the
Australian population is the parent this might be obscured by the recent
bottleneck it is going through.

Re separation of Australia from Anatartica - The spreading ridge appeared to
the west during the Early Cretaceous, and eventually extended eastwards.  By
the Palaeocene there are marine basalts separating the two as far east as
the SA - Victoria border.  Continuous marine basalts from west to east
appear during the late Eocene / early Oligocene (although they remain narrow
south of Tasmania).  However, sea level stands during the early Tertiary
appear to have been reasonably high, and a continuous sea-way was likely to
have been present since at least the Palaeocene and perhaps even the
uppermost Cretaceous.  Unfortunately, the maps I have show only the margins
of the continental shelves, and not the palaeoshorelines - I'll see if I can
find a more precise date for the formation of the sea way.  At the moment,
some time between 85Ma and 45Ma is as precise as I can find....

Bear in mind, though, that sea way or no, Tasmania remained pretty close to
Antarctica until the Miocene.

Ref:  J. J. Veevers, 2001, Billion year earth history of Australia and
neighbours in Gondwanaland.

Cheers
Colin