Thursday 27 November 2014

Fossil Coryphoid Palm leaves from the Deccan Intertrappean beds of Madhya Pradesh, India.

Palms are an important component of modern tropical ecosystems, with the majority of species (~90%) restricted to tropical rainforests, where they are important understory plants. Palms reach their maximum diversity today in Asia (over 1200 species) and the Americas (about 730 species), but are much less diverse in Africa (about 65 species, less than Madagascar), with only one species native to Europe, though it is thought that they were more diverse in these areas prior to the cooling and aridification of the Plio-Pleistocene. Palms are poorly adapted to cooler or drier climates, as they have large evergreen leaves and are incapable of going through dormant periods like many plants, so that they tend to require high levels of sunlight and water all year round and are unable to cope with frost or snow. However one group within the Palms, the Coryphoideae, is more tolerant of cooler and drier conditions than others, with many species found in arid or warm temperate climates.

In a paper published in the journal PLoS One on 13 November 2014, Rashmi Srivastava and Gaurav Srivastava of the Cenozoic Palaeoflorist Laboratory at the Birbal Sahni Institute of Palaeobotany and David Dilcher of the Department of Geology at Indiana University describe a new species of fossil Coryphoid Palm leaves from the Deccan Intertrappean beds of Madhya Pradesh, India.

The Deccan Traps are a massive are of flood basalts covering about 500 000 km2 of modern India, and thought to have originally covered around 1 500 000 km2 of what is now India and the western Indian Ocean. These basalts were produced by extensive volcanic eruptions that began about 69 million years ago and persisted to about 61 million years ago, peaking between 67 and 65 million years ago. The timing of this volcanic activity leads to the inevitable conclusion that it must have played a role in the End Cretaceous Extinction, about 65.5 million years ago, although scientists differ in the weight they give to this and the major impact event that took place at Chixulub (check) on the Yucatan Peninsula at the end of the Cretaceous as causes of this extinction.

The Deccan Intertrappean beds are layers of sedimentary rocks between different layers of basalts within the Deccan Traps. These often represent highly fossiliferous lacustrine (lake) and fluvial (river) environments, which produce large numbers of plant fossils. Clearly these fossils present a potential wealth of knowledge for understanding extinction patterns at the end of the Cretaceous, although interpreting this has proven to be difficult, as there appears to be no direct correlation between episodes of volcanism and plant extinctions.

 Map of India showing fossil locality. (A) Map of India showing extent of Deccan traps. (B) High resolution map showing the fossil locality. Srivastava et al. (2014).

The Palm leaves described by Srivastavaet al. are place in the organ genusSabalites (in palaeobotany an organ genus is applied to a part of an extinct plant, such as a leaf or a root, in the accepted knowledge that other parts of the plant may be named separately; this is because whole, intact plant fossils are a rarity, and palaeobotanists need to be able to name the separate organs to havereference points for other work), and given the specific name dindoriensis, meaning ‘from Dindori’; they were collected from the Ghughua Fossil National Park in the Dindori District of Madhya Pradesh. These beds are thought to be from the end of the Cretaceous to the beginning of the Palaeocene (Maastrichtian–Danian).

Sabalites dindoriensis.(A) Basal portion showing thick costa. (B) Drawing of the same fossil. (C) Middle portion of the fossil leaf showing leaf segments attached to costa. Srivastava et al. (2014).

The species is described from five specimens, the largest of which is roughly 45 x 13.5 cm. None of these specimens are of whole leaves, with one specimen (the largest) showing part of the base of the leaf and the stem, two from the middle part of the leaf and two from the tip.

Sabalites dindoriensis.(A) Specimen seems to be of apical portion showing faint impressions ofrachilla like structure (white arrows). (B) Enlarged portion of the same specimen showing rachilla like structure (white arrows). (C) Specimen seems to be of middle portion. (D) Enlarged portion showing high order venation. Srivastava et al. (2014).

The first Palms are thought to have appeared in Laurasia (Eurasia and North America) about 100 million years ago, with the Coryphoids originating about 87 million years ago. India has a high number of endemic Palm species today, but was an island continent in the Late Cretaceous, and it has generally been thought that Palms first reached India in the Miocene, when it began to collide with Eurasia. The presence of Palm fossils in the Deccan Intertrappean beds clearly indicates that this was not the case, and instead Srivastava et al. suggest an alternative scenario, in which Palms dispersed from Europe into Africa and then across the (narrower) Indian Ocean to India by the end of the Cretaceous.

Palaeogeographic map at 65.5 Ma showing possible dispersal path of Coryphoideae from Europe to India via Africa (red broken line). Srivastava et al. (2014).

See also…

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