Letter to the Editor
Geological data indicate that the interpretation for the age-calibrated phylogeny for the Kurixalus-genus frogs of South, South-east and East Asia (Lv et al., 2018) needs to be rethought

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Abstract

Recently, Lv et al. (2018) published an age-calibrated phylogenetic tree for the Kurixalus frogs, members of which occur across parts of South, South-east and East Asia. A clade on Taiwan, represented by Kurixalus idiootocus and the Kurixalus eiffingeri species complex, is deemed to have been resident since the middle Cenozoic; its closest congeners are in southern Indochina (not in the adjacent parts of south-east China), and the split between the two is dated at 32.8 Ma. Furthermore, a sub-population of Kurixalus eiffingeri is believed to have colonized islands in the western Ryukyus c. 13.5 Ma. There is, however, a problem with this scenario: the landmass regarded as modern-day Taiwan has existed only for 4–5 million years (it results from a young and ongoing tectonic-plate collision). Assuming the Kurixalus phylogeny and the dating of its branchings are correct, then a palaeobiogeographical scenario involving an older, alternative land surface with later transfer to Taiwan, possibly involving over-water dispersal, would reconcile the biology, but testing this may be difficult/impossible. If the ages of the nodes in the proposed tree are found to be significantly overestimated, the geology and biology might more easily be accommodated.

Introduction

Kurixalus is a monophyletic genus of rhacophorid frogs (mainly tree dwellers) whose sixteen constituent species inhabit a broad swath encompassing the eastern Himalayas, Myanmar, Indochina, southern China, Sundaland, the Philippines, Taiwan, and Japan’s Ryukyu islands. Recently, Lv et al. (2018) presented a dated molecular phylogeny for the group based on a synthesis of sub-regionally focused investigations (Frost et al., 2006; Li et al., 2008, Li et al., 2009, Li et al., 2013; Yu et al., 2010, Yu et al., 2013; Rowley et al., 2011; Hertwig et al., 2013; Nguyen et al., 2014a, Nguyen et al., 2014b). Following their analysis, Lv et al. (2018) proposed that the genus originated in southern Indochina-Taiwan c. 36.8 Ma (Late Eocene). The deepest split, estimated at 32.8 Ma (Early Oligocene), produced lineages ancestral to clades now separated by a great circle distance ≥1,400 km. Interestingly this scenario is congruent, at least superficially, with the well-established geological model for the formation of the South China Sea (Briais et al., 1993; Hall, 2012; Sibuet et al., 2016). At about 32 Ma the Palawan block (c. 100-km-wide, 650-km-long) rifted from southern China; sea-floor spreading until sometime between 20 Ma (Early Miocene) and 15 Ma (Middle Miocene) created the basin as the terrane drifted c. 900 km south to its present location to the north-east of Borneo and to the west of the main Philippine archipelago. The fragment, when it was attached to southern China, is thought by Lv et al. (2018) to have formed part of a land surface that linked Taiwan to Indochina.

On Taiwan, two sister species of Kurixalus are recognized: Kurixalus idiootocus and the Kurixalus eiffingeri species complex. Various types of information (morphologies of the eggs, tadpoles and adults; mating calls; molecular data; ecology etc) led Wu et al. (2016) to propose that the latter comprises three forms: K. eiffingeri, Kurixalus berylliniris and Kurixalus wangi, with each occupying discrete geographical areas within the lowlands and foothills (up to 1,000 m) that ring Taiwan’s mountainous east (Wu et al., 2016). Lv et al. (2018) estimated the separation between K. idiootocus and the K. eiffingeri species complex as c. 22.0 Ma (Early Miocene). Populations of K. eiffingeri are also present on Iriomote and Ishigaki at the south-west end of Japan’s Ryukyu chain. The two islands sit atop a shallow platform and are respectively 180 km and 220 km east of northern Taiwan; this colonization was dated at c. 13.5 Ma (Middle Miocene; Lv et al., 2018).

Section snippets

Taiwan Island is considered to be just 4–5 million years old

The scenario presented by Lv et al. (2018) is, however, problematic because Taiwan is a young landmass that became emergent only 4–5 million years ago (Early Pliocene). The uprising results from spectacular crustal thickening due to the Luzon Arc on the western Philippine Sea Plate colliding with and overriding the passive margin of the Eurasian continent off south-east China (e.g. Teng, 1990; Sibuet and Hsu, 2004); in a newly proposed scheme for classifying marine landmass types, Taiwan is

Problems with the Lv et al. (2018) proposal

I therefore contend that there are major issues with the scenario proposed by Lv et al. (2018). One solution invokes another landmass, perhaps in the Ryukyu chain or elsewhere, with later transfer to Taiwan (Nishizawa et al., 2011, explored this briefly in explaining the 26.3 Ma [Late Oligocene] branching between Buergeria buergeri in mainland Japan and its sister species, Buergeria robusta, in Taiwan). However, this would be highly speculative and possibly not testable, either biologically

Other relevant information

Evolutionary features exhibited by Taiwan’s other land-locked vertebrates are also pertinent to the discussion, in particular evidence for other “old” lineages. As a consequence of the island having shallow connections to mainland Asia (70 m below the present-day sea-level datum; verified using Ryan et al., 2009), the sizeable assemblage is dominated by non-endemics (e.g. Xiang & Li, 2000; IUCN Red List, 2018; Lue, 2018; Taiwan Biodiversity National Information Network, 2018) that likely

Summary

In view of Taiwan’s considerable area and ecological-niche range, which should facilitate biodiversity expansion given a reasonable amount of geological time (say, 10–20 million years), several features of its native biota offer strong support to the young-landmass model. Firstly, although the land-locked vertebrate suite comprises at least 135 native species, the proportion of exclusives is relatively low. Secondly, those endemic species that have emerged have done so largely within the last

Acknowledgements

Formal critiques provided by Sebastian Klaus and two anonymous reviewers helped improve the document. Allan Larson is thanked for his editorial guidance.

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