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).
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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 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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