Tuesday, 11 November 2014

Wood fossils from the Plio-Pleistocene of northwest India with African affinities.


India separated from Africa about 130 million years ago, and was effectively an island continent until its collision with Eurasia in the Middle Cenozoic. Nevertheless the modern flora and fauna of India show strong affinities with that of Africa, and while there was probably some exchange of species between the two landmasses while they were separate islands, it is thought likely that most of this exchange has happened since Africa also collided with Eurasia, about 15 million years ago, creating a land-bridge between the two continents. There is a continuous climatic zone reaching from North Africa to northwest India today, along which species could potentially migrate, but this is a zone of extremely arid conditions, quite unsuitable for many of the wet tropical animals and plants found on both landmasses, suggesting that there must have once been a connecting zone with a moister climate than today.

In a paper published in the journal Alcheringa on 3 August 2012, Anumeha Shukla and R. C. Mehrotra of the Birbal Sahni Institute of Palaeobotany and J. S. Guleria of Lucknow, India, describe a series of mineralised wood specimens from the Plio-Pleistocene of Rajasthan and Gujarat States in northwest India, which show clear affinities to the wood of various tropical African trees. Identifying wood fossils from their gross morphology is usually somewhat hopeless (at best they look like sticks or branches), unlike fossils of leaves or fruit, but it is often possible to make thin sections of such wood, which reveals the distinctive inner structure, a useful diagnostic tool.

The first specimen comes from the Plio-Pleistocene Shumar Formation at Hema Ki Dhani, near Habur in the Jaisalmer District of Rajasthan. It is assigned to the species Baphioxylon dechampsii, which was originally described from Cenozoic wood samples from the Congo in 1970. The wood is thought to resemble closely that of trees and shrubs of the genus Baphia, of which there are about 60 extant members, of which about 25 are found in the Congo Republic, and in particular that of Baphia nitida, the Camwood or African Sandlewood, an understory tree reaching about 9 m in height, from the wet coastal forests of west Central Africa.

Baphioxylon dechampsii. Transverse Section of the fossil wood showing vessel arrangement and pattern of parenchyma bands. Shukla et al. (2012).

Baphia nitida, the Camwood or African Sandlewood. Wikimedia Commons.

The next two samples described come from the Pliocene Sandhan Formation at Vinjhan in the Kachchh District of Gujarat and the Plio-Pleistocene Shumar Formationnear Habur in the Jaisalmer District of Rajasthan. Both are referred to the species Brachystegioxylon premicrophyllum, which was first used to describe specimens from the Cenozoic of the Congo; two other species in the same genus have since been erected, both to describe wood fossils from Africa. The wood of Brachystegioxylon premicrophyllum is thought to resemble that of the modern Miombo tree, Brachystegia microphylla, which gives its name to the Miombo woodlands of Central and Southern Africa. The Miombo is a 12-30 m tree, which grows abundantly in open forests and savannah woodlands from Uganda to Mozambique.

(A) Transverse section of the fossil wood of Brachystegioxylon premicrophyllum showing growth ring limit (marked by arrow), vessel distribution and vasicentric to aliform to locally confluent axial parenchyma. (B) Transverse section of the modern wood of Brachystegia microphylla (stained with safranin) showing similar anatomical characters to the fossil. Shukla et al. (2012).

The Miombo tree, Brachystegia microphylla. Flora of Zimbabwe.

The next specimen described is assigned to the species Erythrophloeoxylon feistmanteli, and comes from the Plio-Pleistocene Shumar Formation at Hema Ki Dhani, near Habur in the Jaisalmer District of Rajasthan. Erythrophloeoxylon feistmanteli has bee previously described from Tamil Nadu (India). The wood of Erythrophloeoxylon feistmanteli is thought to resemble what of Erythrophleum densiflorum, a Leguminous African tree occurring in moist semi-deciduous forests, gallery forest and wooded grasslands.

Erythrophloeoxylon feistmantali. (A, B) Transverse section of the fossil wood showing vessel arrangement andparenchyma pattern.(C) Transverse section of the fossil showing marginal parenchyma (marked by an arrow). Shukla et al. (2012).

The next specimen described is assigned to the species Entandrophragminium aegyptiacum and comes from the Plio-Pleistocene Shumar Formation at Hema Ki Dhani, near Habur in the Jaisalmer District of Rajasthan. Six species of Entandrophragminium have been described to date, all from Africa and Europe. All are thought to resemble woods of the modern genus Entandrophragma, which contains 11 deciduous trees found in African rainforests and deciduous forests. Entandrophragminium aegyptiacum is though to most closely resemble the modern Entandrophragma angolense or Entandrophragma congoense.

Entandrophragminium aegyptiacum. (A, B) Transverse section of the fossil wood showing solitary and radial multiples of vessels andparenchyma bands (marked by arrows). Shukla et al. (2012).

The next specimen described is assigned to the modern genus Khaya and given the specific name palaeoindica, meaning ‘ancient Indian’. It comes from the Plio-Pleistocene Shumar Formation at Hema Ki Dhani, near Habur in the Jaisalmer District of Rajasthan. The genus Khaya includes seven extant species from tropical Africa and Madagascar, plus a fossil species from the Pliocene of Kenya. The wood of Khaya palaeoindica most closely resembles that of Khaya senegalensis, the African Mahogany, a deciduous evergreen tree reaching 15-30 m in height, which is found in riverine forests and high rainfall savannah woodlands.

Khaya palaeoindica. (A, B) Transverse sectionof fossil wood showing marginal parenchyma (marked by arrows),vessel arrangement and vasicentric parenchyma. Shukla et al. (2012).

The African Mahogany, Khaya senegalensis. Wikimedia Commons

The final specimen is assigned to a new species and genus as Milicioxylon kachchhensis, where ‘Milicioxylon’ means ‘Milicia-wood’ and ‘kachchhensis’ means ‘from Kachchh’. The specimen comes from the Pliocene Kankawati Series at Dhaneti in the Kachchh District of Gujarat. The wood of Milicioxylon kachchhensis resembles that of the modern Mvele tree, Milicia excelsa, an evergreen or deciduous tree reaching 35-60 m in height, which is found throughout tropical Africa.

(A) Transverse section of the fossil wood of Milicioxylon kachchhensis,showing tylosed vesselsand paratracehal banded parenchyma. (B) Transverse section of the modern wood of Milicia excelsa (stained with safranin) showing similar vessels and parenchyma to the fossil. Shukla et al. (2012).

The Mvule tree, Milicia excelsa. Wikimedia Commons.

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