Re: MYSTICETE EVOLUTION

[christian farrell kammerer <cfkammer@midway.uchicago.edu>]

Darren, 
Thanks for pushing this thread back on topic. You've made some excellent
points, which I'll try to address as best I can.

On Wed, 1 Aug 2001 darren.naish@port.ac.uk wrote:
> The most basal mysticetes do have a strong 'southern signal', e.g. 
> _Mammalodon_, perhaps the most basal mysticete known (and 
> originally regarded, despite its early Oligocene age, as an archaeocete-
> grade form)

I'm not arguing against a southern origin for mysticetes--there is indeed
strong evidence there for the earliest members of the group (in addition
to Mammalodon, you have things like Kekenodon from N.Z., and if Llanocetus
is a mysticete, well, it's from the Eocene of Antarctica! Can't get a
stronger "southern signal" than that...). What I'm arguing against is a
southern origin for the balaeniferous (still don't know if that's a
word) mysticetes, and the Antarctic "catalyst" for their evolution and
diversification. The aetiocetids, kekenodontids and such had some pretty
impressive teeth for mysticetes, so the cold water krill-bloom wouldn't
explain their diversity (Well, since these whales were likely still
piscivores, it could indirectly). 

> Even so, IIRC the
> earliest cetotheres are from New Zealand.

You must mean Mauicetus, which is indeed one of the "exceptions" I was
talking about when I mentioned a paucity of southern cetotheres. It is
indeed one of the earliest cetotheres, being Late Oligocene if I'm not
mistaken. However, I'd counter with the L. Oligocene presence of
Cetotheriopsis in Europe, as well as with the supposed presence of
Mauicetus itself in Asia. 

> Cetotheres 
> (almost certainly a paraphyletic grade - see work by Luo and Geisler)

Indeed. In fact, the paraphyly of the "Cetotheriidae" is central to my
argument. Let's see, we have 30-odd genera of Northern Hemisphere
cetotheres, as opposed to the aforementione Mauicetus plus Piscocetus and
Piscobalaena in Peru (which are Pliocene, well after the diversification
of the extant mysticete families) and Aglaocetus from the Miocene of South
America and Australia. Now, if the cetotheres were monophyletic then it
could be argued that they were a mostly northern branch of a mainly
southern radiation. However, given their paraphyly, isn't it
unparsimonious to suppose that each cetothere "on up the tree" (so to
speak) represents a northern invasion from a hypothetical southern
ancestor? Wouldn't the better explanation be that the few southern
representatives are for the most part late invasions from the
north? Furthermore, many of the earliest fossil representatives of the
extant whale families are from the northern hemisphere, and those that are
from the south (e.g. Plesiocetus) have also been found in Europe 
(Notiocetus is an exception to this, admitted.) Why aren't the southern
members of these groups being found, if this was the center of mysticete
diversification? Now, sampling is certainly a problem, as is the ability
of whales to cross oceans with extreme rapidity (which you address below),
but given the existing fossil record, I tend towards equating "high
diversity" with "probable center of radiation." Future finds could
certainly topple this argument, but for the moment, the handful of
southern cetotheres doesn't cut it.
 
> The problem with marine taxa is that, 
> once they evolve the ability to exploit the pelagic realm, they only need 
> a few million years (or less) to cross entire oceans. 

Very true, and in the case of things like whales that are capable of truly
epic migrations, a mere handful could cross an ocean and start a new
population after a few months of travel. Nonetheless, if the cetotheres
were behaving as such, one would expect to find them in equal amounts
cosmopolitanly. Those genera that are known from multiple continents are
usually the trio of North American, Europe, and Asia (e.g. Cetotheriopsis,
Cetotherium proper, Herpetocetus, Mesocetus.) (Aglaocetus is, once again,
the exception to this rule.) Though I maintain that cetothere diversity
(and thus, basal balaenopterid and balaenid) in the southern hemisphere
might have been considerably less than in the northern, the incompleteness
of the fossil record hamstrings a more definite statement on this point.

> While a true Circum-Antarctic current was not established until South 
> American broke away from Antarctica in the (?) Eocene, cool southern 
> hemisphere currents in the Cretaceous may have contributed to the 
> evolution of large (?) filter-feeding cryptoclidids like the new form 
> from New Zealand and South American _Aristonectes_ (which, despite 
> Bardet et al., I still think is a cryptoclidid and not an elasmosaur...). 
> This model was proposed by Fordyce and Cruickshank in _J. African 
> Earth Sci._.

Of course, the feeding in Aristonectes wouldn't be akin to mammal
filter-feeding, correct? It would be more like "sifting" (accursed
reliance on vernacular descriptors!), if I understand correctly.

>The NZ cryptoclidid has skin preserved BTW...

Awesome! I can't wait until this one is described. Is this only the second
Mesozoic skin impression from the Australia-New Zealand area? I know
there's opalized (!!!) ?dinosaur skin known from Lightning Ridge
(beautifully figured on page 85 of _The Black Opal Fossils of Lightning
Ridge_, by the way. If you haven't taken a look at this book yet, it's
worth a gander), but that's all I'm familiar with.

Oh, and in case anyone was interested in that krill evolution ref, its:
Jarman, S.N. 2001. The evolutionary history of krill inferred from nuclear
large subunit rDNA sequence analysis. Biological Journal of the Linnaean
Society. Vol. 73, No. 2.

-Christian Kammerer