MYSTICETE EVOLUTION

[<darren.naish@port.ac.uk>]

The idea that mysticete evolution is tied to the opening of Circum-
Antarctic current was first proposed by Ewan Fordyce in 1981 or 
thereabouts. I suspect Carl Zimmer's discussion is based on this work.

Christian Kammerer wrote (re: southern origin of mysticetes)...

> I've read this too, and while it's an interesting idea, I'm quite
> skeptical. If this were the case, one would expect the most primitive
> mysticetes to be found in the Southern Hemisphere, but with only a few
> exceptions 

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), is from New Zealand. Basal toothed aetiocetid mysticetes 
apparently soon spread to the northern Pacific however. Cetotheres 
(almost certainly a paraphyletic grade - see work by Luo and Geisler) 
first appeared in the Oligocene and became increasingly widespread 
during the Miocene: they are toothless and clearly more closely related 
to crown-group mysticetes than to most basal forms. Even so, IIRC the 
earliest cetotheres are from New Zealand.

There are a stack of papers that address these issues but I do not have 
the refs to hand. I think that a southern origin is most likely based on 
the evidence to hand, but I would agree that it is not decisive and there 
are several potential problems, e.g. _Chrysocetus_, the dorudontid 
most recently posited as morphologically closest to odontocetes + 
mysticetes, is from N. America. 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. Stig Walsh and I 
recently addressed this in a paper on the evolution of monachine seals - 
it seems that pretty much as soon as they evolved, they swam all over 
the place (incidentally, this paper is proof that I am not a complete 
austrophile - we favoured multiple invasions from the north rather 
than endemic southernism as proposed by... Ewan Fordyce).

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._. The NZ cryptoclidid has skin preserved BTW...

DARREN NAISH 
PALAEOBIOLOGY RESEARCH GROUP
School of Earth & Environmental Sciences
UNIVERSITY OF PORTSMOUTH
Burnaby Building
Burnaby Road                           email: darren.naish@port.ac.uk
Portsmouth UK                          tel (mobile): 0776 1372651     
P01 3QL                                tel (office): 023 92842244
                                       www.palaeobiology.co.uk