[Taxacom] pre-drift vicariance

John Grehan calabar.john at gmail.com
Tue Dec 20 23:14:22 CST 2016

One of the important lessons presented in *Biogeography and Evolution in
New Zealand* is that there can be numerous examples illustrating vicariance
that took place before ‘continental’ drift. There is a tendency to think of
vicariance as something related to the displacement of continents, but the
Mesozoic continents were not stable entities and were subject to pre-drift
extension and other geological process that continue to leave their mark on
modern day distributions. An example below:

“The plant family Loganiaceae includes an interesting case involving
endemism around the Paparoa and Papahaua Ranges, and also trans-Tasman
disjunction (Gibbons et al., 2014). In this example, *Schizacme helmsii*
(Loganiaceae) is endemic to the northern and southern Paparoa Range,
Denniston Plateau, and Stockton Plateau. It is sister to the two Australian
species of *Schizacme* (found in Tasmania and Victoria) rather than the two
other New Zealand species. *Schizacme* as a whole is sister to *Phyllangium*,
and the two genera have the following distributions (Fig. 7.6a, from
Gibbons et al., 2014; GBIF, 2016):

*Schizacme*: New Zealand, W Tasmania, and Victoria.

*Phyllangium*: SW Australia, SE Australia and E. Tasmania, and NE

The two genera show a high level of allopatry, and any geographic overlap
is restricted to local areas of Tasmania and Victoria. Gibbons et al.
(2014) concluded: “The phylogenies inferred in this study suggest
*Schizacme* might have originated in New Zealand as a result of long
distance dispersal of a common ancestor of *Schizacme* and *Phyllangium*
from Australia to New Zealand [followed by dispersal of Schizacme back to
Australia]”. Nevertheless, the phylogenies are also consistent with an
origin of the two genera by vicariance. This would have taken place in a
widespread, prebreakup, Australia-New Zealand ancestor somewhere near the
Great Dividing Range of Australia, followed by local overlap within
Victoria and Tasmania. A vicariance model for *Phyllangium* and *Schizacme*
predicts that the pattern should be repeated in other groups, and this is
seen, for example, in *Monotoca *and its sister *Montitega *(Ericaceae)
(Figure 7.6b; Heads, 2014).

The primary phylogenetic break in *Schizacme* is not at the Tasman Sea, but
between the clade of Victoria, Tasmania, Paparoa Range, Denniston Plateau,
and Stockton Plateau on one hand and the clade of western South Island and
Stewart Island on the other. There is minor overlap only in the northern
Paparoa Range (Gibbons et al., 2014). Thus, the main break in *Schizacme*,
somewhere around the Paparoa Range, had taken place before the break in the
Tasman Basin rifted the Australia-Paparoa Range clade apart. The latter
event occurred at 85 Ma, with the opening of the Tasman, suggesting that
the former event occurred with the extension and uplift of the Paparoa
metamorphic core complex at ~100 Ma.”

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