[Taxacom] tree shrew vicariance and tectonics

[John Grehan]

Below and excerpt (allowing for typos) from Heads (2012) for those curious
about the relationship between vicariance and tectonics. A nice
illustrative example for the Scandentia (tree shrews) showing patterns of
vicariance and dispersal (sympatry) in tree shew distribution and phylogeny
show tectonic concordance and evidence of primary allopatry as the result
of vicariance. Sorry not to be able to reproduce the map here.

"The Southeast Asian order Scandentia (Fig. 5-17) is closely related to
primates (Fig. 5-1). In the main clade of Scandentia (Olsonet al., 20005),
Anathana (Indian Plate) is allopatric with its sister group Tupaia
(Eurasian plates) + Urogale (Philippine mobile belt). These breaks
correlated with the India/Eurasia plate boundary and the Eurasia
plate/Philippine mobile belt boundary. Olsen et al. (2005: 666) wrote that
the circumstances leading to the disjunction between Anathana and the other
genera across the Bay of Bengal “remain a mystery fo which our results
offer no clear explanation.” The same break occurs in lorisid primates
(Fig. 5-18) and could be the result of vicariance. As Olson et al. (2005:
668) noted, the absence of Scandentia from such proximatee islands as the
Andamans “suggests severe limitations to overwater dispersal [and]
vicariance can almost certainly be assumed to have played a prominent role
in the past diversification and resulting distribution of tree shrews.” The
same conclusion is reached here for primates.

The two basal clades in the Scandentia, Ptilocercus and Dendrogale, have
different overall distributions but meet and overlap in Borneo. The main
clades in the widespread Tupaia also have their main breaks in Borneo:

Tupaia group 1: Borneo species basal to others which range to Nepal.
Tupaia group 2: Borneo species basal to others which range to Thailand>
Tupaia group 3: (T. gracilis): Borneo.

Although these phylogenetic breaks all involve Borneo, this does not
necessarily imply a center of origin there. (Likewise, the oldest fossils
of Scandentia are found in Thailand, but this does not necessarily mean the
area was a center of origin). Borneo is a geological composite, and several
clades may have been juxtaposed there with terrane accretion of
differentiated during accretion. Thus Borneo – or rather the terranes that
became Borneo – could represent sites of differentiation in an already
widespread proto-Scandentia. The distribution of Ptilocercus is centered on
the Riau (Riouw) Islands region (off Singapore), while that of Dendrogale
is based further north around the central South China Sea, and the two are
almost completely allopatric. (The South China Sea basins opened with the
Late Cretaceous and Cenozoic rifting of continental crust). The allopatry
of the two genera suggests early zones of differentiation around the two
regions before the opening of the South China Sea and before the plate
boundary breaks in Anathana/Tupaia/Urogale. Within Borneo, Ptilocerus and
Dendrogale are restricted to the north and northwest of the island (north
of the Lupar line, cf. Heads, 2003, and west of the Mangkalihat terrane).
Within this region they remain largely allopatric. Tupaia has complex
diversity in different parts of Borneo, including the south and east, and
so perhaps this was its original sector in “Borneo”. When the terranes of
Borneo were juxtaposed, so were the genera. Tupaia has expanded its range
to overlap that of the two basal genera, and the two basal genera
themselves show minor overlap, but apart from this, the biogeography of the
Scandentia genera reflects their original evolution by simple vicariance.