Batagur Origins

Batagur Origins:

where did turtles of the genus Batagur originally evolve and how do we know?

 

Prologue

A large river turtle powers its way smoothly against the current of a voluminous river. The temperate climate of its surrounds provide it a rich bounty that make its ability to swim in strong currents pay off. Its river is changing, slowly but surely, as the Indian subcontinent contacts and begins to push against the Eurasian continent. Rivers change course and a section that was previously near the coast is now far inland. Having evolved to deal with river mouth tides and strong currents, our turtle is now in a deep, upriver section. The environment is warm and forests abound.

So may have been the evolution of the shared ancestor of the large river turtles of the genera Batagur and Hardella. 

Today these genera exist, from India, eastward through Myanmar, Thailand, Cambodia, and Vietnam; and south through Malaysia and Indonesia, to Borneo. They are strong swimmers, deep divers, and can tolerate salt water for some time even though they don't actually have specialised salt glands to expel salt from their systems. This is an examination of their evolutionary origins, as a starting point to examine their biogeography, or, an explanation as to how they came to be distributed the way that they are.

Introduction

There has not been any great mystery as to how Batagur came to be distributed through many of Asia's large rivers as it is: the main species that are outside of India and Myanmar are very capable swimmers that are moderately tolerant of life in salt water. But is it a reasonable assumption that Batagur’s evolution began in India? Or could the genus have found its beginnings in Southeast Asia, where they are also well-represented?

I'll use several methods of analysis to illuminate a theory of Batagur's origin. Firstly, current distributions of species within the genus and their richness throughout their range will be assessed. The fossil record of current and extinct species will be viewed. Lastly, the phylogenies of relationships between species discerned from genetic analysis will be examined, together with molecular clock techniques that allow us to provide reasonable estimates of when such splits in relationships occurred, so we can give an idea of when this origin appears to have been and see if the current fossil record agrees.

Biogeographically, India has long been thought to be the origin for the family Geoemydidae as a whole, based largely on the assumption that the longer a lineage is in an area, the more chance it has to speciate. Therefore it follows that 70 Geoemydid species worldwide should be represented by more species in India than elsewhere. India contains 23 species of Geoemydid, which amounts to 23% of the worldwide count. This is more illuminating when you look at numbers of genera as it represents a longer timescale of diversification than just species. Looked at this way, India has representatives of 16 of the 19 Geoemydid genera, which represents 84% of the global genera.

Modern distributions, prior to human-induced population declines, have been reasonably well documented. First to have its description published was Batagur baska (Gray 1830), which ranged from Orissa in India, through the Ganges Delta and the Sundarbans of India and Bangladesh, through to Myanmar. B. dhongoka and B. kachuga were also the subjects of Gray's descriptions from materials sent from India (Gray 1832; Gray 1831, respectively) are confined to the Ganges drainage and stay in the higher reaches than does B. baska. Another riverine species, B. trivittata (Dumeril and Bibron, 1835), is confined to Myanmar in the middle reaches of the Irrawaddy. Beyond the India to Myanmar distribution, we then see B. affinis that was first noted by Cantor in 1847 and ranges from the south of Myanmar (ref) through the west coast of Thailand and Malaysia through Sumatra and historically to Java (Rhodin et al, 2015). B. affinis then continues to range up as a different subspecies, B. a. edwardmollii up through the east coast of Malaysia, probably Thailand, Cambodia, to the Mekong River Delta in Vientam (Bourret, 1931?). Lastly, B. borneoensis (Schlegel and Muller, 1845) is found on the peninsula of Malaysia, Sumatra, and Borneo's west coast.

 

Fossils

Fossils found by Jaekel, Lydekker, and West have variously been attributed to other species but were later synonymised by Karl (1987) and Garbin (2020).

The ranges of B. kachuga and B. dhongoka extend into the fossil sites of the Siwalik hills in Nepal. This is also where older, indeterminate species occur, earlier in their evolution.

Tidaker and Sharma (1985) state that prehistoric remains of Batagur baska indicate that it was widely distributed in the Ganga river system up to the twelfth century A.D.  Fragments of this species have also been recorded from the Mohanjodaro and Harappa sites (Tidaker and Sharma, 1985, p52.)

 

Table 1: Fossil records of Batagur and their close relatives.

Genus	Range
	Holocene	Pleistocene	Pliocene	Miocene
Batagur	Bangladesh, Borneo, Cambodia, India, Malay Peninsular, Myanmar, Nepal, Sumatra, Thailand, Vietnam 4	? Java 2, 5	B. kachuga, B. dhongoka Punjab, India 3	? Kachuga sp. indet. Nepal 1** Cf. Batagur sp. Nepal 1
Pangshura	India,  Bangladesh 4		P. tecta,  Punjab, India 3
Hardella	India 4		Punjab, Perim Island (Mayyum) India 3
Morenia	India 4

Sources:

_1. West et al, 1991

_2. Jaekel, 1911 (in West et al, 1991)

_3. Lydekker, 1889

_4. Iverson, 1992

_5. Karl et al., 2019

 

_**. Possibly also cf. Batagur sp.

