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Penguin post script



Once upon a thread there was a discussion of why penguins are almost
exlusively southern hemisphere species.  I came across a new book called
Aquagenesis (The Origin and Evolution of Life in the Sea) by Richard
Ellis.  He reviews the penguin fossil record and notes that putative
penguin ancestors of the Oligocene did exist in the northern
hemisphere.  He also quotes Olson and Hasegawa (1996--ref. if needed) as
saying that "there is a strong possibility that the disappearance of
(large penguins) is linked with the ascendancy of seals and porpoises."  I
thought I would plug these ideas into the competition between the
following hypotheses for the strange distribution of extant penguins:
1) Penguins are unable to access northern waters because they evolved in
the south and are completely dependent on cold water currents, none of
which ever cross the equator.  The equator thus serves as a barrier to
penguin radiation northward.  If penguins or their close ancestors once
lived and bred in the north, this is a questionable hypothesis.  But this
depends on the status of _Tonsala hildegardae_ and _Copeptryx titan_.
2) Southern continents have fewer and/or less effective predators.  This
enables the existence of these highly vulnerable species (they are the
only obligatory terrestrial egg-laying species that depends entirely on
the sea for its food--true?).  Antarctica and NZ have no land-based
predators (I'm excluding avian predators because they are roughly
constant for N. and S.hemispheres).  With a couple of exceptions, other
southern continental breeding sites are on offshore or off-continent
islands.  In the northern hemisphere, the coast of Asia, Europe, and
North America, all possess effective predators.  However, there are also
off-shore islands in the north.  This hypothesis must account for penguin
absence here as well.  And so...another hypothesis: during ice ages,
northern ice sheets extend to islands off those continents and allow
predator access from source continents.  Southern ice sheets must extend
further to reach continents.  This provides an isolated refuge that
(since the Miocene????) has been consistently available through
evolutionary time.  In times of warming, marine access to predatorless
terrestrial continental regions persists in the south (i.e., Antarctica
has inhospitable terrain further south than northern hemisphere islands
are north).  In this respect, I would like to invoke seals and co.  Is it
possible that seals and penguins are in direct competition for breeding
sites, but that penguins parents, eggs and chicks can tolerate harsher
conditions than seal neonates?

To encapsulate my favored hypothesis: In terms of evolutionary time,
penguin habitat is ephemeral in the Nth.  This is reflected both in the
lack of diversity in penguin analogues (e.g., the Great Auk), and in the
absence of northern radiation of penguins themselves.  And the global
distribution of seals is a problem for the equatorial barrier
hypothesis.  It assumes that the swimming, foraging, and navigational
abilities of aquatic dinosaurs are inferior to that of sea-going
mammals: seals are able to cross the equatorial barrier, penguins are
not.  This hypothesis assumes that mammals are superior to
dinosaurs.  Such an absurd claim should be rejected out of hand.  Can I
get a witness?