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Barbourula kalimantanensis Photo from WWF International, © David Bickford

The frog above is the Barbourula kalimantanesis, endangered and can be found only in the remote forest of Kalimantan in the Indonesian part of Borneo.  This frog is special because it has no lungs and breaths only through its skin [1].  While the frog below is Barbourula busuangensis, its cousin.  It is classified as vulnerable and can be found only in scattered places in Palawan.

Barbourula busuangensis (wiki commons)

Recently, a team of scientists from the US, Singapore, Indonesia, and the Philippines studied these frogs and their very distant relatives from mainland Eurasia to try to gain insights on species diversification in Southeast Asia that is much older than the Pleistocene period (2.588 million to 12,000 years before present or the land bridge period).

As they write in their paper in PLoS ONE this August [2], they have uncovered the oldest estimate of cladogenic event (evolutionary splitting) with still living species, that is consistent with a relatively ignored biogeographic hypothesis, the Palawan Ark Hypothesis.

The Palawan Ark Hypothesis says that because of the geological history of Palawan, it acts as a place or a raft (as in Noah’s Ark)  in which species from mainland Eurasia where able to go to Sundaland (present day Borneo, Sumatra, Malay peninsula, and Java).  This is different from the ‘usual’ diversification of animals via land bridge from Sundaland to Palawan.  The authors however say that this does not mean that the Palawan Ark Hypothesis happens for all species but it is an alternative route for some species. They said the Barbourula frogs are the best example.

The history of these frogs starts from its separation from the Bombina in the Paleogene period (~50 million years ago (mya)) according to the paper [2].  The separation is congruent with the rifting of the northern Palawan from the Asian continent 32  to 17 mya by the opening of the South China sea. At less than 15 mya, the frogs were able to migrate to Borneo via land bridge. During this time, Palawan is closest to Borneo. This agrees with the divergence of the two species of Barbourula in the Late Miocene (12~5 mya).  The Barbourula has been part of the Sundaland only about 10 mya.

The authors used a combo of tests: mitochondrial and nuclear genetic data, multiple fossil calibration points, and likelihood and Bayesian methods.

Palawan ark hypothesis and the Barbourula frogs. The pale arrows show the opening of the South China Sea while the pale red is the continental margin. From D. Bickford, DOI:10.1016/j.cub.2008.03.010.

Wouldn’t it be possible that the frogs migrated from the Sundaland to Palawan? According to the authors, yes it is but their evidences suggest that if this is the case, the Barbourula is  a ‘‘relic’’ which became extinct throughout the Sundaland.  Also, it will not be able to explain why there is an intraspecific divergence (estimated to be 0.7 mya) within B. busuangensis, and why there is limited area of  distribution and apparent lack of diversity for B. kalimantanesis.

The geological history of Palawan is very complex.  This maybe the reason why there is a very high level of species endemicity in Palawan according to the author. This they say, suggests that northern Palawan may have been “connected” to the mainland Asia via small island hops or is connected by a land bridge, and then for a long time become an oceanic island before the uplift of southern Palawan producing a virtual land bridge to northern Borneo.

And this is just Palawan.  What more with the colonialization of oceanic Philippine islands?

[1] D. Bickford, D. Iskandar and A.  Barlian, “A lungless frog discovered on Borneo”, Current Biology 18 R374–R375  (2008). DOI:10.1016/j.cub.2008.03.010.

[2] D. Blackburn, D. Bickford, A. Diesmos, D. Iskandar, R. Brown, “An Ancient Origin for the Enigmatic Flat-Headed Frogs (Bombinatoridae: Barbourula) from the Islands of Southeast Asia,” PLoS ONE August 5 e12090 (2010). DOI: 10.1371/journal.pone.0012090.