 

Phylogenetic analyses

(India-Myanmar)(Southeast Asia). Nomenclature follows current taxonomy, while the original papers may have used other generic names.

Praschag et al 2007

(((B. baska (B. kachuga, B. affinis))(B. dhongoka (B. trivittata, B. borneoensis)))

(Hardella, Pangshura)

This study only included the species shown.

Le et al 2007

            ((((((B. baska, B. kachuga)((B. borneoensis, B. trivittata) B. dhongoka)) 

Hardella) Pangshura) Morenia) Geoclemmys)(Orlitia, Malayemys)

In agreement with Guillon et al 2012 but does not include B. affinis.

Guillon et al 2012.

(((((((B. baska, B. kachuga, B. affinis) (B. dhongoka (B. trivittata, B. borneoensis))) 

Hardella ) Pangshura) Morenia) Geoclemmys)(Orlitia, Malayemys))

4 of the 6 Batagur spp are India-Myanmar natives, as are all of the other genera of the clade including Batagur, Hardella, Pangshura, Morenia, and Geoclemys.

Pereira et al, 2017

(((((B.baska, B.affinis)B.kachuga(B.dhongoka (B.trivittata, B.borneoensis))) 

 Hardella) Pangshura)

From the three phylogenies above, it can be seen that the bulk of species have a range in India to Myanmar. Further, of the Batagur species in Southeast Asia, both are estuarine species, which makes them appear more suited to oceanic dispersal than the fully freshwater species. It would appear from this that the common ancestor of the clade bounded by Batagur and Geoclemys was found in the India-Myanmar region and that B. affinis and B. borneoensis have dispersed since that time. Further, Pereira et al (2017) postulate the relationships which they then overlay on the geological timeline, providing estimated times of divergence as follows:

Clades diverged		Date of divergence (MYBP)	Probable location of divergence
B. baska	B. affinis	12 Mid Miocene (Serravallian)	Oriental (Myanmar/ Thailand)
B. borneoensis	B. trivittata	13 Mid Miocene (Serravallian)	Oriental (Myanmar)
B. baska/ affinis	B. kachuga	14 Mid Miocene (Langhian)	India
B. borneoensis/ trivittata	B. dhongoka	19 Early Miocene (Burdigalian)	India
B. baska/ affinis/ kachuga	B. dhongoka/ borneoensis/ trivittata/	25 Oligocene (Chattian)	India
Batagur	Hardella	30 Oligocene	India

 Conclusion

We can be fairly confident that Batagur originated in India, likely in the Ganges drainage or further north toward Nepal. Current distributions put three of the six species in India; the Ganges drainage specifically. Phylogenetic studies place the lineages outside India peppered among those from within. This, along with the two farthest-dispersed species being estuarine, makes their recent dispersal seem quite obvious. Fossil studies put the oldest Batagur in Nepal, providing further evidence for this centre of origin. Such then, may be the origin of Batagur as my sketch at the beginning. 

Next: how did Batagur disperse and what vicariance events and dispersal boundaries have affected the animals of this genus?

 

Literature Cited

Garbin, R., S. Bandyopadhyay, and W. G. Joyce. 2020. A taxonomic revision of geoemydid turtles from Siwalik-age of India and Pakistan. European Journal of Taxonomy 652:1-67 [E. Vlachos/E. Vlachos]

Guillon, J-M, L. Guéry, V. Hulin, and M. Girondot. 2012. A large phylogeny of turtles (Testudines) using molecular data. Contributions to Zoology, 81 (3) 147-158.

Karl, H.-V. , A. Safi, and H.-D. Philippen. 2019. Evidences of cheloniophagy by early hominid (Homo erectus) during middle of pleistocene from beds of trinil's layers in central Java (Indonesia), with an updated list of trinil's testudines, and a redescription of Duboisemys isoclina (Dubois, 1908). International Journal of Zoological Studies 4(5):73-84. Le, M., McCord, W.P., and Iverson, J.B. 2007. On the paraphyly of the genus Kachuga (Testudines: Geoemydidae). Molecular Phylogenetics and Evolution 45:398–404.

Pereira, A. G., J. Sterli, F. R. R. Moreira, and C. G. Schrago. 2017. Multilocus phylogeny and statistical biogeography clarify the evolutionary history of major lineages of turtles. Molecular Phylogenetics and Evolution 113: 59-66.

Praschag, P., Hundsdörfer, A.K., and Fritz, U. 2007. Phylogeny and taxonomy of endangered South and South-east Asian freshwater turtles elucidated by mtDNA sequence variation (Testudines: Geoemydidae: Batagur, Callagur, Hardella, Kachuga, Pangshura). Zoologica Scripta 36:429–442.