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Eocene out-of-India dispersal of Asian dipterocarps

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Review of Palaeobotany and Palynology

Research papers

Eocene out-of-India dispersal of Asian dipterocarps

Abstract

The Dipterocarpaceae, a well known and economically important family of trees of the tropical rain forests of Asia, comprise over 470 species. These angiosperm trees contribute to 30% of the total area in typical lowland evergreen forests in Southeast Asia. Despite their remarkable diversity and regional ecological dominance, the origins and phytogeographical evolution of the family are poorly understood. The earliest dipterocarp fossils recorded in SE Asia come from Oligocene (34–23

Ma) sediments of Borneo. Here, we report an occurrence of Asian dipterocarps from approximately 53

 

Ma old sediments from western India based on fossil resin chemistry and palynological data. An important implication of our finding is that Asian dipterocarps must have originated in Gondwana and dispersed from India into Asia once the land connection between the Indian and Asian plate was well established during the middle Eocene (49–41

 

Ma). Moreover, the present study supports the hypothesis which suggests that many angiosperms did not originate in the SE Asian region, but dispersed into the area from western Gondwanaland.

Research highlights

► Dipterocarpaceae contribute to 30% of the total area in lowland evergreen forests in Southeast Asia. ► The earliest dipterocarp fossils recorded in SE Asia come from Oligocene. ► We report an occurrence of dipterocarps from 53 Ma sediments of western India based on resin chemistry and pollen data. ► Asian dipterocarps were originated in Gondwana and dispersed out-of-India into Asia during the middle Eocene.

Introduction

The Dipterocarpaceae comprise large trees that dominate the canopy of lowland equatorial forests. They typically contribute to 30% of the total area in lowland evergreen forests in Southeast Asia (Aiba and Kitayama 1999) and play a dominant role in Asian rain forest ecology (Ashton, 1982, Ashton, 1988). There is no equivalent elsewhere in the tropics of a single, highly diverse, family dominating large-tree forests over such large areas (Corlett 2007). The Dipterocarpaceae comprise three subfamilies: the Dipterocarpoideae in Asia, the Pakaraimoideae in South America and the Monotoideae in Africa. There are approximately 520 species in 17 genera amongst which the Asian dipterocarps include 470 species alone. This family is well known as one of the major sources of valuable commercial hardwood timber while its resins and leaves contribute significantly to the rural economies of SE Asia. Two opposing hypotheses have been proposed to explain the origin of the Asian dipterocarps. Some hypothesize that the family originated in Southeast Asia, most probably from West Malaysia in the late Mesozoic (Lakhanpal, 1970, Sasaki, 2006) and migrated into India during the late Cenozoic Era. The occurrence of bicadinanes diagnostic of Dipterocarpaceae resins in late Cenozoic fluvio-deltaic oils from across SE Asia represented a further connection with this age (Stout 1995). Others suggest that Dipterocarpaceae have a Gondwanan origin and reached Asia by rafting on the Indian plate (Ashton, 1982, Ashton and Gunatilleke, 1987, Dayanandan et al., 1999, Ducousso et al., 2004). Here, we report the occurrence of Dipterocarpaceae from more than 53

 

Ma old sediments from India based on fossil resin chemistry and palynological data. Our findings suggest that Asian dipterocarps migrated from India into Asia as the land connection between the Indian and the Asian plate was established at ca. 50

 

Ma ago (Scotese et al., 1988, Rowley, 1996).

Section snippets

Samples

Fossil resins, lignite and carbonaceous shale samples were collected from the Vastan lignite mine (21°25′47″ N: 73°07′30″ E) Cambay Basin, western India (Fig. 1). The rock strata are referred to as the Cambay Formation. The studied section is 30

 

m thick and is composed of shale, clay, calcareous clay and lignite beds. The sediments of the lower half of the Vastan lignite mine section were deposited in estuarine to lagoonal environments whereas the upper half was deposited under shallow marine

Results

We recovered well-preserved macroscopic pieces of fossil resin from 53

 

Ma old sediments in the Vastan lignite mine (Sahni et al., 2006, Punekar and Saraswati, 2010). We previously reported cadinane and bicadinane distributions in a preliminary study of the Vastan resin (Mallick et al. 2009). Here we examine this chemistry including a comparison with extant dammar resin. Both fossil and extant resins were analyzed by pyrolysis-gas chromatography-mass spectrometry (Py–GC–MS) and the major products

Discussion and conclusions

In India, the most prolific fossil records of Dipterocarpaceae are found from the Miocene onwards (Prasad, 1993, Guleria, 1996, Khan and Bera, 2010). Some researchers hypothesised that the Dipterocarps migrated into India from SE Asia once the land connection between Indian and Asian plate was established (Lakhanpal, 1970, Sasaki, 2006). This proposition is supported by the remarkable species richness of the family in SE Asia. In contrast, the phylogenetic studies comparing Dipterocarpaceae and

Acknowledgements

DST (SR/FTP/ES-19/2008) is acknowledged for providing financial support to S. Dutta. Authors are thankful to Peter Ashton for valuable discussions. Work at MIT was supported by the NASA Exobiology Program (NNX09AM88G). C. Colonero is acknowledged for providing technical support during resin analyses. S. Bera is acknowledged for providing the modern pollen of Dipterocarpus indicus.

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