millipede biogeography and "Pacifica"

[John Grehan]

I was interested to read the millipede review by Bill Shear in American
Scientist 89
in which he noted that the geographic distribution of the
Heterochordeumatoidea along the
edge of the Pacifc rim fitted well with the notion of a lost Pacific
continent. He stated that
this idea was first proposed by Nur and Ben Avraham in 1977. This is
actually not quite
correct.

Nur and Ben Avraham stated that the concept of a "Pacifica" was first
introduced by
biogeographers. They cited Melville (1966), Mc Gowran (1973), Croizat
(1958) and Martin
(1976). Croizat stands out, therefore, as possibly the earliest originator
of an explict
"Pacifica" model, particularly in his 1961 book where he presents this in
the context
of continental drift. Croizat's model is even more significant in that it
made explicit predictions
on the composte tectonic structure of the Americas - a fact that has since
received geological
corroboration.

Shear noted in an earlier TAXACOM posting that "Mayr's contributions to
evolutionary biology
and to the history of biology need no defence. They will remain standard
works when Croizat
is a mere footnote". Interestingly, when it comes to Pacific millipedes,
Shear gives (indirectly)
more credence to Croizat's biogeography than to Mayrian biogeography. By
asserting a possible connection between Pacific tectonics and millipede
distribution Shear is invoking the panbiogeographic methodological
principles of main massing, minimum spanning distance, and baseline. All
three concepts are necessary to assert a Pacific history for this kind of
distribution. So perhaps a footnote, but nevertheless perhaps a very
significant footnote indeed. And as for Shear's article, it shows that he
is not afraid to step outside conventional boundaries in biogeography. I
applaud.

On a final note, Shear also comments on the importance of earthworms in
biogeography. The
pertiochaetine earthworms also show a Pacific pattern, being widely
distributed in east Asia-
Australasia and western Columbia/Ecuador.

John Grehan