[Taxacom] Overdoing vicariance again? (tree shrews)

[Kenneth Kinman]

Dear All,

       Heads (2012) says "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."  And he makes reference to a "widespread proto-Scandentia".

      Although I would agree with Heads that there was some vicariance involved in the evolution of tree shrews, I believe dispersal was the dominant factor (dispersal over land or temporary land bridges, but no evidence of any transoceanic dispersal in this case).  And I especially see no compelling evidence of a "widespread proto-Scandentia" splitting up as a "simple vicariance".  A more limited center of origin followed by dispersals seems more probable (and also more interesting).

       It looks like tree shrews most likely had their center of origin in or near the area of Myanmar or Yunnan Province (China).  The oldest known fossil tree shrew (Ptilocercus kylin of the Early Oligocene) was recently discovered in Yunnan (note: whether or not the older Eodendrogale fossil is a tree shrew is controversial).  The dispersal of Ptilocercus southward towards Borneo could very well have begun early in the Cenozoic since it split off so early.  It was presumably well-established across a large area before those other genera began to split away from one another.   Dendrogale and Tupaia would have expanded southward much later.  Anathana instead dispersed westward into India.

      Therefore, the present distribution of the basal genus Ptilocercus is probably just a relict of a much broader distribution before genus Tupaia later won the competition for much of that broad territory.  Dendrogale no doubt lost territory to Tupaia as well.   Anathana met no such competition once it expanded into southern India (although it was perhaps competition that eliminated it from the territory where it originated).  Lucky break for Anathana that India had crashed into Asia earlier.    Tupaia vs. Ptilocercus (and Dendrogale) have probably managed to coexist where they do because Ptilocercus and Dendrogale are more arboreal than Tupaia, and Ptilocercus is the only tree shrew that is nocturnal.

      So I don't see any need to invoke "simple vicariance" (much less a widespread proto-Scandentia) in a case where there was a combination of both vicariance and dispersal (perhaps a lot more of the latter than the former).  As with primates, I think that male aggression could have played a role in Tupaia's expansion at the expense of Ptilocercus and Dendrogale.  But in the case of tree shrews, it was the younger genus Tupaia that largely prevailed, probably because it dispersed over land (not a chance dispersal over water).  And also the fact that Tupaia species are mostly larger and heavier (and have longer, sharper claws) would tend to make them physically dominant.  I would predict that carefully designed studies would provide evidence of this,  But unfortunately, the research that has been done regarding Tupaia aggression has been on conspecific males.  Research on Tupaia males vs. Ptilocercus or Dendrogale males would be more helpful and interesting.

                        ---------------Ken

P.S.   Anyway, I found Roberts et al. (2011) to be much more informative, so here is a weblink to that paper  ( http://linkolson.org/research/publications/My%20pubs/Roberts%20et%20al.%202011.pdf ).

And here is a weblink to the paper describing the earliest known fossil tree shrew:  https://www.nature.com/articles/srep18627

________________________________
From: Taxacom <taxacom-bounces at mailman.nhm.ku.edu> on behalf of John Grehan <calabar.john at gmail.com>
Sent: Tuesday, June 26, 2018 3:24 PM
To: taxacom
Subject: [Taxacom] tree shrew vicariance and tectonics

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.
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