Thursday, 13 November 2014

Diptocarp wood from the northwest of India.


Diptocarps, Dipterocarpaceae, are the dominant trees of modern South and Southeast Asian rainforests, and are also found in South America, Africa and Madagascar. The group reach their maximum diversity today on the island of Bornea, where there are over 280 described species of Diptocarp, but the earliest fossil Diptocarps from Southeast Asia date back only as far as the Oligocene, whereas fossil Diptocarps appear in western India in the Early Eocene.

Modern Diptocarps are now found only in the south and east of India, though they have a fossil record in the north and west of the country. This is thought to be due to the relatively narrow climate tolerance of Diptocarps, which strongly favour areas with high rainfall year round, while many areas of modern India have a monsoonal climate, with a long dry season and a shorter wet season. This monsoon climate is thought to have evolved as a result of uplift in the Himalayas, which in turn has been driven be the collision between the Indian and Eurasian continental plates. The narrow climatic tolerance of Diptocarps makes them very useful in palaeoclimatic reconstructions, which is particularly useful when trying to understand the evolution of the modern monsoonal climate.
 
In a paper published in the Journal of Earth System Science in October 2013, Anumeha Shukla, RC Mehrotra and JS Gularia of the Birbal Sahni Institute ofPalaeobotany describe a series of Diptocarp wood specimens from the Oligocene and Plio-Pleisctocene of northwest India.

The first specimen described is assigned to the species Dipterocarpoxylon jammuense, and comes from the Plio-Pleistocene Raja Shumar Formation near Habur Village in the Jaisalmer District of Rajasthan. Fossil woods of the genus Dipterocarpoxylon are thought to resemble woods of the extant genus Dipterocarpus, which is currently found in the Western Ghats of southern India and Assam in the northeast, as well as in the Andaman Islands, and Southeast Asia as far east as the Philippines, reaching its maximum diversity on the Malaysian Peninsula, Sumatra and Borneo. The species Dipterocarpoxylon jammuense, which has previously been recorded from Late Tertiary and Miocene deposits in the Himalayan region, is thought to resemble the modern Dipterocarpus lowii, which is now found in the Malaysian region.

Dipterocarpoxylon jammuense (a), (b) transverse sections (T.S.) of the fossil showing exclusively solitary vessels and scattered gum canals (marked by arrow), (c) tangential longitudinal section(T.L.S.) of the fossil showing multiseriate rays with long tails (marked by arrow), (d) T.L.S. showing sheath cells onthe flanks of a multiseraite ray (marked by arrows) and (e) radial longitudinal section (R.L.S.) showing heterogeneousray tissue. Shukla et al. (2014).

The second specimen described is assigned to the species Hopenium pondicherriense, and comes from the Miocene Bhumbli Conglomerate at Lakhanka-Mithi-Viri on the east coast of Bhavnagar District in Gujarat. Fossil woods of the genus Hopenium are thought to resemble those of members of the modern genus Hopea, and Hopenium pondicherriense is thought to particularly resemble the wood of Hopea glabra, a medium-sized tree found today in southern India, and Hopea helferi, a large tree found today in the Andaman Islands, Myanmar, Malaysia, Thailand and Cambodia.

Hopenium pondicherriense (a) T.S. of the fossil showing solitary as well as grouping of vessels, (b) T.S. showing tangentially arranged gum canals (marked by arrows), (c) T.S. showing gum canals enclosed in parenchyma bands(marked by arrows), (d), (e) T.L.S. of the fossil showing 3–4 seriate rays with crystalliferous upright cells interspersed amongthe procumbent cells (marked by arrows), and (f) R.L.S. of the fossil showing heterogeneous ray tissues with crystalliferousupright cells (marked by arrows). Shukla et al. (2014).

The third specimen described is assigned to the species Shoreoxylon burmense, and comes from the Early Miocene Kand Formation on the Ratanpurniriver bed at Ratanpor in the Bharuch District of Gujarat. Fossil woods of the genus Shoreoxylon are thought to resemble those of the modern genus Shorea, which is largely confined to rainforests today, and Shoreoxylon burmenseis thought to particularly resemble the wood of the modern Shorea ovalis, which is found in Malaysia. Shoreoxylon burmense has previously been reported from Tertiary deposits in Myanmar and northeast India.

Shoreoxylon burmense (a) T.S. of the fossil showing predominantly solitary vessels and tangentiallyarranged gum canals (marked by arrows), (b) T.S. showing paratracheal parenchyma around the vessels (marked by arrow),(c), (d) T.L.S. of the fossil showing 2–3 seriate rays, (e) R.L.S. showing weakly heterogeneous ray tissue, and (f) R.L.S.showing alternate intervessel pits. Shukla et al. (2014).

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